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Faraday shields and radiation and misinterpretations
I have been reading the groups archives on shield antennas and Faraday
shields and the different auguments regarding how shielding or the Faraday shield works. Frankly it is a total mess and should be removed so that hams are not mislead. Shielding is very simple. A particle with a electromagnetic field strikes the outside of the shield. The magnetic field of same passes thru the shield some might say it is coupled to the inside of the shield. The magnetic vector component is out of phase with the electrical field so it will be just a static particle at rest on the inside but no inline with the electrical field vector which is now a staic particle at rest on the outside We now have a arbitrary boundary as discused by Gauss For equilibrium all vectors impinging on the boundary must be aligned such that they cancel. To accomplish this the inner vector or charge MUST move sideways THE CHARGE WHEN ACCELERATED CREATES A TIME VARYING CURRENT ALONE WHILE THE OTHER FIELD VECTORS CANCEL OUT ( I believe that this was the object intended in the cross field antenna) As with a applied varying current leaves a xmitter to create radiation, so must the receiver obtain a time varying current. Maxwells equations show equations with the electric field, the magnetic field and a time varying current. When you have a electrical field or vector of a static particle at rest outside the boundary opposing the static vector on the inside of the boundary you have nothing left EXCEPT a time varying current in the closed circuit. For informative descriptions of how radiation occurs view the QRZ forum of ( antenna construction and design ) threads (3) on the double helix antenna ( see you there) Somebody some where should re write the above such that a definition is left for those who follow and remove the garbage which is now in place |
Faraday shields and radiation and misinterpretations
On Nov 30, 8:10*pm, Art Unwin wrote:
I have been reading the groups archives on shield antennas and Faraday shields and the different auguments regarding how shielding or the Faraday shield works. Frankly it is a total mess and should be removed so that hams are not mislead. Shielding is very simple. A particle with a electromagnetic field strikes the outside of the shield. The magnetic field of same passes thru the shield some might say it is coupled to the inside of the shield. The magnetic vector component is out of phase with the electrical field so it will be just a static particle at rest on the inside but no inline with the electrical field vector which is now a staic particle at rest on the outside We now have a arbitrary boundary as discused by Gauss For equilibrium all vectors impinging on the boundary must be aligned such that they cancel. To accomplish this the inner vector or charge MUST move sideways *THE CHARGE WHEN ACCELERATED *CREATES A TIME VARYING CURRENT ALONE WHILE THE OTHER FIELD VECTORS CANCEL OUT ( I believe that this was the object intended in *the cross field antenna) As with a applied varying current leaves a xmitter to create radiation, so must the receiver obtain a time varying current. Maxwells equations show equations with the electric field, the magnetic field and a time varying current. When you have a electrical field or vector of a static particle at rest outside the boundary opposing the static vector on the inside of the boundary you have nothing left EXCEPT a time varying current in the closed circuit. For informative descriptions of how radiation occurs view the QRZ forum of *( antenna construction and design ) threads (3) on the double helix antenna ( see you there) Somebody some where should re write the above such that a definition is left for those who follow and remove the garbage which is now in place hey there groups archivist, if there is such a thing... remove this post in accordance with his own request that such garbage be removed. |
Faraday shields and radiation and misinterpretations
Art Unwin wrote:
I have been reading the groups archives on shield antennas and Faraday shields and the different auguments regarding how shielding or the Faraday shield works. Frankly it is a total mess and should be removed so that hams are not mislead. Shielding is very simple. A particle with a electromagnetic field strikes the outside of the shield. The magnetic field of same passes thru the shield some might say it is coupled to the inside of the shield. The magnetic vector component is out of phase with the electrical field so it will be just a static particle at rest on the inside but no inline with the electrical field vector which is now a staic particle at rest on the outside We now have a arbitrary boundary as discused by Gauss For equilibrium all vectors impinging on the boundary must be aligned such that they cancel. To accomplish this the inner vector or charge MUST move sideways snip crap, but left plenty WOW!!! Look everybody he's totally making up physics again! Art - get back on your meds, you're nuts again. tom K0TAR |
Faraday shields and radiation and misinterpretations
Art Unwin wrote:
I have been reading the groups archives on shield antennas and Faraday shields and the different auguments regarding how shielding or the Faraday shield works. Frankly it is a total mess and should be removed so that hams are not mislead. Shielding is very simple. A particle with a electromagnetic field strikes the outside of the shield. The magnetic field of same passes thru the shield some might say it is coupled to the inside of the shield. The magnetic vector component is out of phase with the electrical field so it will be just a static particle at rest on the inside but no inline with the electrical field vector which is now a staic particle at rest on the outside We now have a arbitrary boundary as discused by Gauss For equilibrium all vectors impinging on the boundary must be aligned such that they cancel. To accomplish this the inner vector or charge MUST move sideways THE CHARGE WHEN ACCELERATED CREATES A TIME VARYING CURRENT ALONE WHILE THE OTHER FIELD VECTORS CANCEL OUT ( I believe that this was the object intended in the cross field antenna) As with a applied varying current leaves a xmitter to create radiation, so must the receiver obtain a time varying current. Maxwells equations show equations with the electric field, the magnetic field and a time varying current. When you have a electrical field or vector of a static particle at rest outside the boundary opposing the static vector on the inside of the boundary you have nothing left EXCEPT a time varying current in the closed circuit. For informative descriptions of how radiation occurs view the QRZ forum of ( antenna construction and design ) threads (3) on the double helix antenna ( see you there) Somebody some where should re write the above such that a definition is left for those who follow and remove the garbage which is now in place TROLL! |
Faraday shields and radiation and misinterpretations
orfus wrote:
Art Unwin wrote: I have been reading the groups archives on shield antennas and Faraday shields and the different auguments regarding how shielding or the Faraday shield works. Frankly it is a total mess and should be removed so that hams are not mislead. Shielding is very simple. A particle with a electromagnetic field strikes the outside of the shield. The magnetic field of same passes thru the shield some might say it is coupled to the inside of the shield. The magnetic vector component is out of phase with the electrical field so it will be just a static particle at rest on the inside but no inline with the electrical field vector which is now a staic particle at rest on the outside We now have a arbitrary boundary as discused by Gauss For equilibrium all vectors impinging on the boundary must be aligned such that they cancel. To accomplish this the inner vector or charge MUST move sideways THE CHARGE WHEN ACCELERATED CREATES A TIME VARYING CURRENT ALONE WHILE THE OTHER FIELD VECTORS CANCEL OUT ( I believe that this was the object intended in the cross field antenna) As with a applied varying current leaves a xmitter to create radiation, so must the receiver obtain a time varying current. Maxwells equations show equations with the electric field, the magnetic field and a time varying current. When you have a electrical field or vector of a static particle at rest outside the boundary opposing the static vector on the inside of the boundary you have nothing left EXCEPT a time varying current in the closed circuit. For informative descriptions of how radiation occurs view the QRZ forum of ( antenna construction and design ) threads (3) on the double helix antenna ( see you there) Somebody some where should re write the above such that a definition is left for those who follow and remove the garbage which is now in place TROLL! Nope. Local loony. You, however, are a troll until proven otherwise. tom K0TAR |
Faraday shields and radiation and misinterpretations
tom wrote in
. net: orfus wrote: Art Unwin wrote: I have been reading the groups archives on shield antennas and Faraday shields and the different auguments regarding how shielding or the Faraday shield works. Frankly it is a total mess and should be removed so that hams are not mislead. Shielding is very simple. A particle with a electromagnetic field strikes the outside of the shield. The magnetic field of same passes thru the shield some might say it is coupled to the inside of the shield. The magnetic vector component is out of phase with the electrical field so it will be just a static particle at rest on the inside but no inline with the electrical field vector which is now a staic particle at rest on the outside We now have a arbitrary boundary as discused by Gauss For equilibrium all vectors impinging on the boundary must be aligned such that they cancel. To accomplish this the inner vector or charge MUST move sideways THE CHARGE WHEN ACCELERATED CREATES A TIME VARYING CURRENT ALONE WHILE THE OTHER FIELD VECTORS CANCEL OUT ( I believe that this was the object intended in the cross field antenna) As with a applied varying current leaves a xmitter to create radiation, so must the receiver obtain a time varying current. Maxwells equations show equations with the electric field, the magnetic field and a time varying current. When you have a electrical field or vector of a static particle at rest outside the boundary opposing the static vector on the inside of the boundary you have nothing left EXCEPT a time varying current in the closed circuit. For informative descriptions of how radiation occurs view the QRZ forum of ( antenna construction and design ) threads (3) on the double helix antenna ( see you there) Somebody some where should re write the above such that a definition is left for those who follow and remove the garbage which is now in place TROLL! Nope. Local loony. You, however, are a troll until proven otherwise. tom K0TAR Ok, at the risk of stirring muddy water, I'm curious now, I'm new to this group, and the subject as there clearly seems to be more to it than I knew. I also don't know of those archives mentioned so I haven't seen the context. So in simple terms (hopefully) what is the truth of it? As far as I knew, a photon at RF with energy but no mass will produce a current that changes over time in a metal that it hits, though I imagine that as metal has resistance there must also be a voltage too. I've also heard of the 'skin effect' that means that at high RF frequencies, current flow tends to stay on the surface, so clearly the picture isn't as simple as DC and Ohm's law. I also know that when photons in optical fibres meet boundaries between layers they don't reflect simply on one side, within one region of specific refractive index, there's apparently some more complex information exchange that amounts to the photon crossing the border before returning. Which makes me suspect that equally exotic action happens when RF photons hit metal sheilds. So what IS correct? And even if there is more to it, does the aggregate of many photons, and the wave analysis of their behaviour, reduce to a simple model that makes the OP correct? I'm asking this because calls of 'troll' and 'loony' aren't working for me. |
