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Jim Kelley wrote:
Personally, I think some actual measurements would help enormously in formulating/evaluating a model. Otherwise all we have is hand waving and proselytizing. I have reported my 25 nS actual measurements using my dual-trace 100 MHz o'scope. I assume that your physics lab has the ability to perform such a simple measurement. May I suggest that you simply perform the experiment and report the results - or get one of your students to do it. I suspect that others have done such experiments, measured what I measured, and are withholding the results for obvious reasons. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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Art wrote:
"Thus Kraus`s antennas are not in equilibrium and thus deviated away from Maxwell`s laws." Impossible. Maxwell`s laws are all that is nscessary and sufficient to describe radiation from any antenna. On page 37 of Kraus & Marthelka`s "Antennas for All Applications" one can read: "Although a charge moving with uniform velocity along a sreaighr conductor does not radiate, a charge moving back and forth in simple harmonic motion along the conductor is subject to aceleration (and deceleration) and radiates." To better understand Maxwell and radiation, I recommend "Radio-Electronic Transmission Fundamentals" by B. Whitfield Griffith, Jr (now reprinted by Scitech Publishing Inc.. See "Directive Patterns Over Real Groind" in the "ARRL Antenna Book" for how rays combine to make a pattern. Best regards, Richard Harrison, KB5WZI |
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Cecil Moore wrote:
I assume that your physics lab has the ability to perform such a simple measurement. May I suggest that you simply perform the experiment and report the results - or get one of your students to do it. I'm not the one hawking the tonic, Cecil. But send me the coil, and I'll happily test it, report back, and return it to you after. ac6xg |
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Jim,
I don't have the measurement capabilities that some of you folk have, so I'm just using EZNEC as my "experimental equipment". I'm being assured that it's representative of what I would see "for real", and it's easier pressing keys than building new test equipment :) Are you suggesting that "real world" current measurements would be significantly different than the EZNEC predictions? I gather from your response that you feel the Corum model is inappropriate for the "bugcatcher", but you're not advocating an alternative? That's the part I struggle with - if a model gets close (albeit not perfect) to predicting "real world" performance, and there's no other alternative being put forward, I don't see the reason for rejecting it so forcefully? 73, Steve G3TXQ On May 8, 8:21*pm, Jim Kelley wrote Hi Steve, Personally, I think some actual measurements would help enormously in formulating/evaluating a model. *Otherwise all we have is hand waving and proselytizing. 73, ac6xg |
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Hi Tom,
Well I don't know as much about antennas as I would like :) I take your response to mean that you think only MoM can model a "bugcatcher" coil accurately, and that you are dismissing the apparent accuracy with which the Corum model predicts some coil performance parameters? I don't subscribe to the Corum-Moore "label". The genesis of the transmission-line approach to coil analysis seems to go back a long way from what I've read, and I don't think Cecil deserves or claims any recognition for it. Besides the method might suddenly begin to appear in all the text books - think how you'd feel then if it included his name;) Steve G3TXQ On May 8, 10:44*pm, "Tom Donaly" wrote: You know quite a bit about antennas, Steve, so you should know the answer to the following: 1. Mathematically, what does MoM do? 2. Why would anyone use MoM if there were a set of symbolic equations that would work just as well? 3. When are we going to see the Corum-Moore method in the textbooks? 73, Tom Donaly, KA6RUH |
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"Art Unwin" wrote in message ... Atta boy, Keep using that slide rule from your school days, there is absolutely no reason why you should change and update actually, i think i still have one or two of those laying around here somewhere. |
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Art Unwin wrote:
Atta boy, Keep using that slide rule from your school days, there is absolutely no reason why you should change and update Art Your answers are just as wrong with a slide rule, an HP15C, Fortran IV on a 360/65, C on a 64 bit AMD or anything else you can find. And denigrating slide rules is silly. Most of the world that surrounds you was calculated with a slide rule's resolution. When used properly they give answers that are as accurate as is needed for engineering. You obviously have no clue as to what it takes to do engineering calculations. Richard, if I used terms improperly, I ask forgiveness. tom K0TAR |
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Tom Ring wrote:
