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#51
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Richard Harrison wrote:
Ian Jackson wrote: "Maybe 377 ohms?" Arnold B. Bailey in "TV and Other Receiving Antennas" shows his calculations of radiation resistance at the current maximum point for a center-fed thin dipole at its various resonances: 1st--------------------------------72 ohms 2nd------------------------------200 ohms 3rd-------------------------------102 ohms 4th-------------------------------260 ohms 5th-------------------------------117 ohms 6th-------------------------------295 ohms 7th-------------------------------127 ohms 8th-------------------------------321 ohms 9th-------------------------------135 ohms 10th-----------------------------340 ohms Note that those are not the feedpoint impedances. They are based on the current maximum points which often occur somewhere besides the feedpoint. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
#52
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| "Roy Lewallen"
| wrote in message ... | [...] | The Z of an | infinite length antenna is indicated by locating the centers of the | circles and noting that the center converges. | [...] | Roy Lewallen, W7EL If we discuss here the impedance referenced to the input (base) current - and not to the maximum one - then IMHO: The quoted text above does not prove convergence. The convergence must be independent of the way the length goes to infinity. The centers of whatever circles may converge to a finite complex number but their radii have to simultaneously converge to zero, to have convergence. But the limit for Z exists if and only if both the limits for R and X exist. Therefore if the limit for R is dependent on the way the length goes to infinity then its limit does not exist. A guess for either a non-existent limit for R or an infinite one comes out from: http://antennas.ee.duth.gr/ftp/visua...s/fu010100.zip [850 KB] If either of the above is true for R then the corresponding is true for Z: The limit for Z does not exist or is (in general) the complex infinity. But always and only for the the impedance referenced to the input (base) current. Sincerely, pezSV7BAXdag |
#53
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pezSV7BAXdag wrote:
The limit for Z does not exist or is (in general) the complex infinity. As the length of a dipole is increased, for the same power input, more energy is radiated during the first transcient cycle and less is available for reflection from the ends of the dipole. Reflected energy is what is causing the feedpoint impedance to change. As the length of the dipole is incrementally increased, the magnitude of the reflected energy is incrementally decreased. I believe Balanis alludes to this characteristic of standing-wave antennas. The feedpoint impedance is Zfp = (Vfor+Vref)/(Ifor+Iref) using phasor addition. The limit of that equation as Vref and Iref go to zero is Vfor/Ifor. That's what happens for an infinitely long dipole. That's also what happens during the transient phase of a finite dipole. Thus, Vfor/Ifor can be thought of as the characteristic impedance of the dipole. Seems to me, Vfor/Ifor could actually be measured during the transient phase of a long finite dipole. Will a TDR report the ratio of V/I for an RF pulse? -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#54
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Cecil Moore wrote:
Roy Lewallen wrote: The theoretical values converge at 214 - j189 ohms, and the measured values at 218 - j174 ohms. Free space? As a data point, I pushed EZNEC to the limit on 40m with a 9000 ft. dipole. Resonant feedpoint resistance at 7.152 is 390 ohms. Anti-resonant feedpoint resistance at 7.092 is 1980 ohms. It appears that EZNEC would converge to something in between those two values for an infinite dipole in free space. Forgot to add, EZNEC would also converge to approximately the same reactance value as above. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#55
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Cecil, W5DXP wrote:
"Note that those are not the feedpoint impedances.". Well, the imprdance I qupted was the radiation resistance, which is the voltage to current ratio of an antenna at the maximum current point in the antenna. I was just too lazy to post the complete Table given by Bailey. Only when the order of resonance in a center-fed dipole is odd, 1st, 3rd, 5th, etc, is the feedpoint resistance the same as the radiation resistance. In odd-ordered resonances, the antenna feedpoint resistance is te same as the radiation resistance. In odd-orfered resonances, the antenna feedpoint is at s current loop. In even-ordered resonances, the value of the feedpoint resistance is equal to the feedpoint impedance squared, divided by the radiation resistance value. Bailey has worked it all out for us. Best regards, Richard Harrison, KB5WZI |
#56
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| -----------------------------------------------------------------
