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#11
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Tom Bruhns wrote:
"What`s the reactance at the anti-resonance point? Is highest SWR at anti-resonance or at maximum reactance or at some point between?" I don`t quite understand the questions but that`s no inhibition. The anti-resonance point must be at the drivepoint of an end-fed 1/2-wave element because anti-resonance is the frequency at which the impedance of the system is very high. If the system is anti-resonant, reactance is zero. Highest SWR comes from greatest mismatch. It gets no worse that from an open or short circuit. The original question concerned 1/4-wave and 1/2-wave resonances. SWR is highest at the reflection point because it is a creature of the reflected wave`s reaction with the incident wave, and nowhere can the reflected wave be stronger than at the tip of the antenna. The open circuit at the tip has some capacitance, but the only effect of this reactance is to shift the standing wave pattern along the antenna according to transmission line analogy. Unlike a transmission line, the impedance seen by a wave traveling in either direction along an antenna varies from one end to the other. As usual, I hope to learn something from this thread. Best regards, Richard Harrison, KB5WZI |
#12
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Reg Edwards wrote:
Cec, & Co. Zo of an antenna wire seems to be an important parameter in your discussions. Why do you say that? The only thing I have dealt with is a constant feedpoint impedance at any one frequency. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#13
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Reg Edwards wrote:
Cec, & Co. Zo of an antenna wire seems to be an important parameter in your discussions. Here's an example of what I have been talking about. The feedpoint impedance of my 130 ft. dipole (according to EZNEC) is: 60+j0 ohms at 3.63 MHz - resonance 2513+j2314 at 6.575 MHz - maximum reactance 5000+j0 at 7.118 MHz - anti-resonance Assuming the antenna is fed with Z0 = 550 ohm open-wire line, the SWR ranges from 8.5:1 to 9.2:1 and the impedance at a current maximum point on the ladder-line ranges from 60 ohms to 65 ohms. The feedpoint impedances from 3.63 MHz (resonance) to 7.118 MHz (anti-resonance) describe an imperfect semi-circle on a Smith Chart normalized for 550 ohms. If one plots the feedpoint impedances for all the different bands on a Smith Chart normalized for SQRT(Rresonant * Ranti-resonant), it will resemble an imperfect SWR circle (or imperfect spiral). By feeding my dipole only at the current-maximum points, I achieve a near-perfect match on all amateur HF bands without an antenna tuner. At least for my multi-band dipole, it appears that the anti-resonant feedpoint impedance is about 100 times the resonant feedpoint impedance. The feedpoint impedance at the maximum reactance point is about 5000/2+j5000/2, i.e. the R is about half the anti-resonant resistance and the Xmax is about half the anti-resonant resistance times 'j'. These are my rules-of-thumb for my dipole. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#14
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"Cecil Moore" wrote in message ... 5000+j0 at 7.118 MHz - anti-resonance making up terms again cecil?? I have never heard of 'anti-resonance' and don't see it in my references here... i would say it is just resonant at the second harmonic, just because the impedance is high doesn't mean it is 'anti' anything, or anything else special. it is just plain resonant at 7118. |
#15
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making up terms again cecil?? I have never heard of 'anti-resonance' and
don't see it in my references here I have heard the term used before. I typed anti resonance into Google and got 296,000 hits. I don't think the term was made up. 73 Gary N4AST |
#16
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"JGBOYLES" wrote in message ... making up terms again cecil?? I have never heard of 'anti-resonance' and don't see it in my references here I have heard the term used before. I typed anti resonance into Google and got 296,000 hits. I don't think the term was made up. 73 Gary N4AST it would appear from a quick check that many of those 'anti-resonance' references are mechanical (preventing resonance), quantum mechanical(some kind of spin resonance), optical(something about lithography masks), or at best electro-mechanical (lots of piezo-electric references in there), i did see one that was explaining electrical resonance as mechanical analogies that used that term. and, oh yes, there was another discussion group that had latched onto it as an alternate term for a 'parallel resonant' circuit. |
