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Short antenna = reduced power
Quoting from Electromagnetism
By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... |
Short antenna = reduced power
"gareth" wrote in message ... Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... Damned typo! ISBN 0 19 851801 3 |
Short antenna = reduced power
"gareth" wrote in news:m1g1n8$39o$1@dont-
email.me: (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. Ok, but again, doesn't this just mean the system, as in taking into account feeding it? I'm not up to the maths of it, I'm just imaging a kind of logical extreme where you have a tiddly bit of wire stub out of the end of a coax instead of a 9m tall vertical whip. It seems obvious to me that to get the same efficiency, same power, you have a vastly increased energy density, so even without the maths I have no problem seeing the relevance of comments like Jim's (Jeff's?) allusion to room temperature superconductors and such. In other words, any actual reduction is based on practical limits, not theory itself. It's not so different with laser diodes, in that a diffraction limited spot may be obtained easily with a simple aspheric lens from any size apeture so long as it's a signle lattidutinal mode emitter, but try actually MAKING an emitter that size. Theory says sure, no problem, energy density and nature of materials says otherwise. |
Short antenna = reduced power
"Jeff" wrote in message ... Gareth, please have a look around the web and find a copy of Kraus to download; then read, in particular, chapters 3 and 5. In particular note the following in relation to short dipoles: "Assuming no losses it [the power radiated] is also equal to the power delivered to the [short] dipole" "The maximum effective aperture of the 1/2 wavelength antenna is about 10% greater than that of the short dipole" The gains of a short and 1/2 wave dipole is also quoted as 1.76 and 2.14dBi respectively. So can we now put this to bed, the short dipole radiates well it is the practicabilities that make it a poor antenna. Jeff And along the same lines, antennas are often described in terms of isotropic (point) antennas. With radiation being related to length, isotropic antennas would not radiate. Also with effective aperture, the 10% greater you mention is a result of orientation of the aperture with respect to the maximum part of the individual antenna pattern. Considering the entire pattern of both antennas, reciprocity is maintained. |
Short antenna = reduced power
On 13/10/14 12:34, Jeff wrote:
On 13/10/2014 09:15, gareth wrote: Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... That makes no sense, at least quoted out of context, as it would imply that the power radiated was independent of the power applied. So an infinitely long antenna would radiate infinite power !!!! Jeff His problem is he is not considering the Radiation Resistance, Loss Resistance, and reactive element which determine the eff., and Zo. (The reactive element represents the energy 'stored' in the field around the antenna- just like the energy store in an inductor or capacitor, both reactive components.) A short dipole, for example, will be a poor match but RRLR. Provided the feeder loss is low, either by good matching or the use of low loss feeder (assuming the PA is 'happy') then the overall losses are low and the RF only has one place to go, to be radiated. A short dipole has other issues, in particular if matching is used to overcome the issue of the Zo, then the matching network plus antenna will have a very narrow bandwidth (compared to a full sized dipole) and adjustment will be essential to maintain efficiency if the frequency of operation is changed. Remember, the use of 'standard' Zo of 50 or 75 ohm is not essential, nor is maintaining a feeder SWR of 1.5, provided the PA can cope and feeder loss can be tolerated/reduced (eg by using open wire feeder). No doubt he will dismiss this with his usual tirade of abuse etc, but that is his normal response when corrected. |
Short antenna = reduced power
"Brian Reay" wrote in message
... His problem is he is not considering the Radiation Resistance, On the same page, the radiation resistance is defined, also including that term, for the radiation resistance is derived from the firat equation quoted No doubt he will dismiss this with his usual tirade of abuse etc, but that is his normal response when corrected. As usual, the only origination of abuse comes from you. |
Short antenna = reduced power
On 10/13/2014 11:00 AM, Wayne wrote:
"Jeff" wrote in message ... Gareth, please have a look around the web and find a copy of Kraus to download; then read, in particular, chapters 3 and 5. In particular note the following in relation to short dipoles: "Assuming no losses it [the power radiated] is also equal to the power delivered to the [short] dipole" "The maximum effective aperture of the 1/2 wavelength antenna is about 10% greater than that of the short dipole" The gains of a short and 1/2 wave dipole is also quoted as 1.76 and 2.14dBi respectively. So can we now put this to bed, the short dipole radiates well it is the practicabilities that make it a poor antenna. Jeff And along the same lines, antennas are often described in terms of isotropic (point) antennas. With radiation being related to length, isotropic antennas would not radiate. Also with effective aperture, the 10% greater you mention is a result of orientation of the aperture with respect to the maximum part of the individual antenna pattern. Considering the entire pattern of both antennas, reciprocity is maintained. Yes, but an isotropic source is an imaginary tool used for comparisons. It obviously cannot exist in the real world, but it's spherical radiation pattern can be used as a standard for comparisons. Similar to an inductor or capacitor with no resistance - only reactance. They don't exist in the real world, but are used to simplify calculations. Once you get an answer, you can tweak the results for the resistance. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
Short antenna = reduced power
gareth wrote:
Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... You do that and while you are at it take note of the fact that the expression you give is unitless and can not be power. You will also find that the total power radiated by an antenna is the surface integral of the average Poynting vector over a surface enclosing the antenna. The surface usually chosen is a sphere in the far field to keep the equations "simple". -- Jim Pennino |
Short antenna = reduced power
|
Short antenna = reduced power
On 10/13/2014 11:45 PM, wrote:
rickman wrote: On 10/13/2014 1:36 PM, wrote: gareth wrote: Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... You do that and while you are at it take note of the fact that the expression you give is unitless and can not be power. You will also find that the total power radiated by an antenna is the surface integral of the average Poynting vector over a surface enclosing the antenna. The surface usually chosen is a sphere in the far field to keep the equations "simple". He is taking a portion of the equation and presenting it out of context assuming that this is a valid way to consider what he wishes to show. I would like to see the full equation. The devil is in the details. Actually, there is no "the" equation for the power radiated by an antenna other than the surface integral of the average Poynting vector over a surface enclosing the antenna. There are some approximate rules for specific cases and limiting conditions, but this isn't one of them. What he presented is for a 1/2 wavelegth antenna 9.87 and a full wave antenna 39.48. WTF is that?? I have no idea what you are talking about. Where did you get these numbers? 9.87 what? -- Rick |
Short antenna = reduced power
rickman wrote:
On 10/13/2014 11:45 PM, wrote: rickman wrote: On 10/13/2014 1:36 PM, wrote: gareth wrote: Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... You do that and while you are at it take note of the fact that the expression you give is unitless and can not be power. You will also find that the total power radiated by an antenna is the surface integral of the average Poynting vector over a surface enclosing the antenna. The surface usually chosen is a sphere in the far field to keep the equations "simple". He is taking a portion of the equation and presenting it out of context assuming that this is a valid way to consider what he wishes to show. I would like to see the full equation. The devil is in the details. Actually, there is no "the" equation for the power radiated by an antenna other than the surface integral of the average Poynting vector over a surface enclosing the antenna. There are some approximate rules for specific cases and limiting conditions, but this isn't one of them. What he presented is for a 1/2 wavelegth antenna 9.87 and a full wave antenna 39.48. WTF is that?? I have no idea what you are talking about. Where did you get these numbers? 9.87 what? (2 * 3.14 * 5 meters / 10 meters) ^ 2 9.87 nothing; the expression is unitless, i.e. a pure number without units. In the expression you have a length divided by a length, which cancels into a unitles number. As my old physics professor used to say, always check the units of your answer; the arithmatic may be correct but it is meaningless unless the units are correct. Sample problem: You drove 100 miles and used 5 gallons of gas. What was your mileage? 5 gallons * 100 miles = 500 gallon-miles --- wrong units. 5 gallons / 100 miles = .05 gal/mile --- wrong units. 100 miles / 5 gallons = 20 miles/gal --- correct units and correct answer. -- Jim Pennino |
Short antenna = reduced power
On 13/10/2014 09:15, gareth wrote:
Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... Something is missing here, I suspect that you have not quoted all the formulae as this does not make sense in isolation. ISTR from my Electromagnetics that there is a polynomial relationship and not a linear relationship. It produces peaks of radiated power as the length of the apparent radiator approaches the point where wave superposition occurs. Can you give more context? However it has been a while since antenna theory and electromagnetism; about 30 years so it is likely that I have forgotten a lot. Andy |
Short antenna = reduced power
|
Short antenna = reduced power
On 10/14/2014 1:21 AM, wrote:
rickman wrote: On 10/13/2014 11:45 PM, wrote: rickman wrote: On 10/13/2014 1:36 PM, wrote: gareth wrote: Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... You do that and while you are at it take note of the fact that the expression you give is unitless and can not be power. You will also find that the total power radiated by an antenna is the surface integral of the average Poynting vector over a surface enclosing the antenna. The surface usually chosen is a sphere in the far field to keep the equations "simple". He is taking a portion of the equation and presenting it out of context assuming that this is a valid way to consider what he wishes to show. I would like to see the full equation. The devil is in the details. Actually, there is no "the" equation for the power radiated by an antenna other than the surface integral of the average Poynting vector over a surface enclosing the antenna. There are some approximate rules for specific cases and limiting conditions, but this isn't one of them. What he presented is for a 1/2 wavelegth antenna 9.87 and a full wave antenna 39.48. WTF is that?? I have no idea what you are talking about. Where did you get these numbers? 9.87 what? (2 * 3.14 * 5 meters / 10 meters) ^ 2 9.87 nothing; the expression is unitless, i.e. a pure number without units. In the expression you have a length divided by a length, which cancels into a unitles number. As my old physics professor used to say, always check the units of your answer; the arithmatic may be correct but it is meaningless unless the units are correct. Sample problem: You drove 100 miles and used 5 gallons of gas. What was your mileage? 5 gallons * 100 miles = 500 gallon-miles --- wrong units. 5 gallons / 100 miles = .05 gal/mile --- wrong units. 100 miles / 5 gallons = 20 miles/gal --- correct units and correct answer. I have no idea what you are going on about. Ok, 9.87 is a unitless number. So is 33.043. Now what? -- Rick |
Short antenna = reduced power
|
Short antenna = reduced power
"gareth" wrote in message
... Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... Ramo, Whinnery and Van Duzer gives the same derivation, including the derivation of radiation resistance. |
Short antenna = reduced power
On 10/14/2014 4:27 AM, gareth wrote:
"gareth" wrote in message ... Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... Ramo, Whinnery and Van Duzer gives the same derivation, including the derivation of radiation resistance. Are you ever going to share the equations with us? -- Rick |
Short antenna = reduced power
On Tue, 14 Oct 2014 04:40:19 -0400, rickman wrote:
On 10/14/2014 4:27 AM, gareth wrote: "gareth" wrote in message ... Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... Ramo, Whinnery and Van Duzer gives the same derivation, including the derivation of radiation resistance. Are you ever going to share the equations with us? I wouldn't have thought so, as they're likely to compound his embarrassment. In a year or so he'll claim to have resolved the matter to his satisfaction and declare that the posters who disagreed with him were idiots. |
Short antenna = reduced power
On 10/13/2014 3:15 AM, gareth wrote:
Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... Read this, Gareth: http://www.antenna-theory.com/basics/friis.php |
Short antenna = reduced power
|
Short antenna = reduced power
"Wayne" wrote in :
There is no loss due to distance itself, but to the radiation spreading. I'm wondering now if what Gareth is concerned with is the same as divergence in an asperic lens output with a laser diode. Assuming the diode has an emitter width of a very few microns (is already usually only one micron in one axis even in a multimode diode with a single 'stripe' emitter pattern) then a large enough single asperic lens will make a finely directed but broad beam, but if you want it very narrow as well, it diverges more widely and various optic methods will tame it a bit, but there's no real substitute for a single mode diode if possible to use one for the wanted power. Assuming it is NOT possible, the multimode diode needed will demand a bigger lens to match its power efficiently into a well directed, 'collimated' beam. It seems to me that this is more than just an analogy, but maybe fundmentally similar to the difficulties with energy density, accuracy of form, low loss of materials used, aperture size for emission, and maybe several other things I've seen mentioned recently about this subject of small antennas. Including the fact that eben if the laser beam IS highly divergent, the small aspheric lens is just as efficnt at prjecting the power as the larger one, so long as all light from the diode gets coupled through it without spill or reflection. I hope that's not too off-topic, but it seems to be that I might get some learning from responses to this one... |
Short antenna = reduced power
rickman wrote:
On 10/14/2014 1:21 AM, wrote: rickman wrote: On 10/13/2014 11:45 PM, wrote: rickman wrote: On 10/13/2014 1:36 PM, wrote: gareth wrote: Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... You do that and while you are at it take note of the fact that the expression you give is unitless and can not be power. You will also find that the total power radiated by an antenna is the surface integral of the average Poynting vector over a surface enclosing the antenna. The surface usually chosen is a sphere in the far field to keep the equations "simple". He is taking a portion of the equation and presenting it out of context assuming that this is a valid way to consider what he wishes to show. I would like to see the full equation. The devil is in the details. Actually, there is no "the" equation for the power radiated by an antenna other than the surface integral of the average Poynting vector over a surface enclosing the antenna. There are some approximate rules for specific cases and limiting conditions, but this isn't one of them. What he presented is for a 1/2 wavelegth antenna 9.87 and a full wave antenna 39.48. WTF is that?? I have no idea what you are talking about. Where did you get these numbers? 9.87 what? (2 * 3.14 * 5 meters / 10 meters) ^ 2 9.87 nothing; the expression is unitless, i.e. a pure number without units. In the expression you have a length divided by a length, which cancels into a unitles number. As my old physics professor used to say, always check the units of your answer; the arithmatic may be correct but it is meaningless unless the units are correct. Sample problem: You drove 100 miles and used 5 gallons of gas. What was your mileage? 5 gallons * 100 miles = 500 gallon-miles --- wrong units. 5 gallons / 100 miles = .05 gal/mile --- wrong units. 100 miles / 5 gallons = 20 miles/gal --- correct units and correct answer. I have no idea what you are going on about. Ok, 9.87 is a unitless number. So is 33.043. Now what? Exactly. Since it is a unitless number, it can not be power as claimed. Nor can it be a rule of thumb for power versus antenna length as a full wave antenna does not radiate 4 (39.48/9.87) more power than a 1/2 wave antenna. It is just nonsense. -- Jim Pennino |
Short antenna = reduced power
Lostgallifreyan wrote:
wrote in : 5 gallons / 100 miles = .05 gal/mile --- wrong units. Context is everything. :) There are cases where this reciprocated form gets used to good effect. Maybe convention demands 1/20 gal/mile just to indicate reciprocation of a normal convention. True but it is the answer to a different question. -- Jim Pennino |
Short antenna = reduced power
"Lostgallifreyan" wrote in message . .. "Wayne" wrote in : There is no loss due to distance itself, but to the radiation spreading. I'm wondering now if what Gareth is concerned with is the same as divergence in an asperic lens output with a laser diode. Assuming the diode has an emitter width of a very few microns (is already usually only one micron in one axis even in a multimode diode with a single 'stripe' emitter pattern) then a large enough single asperic lens will make a finely directed but broad beam, but if you want it very narrow as well, it diverges more widely and various optic methods will tame it a bit, but there's no real substitute for a single mode diode if possible to use one for the wanted power. Assuming it is NOT possible, the multimode diode needed will demand a bigger lens to match its power efficiently into a well directed, 'collimated' beam. It seems to me that this is more than just an analogy, but maybe fundmentally similar to the difficulties with energy density, accuracy of form, low loss of materials used, aperture size for emission, and maybe several other things I've seen mentioned recently about this subject of small antennas. Including the fact that eben if the laser beam IS highly divergent, the small aspheric lens is just as efficnt at prjecting the power as the larger one, so long as all light from the diode gets coupled through it without spill or reflection. I hope that's not too off-topic, but it seems to be that I might get some learning from responses to this one... ############ Well, it may be slightly off topic and certainly out of my field of experience, but I find it more interesting than Gareth's misused equations :) |
Short antenna = reduced power
gareth wrote:
"gareth" wrote in message ... Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... Ramo, Whinnery and Van Duzer gives the same derivation, including the derivation of radiation resistance. What you have posted is a unitless number with no relationship to anything. -- Jim Pennino |
Short antenna = reduced power
On 10/14/2014 12:58 PM, wrote:
rickman wrote: On 10/14/2014 1:21 AM, wrote: rickman wrote: On 10/13/2014 11:45 PM, wrote: rickman wrote: On 10/13/2014 1:36 PM, wrote: gareth wrote: Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... You do that and while you are at it take note of the fact that the expression you give is unitless and can not be power. You will also find that the total power radiated by an antenna is the surface integral of the average Poynting vector over a surface enclosing the antenna. The surface usually chosen is a sphere in the far field to keep the equations "simple". He is taking a portion of the equation and presenting it out of context assuming that this is a valid way to consider what he wishes to show. I would like to see the full equation. The devil is in the details. Actually, there is no "the" equation for the power radiated by an antenna other than the surface integral of the average Poynting vector over a surface enclosing the antenna. There are some approximate rules for specific cases and limiting conditions, but this isn't one of them. What he presented is for a 1/2 wavelegth antenna 9.87 and a full wave antenna 39.48. WTF is that?? I have no idea what you are talking about. Where did you get these numbers? 