Faraday shields and radiation and misinterpretations
On Nov 30, 9:05*pm, Lostgallifreyan wrote:
tom wrote e.net: orfus wrote: Art Unwin wrote: I have been reading the groups archives on shield antennas and Faraday shields and the different auguments regarding how shielding or the Faraday shield works. Frankly it is a total mess and should be removed so that hams are not mislead. Shielding is very simple. A particle with a electromagnetic field strikes the outside of the shield. The magnetic field of same passes thru the shield some might say it is coupled to the inside of the shield. The magnetic vector component is out of phase with the electrical field so it will be just a static particle at rest on the inside but no inline with the electrical field vector which is now a staic particle at rest on the outside We now have a arbitrary boundary as discused by Gauss For equilibrium all vectors impinging on the boundary must be aligned such that they cancel. To accomplish this the inner vector or charge MUST move sideways *THE CHARGE WHEN ACCELERATED *CREATES A TIME VARYING CURRENT ALONE WHILE THE OTHER FIELD VECTORS CANCEL OUT ( I believe that this was the object intended in *the cross field antenna) As with a applied varying current leaves a xmitter to create radiation, so must the receiver obtain a time varying current. Maxwells equations show equations with the electric field, the magnetic field and a time varying current. When you have a electrical field or vector of a static particle at rest outside the boundary opposing the static vector on the inside of the boundary you have nothing left EXCEPT a time varying current in the closed circuit. For informative descriptions of how radiation occurs view the QRZ forum of *( antenna construction and design ) threads (3) on the double helix antenna ( see you there) Somebody some where should re write the above such that a definition is left for those who follow and remove the garbage which is now in place TROLL! Nope. Local loony. You, however, are a troll until proven otherwise. tom K0TAR Ok, at the risk of stirring muddy water, I'm curious now, I'm new to this group, and the subject as there clearly seems to be more to it than I knew. I also don't know of those archives mentioned so I haven't seen the context.. So in simple terms (hopefully) what is the truth of it? As far as I knew, a photon at RF with energy but no mass will produce a current that changes over time in a metal that it hits, though I imagine that as metal has resistance there must also be a voltage too. I've also heard of the 'skin effect' that means that at high RF frequencies, current flow tends to stay on the surface, so clearly the picture isn't as simple as DC and Ohm's law. I also know that when photons in optical fibres meet boundaries between layers they don't reflect simply on one side, within one region of specific refractive index, there's apparently some more complex information exchange that amounts to the photon crossing the border before returning. Which makes me suspect that equally exotic action happens when RF photons hit metal sheilds. So what IS correct? And even if there is more to it, does the aggregate of many photons, and the wave analysis of their behaviour, reduce to a simple model that makes the OP correct? I'm asking this because calls of 'troll' and 'loony' aren't working for me. It's fairly straightforward, actually, if you believe in Faraday's law of magnetic induction. That law says that for any closed loop (through air, through a conductor, through anything), there is an electromotive force (a voltage source, if you will) whose magnitude is proportional to the rate of change of magnetic flux enclosed by the loop. As there is no voltage drop along a perfect conductor, if your closed loop follows the path of a perfect conductor, there is no voltage drop around that loop, and therefore the rate of change of the total magnetic flux enclosed by that loop must be zero. If the perfect conductor is a closed box, then you can draw loops anywhere through that conductor and you will never see a changing magnetic field enclosed by that loop. Thus, the inside of the box and the outside are magnetically independent; things happening on one side (magnetically) are not sensed on the other side. You can understand how this works if you realize that a changing magnetic field outside the box that would penetrate the box if it weren't there will induce currents in the conducting box (or even just in a closed loop of wire). Those currents will (in a perfect conductor) be exactly the right magnitude to cause a magnetic field that cancels the external one everywhere inside the closed box (or the net flux enclosed by a loop of wire). An example: if you short the secondary of a mains transformer, the primary will draw lots of current at its rated voltage: it's very difficult for the primary to change the magnetic flux in the core. Does the electric field shielding from a perfect conductor need any explanation? Of course, an imperfect conductor will be an imperfect magnetic shield. But a perfect conductor won't let any change of field through, no matter how slow (no matter how low an EMF it generates), so a perfect conductor works as a shield all the way down to DC. A box made with an imperfect conductor is essentially a perfect shield if the box's wall thickness is at least many skin-depths thick at the frequency of interest. That's a quick beginning. You can find lots more about this in E&M texts. There's even useful stuff about it on the web. ;-) Cheers, Tom |
Faraday shields and radiation and misinterpretations
K7ITM wrote:
I'm asking this because calls of 'troll' and 'loony' aren't working for me. It's fairly straightforward, actually, if you believe in Faraday's law of magnetic induction. That law says that for any closed loop (through air, through a conductor, through anything), there is an electromotive force (a voltage source, if you will) whose magnitude is proportional to the rate of change of magnetic flux enclosed by the loop. As there is no voltage drop along a perfect conductor, if your closed loop follows the path of a perfect conductor, there is no voltage drop around that loop, and therefore the rate of change of the total magnetic flux enclosed by that loop must be zero. If the perfect conductor is a closed box, then you can draw loops anywhere through that conductor and you will never see a changing magnetic field enclosed by that loop. Thus, the inside of the box and the outside are magnetically independent; things happening on one side (magnetically) are not sensed on the other side. You can understand how this works if you realize that a changing magnetic field outside the box that would penetrate the box if it weren't there will induce currents in the conducting box (or even just in a closed loop of wire). Those currents will (in a perfect conductor) be exactly the right magnitude to cause a magnetic field that cancels the external one everywhere inside the closed box (or the net flux enclosed by a loop of wire). An example: if you short the secondary of a mains transformer, the primary will draw lots of current at its rated voltage: it's very difficult for the primary to change the magnetic flux in the core. Does the electric field shielding from a perfect conductor need any explanation? Of course, an imperfect conductor will be an imperfect magnetic shield. But a perfect conductor won't let any change of field through, no matter how slow (no matter how low an EMF it generates), so a perfect conductor works as a shield all the way down to DC. A box made with an imperfect conductor is essentially a perfect shield if the box's wall thickness is at least many skin-depths thick at the frequency of interest. That's a quick beginning. You can find lots more about this in E&M texts. There's even useful stuff about it on the web. ;-) Here is a link to a generalized proof of the skin effect: http://www.ifwtech.co.uk/g3sek/misc/skin.htm This is exactly equivalent to Tom's explanation above. The detailed proof is quite mathematical but it is solidly based in classical physics - Faraday's Law and Ampere's theorem (both of which are embodied in Maxwell's equations). This derivation produces the well-known equations for current density as a function of depth, conductivity and permeability. The special feature of this particular proof is that it's much more general than the ones you see in better-known textbooks - and therefore much more powerful. It shows that if RF current is flowing in/on *any* conducting surface, for *any* reason, then the skin effect will be present. The possible reasons why RF current may be flowing can be divided into two main groups: * "Circuit conditions" - the conductor is part of a circuit that makes RF current flow. * "Electromagnetic induction" - the conductor is intercepting an incident electromagnetic wave which induces a current. In either case, an RF current flows... and wherever that happens, there you will also find the skin effect. -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek |
Faraday shields and radiation and misinterpretations
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Faraday shields and radiation and misinterpretations
Ian White GM3SEK wrote in
: http://www.ifwtech.co.uk/g3sek/misc/skin.htm This is exactly equivalent to Tom's explanation above. The detailed proof is quite mathematical but it is solidly based in classical physics - Faraday's Law and Ampere's theorem (both of which are embodied in Maxwell's equations). This derivation produces the well-known equations for current density as a function of depth, conductivity and permeability. Thanks, that linking of laws and theories will help me (and the confirmation that classical physics will be enough to describe it, as I hoped). The OP mentioned Maxwell too... so did he make some error I have yet to grasp? Other than taking pot shots at an establishment, that is... :) The special feature of this particular proof is that it's much more general than the ones you see in better-known textbooks - and therefore much more powerful. It shows that if RF current is flowing in/on *any* conducting surface, for *any* reason, then the skin effect will be present. That appeals to me. I think the more something can be seen to apply generally, the more it helps. Proportion can't be gauged with a model that denies it. |
Faraday shields and radiation and misinterpretations
Lostgallifreyan wrote:
Conversely, I found some nice coax in a skip once that had two heavy braids amounting to almost complete coverage around a single fine stranded core. (Found outside a telephone exchange, but I don't know what frequency they were intended for, though I used some for an outdoor VHF receiving quarter wave dipole with good results, and I suspect it will do for a SW longwire once I get a matching transformer for it). If it was a UK (BT) telephone exchange, then it probably was "Cable coaxial 2003". Used for critical video and general HF use. I don't know what it's officially spec'd to, but it would work well up to several hundred MHz. Characteristic impedance of 75ohms, and easily capable of 100W into a decent match. |
Faraday shields and radiation and misinterpretations
Gaius wrote in
