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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 |
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