Art Unwin wrote: Atta boy, Keep using that slide rule from your school days, there is absolutely no reason why you should change and update Art Your answers are just as wrong with a slide rule, an HP15C, Fortran IV on a 360/65, C on a 64 bit AMD or anything else you can find. And denigrating slide rules is silly. Most of the world that surrounds you was calculated with a slide rule's resolution. When used properly they give answers that are as accurate as is needed for engineering. You obviously have no clue as to what it takes to do engineering calculations. Richard, if I used terms improperly, I ask forgiveness. tom K0TAR There's no point in asking forgiveness from Richard. He's read _Through the Looking Glass_: "I don't know what you mean by 'glory,'" Alice said. Humpty Dumpty smiled contemptuously. "Of course you don't - till I tell you. I meant 'there's a nice knock-down argument for you!'" "But 'glory' doesn't mean 'a nice knock-down argument,'" Alice objected. "When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean, neither more nor less." "The question is," said Alice, "whether you _can_ make words mean so many different things." "The question is," said Humpty Dumpty, "which is to be master -- thats all." 73, Tom Donaly, KA6RUH |
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Tom,
I thought I'd quoted some numbers in the related "Dish reflector" thread - apologies if I did not. Here are some mo * I modelled a coil as a spiral in EZNEC (40T, diameter=6", length=12", #14 copper wire) * I added a 6ft "stinger" and found the frequency where the combination was resonant: 3.79 MHz * I checked the feedpoint impedance without the coil present: 0.46- j2439 * That tells me the "lumped circuit equivalent" reactance of the coil at 3.79 MHz is +j2439 ohms * I found the frequency where the coil was resonant with no "stinger": 6.2 MHz Now I look at what ON4AA's "Corum method" inductance calculator tells me: * "Lumped circuit equivalent" reactance at 3.79 MHz: +j2449 * Self-resonant frequency: 6.3 MHz Unless I'm missing an option, if I want to predict the RF characteristics of a "bugcatcher" it seems I have 3 choices: * Use Wheeler's formula * Build a helical model in EZNEC * Use the Corum method Wheeler's formula is inappropriate at frequencies close to a coil's SRF. EZNEC and the Corum method give very close results. The Corum formulas are not difficult to use; even if they were, there is an on-line calculator which removes the need for any maths. So it seems to me the Corum formulas would be the more convenient tool to use, at least for a "first look". 73, Steve G3TXQ On May 9, 7:06*am, "Tom Donaly" wrote: You know, you haven't shown that the Corum model accurately measures the bugcatcher coil. You have stated - and I have no reason to disbelieve you - that the Corum model agrees with EZNEC. If that's the case, it's just as easy to use EZNEC, right or wrong. MoM is a method of obtaining numerical solutions to integral equations. The only reason to do that is if symbolic solutions are either too difficult or impossible to puzzle out of those same integral equations. In other words, some very deep thinkers decided that MoM would give results superior to algebraic approximations and hand waving, so they applied it to antenna analysis. I don't think it's perfect. It's certainly useful. If you think Corum is good enough for your purposes, though, go for it. |
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Tom Donaly wrote:
3. When are we going to see the Corum-Moore method in the textbooks? "Transmission Lines and Networks", by Johnson copyright 1950 "Fields and Waves in Modern Radio", Ramo and Whinnery, Copyright 1944, 1953 - my fields and wave textbook in ~1957 at Texas A&M. The fundamentals of everything I have presented have been in that textbook for 65 years and I'm sure it was not the first textbook on the subject. "Reflection and Transmission at a Discontinuity" Equations for traveling waves vs standing waves "Energy Theorems for Transmission Lines" "The Idealized Helix and Other Slow-Wave Structures" Separate forward and reflected Poynting vectors whose ratio is rho^2 "Quarter-wave coating for Eliminating Reflections" "Elimination of Reflections from Dielectric Slabs" "Scattering and Transmission Coefficients" "Directional Couplers" Add "Antennas ..." by Kraus and Balanis Add "Optics ..." by Hecht and Born and Wolf Add "Traveling Wave Engineering", by Moore Add "Reflections", by Walter Maxwell One cannot blame one's ignorance on a lack of textbooks. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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wrote:
... I don't think Cecil deserves or claims any recognition for it. I've been accused of inventing theories while at the same time being accused of quoting others too much. All I have ever done is borrow some technical water from the experts who came before me to use in wearing away some of the mythical stones presented as technical facts by the misinformed on this newsgroup and others. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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Jim Kelley wrote:
I have no particular knowledge or opinion on Cecil's EZNEC files, other than his rather odd take standing wave current phase shift - whatever that is. It's not my take, Jim, it is EZNEC's take. EZNEC reports current on a standing-wave antenna that closely matches the standing wave equations for current whose phase cannot be used to measure delay through a wire or through a loading coil. Both w7el and w8ji used the current on a standing wave antenna to measure phase shift and predictably, there was negligible phase shift. They erroneously attributed the lack of phase shift with a lack of delay, i.e. they assumed the proof - a well known logical fallacy. There is no relationship between phase shift and delay in the current on standing wave antennas either through the wire or through a loading coil. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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On May 9, 7:48*am, Cecil Moore wrote:
Jim Kelley wrote: I have no particular knowledge or opinion on Cecil's EZNEC files, other than his rather odd take standing wave current phase shift - whatever that is. It's not my take, Jim, it is EZNEC's take. EZNEC reports current on a standing-wave antenna that closely matches the standing wave equations for current whose phase cannot be used to measure delay through a wire or through a loading coil. Both w7el and w8ji used the current on a standing wave antenna to measure phase shift and predictably, there was negligible phase shift. They erroneously attributed the lack of phase shift with a lack of delay, i.e. they assumed the proof - a well known logical fallacy. There is no relationship between phase shift and delay in the current on standing wave antennas either through the wire or through a loading coil. -- 73, Cecil, IEEE, OOTC, *http://www.w5dxp.com Cecil I believe Steve has vindicated you in your struggle so you should feel good. I would avoid the tactics if I were you when attempts are made to reserect this debate.Corum are acknowledged experts with respect to coils and tho not as absolute as Maxwells equations one should feel confident with respect to their works so I feel you have finally made your point. Well done Art |
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On May 9, 1:40*am, wrote:
Tom, I thought I'd quoted some numbers in the related "Dish reflector" thread - apologies if I did not. Here are some mo * I modelled a coil as a spiral in EZNEC (40T, diameter=6", length=12", #14 copper wire) * I added a 6ft "stinger" and found the frequency where the combination was resonant: 3.79 MHz * I checked the feedpoint impedance without the coil present: 0.46- j2439 * That tells me the "lumped circuit equivalent" reactance of the coil at 3.79 MHz is +j2439 ohms * I found the frequency where the coil was resonant with no "stinger": 6.2 MHz Now I look at what ON4AA's "Corum method" inductance calculator tells me: * "Lumped circuit equivalent" reactance at 3.79 MHz: +j2449 * Self-resonant frequency: 6.3 MHz Unless I'm missing an option, if I want to predict the RF characteristics of a "bugcatcher" it seems I have *3 choices: * Use Wheeler's formula * Build a helical model in EZNEC * Use the Corum method Wheeler's formula is inappropriate at frequencies close to a coil's SRF. EZNEC and the Corum method give very close results. The Corum formulas are not difficult to use; even if they were, there is an on-line calculator which removes the need for any maths. So it seems to me the Corum formulas would be the more convenient tool to use, at least for a "first look". 73, Steve G3TXQ On May 9, 7:06*am, "Tom Donaly" wrote: You know, you haven't shown that the Corum model accurately measures the bugcatcher coil. You have stated - and I have no reason to disbelieve you - that the Corum model agrees with EZNEC. If that's the case, it's just as easy to use EZNEC, right or wrong. MoM is a method of obtaining numerical solutions to integral equations. The only reason to do that is if symbolic solutions are either too difficult or impossible to puzzle out of those same integral equations. In other words, some very deep thinkers decided that MoM would give results superior to algebraic approximations and hand waving, so they applied it to antenna analysis. I don't think it's perfect. It's certainly useful. If you think Corum is good enough for your purposes, though, go for it. Steve, this is fine for a base loading coil, but I'd suggest you try your experiment with a loading coil well up the antenna, where the coil is significantly larger diameter than the straight conductor in which it's placed. The same size coil you described (though presumably a different number of turns), placed at least half way up something like a 15 or 20 foot long thin wire, should illustrate the point. Is the EZNEC model then in such good agreement with placing a reactive load at that point in the antenna, where the reactance is from ON4AA's online calculator? If I trusted NEC to handle large steps in conductor diameter accurately, I'd suggest putting a segment in the antenna description to represent the length and diameter of the coil, with the calculated reactance placed as a load in that segment. As I understand it, though, NEC has trouble with large diameter steps. Cheers, Tom |
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K7ITM wrote:
Steve, this is fine for a base loading coil, but I'd suggest you try your experiment with a loading coil well up the antenna, where the coil is significantly larger diameter than the straight conductor in which it's placed. The same size coil you described (though presumably a different number of turns), placed at least half way up something like a 15 or 20 foot long thin wire, should illustrate the point. Is the EZNEC model then in such good agreement with placing a reactive load at that point in the antenna, where the reactance is from ON4AA's online calculator? The key to understanding this question and its logical answer lies in the phase shift that occurs at impedance discontinuities. For a base-loading coil, there is only one impedance discontinuity in the system, a hi-Z0 coil to a low-Z0 stinger. That single discontinuity provides a positive phase shift at the '+' junction of the coil and stinger. coil stinger