: Lostgallifreyan wrote: Conversely, I found some nice coax in a skip once that had two heavy braids amounting to almost complete coverage around a single fine stranded core. (Found outside a telephone exchange, but I don't know what frequency they were intended for, though I used some for an outdoor VHF receiving quarter wave dipole with good results, and I suspect it will do for a SW longwire once I get a matching transformer for it). If it was a UK (BT) telephone exchange, then it probably was "Cable coaxial 2003". Used for critical video and general HF use. I don't know what it's officially spec'd to, but it would work well up to several hundred MHz. Characteristic impedance of 75ohms, and easily capable of 100W into a decent match. Sounds like the same stuff, though I have 2002 on mine (which in absence of other markings was cryptic enough that I was unsure of it, though I think it might have been earlier than 2002 when I found it. :) I thought it might be 75 ohm but I had no idea it might efficiently carry high power. But I knew it was well over-spec'd for the uses I put it to. I was lucky to find it. The staff there were happy enough for me to raid the skip, too... Should try it again sometime. That stuff seems to last forever even outside in strong daily temperature changes and direct sunlight. |
Faraday shields and radiation and misinterpretations
Lostgallifreyan wrote in
: Gaius wrote in : Lostgallifreyan wrote: Conversely, I found some nice coax in a skip once that had two heavy braids amounting to almost complete coverage around a single fine stranded core. (Found outside a telephone exchange, but I don't know what frequency they were intended for, though I used some for an outdoor VHF receiving quarter wave dipole with good results, and I suspect it will do for a SW longwire once I get a matching transformer for it). If it was a UK (BT) telephone exchange, then it probably was "Cable coaxial 2003". Used for critical video and general HF use. I don't know what it's officially spec'd to, but it would work well up to several hundred MHz. Characteristic impedance of 75ohms, and easily capable of 100W into a decent match. Sounds like the same stuff, though I have 2002 on mine (which in absence of other markings was cryptic enough that I was unsure of it, though I think it might have been earlier than 2002 when I found it. :) I thought it might be 75 ohm but I had no idea it might efficiently carry high power. But I knew it was well over-spec'd for the uses I put it to. I was lucky to find it. The staff there were happy enough for me to raid the skip, too... Should try it again sometime. That stuff seems to last forever even outside in strong daily temperature changes and direct sunlight. I forgot to mention that I also used some for a pair of DIY scope leads for a 100 MHz scope, and they worked right even without the little capacitative adjuster usually supplied on properly made probes. A bit clumsy, but a nice find all the same. |
Faraday shields and radiation and misinterpretations
Lostgallifreyan wrote:
Sounds like the same stuff, though I have 2002 on mine (which in absence of other markings was cryptic enough that I was unsure of it, though I think it might have been earlier than 2002 when I found it. :) I thought it might be 75 ohm but I had no idea it might efficiently carry high power. But I knew it was well over-spec'd for the uses I put it to. I was lucky to find it. The staff there were happy enough for me to raid the skip, too... Should try it again sometime. That stuff seems to last forever even outside in strong daily temperature changes and direct sunlight. If it has 2002 printed on it, that means it's "Cable coaxial 2002", which is one down the scale in loss terms from 2003. 2002 (nothing to do with the date!) is as good quality as 2003 (which is thicker), but the loss is a bit higher. The three usual types were AFAIR - 2001 - single screened, foam dielectric. Quite thin - used for short runs and jumpers. 2002 - General purpose, high quality. Solid dielectric. 2003 - Top quality (in loss terms). Solid dielectric. All are 75ohms - like pretty well all telecom coax. (50 ohm is usually only found in antenna feeders and traditional ethernet). The normal PVC jacket colour was "Light straw" (yellowish cream), but other colours were occasionally used for special purposes. |
Faraday shields and radiation and misinterpretations
Gaius wrote in
: Lostgallifreyan wrote: Sounds like the same stuff, though I have 2002 on mine (which in absence of other markings was cryptic enough that I was unsure of it, though I think it might have been earlier than 2002 when I found it. :) I thought it might be 75 ohm but I had no idea it might efficiently carry high power. But I knew it was well over-spec'd for the uses I put it to. I was lucky to find it. The staff there were happy enough for me to raid the skip, too... Should try it again sometime. That stuff seems to last forever even outside in strong daily temperature changes and direct sunlight. If it has 2002 printed on it, that means it's "Cable coaxial 2002", which is one down the scale in loss terms from 2003. 2002 (nothing to do with the date!) is as good quality as 2003 (which is thicker), but the loss is a bit higher. The three usual types were AFAIR - 2001 - single screened, foam dielectric. Quite thin - used for short runs and jumpers. 2002 - General purpose, high quality. Solid dielectric. 2003 - Top quality (in loss terms). Solid dielectric. All are 75ohms - like pretty well all telecom coax. (50 ohm is usually only found in antenna feeders and traditional ethernet). The normal PVC jacket colour was "Light straw" (yellowish cream), but other colours were occasionally used for special purposes. Thanks. That matches closely except the dielectric, which I think is foam (is certainly foamy or foamish). Colour is same too, though closer to white than yellow. As far as I know the impedance is purely based on the scale and geometry of the cross-section, and if so, I guess the central conductor of 2003 is also thicker. I think there were seven strands of very thin copper. If you or anyone reading this really wants to know I'll get a vernier gauge and find a bare cable end somewhere... External diameter is approx 5mm on the 2002 type. Do you know if it's only BT internal use? If I can buy it economically, I'd consider it. I like working with it, when I'm in the mood for picking apart cable braids. |
Faraday shields and radiation and misinterpretations
On Nov 30, 11:05*pm, Lostgallifreyan wrote:
tom wrote e.net: orfus wrote: Art Unwin wrote: I have been reading the groups archives on shield antennas and Faraday shields and the different auguments regarding how shielding or the Faraday shield works. Frankly it is a total mess and should be removed so that hams are not mislead. Shielding is very simple. A particle with a electromagnetic field strikes the outside of the shield. The magnetic field of same passes thru the shield some might say it is coupled to the inside of the shield. The magnetic vector component is out of phase with the electrical field so it will be just a static particle at rest on the inside but no inline with the electrical field vector which is now a staic particle at rest on the outside We now have a arbitrary boundary as discused by Gauss For equilibrium all vectors impinging on the boundary must be aligned such that they cancel. To accomplish this the inner vector or charge MUST move sideways *THE CHARGE WHEN ACCELERATED *CREATES A TIME VARYING CURRENT ALONE WHILE THE OTHER FIELD VECTORS CANCEL OUT ( I believe that this was the object intended in *the cross field antenna) As with a applied varying current leaves a xmitter to create radiation, so must the receiver obtain a time varying current. Maxwells equations show equations with the electric field, the magnetic field and a time varying current. When you have a electrical field or vector of a static particle at rest outside the boundary opposing the static vector on the inside of the boundary you have nothing left EXCEPT a time varying current in the closed circuit. For informative descriptions of how radiation occurs view the QRZ forum of *( antenna construction and design ) threads (3) on the double helix antenna ( see you there) Somebody some where should re write the above such that a definition is left for those who follow and remove the garbage which is now in place TROLL! Nope. Local loony. You, however, are a troll until proven otherwise. tom K0TAR Ok, at the risk of stirring muddy water, I'm curious now, I'm new to this group, and the subject as there clearly seems to be more to it than I knew. I also don't know of those archives mentioned so I haven't seen the context.. So in simple terms (hopefully) what is the truth of it? As far as I knew, a photon at RF with energy but no mass will produce a current that changes over time in a metal that it hits, though I imagine that as metal has resistance there must also be a voltage too. I've also heard of the 'skin effect' that means that at high RF frequencies, current flow tends to stay on the surface, so clearly the picture isn't as simple as DC and Ohm's law. I also know that when photons in optical fibres meet boundaries between layers they don't reflect simply on one side, within one region of specific refractive index, there's apparently some more complex information exchange that amounts to the photon crossing the border before returning. Which makes me suspect that equally exotic action happens when RF photons hit metal sheilds. So what IS correct? And even if there is more to it, does the aggregate of many photons, and the wave analysis of their behaviour, reduce to a simple model that makes the OP correct? I'm asking this because calls of 'troll' and 'loony' aren't working for me. If you go back to the arbitary boundary of the Gaussian law of statics and view it as a Faraday shield it all becomes quite simple. If one adds a time varying field you have the duplicate of Maxwells laws for radiation, where the outside of the boundary is the radiator. The Faraday shield supplies the transition from a static to a dynamic field for xmission and the reverse action for receiving. Very basic my dear Watson, and a vindication that particles and not waves create radiation which puts it in line with deductions when other methods are applied. |
Faraday shields and radiation and misinterpretations
Lostgallifreyan wrote:
Thanks. That matches closely except the dielectric, which I think is foam (is certainly foamy or foamish). Colour is same too, though closer to white than yellow. As far as I know the impedance is purely based on the scale and geometry of the cross-section, and if so, I guess the central conductor of 2003 is also thicker. I think there were seven strands of very thin copper. If you or anyone reading this really wants to know I'll get a vernier gauge and find a bare cable end somewhere... External diameter is approx 5mm on the 2002 type. Do you know if it's only BT internal use? If I can buy it economically, I'd consider it. I like working with it, when I'm in the mood for picking apart cable braids. You're right - my memory must be porous. 2002 has a FOAM dielectric. Also, 2003 has a single strand inner conductor (spec must have changed - used to be stranded). You can buy 2002 from RS - it's a BT spec, but available for anyone. Have a look at : http://uk.rs-online.com/web/search/s...ct&R=520306 8 It's only £58 for 100m, and the loss is reasonable at 3.61dB/100m @ 4MHz. (2003 cable is 2.33dB/100m @ 4MHz) |
Faraday shields and radiation and misinterpretations
Gaius wrote in
: Lostgallifreyan wrote: Thanks. That matches closely except the dielectric, which I think is foam (is certainly foamy or foamish). Colour is same too, though closer to white than yellow. As far as I know the impedance is purely based on the scale and geometry of the cross-section, and if so, I guess the central conductor of 2003 is also thicker. I think there were seven strands of very thin copper. If you or anyone reading this really wants to know I'll get a vernier gauge and find a bare cable end somewhere... External diameter is approx 5mm on the 2002 type. Do you know if it's only BT internal use? If I can buy it economically, I'd consider it. I like working with it, when I'm in the mood for picking apart cable braids. You're right - my memory must be porous. 2002 has a FOAM dielectric. Also, 2003 has a single strand inner conductor (spec must have changed - used to be stranded). You can buy 2002 from RS - it's a BT spec, but available for anyone. Have a look at : http://uk.rs-online.com/web/search/s...method=getProd uct&R=5203068 It's only £58 for 100m, and the loss is reasonable at 3.61dB/100m @ 4MHz. (2003 cable is 2.33dB/100m @ 4MHz) Nice. Given what RS are charging for RG59 with a double braid that appears similar, it looks very good. I don't know enough to choose between them though, especially given the huge variety of cables RS show for RG59 with costs varying more than tenfold per metre. From what I've seen of it I'd go for that BT cable at their price. (They add VAT though..) I guess BT's economies of large scale help this stuff to be cheaper than it otherwise would be. |