9.87 what? (2 * 3.14 * 5 meters / 10 meters) ^ 2 9.87 nothing; the expression is unitless, i.e. a pure number without units. In the expression you have a length divided by a length, which cancels into a unitles number. As my old physics professor used to say, always check the units of your answer; the arithmatic may be correct but it is meaningless unless the units are correct. Sample problem: You drove 100 miles and used 5 gallons of gas. What was your mileage? 5 gallons * 100 miles = 500 gallon-miles --- wrong units. 5 gallons / 100 miles = .05 gal/mile --- wrong units. 100 miles / 5 gallons = 20 miles/gal --- correct units and correct answer. I have no idea what you are going on about. Ok, 9.87 is a unitless number. So is 33.043. Now what? Exactly. Since it is a unitless number, it can not be power as claimed. Nor can it be a rule of thumb for power versus antenna length as a full wave antenna does not radiate 4 (39.48/9.87) more power than a 1/2 wave antenna. It is just nonsense. Did you read the OP? He says: "Has in the equation for radiated power the term" He is just giving us a portion of the equation to show the dependence on wavelength vs antenna length. But without the full equation we can't know if there are mitigating factors. Nothing that you have posted makes any sense in this context. -- Rick |
Short antenna = reduced power
|
Short antenna = reduced power
"Wayne" wrote in :
Well, it may be slightly off topic and certainly out of my field of experience, but I find it more interesting than Gareth's misused equations :) Thanks. I do try. :) I figure even if I am imprecise, I can either try to entertain or at least come at it from an angle that might be useful, perhaps not just to me. |
Short antenna = reduced power
rickman wrote:
On 10/14/2014 12:58 PM, wrote: rickman wrote: On 10/14/2014 1:21 AM, wrote: rickman wrote: On 10/13/2014 11:45 PM, wrote: snip 9.87 nothing; the expression is unitless, i.e. a pure number without units. In the expression you have a length divided by a length, which cancels into a unitles number. As my old physics professor used to say, always check the units of your answer; the arithmatic may be correct but it is meaningless unless the units are correct. Sample problem: You drove 100 miles and used 5 gallons of gas. What was your mileage? 5 gallons * 100 miles = 500 gallon-miles --- wrong units. 5 gallons / 100 miles = .05 gal/mile --- wrong units. 100 miles / 5 gallons = 20 miles/gal --- correct units and correct answer. I have no idea what you are going on about. Ok, 9.87 is a unitless number. So is 33.043. Now what? Exactly. Since it is a unitless number, it can not be power as claimed. Nor can it be a rule of thumb for power versus antenna length as a full wave antenna does not radiate 4 (39.48/9.87) more power than a 1/2 wave antenna. It is just nonsense. Did you read the OP? He says: "Has in the equation for radiated power the term" He is just giving us a portion of the equation to show the dependence on wavelength vs antenna length. But without the full equation we can't know if there are mitigating factors. And it is utter nonsense. There is NOTHING about a 1 wavelength antenna that is 4 times that of a 1/2 wave antenna, or 16 times than of a 1/4 wave antenna, which is what the expression evaluates to. The part L/LAMBDA is the fractional size of the antenna, and the rest just numbers and I assume you have a calculator of some kind. It has already been shown by others that, neglecting loss, all power is radiated by an antenna no matter what the size. -- Jim Pennino |
Short antenna = reduced power
On 10/14/2014 3:27 AM, gareth wrote:
"gareth" wrote in message ... Quoting from Electromagnetism By F.N.H.Robinson in the Oxford Physics Series 1973 edition ISBN 0 19 8518913 Chapter 11, Radiation, page 102 Formula 11.11 Has in the equation for radiated power the term (2*PI*L/LAMBDA)**2 where L is the antenna length and LAMBDA the wavelength, thereby showing that the radiated power decreases when the antenna length decreases. I will read up further and report further... Ramo, Whinnery and Van Duzer gives the same derivation, including the derivation of radiation resistance. Please cite book title, edition, and page number. Thanks. |
Short antenna = reduced power
En el artículo , John S
escribió: Ramo, Whinnery and Van Duzer gives the same derivation, including the derivation of radiation resistance. Please cite book title, edition, and page number. You really didn't expect a reply, did you? He's just looking to stir the ****, as evidenced by the new thread he has started. -- (\_/) (='.'=) (")_(") |
Short antenna = reduced power
"Mike Tomlinson" wrote in message
... You really didn't expect a reply, did you? He's just looking to stir the ****, as evidenced by the new thread he has started. Once again, it is only from your keyboard that the gratuitous abuse originates. |
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