FP//////////+------------------- When a straight shaft section is installed under the coil, it introduces one additional impedance discontinuity at 'x' in addition to the '+' top of coil to stinger discontinuity. base coil stinger FP-------x////////////+--------- Because the impedance discontinuity between the base section is a low-Z0 to hi-Z0 transition, the phase shift is negative, i.e. the antenna *loses electrical degrees* at that junction. Therefore, more turns must be added to the inductor to supply the number of negative degrees lost at the base section to coil impedance discontinuity. This might best be illustrated with pieces of transmission line. Please reference my web page at: http://www.w5dxp.com/shrtstub.htm The following concepts apply to the above antennas but may be easier to understand using transmission lines. Here is a dual-Z0 stub that is physically 44.4 degrees long but is 90 degrees (1/4WL) long electrically, i.e. it is functionally a 1/4WL open-circuit stub. ---22.2 deg 300 ohm line---+---22.2 deg 50 ohm line--- The Z0=300 ohm to Z0=50 ohm transition provides for +45.6 degrees of phase shift. This is akin to the base- loaded antenna above. Here is a dual-Z0 stub with 11.1 degrees (half) of the 50 ohm line moved to the left. (The words are abbreviated because of space on the line.) --11.1 deg 50--+--22.2 deg 300--+--11.1 deg 50-- Who can tell me how long electrically is this stub using the identical feedlines from the above example? This reconfigured stub with half of the 50 ohm feedline moved to the bottom is now electrically only ~80.6 degrees long. What has happened? The new impedance discontinuity from the base section at the bottom of the coil has cost us electrical degrees by providing a *negative phase shift*. How do we solve the problem? Add some length (degrees) to the Z0=300 ohm section. If we make the 300 ohm section 38.5 degrees long, the stub will be electrically 90 degrees long once again. This is conceptually the same problem we encounter when we move the loading coil from the base location to the center location. When we move the coil up the shaft, we introduce a negative phase shift at the bottom of the coil. Therefore, we must increase the number of turns to make the loading coil electrically longer. Incidentally, w8ji knows about the coil to stinger positive phase shift and describes it on his web page. He apparently doesn't know about the opposite negative phase shift at the bottom of the coil where the shaft attaches. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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Cecil Moore wrote:
"One cannot blame one`s ignorance on a lack of textbooks." True. The most reliable in my opinion is Terman`s 1955 opus "Electronic and Radio Engineering". Terman agrees with Cecil. On page 854 Terman writes: "The laws governing such radiation are obtained by using Maxwell`s equations to express the fields associated with the wire; when this is done there is found to be a component, termed the radiation field, having a strength that varies inversely with distance." Terman then gives the formula for the electric field strength in terms of distance from the elementary doublets in the wire that make up the antennna to a distant observing point P, and angle of the direction of point P with respect to a plane perpendicular to the axis of the elementary doublet. The strength of the radiated field is distributed in space in accordance with the doughnut pattern for a thin wire which is short compared with wavelength and has a figure-of-8 cross section. Illustrations are provided on page 865. On page 866 Terman illustrates current distribution on an antenna open circuited at both ends and made up of elementary doublets. On page 867 Terman says: "A wire antenna is a circuit with distributed constants; hence the current distribution in a wire antenna that results from the application of a localized voltage follows the principles discussed in Chap. 4 (Transmission Lines), and depends upon the antenna length, measured in wavelengths; the terminations at the ends of the antenna wire; and the losses in the system. The current distribution is also affected by the ratio of the wire length to diameter in situations where the wire is unusually thick. Under most circumstances the losses are sufficiently low and the ratio of wire length to diameter sufficiently great so that to a first approximation very closely the current distribution can be taken as that for a line with zero losses; it then has the characteristics discussed in 4-5. Best regards, Richard Harrison. KB5WZI |
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Richard Harrison wrote:
"A wire antenna is a circuit with distributed constants; hence the current distribution in a wire antenna that results from the application of a localized voltage follows the principles discussed in Chap. 4 (Transmission Lines), and depends upon the antenna length, measured in wavelengths; the terminations at the ends of the antenna wire; and the losses in the system." In other words, an antenna acts a lot like a lossy transmission line where the loss from the system is radiation. Here's a transmission line example using resistance wire in a transmission line to emulate the losses normally due to radiation in an antenna. Note that the feedpoint impedance is around 35 ohms. http://www.w5dxp.com/stub_dip.EZ -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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Tom,