Faraday shields and radiation and misinterpretations
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Faraday shields and radiation and misinterpretations
On Tue, 01 Dec 2009 03:42:13 -0600, Lostgallifreyan
wrote: why is it often ok for a Faraday cage to have holes in it? :) Braided screens, meshes, perforated metal sheets, etc, I've seen many shields that are not a complete 'seal'... UHF TV cables especially seem to be very loosely shielded but they work. This can be explained at super high frequency and at DC as easily. However, before that it should be pointed out that the coverage (the ratio of what is conductor to what is not - the air space) defines how "good" the faraday shield will be. Not surprisingly, coverage is wavelength dependant. To cut to the chase, a wide mesh will allow increasingly higher frequencies (shorter waves) through. Now, as to the how. With a separation in the mesh, and for very large wavelength (in proportion to the opening size), you will have a very, very small potential difference across any of the mesh openings. Very little potential voltage across the mesh opening means very little current flow around the mesh opening that is specifically due to that potential difference. This is not to say there isn't a very, very large current flow by virtue of some very, very long wave. No, there's no denying that, but to get through the mesh you have to satisfy local conditions that demand what amounts to leakage (and this is exactly the term that correlates to coverage when discussing coax weave). If that huge current cannot induce a significant voltage across the mesh opening, then the mesh opening loop current cannot induce a field through to the other side. Now, if you examine the context of "huge current" in a resistive conductor, then obviously a potential difference can occur. Point is that reality (and science) allow for poor grade shields, but as a one knock-off proof you can summon up any failure, ignore simple contra-examples and create a new theory. However, returning to what is well known. If you increase the frequency applied to the mesh, then at some point wavelength will allow a situation where the general current flowing through the whole structure will naturally exhibit a potential difference across some small scale. By this point, abstraction may be wearying. Let's say you have a 10 meter-on-a-side cage with 1 meter mesh openings. If your applied field were exciting the cage at 75MHz (4M), then any spot on the cage could be at a very high potential difference from any spot adjacent and 1 meter away (a simple quarterwave relationship). This works for a solid conductor, it works for a mesh conductor. The 1 meter mesh openings can thus exhibit a substantial potential difference across the opening, and a local current loop associated with that potential difference. The mesh opening becomes a quarterwave radiator (aka slot antenna) and can couple energy from the external field into the interior of the cage (now possibly a resonant chamber, aka RF cavity). In practice and literature, the mesh opening loop exhibits a radiation resistance of 10s of Ohms. That compared to its mesh loop Ohmic path loss, makes it a very efficient coupler of energy. Take this very poor example of mesh, and lower the frequency to 750 KHz. The mesh opening - if we originally likened it to an antenna, we should be able to continue to do that - is now 1/400th Wave. A 1/400th wave radiator has extremely small radiation resistance. The exact value would be 751 nanoOhms. As we are examining a poor mesh, it becomes clear that it must have some resistance over that 1 meter distance (this is a real example, after all). Being generous and constructing that cage out of rebar will give us a path resistance of, luckily, 1 milliOhm. This figure and that of the radiation resistance yield the radiation efficiency (that is, how well the exterior RF will couple into the interior) which reduces to 0.075%. The cage works pretty well, but not perfectly (it was, after all, a poor example). Now, repeat this with a poorer conductor, or a tighter mesh and imagine the shielding effect. The mesh has an opening radius squared-squared relationship driving down the radiation resistance compared to the linear relationship of conductance. ************* Now, expanding the topic to allow for the contribution of ALL openings in the mesh, we must again return to the physical dimension compared to the wavelength dimension. If the cage is truly large, larger than the field exciting it, then you have miniscule radiators along it, each very inefficient. However, each of those radiators is out of phase with a distant neighbor (not so with its close mesh neighbors). Those two wavelength distant mesh radiators will combine somewhere in the interior space and build a field. This is very commonly found in inter-cable cross coupling through leakage that is exhibited in very long cable trays with tightly bound lines. This doesn't improve the efficiency, but sensitive circuits running parallel to power drives can prove to be a poor combination. What to do when conditions condemn the small signal coax to live in proximity to the large signal supply? This introduces the foil shield. The foil shield is a very poor conductor over any significant length, but over the span between mesh openings (e.g. coax shield weave), the resistance is sufficiently low to close the conductance gap. 73's Richard Clark, KB7QHC |
Faraday shields and radiation and misinterpretations
On Dec 1, 11:25*am, Lostgallifreyan wrote:
Art Unwin wrote in news:15904250-69bb-4aba-8a3f- : If you go back to the arbitary boundary of the Gaussian law of statics and view it as a Faraday shield it all becomes quite simple. If one adds a time varying field you have the duplicate of Maxwells laws for radiation, *where the outside of the boundary is the radiator. The Faraday shield supplies the transition from a static to a dynamic field for xmission and the reverse action *for receiving. Very basic my dear Watson, and a vindication that particles and not waves create radiation which puts it in line with deductions when other methods are applied. Doesn't look basic, and I suspect it never will to me. The only thing I can get from this is the idea that a particle model will do what the wave one does, which isn't surprising but I've been told that particle based models are usually best left to situations (usually atomic scale quantum mechanical) where the wave model won't do, and I've never seen anyone suggest that wave-based theories of electromagnetics were inadequate (or inefficient) for scales involving obviously large numbers of particles. The other explanations seemed to grip, but not this one. I'll leave well alone now, but if anyone else takes up the discussion, I'll read it and only comment if I can't stop myself.. Well I didn't tell all in the first place because so much untruths are buried in people"s mind. When the charge or particle hits the outside of the shield both the electric and magnetic fields dissapate leaving just the static particle adheared to the outside. Ofcourse non bound particles in the air are immediatly attracted to the inside of the shield and move along the inside of the shield to align themselves with the outside static particles for equilibrium. Now for the important stuff that will upset hams. The internal particle moves to align itself with the outside particle. By moving it generates a time varying current such that the electric and magnetic fields that disapated on the outside are now REGENERATED on the inside. Most people see or think that the outside magnetic field can pierce the shield, which is why the name magnetic loop came about. Fields do NOT penetrate a Faraday shield. A electromagnetic shield is regenerated by the newly formed internal current which then closes the circuit. To put this with the original explanation would be to much for hams to digest so it is best to split it into two parts. |
Faraday shields and radiation and misinterpretations
"Ian White GM3SEK" wrote in message ... K7ITM wrote: I'm asking this because calls of 'troll' and 'loony' aren't working for me. - snip - Here is a link to a generalized proof of the skin effect: http://www.ifwtech.co.uk/g3sek/misc/skin.htm This is exactly equivalent to Tom's explanation above. The detailed proof is quite mathematical but it is solidly based in classical physics - further snip - -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek Your statement on your web page 'It is temporarily reproduced here, under provisions of the Berne Copyright Convention, to support technical discussions on the rec.radio.amateur.antenna newsgroup' looks interesting because, to the best of my knowledge, the issue of the legality of copying parts of others' published work onto Web sites hasn't been resolved. I can't find any specific provision in the Berne Convention that _allows_ re-publishing on the Internet - it looks more likely to inhibit it because Web sites are automatically worldwide. On the page 'http://www.copyrightservice.co.uk/copyright/p09_fair_use' it is stated: 'Under fair use rules, it may be possible to use quotations or excerpts, where the work has been made available to the public, (i.e. published). Provided that: a.. The use is deemed acceptable under the terms of fair dealing. b.. That the quoted material is justified, and no more than is necessary is included. c.. That the source of the quoted material is mentioned, along with the name of the author.' But also: 'The actual specifics of what is acceptable will be governed by national laws, and although broadly similar, actual provision will vary from country to country.' Presumably you have researched this matter, and I for one would be interested to hear what you've found that appears to cover international www re-publishing. Chris |
Faraday shields and radiation and misinterpretations
On Dec 1, 9:25*am, Lostgallifreyan wrote:
Art Unwin wrote in news:15904250-69bb-4aba-8a3f- : If you go back to the arbitary boundary of the Gaussian law of statics and view it as a Faraday shield it all becomes quite simple. If one adds a time varying field you have the duplicate of Maxwells laws for radiation, *where the outside of the boundary is the radiator. The Faraday shield supplies the transition from a static to a dynamic field for xmission and the reverse action *for receiving. Very basic my dear Watson, and a vindication that particles and not waves create radiation which puts it in line with deductions when other methods are applied. Doesn't look basic, and I suspect it never will to me. The only thing I can get from this is the idea that a particle model will do what the wave one does, which isn't surprising but I've been told that particle based models are usually best left to situations (usually atomic scale quantum mechanical) where the wave model won't do, and I've never seen anyone suggest that wave-based theories of electromagnetics were inadequate (or inefficient) for scales involving obviously large numbers of particles. The other explanations seemed to grip, but not this one. I'll leave well alone now, but if anyone else takes up the discussion, I'll read it and only comment if I can't stop myself.. Yep, that's about right. In fact, my advice if you do get into that situation (where quantization of energy is important), is to NOT think of particles or waves, but realize that quanta of electromagnetic radiation behave exactly as they behave, which is neither exactly like waves nor exactly like particles. One of Richard Feynman's physics lectures covered what I think is a lovely example of this: how you can NOT explain the results of the experiment he sets up, using EITHER wave OR particle behaviour. I highly recommend it, to arm yourself against people who get into the particle-vs-wave battle. I believe it's the sixth of what has been published as Feynman's "Six Easy Pieces." Cheers, Tom |
