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Relation of radiation resistance and terminal resistance
Assume a non-reactive antenna. Start with a half-wave dipole.
What is the radiation resistance and what is the terminal resistance? Thanks, John |
#2
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Relation of radiation resistance and terminal resistance
On May 26, 4:53*pm, John KD5YI wrote:
Assume a non-reactive antenna. Start with a half-wave dipole. What is the radiation resistance and what is the terminal resistance? Thanks, John Radiation resistance can be, and is, defined differently by different writers. Often the term is used without clarifying definition, and of course that leads to trouble. Beware any time an author throws around "radiation resistance" without carefully defining how he means it. There's a nice section (section 17; page 118) on it in King's "Antennas" chapter of King, Mimno and Wing's "Transmission Lines, Antennas and Wave Guides." I highly recommend reading that passage, though I know that not everyone likes King's writing as much as I do. (I may be able to supply a PDF of it, if you can't find the book.) Generally, radiation resistance is associated with power actually radiated by the antenna: i^2 * R(radiation). One possible definition is "that portion of the resistive component of the feedpoint terminal impedance that represents radiation." But when the feedpoint is not at a current maximum, "radiation resistance" is sometimes (often?) taken instead to be the resistance which, when multiplied by the square of the current at the current maximum, would result in the value of the radiated power. Presumably the "terminal resistance" is the resistive component of the impedance seen at the antenna's feedpoint terminals. It's generally a good idea to specify that the current is the same magnitude and opposite direction in the two feedpoint terminals, and that they are closely spaced--a tiny fraction of a wavelength apart. The difference between the two represents power dissipated in the antenna itself. Of course that's generally a pretty low percentage of the total in self-resonant antennas, but in short antennas (loaded dipoles and monopoles, and tuned loops that are very small compared with the wavelength) the power lost in heating the antenna and associated loading reactances can be significant. Cheers, Tom |
#3
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Relation of radiation resistance and terminal resistance
On 5/26/2011 10:37 PM, K7ITM wrote:
On May 26, 4:53 pm, John wrote: Assume a non-reactive antenna. Start with a half-wave dipole. What is the radiation resistance and what is the terminal resistance? Thanks, John Radiation resistance can be, and is, defined differently by different writers. Often the term is used without clarifying definition, and of course that leads to trouble. Beware any time an author throws around "radiation resistance" without carefully defining how he means it. There's a nice section (section 17; page 118) on it in King's "Antennas" chapter of King, Mimno and Wing's "Transmission Lines, Antennas and Wave Guides." I highly recommend reading that passage, though I know that not everyone likes King's writing as much as I do. (I may be able to supply a PDF of it, if you can't find the book.) Generally, radiation resistance is associated with power actually radiated by the antenna: i^2 * R(radiation). One possible definition is "that portion of the resistive component of the feedpoint terminal impedance that represents radiation." But when the feedpoint is not at a current maximum, "radiation resistance" is sometimes (often?) taken instead to be the resistance which, when multiplied by the square of the current at the current maximum, would result in the value of the radiated power. Orfanidis has an explanation in Chapter 15 of his online electromagnetics textbook http://www.ece.rutgers.edu/~orfanidi/ewa/ch15.pdf page 612 Not a very complete discussion, as might be found in Kraus, but at least it's online. There's also Prof. David Jeffreries's website http://personal.ee.surrey.ac.uk/Pers...es/radimp.html "The classical way to calculate the radiation resistance is to surround the antenna with a hypothetical closed surface in the far field, calculate the values of electric field and Poynting vector on this surface in terms of the antenna terminal current I, integrate the power flow per unit area, represented by the Poynting vector, all over this surface, to determine the total outward travelling power in watts, and equate this power to the quantity II*Rrad/2 as discussed above. " |
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Relation of radiation resistance and terminal resistance
On 5/27/2011 12:37 AM, K7ITM wrote:
On May 26, 4:53 pm, John wrote: Assume a non-reactive antenna. Start with a half-wave dipole. What is the radiation resistance and what is the terminal resistance? Thanks, John Radiation resistance can be, and is, defined differently by different writers. Often the term is used without clarifying definition, and of course that leads to trouble. Beware any time an author throws around "radiation resistance" without carefully defining how he means it. There's a nice section (section 17; page 118) on it in King's "Antennas" chapter of King, Mimno and Wing's "Transmission Lines, Antennas and Wave Guides." I highly recommend reading that passage, though I know that not everyone likes King's writing as much as I do. (I may be able to supply a PDF of it, if you can't find the book.) Generally, radiation resistance is associated with power actually radiated by the antenna: i^2 * R(radiation). One possible definition is "that portion of the resistive component of the feedpoint terminal impedance that represents radiation." But when the feedpoint is not at a current maximum, "radiation resistance" is sometimes (often?) taken instead to be the resistance which, when multiplied by the square of the current at the current maximum, would result in the value of the radiated power. Presumably the "terminal resistance" is the resistive component of the impedance seen at the antenna's feedpoint terminals. It's generally a good idea to specify that the current is the same magnitude and opposite direction in the two feedpoint terminals, and that they are closely spaced--a tiny fraction of a wavelength apart. The difference between the two represents power dissipated in the antenna itself. Of course that's generally a pretty low percentage of the total in self-resonant antennas, but in short antennas (loaded dipoles and monopoles, and tuned loops that are very small compared with the wavelength) the power lost in heating the antenna and associated loading reactances can be significant. Cheers, Tom Thanks, Tom. That pretty much answers my question. Thanks also to Jim. 73, John |
#5
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Relation of radiation resistance and terminal resistance
On 5/27/2011 12:37 AM, K7ITM wrote:
On May 26, 4:53 pm, John wrote: Assume a non-reactive antenna. Start with a half-wave dipole. What is the radiation resistance and what is the terminal resistance? Thanks, John Radiation resistance can be, and is, defined differently by different writers. Often the term is used without clarifying definition, and of course that leads to trouble. Beware any time an author throws around "radiation resistance" without carefully defining how he means it. There's a nice section (section 17; page 118) on it in King's "Antennas" chapter of King, Mimno and Wing's "Transmission Lines, Antennas and Wave Guides." I highly recommend reading that passage, though I know that not everyone likes King's writing as much as I do. (I may be able to supply a PDF of it, if you can't find the book.) Generally, radiation resistance is associated with power actually radiated by the antenna: i^2 * R(radiation). One possible definition is "that portion of the resistive component of the feedpoint terminal impedance that represents radiation." But when the feedpoint is not at a current maximum, "radiation resistance" is sometimes (often?) taken instead to be the resistance which, when multiplied by the square of the current at the current maximum, would result in the value of the radiated power. Of course. It all makes sense now. Feeding a 1/2W dipole in the center causes one to measure approximately the Rr. However the dipole can be fed at some other point. Even fed at one end where the feed point resistance can be in the hundreds to thousands of ohms. So, radiation resistance is a convenient device (along with current at that point) that we use to represent power "lost" from the antenna through radiation. Good. I can understand that. Gotta be very careful of the definitions. It was in Kraus' Antennas For All Applications that raised my question. Presumably the "terminal resistance" is the resistive component of the impedance seen at the antenna's feedpoint terminals. It's generally a good idea to specify that the current is the same magnitude and opposite direction in the two feedpoint terminals, and that they are closely spaced--a tiny fraction of a wavelength apart. I understand. The difference between the two represents power dissipated in the antenna itself. Of course that's generally a pretty low percentage of the total in self-resonant antennas, but in short antennas (loaded dipoles and monopoles, and tuned loops that are very small compared with the wavelength) the power lost in heating the antenna and associated loading reactances can be significant. That, too, is understandable. Cheers, Tom Thanks again, Tom, for this. Also, I understand Jim's reference to integrating the volume to get the radiated power and, hence, Rr. Thanks, Jim. Cheers & 73, John |
#6
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Relation of radiation resistance and terminal resistance
On May 28, 3:44*am, John KD5YI wrote:
On 5/27/2011 12:37 AM, K7ITM wrote: On May 26, 4:53 pm, John *wrote: Assume a non-reactive antenna. Start with a half-wave dipole. What is the radiation resistance and what is the terminal resistance? Thanks, John Radiation resistance can be, and is, defined differently by different writers. *Often the term is used without clarifying definition, and of course that leads to trouble. *Beware any time an author throws around "radiation resistance" without carefully defining how he means it. There's a nice section (section 17; page 118) on it in King's "Antennas" chapter of King, Mimno and Wing's "Transmission Lines, Antennas and Wave Guides." *I highly recommend reading that passage, though I know that not everyone likes King's writing as much as I do. (I may be able to supply a PDF of it, if you can't find the book.) Generally, radiation resistance is associated with power actually radiated by the antenna: *i^2 * R(radiation). *One possible definition is "that portion of the resistive component of the feedpoint terminal impedance that represents radiation." *But when the feedpoint is not at a current maximum, "radiation resistance" is sometimes (often?) taken instead to be the resistance which, when multiplied by the square of the current at the current maximum, would result in the value of the radiated power. Of course. It all makes sense now. Feeding a 1/2W dipole in the center causes one to measure approximately the Rr. However the dipole can be fed at some other point. Even fed at one end where the feed point resistance can be in the hundreds to thousands of ohms. So, radiation resistance is a convenient device (along with current at that point) that we use to represent power "lost" from the antenna through radiation. Good. I can understand that. Gotta be very careful of the definitions. It was in Kraus' Antennas For All Applications that raised my question. Presumably the "terminal resistance" is the resistive component of the impedance seen at the antenna's feedpoint terminals. *It's generally a good idea to specify that the current is the same magnitude and opposite direction in the two feedpoint terminals, and that they are closely spaced--a tiny fraction of a wavelength apart. I understand. The difference between the two represents power dissipated in the antenna itself. *Of course that's generally a pretty low percentage of the total in self-resonant antennas, but in short antennas (loaded dipoles and monopoles, and tuned loops that are very small compared with the wavelength) the power lost in heating the antenna and associated loading reactances can be significant. That, too, is understandable. Cheers, Tom Thanks again, Tom, for this. Also, I understand Jim's reference to integrating the volume to get the radiated power and, hence, Rr. Thanks, Jim. Cheers & 73, John Blah de blah How many amateur radio operators use this kind of academic preening when they are putting up a dipole. Too much hand-waving here to be useful to most folks. |
#7
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Relation of radiation resistance and terminal resistance
On 5/28/2011 4:25 PM, Frank wrote:
Blah de blah How many amateur radio operators use this kind of academic preening when they are putting up a dipole. Too much hand-waving here to be useful to most folks. Then do us all a favor, Frank, and don't participate in the discussion. Your hand-waving is not useful either. |
#8
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Relation of radiation resistance and terminal resistance
Then do us all a favor, Frank, and don't participate in the discussion. Your hand-waving is not useful either. So you figured out a dipole should be fed at the mid-point? You are a ****ing genius :-) Please continue with your penetrating questions; we are hanging on every word.... |
#9
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Relation of radiation resistance and terminal resistance
On 5/28/2011 5:11 PM, Frank wrote:
Then do us all a favor, Frank, and don't participate in the discussion. Your hand-waving is not useful either. So you figured out a dipole should be fed at the mid-point? You are a ****ing genius :-) Please continue with your penetrating questions; we are hanging on every word.... You may kiss my dipole, Frankenstein. |
#10
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Relation of radiation resistance and terminal resistance
"Frank" wrote in message
... How many amateur radio operators use this kind of academic preening when they are putting up a dipole. Not many, certainly... but I would offer that those who do are able to enjoy the hobby more thoroughly than those who don't. Whether or not that additional enjoyment makes up for the extra time needed for learning, well, that's up to each individuals. I personally care about these things, and even for hams who think they don't... most all of them have probably thoughts to themselves, at one point or another, "Hey, what happens if you feed a dipole off-center?," and it's nice that someone else has already gone through the effort to figure it out such that the answers are readily Google-able! :-) ---Joel |
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