#17
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Dave wrote:
"Cecil Moore" wrote: 5000+j0 at 7.118 MHz - anti-resonance making up terms again cecil?? I have never heard of 'anti-resonance' and don't see it in my references here... i would say it is just resonant at the second harmonic, just because the impedance is high doesn't mean it is 'anti' anything, or anything else special. it is just plain resonant at 7118. There's lots of thing of which I am ignorant but I don't make postings about those things. The first four books I picked up had information on anti-resonance. Are you familiar with books? :-) A web search for "anti-resonance" turned up 700 hits. Perhaps you don't need a book. _Radio_Handbook_, 20th edition, by Bill Orr: "In radio circuits, parallel resonance (MORE CORRECTLY TERMED ANTIRESONANCE) is more frequently encountered than series resonance; ..." emphasis mine _The_ARRL_Handbook_, 17th edition, page 14-5: "A capacitor connected across a cavity provides an ANTI-RESONANT notch below the resonant frequency (f0),..." emphasis mine _Electronic_Fundamentals_and_Applications_, John D. Ryder, page 287 under section 9-13. Parallel-Resonant Circuits: "The total admittance of the parallel-resonant (SOMETIMES CALLED ANTIRESONANT) circuit is ..." emphasis mine _IEEE_Dictionary_: "antiresonant frequency - Usually in reference to a crystal unit or the parallel combination of a capacitor and inductor." Since the resonant feedpoint impedance of a 1/2WL dipole acts somewhat like a series resonant circuit, at Texas A&M we called that the resonant frequency. Since the resonant feedpoint impedance of a one-wavelength dipole acts somewhat like a parallel resonant circuit, at Texas A&M we called that the anti-resonant frequency. Differentiating between the resonant frequency and anti-resonant frequency gives one additional information about the impedance to be matched. "Resonant" = low, "Anti-resonant" = high. It's a useful concept. You really should try it. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#18
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Dave wrote:
... and, oh yes, there was another discussion group that had latched onto it as an alternate term for a 'parallel resonant' circuit. According to the IEEE Dictionary, that's exactly what it is. And the feedpoint of a one-wavelength center-fed dipole acts a lot like a parallel resonant circuit. Somehow, saying that the resonant feedpoint impedance of your dipole is 6000 ohms leaves something to be desired. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#19
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Cecil Moore wrote in message ...
Tom Bruhns wrote: Cecil Moore wrote: alhearn wrote: My question is what determines where that peak occurs? Mathematically, it will be where the SWR circle is tangent to the reactance arc. That is not in general true; do you have reason to believe it's true for the impedance of a dipole? When the dipole is at maximum feedpoint reactance, can the SWR be calculated? Of course. Will that point, when plotted on a Smith Chart lie on the SWR circle? Of course. Will it also lie on a reactance arc? of course. Will the SWR circle be tangent to that reactance arc? Of course. I'm sorry, Cecil, but you lost me there. For any given SWR circle, there are only two (complex conjugate) points at which reactance arcs are tangent. Why would we think that the point of max reactance on the antenna impedance curve will necessarily be at the point of tangency? The antenna impedance arc of the simple dipole I modelled indeed does not lie tangent to the max reactance arc at the same point as the SWR circle that's tangent that reactance arc. In any event, I don't see that this tells us anything about _why_ the dipole shows max reactance at that particular frequency. |
#20
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Nope! Cecil's not making up terms.
You just missed a point in your education. Dave wrote: "Cecil Moore" wrote in message ... 5000+j0 at 7.118 MHz - anti-resonance making up terms again cecil?? I have never heard of 'anti-resonance' and don't see it in my references here... i would say it is just resonant at the second harmonic, just because the impedance is high doesn't mean it is 'anti' anything, or anything else special. it is just plain resonant at 7118. |
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