Faraday shields and radiation and misinterpretations
Lostgallifreyan wrote:
I've been told that particle based models are usually best left to situations (usually atomic scale quantum mechanical) where the wave model won't do, and I've never seen anyone suggest that wave-based theories of electromagnetics were inadequate (or inefficient) for scales involving obviously large numbers of particles. Consider that man's most ancient exposure to waves was sea/ocean waves which, incidentally, consist of H2O molecule particles. Seems to me that everything that physically exists must exist as a particle. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
Faraday shields and radiation and misinterpretations
Lostgallifreyan wrote:
I've been told that particle based models are usually best left to situations (usually atomic scale quantum mechanical) where the wave model won't do, and I've never seen anyone suggest that wave-based theories of electromagnetics were inadequate (or inefficient) for scales involving obviously large numbers of particles. Consider that man's most ancient exposure to waves was sea/ocean waves which, incidentally, consist of H2O molecule particles. Seems to me that everything that physically exists must exist as a particle. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
Faraday shields and radiation and misinterpretations
On Dec 1, 3:42*am, Lostgallifreyan wrote:
K7ITM wrote in news:c52a1b1d-ef32-4d69-bf61- : It's fairly straightforward, actually, if you believe in Faraday's law of magnetic induction. *That law says that for any closed loop (through air, through a conductor, through anything), there is an electromotive force (a voltage source, if you will) whose magnitude is proportional to the rate of change of magnetic flux enclosed by the loop. *As there is no voltage drop along a perfect conductor, if your closed loop follows the path of a perfect conductor, there is no voltage drop around that loop, and therefore the rate of change of the total magnetic flux enclosed by that loop must be zero. *If the perfect conductor is a closed box, then you can draw loops anywhere through that conductor and you will never see a changing magnetic field enclosed by that loop. *Thus, the inside of the box and the outside are magnetically independent; things happening on one side (magnetically) are not sensed on the other side. You can understand how this works if you realize that a changing magnetic field outside the box that would penetrate the box if it weren't there will induce currents in the conducting box (or even just in a closed loop of wire). *Those currents will (in a perfect conductor) be exactly the right magnitude to cause a magnetic field that cancels the external one everywhere inside the closed box (or the net flux enclosed by a loop of wire). *An example: *if you short the secondary of a mains transformer, the primary will draw lots of current at its rated voltage: *it's very difficult for the primary to change the magnetic flux in the core. Does the electric field shielding from a perfect conductor need any explanation? Of course, an imperfect conductor will be an imperfect magnetic shield. *But a perfect conductor won't let any change of field through, no matter how slow (no matter how low an EMF it generates), so a perfect conductor works as a shield all the way down to DC. *A box made with an imperfect conductor is essentially a perfect shield if the box's wall thickness is at least many skin-depths thick at the frequency of interest. That's a quick beginning. *You can find lots more about this in E&M texts. *There's even useful stuff about it on the web. *;-) Cheers, Tom Thanks, that helps, especially the paragraph about creating a magnetic field in response that tends to cancel the original one, and the thickness of metal with regard to frequency. The OP (Art Unwin) mentioned cancellation in more complex terms, so I'm still not clear if this validates what he said or not. It appears to but he mentions stuff I'm not likely to grasp in just an hour or two of effort.. What I'm getting at is that I'm not sure if his calling orthodoxy into question is all that drew the flak, or if there's something obviously wrong in his post that I'm missing. When you feed a time varying current to the mesh it is best to view it in small parts, say a square in the mesh. The hole is a static field alongside the applied current flows. This same current generates a displacement current which encircles the static field as it returns to the initial current flow. Of course this section is a microcosm of the flow pattern of the applied varying current which is continually flowing. The initial current flow generates a field at right angles to its axis. This field thus bisects the enclosed static field and accellerates a particle thru this intersection in the same way a particle is accelerated in a cathode ray tube. The particle that was accellerated, by the way, came from the surface of the conducting wire which is diamagnetic upon which particles or free electrons rest without being absorbed into the matrix of the material upon which it rests.The speed that the charge or particle attains is that of the speed of light. So when Einstein gave up his search regarding the standard model it seems rather natural that he came up with E=mc sqd as it was obvious to him that light itself was generated by the same particle or free electron that occupied his mind for so long and not of waves that appeared to persist in the minds of physicists to this very day. Hope that helps you out Regards Art Also (though I'll likely find out about this when I look deeper), why is it often ok for a Faraday cage to have holes in it? :) Braided screens, meshes, perforated metal sheets, etc, I've seen many shields that are not a complete 'seal'... UHF TV cables especially seem to be very loosely shielded but they work. Conversely, I found some nice coax in a skip once that had two heavy braids amounting to almost complete coverage around a single fine stranded core. (Found outside a telephone exchange, but I don't know what frequency they were intended for, though I used some for an outdoor VHF receiving quarter wave dipole with good results, and I suspect it will do for a SW longwire once I get a matching transformer for it). |
Faraday shields and radiation and misinterpretations
On Tue, 1 Dec 2009 13:06:30 -0800 (PST), Art Unwin
wrote: When you feed a time varying current to the mesh it is best to view it in small parts, say a square in the mesh. The hole is a static field alongside the applied current flows. This same current generates a displacement current which encircles the static field as it returns to the initial current flow. Of course this section is a microcosm of the flow pattern of the applied varying current which is continually flowing. Is this true of a discone? I'm under the impression the current flow is identical whether metal rods or wire mesh is used in the antenna's construction. |
Faraday shields and radiation and misinterpretations
On Dec 1, 3:22*pm, Registered User wrote:
On Tue, 1 Dec 2009 13:06:30 -0800 (PST), Art Unwin wrote: When you feed a time varying current to the mesh it is best to view it in small parts, say a square in the mesh. The hole is a static field alongside the applied current flows. This same current generates a displacement current *which encircles the static field as it returns to the initial current flow. Of course this section is a microcosm of the flow pattern of the applied varying current which is continually flowing. Is this true of a discone? I'm under the impression the current flow is identical whether metal rods or wire mesh is used in the antenna's construction. I am under the understanding that for a Faraday shield it doesn't matter whether it is a mesh or solid. When the displacement current flows in terms of an eddy current it produces a vortice which holds the static field Dinner has arrived Art |
Faraday shields and radiation and misinterpretations
Lostgallifreyan wrote:
Doesn't look basic, and I suspect it never will to me. The only thing I can get from this is the idea that a particle model will do what the wave one does, which isn't surprising but I've been told that particle based models are usually best left to situations (usually atomic scale quantum mechanical) where the wave model won't do, and I've never seen anyone suggest that wave-based theories of electromagnetics were inadequate (or inefficient) for scales involving obviously large numbers of particles. The other explanations seemed to grip, but not this one. I'll leave well alone now, but if anyone else takes up the discussion, I'll read it and only comment if I can't stop myself.. It's not basic, and it's not real. Art has made up a whole new wing of physics that has only the slightest ties to reality. It involves neutrinos leaping from diamagnetic materials to radiate. And only diamagnetic materials can radiate, unless he revised his theories, which he does regularly. And there are NO waves, just particles And antennas don't work properly unless they are a multiple of a wavelength, but it's OK to roll all that wire up in a ball so that a 160m antenna fits in a shoebox. And then you can use that with a teeny Dish network dish for directionality. Despite the fact that those dishes won't work reasonably at anything less than low GHz frequencies. He is, to put it very plainly, nuts. tom K0TAR |
Faraday shields and radiation and misinterpretations
Richard Clark wrote:
On Tue, 01 Dec 2009 03:42:13 -0600, Lostgallifreyan wrote: why is it often ok for a Faraday cage to have holes in it? :) Braided screens, meshes, perforated metal sheets, etc, I've seen many shields that are not a complete 'seal'... UHF TV cables especially seem to be very loosely shielded but they work. This can be explained at super high frequency and at DC as easily. However, before that it should be pointed out that the coverage (the ratio of what is conductor to what is not - the air space) defines how "good" the faraday shield will be. Not surprisingly, coverage is wavelength dependant. To cut to the chase, a wide mesh will allow increasingly higher frequencies (shorter waves) through. Now, as to the how. With a separation in the mesh, and for very large wavelength (in proportion to the opening size), you will have a very, very small potential difference across any of the mesh openings. Very little potential voltage across the mesh opening means very little current flow around the mesh opening that is specifically due to that potential difference. This is not to say there isn't a very, very large current flow by virtue of some very, very long wave. No, there's no denying that, but to get through the mesh you have to satisfy local conditions that demand what amounts to leakage (and this is exactly the term that correlates to coverage when discussing coax weave). If that huge current cannot induce a significant voltage across the mesh opening, then the mesh opening loop current cannot induce a field through to the other side. Now, if you examine the context of "huge current" in a resistive conductor, then obviously a potential difference can occur. Point is that reality (and science) allow for poor grade shields, but as a one knock-off proof you can summon up any failure, ignore simple contra-examples and create a new theory. However, returning to what is well