| Subject...: 73 Ohms, How do you get it? | Sent......: Monday, September 19, 2005 5:27 PM | Newsgroups: rec.radio.amateur.antenna | From......: "Cecil Moore" | ----------------------------------------------------------------- | [...] | As the length of a dipole is increased, for the same | power input, more energy is radiated during the first | transcient cycle and less is available for reflection | from the ends of the dipole. Reflected energy is what | is causing the feedpoint impedance to change. As the | length of the dipole is incrementally increased, the | magnitude of the reflected energy is incrementally | decreased. I believe Balanis alludes to this characteristic | of standing-wave antennas. | | The feedpoint impedance is Zfp = (Vfor+Vref)/(Ifor+Iref) | using phasor addition. | | The limit of that equation as Vref and Iref go to zero | is Vfor/Ifor. That's what happens for an infinitely | long dipole. That's also what happens during the transient | phase of a finite dipole. Thus, Vfor/Ifor can be thought | of as the characteristic impedance of the dipole. Seems | to me, Vfor/Ifor could actually be measured during the | transient phase of a long finite dipole. Will a TDR | report the ratio of V/I for an RF pulse? | -- | 73, Cecil http://www.qsl.net/w5dxp | ----------------------------------------------------------------- Do you mean that since the length is infinite there is no reflected wave? But then here it is, once again, one of the most controversial issues... Well, I think we are in the front of a case in which the limit depends on the way we approach it. Every logical way to approach a limit is permissible. And these ways are infinite in number of course. But the convergence is too demanding: "She" wants all these limits to be equal. If just two of them are unequal the convergence simply does not exist. Mathematically this is not a rare case, they say. Sincerely, pezSV7BAXdag |
#57
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"Ian Jackson" wrote in message ... In message , Reg Edwards writes What is the impedance at the centre of an infinitely long dipole (in free space)? =============================== Its not very different from - Zin = 120 * Ln( Wavelength / d ) ohms. where d = conductor diameter, both measured in metres. Thus, at wavelength = 80 metres with 14 gauge copper wire, input impedance = 1300 ohms approx. If you don't believe me, just measure it. ---- Reg. Are you sure it's as high as that, Reg? I once did a Smith Chart plot of the impedance at the centre of a dipole, the valued being taken from a table 'compiled by Wu' (LK Wu?). These only catered for a lengths up to a few wavelengths. As the plot progressed round and round the Smith Chart, it seemed to be heading for something around 350 to 400 ohms. I've just done a search on 'Wu+dipole+impedance', and one of the results is http://www.fars.k6ya.org/docs/antenn...nce-models.pdf I'll have a read of it today. Cheers, Ian. -- |
#58
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"Ian Jackson" wrote - Are you sure it's as high as that, Reg? I once did a Smith Chart plot of the impedance at the centre of a dipole, the valued being taken from a table 'compiled by Wu' (LK Wu?). These only catered for a lengths up to a few wavelengths. As the plot progressed round and round the Smith Chart, it seemed to be heading for something around 350 to 400 ohms. I've just done a search on 'Wu+dipole+impedance', and one of the results is http://www.fars.k6ya.org/docs/antenn...nce-models.pdf I'll have a read of it today. =================================== The characteristic impedance of an infinitely long wire is Zo. If we cut the line and measure between the two ends we obtain an input impedance of twice Zo. Which is the answer to our problem. Zo is a function of wavelength, conductor diameter and conductor resistance R where R includes the uniformly distributed radiation resistance. On a high Zo line the radiation resistance is small compared with Zo and the only effect of the radiation resistance is to give Zo a small negative angle. Which when estimating Zo can be ignored. (It is conductor resistance which at HF gives Zo of ALL lines a very small negative angle). In the problem posed, the current is also uniformly distributed along the low-loss line and radiation resistance is not the value we are familiar with and what we might do with it. And so we get approximately - Rin = 120 * ( Ln( Wavelength / 2 / d ) - 1 ) At a wavelength of 2 metres and a conductor diameter of 10mm the input resistance = 433 ohms. I cannot guarantee the above formula to be correct. But is it low enough for you? ;o) Mr Wu calculates radiation resistance which is not the same as input impedance unless correctly referenced. It is usual in technical papers to calculate Radres at one end of the antenna. Or it may be the distributed value. I havn't the time to find and study the full text. From past experience, with me, it usually ends up as a wild goose chase. ---- Reg. |
#59
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pezSV7BAXdag wrote:
Do you mean that since the length is infinite there is no reflected wave? But then here it is, once again, one of the most controversial issues... Yep, it's my digital logic in action. :-) If there's no end, then there cannot be reflections from the ends. And please note that I am not saying that the characteristic impedance of a dipole is constant all up and down the line. I'm only concerned about the apparent characteristic impedance at the feedpoint, i.e. Vfor/Ifor. Consider an open circuit transmission line. At 1/4WL, it exhibits a very low impedance. At 1/2WL it exhibits a very high impedance. As the transmission line length is increased to infinity, because of losses, the "stub" impedance will spiral into the Z0 characteristic impedance point. A similar concept should apply to a dipole. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#60
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Reg Edwards wrote:
In the problem posed, the current is also uniformly distributed along the low-loss line and radiation resistance is not the value we are familiar with and what we might do with it. Reg, in the real world, an antenna has radiation losses so the current decays along its length. Is there any formula that includes an attenuation factor for a traveling wave antenna? It would be akin to the attenuation factor for a transmission line but presumably higher. I have estimated that, for a 1/2WL dipole, the reflected voltage and current have dropped by approximately 10% below the forward voltage and current during the round trip to the ends of the antenna and back. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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