known. If you increase the frequency applied to the mesh, then at some point wavelength will allow a situation where the general current flowing through the whole structure will naturally exhibit a potential difference across some small scale. By this point, abstraction may be wearying. Let's say you have a 10 meter-on-a-side cage with 1 meter mesh openings. If your applied field were exciting the cage at 75MHz (4M), then any spot on the cage could be at a very high potential difference from any spot adjacent and 1 meter away (a simple quarterwave relationship). This works for a solid conductor, it works for a mesh conductor. The 1 meter mesh openings can thus exhibit a substantial potential difference across the opening, and a local current loop associated with that potential difference. The mesh opening becomes a quarterwave radiator (aka slot antenna) and can couple energy from the external field into the interior of the cage (now possibly a resonant chamber, aka RF cavity). In practice and literature, the mesh opening loop exhibits a radiation resistance of 10s of Ohms. That compared to its mesh loop Ohmic path loss, makes it a very efficient coupler of energy. Take this very poor example of mesh, and lower the frequency to 750 KHz. The mesh opening - if we originally likened it to an antenna, we should be able to continue to do that - is now 1/400th Wave. A 1/400th wave radiator has extremely small radiation resistance. The exact value would be 751 nanoOhms. As we are examining a poor mesh, it becomes clear that it must have some resistance over that 1 meter distance (this is a real example, after all). Being generous and constructing that cage out of rebar will give us a path resistance of, luckily, 1 milliOhm. This figure and that of the radiation resistance yield the radiation efficiency (that is, how well the exterior RF will couple into the interior) which reduces to 0.075%. The cage works pretty well, but not perfectly (it was, after all, a poor example). Now, repeat this with a poorer conductor, or a tighter mesh and imagine the shielding effect. The mesh has an opening radius squared-squared relationship driving down the radiation resistance compared to the linear relationship of conductance. ************* Now, expanding the topic to allow for the contribution of ALL openings in the mesh, we must again return to the physical dimension compared to the wavelength dimension. If the cage is truly large, larger than the field exciting it, then you have miniscule radiators along it, each very inefficient. However, each of those radiators is out of phase with a distant neighbor (not so with its close mesh neighbors). Those two wavelength distant mesh radiators will combine somewhere in the interior space and build a field. This is very commonly found in inter-cable cross coupling through leakage that is exhibited in very long cable trays with tightly bound lines. This doesn't improve the efficiency, but sensitive circuits running parallel to power drives can prove to be a poor combination. What to do when conditions condemn the small signal coax to live in proximity to the large signal supply? This introduces the foil shield. The foil shield is a very poor conductor over any significant length, but over the span between mesh openings (e.g. coax shield weave), the resistance is sufficiently low to close the conductance gap. 73's Richard Clark, KB7QHC Nice explanation Richard. And I had never put together the squared-squared relationship. That's a powerful thing to know. I suppose this is why it ends up that a 1/10 lambda opening is considered the rule of thumb cutoff frequency on a dish. tom K0TAR |
Faraday shields and radiation and misinterpretations
On Tue, 01 Dec 2009 16:22:08 -0500, Registered User
wrote: I'm under the impression the current flow is identical whether metal rods or wire mesh is used in the antenna's construction. A discone does not exhibit any quality of shielding, so it wanders off in that regard. The difference between rods, number of rods, thickness of rods, and mesh all speak to bandwidth. 2, 3, or 4 rods will not be remarkable. 16 rods will closely approximate a cone of sheet metal (as would a grid of similar spacing). The same can be said of the rod/rods/mesh/sheet in the upper section approximating a solid disk. Again, all these "appearances" are a strict function of wavelength to physical length and spacing relationships. 73's Richard Clark, KB7QHC |
Faraday shields and radiation and misinterpretations
On Tue, 01 Dec 2009 18:37:51 -0600, tom wrote:
Nice explanation Richard. And I had never put together the squared-squared relationship. That's a powerful thing to know. I suppose this is why it ends up that a 1/10 lambda opening is considered the rule of thumb cutoff frequency on a dish. tom K0TAR Hi Tom, Radiation resistance certainly plummets quickly. Look at all the tunable loops for HF that are 1 M in size AND made on an herculean scale. I don't think any are rated at 80M (Rr ~ 5 milliOhms), and even less so for 160M (Rr ~ 29 microOhms). This is the principle reason why Art's inventions are doomed to abysmal transmit performance in that band (the shoe-box sized 160M loop). 73's Richard Clark, KB7QHC |
Faraday shields and radiation and misinterpretations
On Dec 1, 6:53*pm, Richard Clark wrote:
On Tue, 01 Dec 2009 18:37:51 -0600, tom wrote: Nice explanation Richard. *And I had never put together the squared-squared relationship. *That's a powerful thing to know. I suppose this is why it ends up that a 1/10 lambda opening is considered the rule of thumb cutoff frequency on a dish. tom K0TAR Hi Tom, Radiation resistance certainly plummets quickly. *Look at all the tunable loops for HF that are 1 M in size AND made on an herculean scale. *I don't think any are rated at 80M (Rr ~ 5 milliOhms), and even less so for 160M (Rr ~ 29 microOhms). *This is the principle reason why Art's inventions are doomed to abysmal transmit performance in that band (the shoe-box sized 160M loop). 73's Richard Clark, KB7QHC I have two Faraday shield antennas at the moment. One of which is a large one sitting on the ground tho sometimes I raise it a foot or so off the ground. This is an all band antenna which the tuner in my solid state radio handles quite well., It is made of mesh on a aluminum frame and at the moment I have not been able to discern any noise difference and the like say on top band. I compare it with a smaller Faraday shield which sits in the roter atop of my tower. The antenna on the ground is square but the one on the tower is a hexigon aluminum frame which is approx from memory about four or five foot long and the hex is approx 3 foot across. This antenna I use for comparison purposes where both antennas are end fed. The smallest radiator that I have made for top band was a 1 inch plastic pipe by about 4 foot tall. The radiator mesh was folded over several times and then wound in helix form on the plastic tube. This was also end fed. I could have folded it over upon itself to make it even smaller but I declined to pursue matters. Now one can accuse me of making up physics, but it was the understanding of physics which the books state is not fully understood that I followed in every step while maintaining equilibrium of the radiator. At the moment I am not inclined to throw away either of these antennas as they are easily confirmed for gain using a NEC with optimizer where, at the same time, the physics that I mention is not in agreement with this group or apparently the many plagerised books on the market today. The bottom line with the pursuit of antennas is to make them small but not electrically small. It is also desirable to make them rotatable and directive with gain. Maximum efficiency of a radiator is determined by how its size fits within a sphere and with the Faraday apparatus the radiator is the inside of the Faraday shield which makes it very efficient. I am continuing with my findings and the antennas and will not be discarding them as a child might say when lacking the knoweledge that is achieved by growing into an adult they attain a modicom of logic that they can some meaning to their outburst The antenna info is all on my page unwin antennas so that amateurs can join me in the joys of antenna design. As for the couch potatoes they can wave their arms as long as they want. I have also discussed it in full on qrz antennas if one wants to delve more into the physics. Nobody over the years I have explained my findings has ever applied existing classical physics to disprove my findings providing only the crying of a child with no physics substantiation applied. |
Faraday shields and radiation and misinterpretations
|
Faraday shields and radiation and misinterpretations
Richard Clark wrote in
: On Tue, 01 Dec 2009 03:42:13 -0600, Lostgallifreyan wrote: why is it often ok for a Faraday cage to have holes in it? :) Braided screens, meshes, perforated metal sheets, etc, I've seen many shields that are not a complete 'seal'... UHF TV cables especially seem to be very loosely shielded but they work. This can be explained at super high frequency and at DC as easily. However, before that it should be pointed out that the coverage (the ratio of what is conductor to what is not - the air space) defines how "good" the faraday shield will be. Not surprisingly, coverage is wavelength dependant. To cut to the chase, a wide mesh will allow increasingly higher frequencies (shorter waves) through. Now, as to the how. With a separation in the mesh, and for very large wavelength (in proportion to the opening size), you will have a very, very small potential difference across any of the mesh openings. Very little potential voltage across the mesh opening means very little current flow around the mesh opening that is specifically due to that potential difference. This is not to say there isn't a very, very large current flow by virtue of some very, very long wave. No, there's no denying that, but to get through the mesh you have to satisfy local conditions that demand what amounts to leakage (and this is exactly the term that correlates to coverage when discussing coax weave). If that huge current cannot induce a significant voltage across the mesh opening, then the mesh opening loop current cannot induce a field through to the other side. Now, if you examine the context of "huge current" in a resistive conductor, then obviously a potential difference can occur. Point is that reality (and science) allow for poor grade shields, but as a one knock-off proof you can summon up any failure, ignore simple contra-examples and create a new theory. However, returning to what is well known. If you increase the frequency applied to the mesh, then at some point wavelength will allow a situation where the general current flowing through the whole structure will naturally exhibit a potential difference across some small scale. By this point, abstraction may be wearying. Let's say you have a 10 meter-on-a-side cage with 1 meter mesh openings. If your applied field were exciting the cage at 75MHz (4M), then any spot on the cage could be at a very high potential difference from any spot adjacent and 1 meter away (a simple quarterwave relationship). This works for a solid conductor, it works for a mesh conductor. The 1 meter mesh openings can thus exhibit a substantial potential difference across the opening, and a local current loop associated with that potential difference. The mesh opening becomes a quarterwave radiator (aka slot antenna) and can couple energy from the external field into the interior of the cage (now possibly a resonant chamber, aka RF cavity). In practice and literature, the mesh opening loop exhibits a radiation resistance of 10s of Ohms. That compared to its mesh loop Ohmic path loss, makes it a very efficient coupler of energy. Take this very poor example of mesh, and lower the frequency to 750 KHz. The mesh opening - if we originally likened it to an antenna, we should be able to continue to do that - is now 1/400th Wave. A 1/400th wave radiator has extremely small radiation resistance. The exact value would be 751 nanoOhms. As we are examining a poor mesh, it becomes clear that it must have some resistance over that 1 meter distance (this is a real example, after all). Being generous and constructing that cage out of rebar will give us a path resistance of, luckily, 1 milliOhm. This figure and that of the radiation resistance yield the radiation efficiency (that is, how well the exterior RF will couple into the interior) which reduces to 0.075%. The cage works pretty well, but not perfectly (it was, after all, a poor example). Now, repeat this with a poorer conductor, or a tighter mesh and imagine the shielding effect. The mesh has an opening radius squared-squared relationship driving down the radiation resistance compared to the linear relationship of conductance. ************* Now, expanding the topic to allow for the contribution of ALL openings in the mesh, we must again return to the physical dimension compared to the wavelength dimension. If the cage is truly large, larger than the field exciting it, then you have miniscule radiators along it, each very inefficient. However, each of those radiators is out of phase with a distant neighbor (not so with its close mesh neighbors). Those two wavelength distant mesh radiators will combine somewhere in the interior space and build a field. This is very commonly found in inter-cable cross coupling through leakage that is exhibited in very long cable trays with tightly bound lines. This doesn't improve the efficiency, but sensitive circuits running parallel to power drives can prove to be a poor combination. What to do when conditions condemn the small signal coax to live in proximity to the large signal supply? This introduces the foil shield. The foil shield is a very poor conductor over any significant length, but over the span between mesh openings (e.g. coax shield weave), the resistance is sufficiently low to close the conductance gap. 73's Richard Clark, KB7QHC Thankyou. That IS a clear picture. I'll have to learn more to understand it well but what I can grasp fits well with things I have already observed. Regarding the other postings today, I can see that if you're receiving a long wave signal a small system will do if the sensivity is good and the noise is low, but transmission is another matter entirely. But whatever the theories propounded might be, I guess the observations are what matters. I haven't the space or equipment to test it, but if anyone manages to transmit a lot of longwave RF from a small directional system such as Art Unwin appears to be describing, then the theory will take care of itself, eventually, but I also get the strong impression that few people, if any, have done it. As far as I know, all low frequency RF transmitting systems are large, powerful things, and not very directional. (I wrote that yesterday but kept clear of the Send button, but it's on topic enough to go for it now.) |
Faraday shields and radiation and misinterpretations
On Dec 3, 2:28*am, Lostgallifreyan wrote:
Richard Clark wrote : On Tue, 01 Dec 2009 03:42:13 -0600, Lostgallifreyan wrote: why is it often ok for a Faraday cage to have holes in it? :) Braided screens, meshes, perforated metal sheets, etc, I've seen many shields that are not a complete 'seal'... UHF TV cables especially seem to be very loosely shielded but they work. This can be explained at super high frequency and at DC as easily. However, before that it should be pointed out that the coverage (the ratio of what is conductor to what is not - the air space) defines how "good" the faraday shield will be. *Not surprisingly, coverage is wavelength dependant. *To cut to the chase, a wide mesh will allow increasingly higher frequencies (shorter waves) through. Now, as to the how. *With a separation in the mesh, and for very large wavelength (in proportion to the opening size), you will have a very, very small potential difference across any of the mesh openings. *Very little potential voltage across the mesh opening means very little current flow around the mesh opening that is specifically due to that potential difference. This is not to say there isn't a very, very large current flow by virtue of some very, very long wave. *No, there's no denying that, but to get through the mesh you have to satisfy local conditions that demand what amounts to leakage (and this is exactly the term that correlates to coverage when discussing coax weave). *If that huge current cannot induce a significant voltage across the mesh opening, then the mesh opening loop current cannot induce a field through to the other side. *Now, if you examine the context of "huge current" in a resistive conductor, then obviously a potential difference can occur. *Point is that reality (and science) allow for poor grade shields, but as a one knock-off proof you can summon up any failure, ignore simple contra-examples and create a new theory. However, returning to what is well known. *If you increase the frequency applied to the mesh, then at some point wavelength will allow a situation where the general current flowing through the whole structure will naturally exhibit a potential difference across some small scale. *By this point, abstraction may be wearying. Let's say you have a 10 meter-on-a-side cage with 1 meter mesh openings. *If your applied field were exciting the cage at 75MHz (4M), then any spot on the cage could be at a very high potential difference from any spot adjacent and 1 meter away (a simple quarterwave relationship). *This works for a solid conductor, it works for a mesh conductor. The 1 meter mesh openings can thus exhibit a substantial potential difference across the opening, and a local current loop associated with that potential difference. *The mesh opening becomes a quarterwave radiator (aka slot antenna) and can couple energy from the external field into the interior of the cage (now possibly a resonant chamber, aka RF cavity). *In practice and literature, the mesh opening loop exhibits a radiation resistance of 10s of Ohms. *That compared to its mesh loop Ohmic path loss, makes it a very efficient coupler of energy. Take this very poor example of mesh, and lower the frequency to 750 KHz. *The mesh opening - if we originally likened it to an antenna, we should be able to continue to do that - is now 1/400th Wave. *A 1/400th wave radiator has extremely small radiation resistance. *The exact value would be 751 nanoOhms. *As we are examining a poor mesh, it becomes clear that it must have some resistance over that 1 meter distance (this is a real example, after all). * Being generous and constructing that cage out of rebar will give us a path resistance of, luckily, 1 milliOhm. *This figure and that of the radiation resistance yield the radiation efficiency (that is, how well the exterior RF will couple into the interior) which reduces to 0.075%. *The cage works pretty well, but not perfectly (it was, after all, a poor example). Now, repeat this with a poorer conductor, or a tighter mesh and imagine the shielding effect. *The mesh has an opening radius squared-squared relationship driving down the radiation resistance compared to the linear relationship of conductance. ************* Now, expanding the topic to allow for the contribution of ALL openings in the mesh, we must again return to the physical dimension compared to the wavelength dimension. *If the cage is truly large, larger than the field exciting it, then you have miniscule radiators along it, each very inefficient. *However, each of those radiators is out of phase with a distant neighbor (not so with its close mesh neighbors). Those two wavelength distant mesh radiators will combine somewhere in the interior space and build a field. *This is very commonly found in inter-cable cross coupling through leakage that is exhibited in very long cable trays with tightly bound lines. *This doesn't improve the efficiency, but sensitive circuits running parallel to power drives can prove to be a poor combination. *What to do when conditions condemn the small signal coax to live in proximity to the large signal supply? This introduces the foil shield. *The foil shield is a very poor conductor over any significant length, but over the span between mesh openings (e.g. coax shield weave), the resistance is sufficiently low to close the conductance gap. 73's Richard Clark, KB7QHC Thankyou. That IS a clear picture. I'll have to learn more to understand it well but what I can grasp fits well with things I have already observed. Regarding the other postings today, I can see that if you're receiving a long wave signal a small system will do if the sensivity is good and the noise is low, but transmission is another matter entirely. But whatever the theories propounded might be, I guess the observations are what matters. I haven't the space or equipment to test it, but if anyone manages to transmit a lot of longwave RF from a small directional system such as Art Unwin appears to be describing, then the theory will take care of itself, eventually, but I also get the strong impression that few people, if any, have done it. As far as I know, all low frequency RF transmitting systems are large, powerful things, and not very directional. (I wrote that yesterday but kept clear of the Send button, but it's on topic enough to go for it now.) You only have to visualise a large solenoid to see that it is quite directional. Modeling shows in the order of 10 dbi when end fed ! If you have a helix antenna in ribbon shape form where all lumped loads are canceled then even Eznec should be able to do the job. No need for cross wires for a single frequency design. A mesh made into a tube and placed on the ground will also get the job done, no hand waving required. Put mesh around a plastic garbage can also does the job and Menards have the mesh for less than $20 and muriatic acis for $4. It is just that hams cannot accept change or even small non electrical antennas. After all, all is known about antennas as they have been studied to death. |
Faraday shields and radiation and misinterpretations
On Thu, 03 Dec 2009 02:28:26 -0600, Lostgallifreyan
wrote: Regarding the other postings today, I can see that if you're receiving a long wave signal a small system will do if the sensivity is good and the noise is low, but transmission is another matter entirely. Reciprocity dominates, but transmit and receive circuits are not always reciprocal (that is, symmetric or identical). If you match at the antenna, you don't lose signal in the loss of the transmission line where SWR would dominate. That topic is best left to other discussion. But whatever the theories propounded might be, I guess the observations are what matters. I haven't the space or equipment to test it, but if anyone manages to transmit a lot of longwave RF from a small directional system such as Art Unwin appears to be describing, then the theory will take care of itself, eventually, but I also get the strong impression that few people, if any, have done it. As far as I know, all low frequency RF transmitting systems are large, powerful things, and not very directional. In logic there is the argument called Reductio Ad Absurdum. With the claim of a resonant small antenna being efficient there exists an obvious example that completely disrupts this. Since the inception of man-made RF radiation, ALL such attempts have been preceded with a resonant coil/capacitor combination. Think of the plate load of the conventional RF transmitter in both amateur and professional applications for the many decades that followed Hertz' work. This small, resonant plate load, is quite specifically designed for RF with low in resistive loss - and yet it is miserable as a propagator of that same RF. The physical size compared to the wavelength size dominates that efficiency with a fourth power law. Hertz' original design was in the VHF where his "plate tank" (so to speak) was physically large in relation to the wavelength he successfully transmitted to a nearby physically large receiving tank. 73's Richard Clark, KB7QHC |
Faraday shields and radiation and misinterpretations
On Dec 3, 12:29*pm, Richard Clark wrote:
On Thu, 03 Dec 2009 02:28:26 -0600, Lostgallifreyan wrote: Regarding the other postings today, I can see that if you're receiving a long wave signal a small system will do if the sensivity is good and the noise is low, but transmission is another matter entirely. Reciprocity dominates, but transmit and receive circuits are not always reciprocal (that is, symmetric or identical). *If you match at the antenna, you don't lose signal in the loss of the transmission line where SWR would dominate. *That topic is best left to other discussion. But whatever the theories propounded might be, I guess the observations are what matters. I haven't the space or equipment to test it, but if anyone manages to transmit a lot of longwave RF from a small directional system such as Art Unwin appears to be describing, then the theory will take care of itself, eventually, but I also get the strong impression that few people, if any, have done it. As far as I know, all low frequency RF transmitting systems are large, powerful things, and not very directional. In logic there is the argument called Reductio Ad Absurdum. *With the claim of a resonant small antenna being efficient there exists an obvious example that completely disrupts this. *Since the inception of man-made RF radiation, ALL such attempts have been preceded with a resonant coil/capacitor combination. *Think of the plate load of the conventional RF transmitter in both amateur and professional applications for the many decades that followed Hertz' work. This small, resonant plate load, is quite specifically designed for RF with low in resistive loss - and yet it is miserable as a propagator of that same RF. *The physical size compared to the wavelength size dominates that efficiency with a fourth power law. *Hertz' original design was in the VHF where his "plate tank" (so to speak) was physically large in relation to the wavelength he successfully transmitted to a nearby physically large receiving tank. 73's Richard Clark, KB7QHC A perfect example of an old man or woman not willing to accept change. For the cost of a few dollars they would not have made such fools of themselves over the years. I have stated many times that adding a time varying current to a Gaussian field of statics represents Maxwells laws for radiation. The group many times over say this is foolish and stupid. I know it is not stated or confirmed in the books. When one accept that statement of mine the next deductions become obvious. A radiator can be any size, shape or elevation as long as it is in a state of equilibrium and is in compliance with Maxwells equation for radiation. I have opreviously shown how static particles are part and parcel how a Faraday shield works. I now have shown again how Maxwell and Gauss also state that particles are part of radiation. In addition, the particle is also part of a CRT mechanism as is the salvage sorting system when sorting aluminum cans. I have also shown how the particle achieves a straight line trajectory with spin unaffected by gravity which is also essential to radio propagation. Yet hams still hang on to the yagi and all its atributes as being the cats whiskers. Thus size has become everything and the volume it occupies instead of distributed loads only as long as it is in equilibrium. The particle responsible for radiation and light is very small and is the perfect example of point radiation at its best. All it takes is a few dollars and a few hours work to make such an antenna, which allows you to stop making idiots of your selves, or a modicum of physics. Instead, you are all so sure that you find no need to get up from a couch. As I have stated many times, all the group has done is the waving of hands with no physics attached or any explanation why it is in total conformance with antenna computer programs of the day in addition to the points I have made. With groups such as this it is no small wonder that radiation has not been fully understood for more than a century. |
Faraday shields and radiation and misinterpretations
On Thu, 03 Dec 2009 10:29:53 -0800, Richard Clark
wrote: This small, resonant plate load, is quite specifically designed for RF with low in resistive loss - and yet it is miserable as a propagator of that same RF. The physical size compared to the wavelength size dominates that efficiency with a fourth power law. To extend this to Art's misinterpretation of Faraday Shields: In the old days, breadboard design was exactly that - your rig was built on (hammered to) a breadboard. It was open wiring with open components. It radiated well with an antenna, and poorly without one. However, as poorly as it radiated without an antenna, if you had a separate receiver, you would hear yourself. This was sometimes useful and gave us what is called "side tone." The monitor was born. Of course, with antennas connected, the receiver was bound to get more than enough of that anyway and if the two were closely spaced, feedback could drive all circuits into saturation. Not a good thing. The Faraday shield for the transmitter was born. It, as many can witness from simple observation, was composed of a fine grid mesh of wire either tied to ground, or to a heavily AC/RF filtered DC potential. As with all Faraday shields that came before it (indeed since Faraday invented it), it completely encapsulated the RF power source. The screen or mesh was simply a contrivance to allow cool air to move in and hot air to move out. Modern implementations use finned constructions and heat wicks - but this is topic drift. With this added to the breadboard, other circuits also came to be shielded, and generally so with the appearance of sheet metal chassis with suitably wavelength small openings for access and heat transfer. As the breadboard went into this RF impenetrable shell for both receivers and transmitters (and with even more care for transceivers), there arose a problem: What about the wires that go in and out? Yes indeed. If those wires were not, in themselves, decoupled; then they became radiators. The lesson to be learned was that those wires had to be held at the same potential as the Faraday shield. This could be accomplished by a simple connection, but with more than one wire this leads to dead shorts between wires. Not a good thing. The solution was to use AC/RF shorts (capacitors) to the shield from the wire and the wire could only penetrate the shield through a very small (in proportion to wavelength) opening. This was not always a good thing. A capacitor could be good, but it exhibits a roll-off of only 6dB per octave, or 10dB per decade isolation. If your line going in and out was a DC control line, and your principle frequency was 1MHz (talking about the old days now); then you had 6 decades of separation between 1Hz and 1MHz - pretty good. If in the intervening years you pushed the technology envelope and added voice modulation and that came through the same wire; then your system shrunk to 3 decades of separation between 10,000Hz and 1MHz. This might work, sometimes it didn't. As the years spun on, more wires penetrated that RF barrier, and they needed to not only be isolated from the RF, but each other; and often they contained very small signals that needed suitable signal to noise ratio (noise being that soup of RF that was stewing inside the shield). Inline bypass filters were born. The lines that penetrate a Faraday shield now appear to be more multi-stage low pass filters with repeating sections of shunt capacitors and series inductors. Their common (ground to the old brass pounder) was the shield which was RF free (as it was decoupled to a sanctioned earth ground). And lest we forget the principle penetration of that old time Faraday shield: The coaxial transmission line was born. By all appearances, this line satisfies the convention of a small opening through the Faraday shield. It's diameter is easily very small in relation to the wavelength of the RF power it reaches into the shield to tap. In a sense, it extends that hole in the shield to some very remote area that is far from the operating position, and then allows a wire(s) to emerge without regard for further shielding: The antenna is born. Funny thing, however, is that presumption of the shield of the coax being inert, un-perturbing, quiescent, invisible, benign - for that presumption is an illusion, a grand delusion. The line is very long with respect to wavelength, it is in the field of excitation that has been drawn out of the soup within the cage, and it is as much an antenna as the wire that emerged from its end. Many familiar problems rise from the ashes of this illusion. The exterior of the coaxial cable appears to the field to be a very long, grounded radiator. However, at any appreciable length (wavelength raises its familiar visage with an ironic grin), this exterior surface ceases to be the familiar DC grounding strap material, and becomes a full-fledge radiator according to its physical length vs. wavelength relationship. Not a very good thing, untill: The transmission line choke is born. To decouple the OUTSIDE of the coaxial line, the convention that has been observed (to widespread validation) is to either wind some sections of the line into Inductive chokes, or to add ferrites which serve the same purpose. These chokes, to be fully useful to their purpose, should be found at not only one point along the line, but at several so as to suppress (wavelength based) couplings along the line, by the line and by the field. When the combination of all these methods are employed, then the Faraday shield does what it has done for these several hundred years while allowing the migration of RF power to a remote drive point, and without allowing that RF power to re-intrude into the shield, nor along the coaxial cable. Thus, the only evidence of RF from inside the Faraday shield is that which arrives over-the-air from the remote antenna. Any other claim is a profanation of Faraday. 73's Richard Clark, KB7QHC |
Faraday shields and radiation and misinterpretations
On Tue, 01 Dec 2009 16:42:00 -0800, Richard Clark
wrote: On Tue, 01 Dec 2009 16:22:08 -0500, Registered User wrote: I'm under the impression the current flow is identical whether metal rods or wire mesh is used in the antenna's construction. A discone does not exhibit any quality of shielding, so it wanders off in that regard. Maybe I'm confused and can't distinguish between Art's all-band mesh antennas and his mesh Faraday shields. I was questioning Art's statement -quote- When you feed a time varying current to the mesh it is best to view it in small parts, say a square in the mesh. The hole is a static field alongside the applied current flows. - end quote - The idea of examining the characteristics of a single square of mesh seems impractical. The impact of adjacent squares should be accounted for otherwise the single square is a loop. Either way I've learned as current varies the fields it produces will vary. If the fields vary they're not static. Too simplistic? What am I missing? The difference between rods, number of rods, thickness of rods, and mesh all speak to bandwidth. 2, 3, or 4 rods will not be remarkable. 16 rods will closely approximate a cone of sheet metal (as would a grid of similar spacing). The same can be said of the rod/rods/mesh/sheet in the upper section approximating a solid disk. IIUC the current flows around the cone of a discone regardless of solid, sheet or mesh construction. This appears to be contrary to the quote above where current flows around each individual hole in the mesh. Again, all these "appearances" are a strict function of wavelength to physical length and spacing relationships. I've built several discones over the years and understand these relationships. How well is subject to conjecture hi. |
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