OK, I tried what you suggested. I put my loading coil midway up a 20ft vertical wire in the EZNEC model. I reduced the number of turns to lift the resonant frequency to 5.6MHz. EZNEC predicted that the magnitude of the current at the top of the coil would be 77% of the magnitude at the bottom. Then I removed the coil in the model, replaced it with a straight wire containing an EZNEC lumped load, and adjusted that load for antenna resonance at 5.6MHz again. I needed +j1630. Given the dimensions of the coil, the Corum calculator predicted a lumped circuit equivalent reactance of +j1573, and it predicted a current fall-off across the coil of 78%. 73, Steve G3TXQ On May 9, 5:35*pm, K7ITM wrote: Steve, this is fine for a base loading coil, but I'd suggest you try your experiment with a loading coil well up the antenna, where the coil is significantly larger diameter than the straight conductor in which it's placed. *The same size coil you described (though presumably a different number of turns), placed at least half way up something like a 15 or 20 foot long thin wire, should illustrate the point. *Is the EZNEC model then in such good agreement with placing a reactive load at that point in the antenna, where the reactance is from ON4AA's online calculator? |
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Tom,
I just modelled that coil in EZNEC and it was self-resonant at 7.5MHz - very close to the Corum model prediction of 7.4137MHz. Interesting that both EZNEC and the Corum are wrong and by the same amount! 73, Steve G3TXQ On May 9, 10:06*pm, "Tom Donaly" wrote: Did you make such a coil and measure its self-resonant frequency? The reason I ask is that I put the dimensions of an old coil I had (D=155mm,length=140mm,wire diameter=1.3mm,N=27 turns) into ON4AA's calculator and got a self resonant number of 7.4137 Mhz. When I measured it, though, it was 8.93 Mhz. Where did I go wrong? Maybe I entered the numbers incorrectly. 73, Tom Donaly, KA6RUH |
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Tom Donaly wrote:
I put the dimensions ... into ON4AA's calculator and got a self resonant number of 7.4137 Mhz. When I measured it, though, it was 8.93 Mhz. Where did I go wrong? The self resonant frequency of a coil varies wildly with the ground provided. What ground did you provide? Perfect? Average? None? -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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wrote:
Tom, I just modelled that coil in EZNEC and it was self-resonant at 7.5MHz - very close to the Corum model prediction of 7.4137MHz. Interesting that both EZNEC and the Corum are wrong and by the same amount! Did you guys agree on the same ground plane? The self resonant frequency of my 75m Texas Bugcatcher coil is very different on my insulated wooden work-bench vs two inches above my GMC pickup. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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Cecil,
I tried mine above Perfect, Mininec Average, Mininec Salt Water. Self resonant frequency was the same in each case. Steve G3TXQ On May 9, 10:49*pm, Cecil Moore wrote: Did you guys agree on the same ground plane? The self resonant frequency of my 75m Texas Bugcatcher coil is very different on my insulated wooden work-bench vs two inches above my GMC pickup. -- 73, Cecil, IEEE, OOTC, *http://www.w5dxp.com |
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Cecil Moore wrote:
wrote: Tom, I just modelled that coil in EZNEC and it was self-resonant at 7.5MHz - very close to the Corum model prediction of 7.4137MHz. Interesting that both EZNEC and the Corum are wrong and by the same amount! Did you guys agree on the same ground plane? The self resonant frequency of my 75m Texas Bugcatcher coil is very different on my insulated wooden work-bench vs two inches above my GMC pickup. I followed Reg's advice and hung mine from the ceiling with a thin thread. 73, Tom Donaly, KA6RUH |
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Cecil Moore wrote:
Tom Donaly wrote: I put the dimensions ... into ON4AA's calculator and got a self resonant number of 7.4137 Mhz. When I measured it, though, it was 8.93 Mhz. Where did I go wrong? The self resonant frequency of a coil varies wildly with the ground provided. What ground did you provide? Perfect? Average? None? If that's true, the Corum brothers should have included that in their formulas. 73, Tom Donaly, KA6RUH |
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Tom Donaly wrote:
I followed Reg's advice and hung mine from the ceiling with a thin thread. That would certainly make it act differently than over a good ground plane. Where was your ground/counterpoise? Which self- resonant frequency did you see? 1/4WL? 1/2WL? -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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Tom Donaly wrote:
If that's true, the Corum brothers should have included that in their formulas. Their formulas assume a near-perfect ground plane. See Figure 2. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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Tom Donaly wrote:
Did you make such a coil and measure its self-resonant frequency? The reason I ask is that I put the dimensions of an old coil I had (D=155mm,length=140mm,wire diameter=1.3mm,N=27 turns) into ON4AA's calculator and got a self resonant number of 7.4137 Mhz. When I measured it, though, it was 8.93 Mhz. Where did I go wrong? Maybe I entered the numbers incorrectly. Please describe your test setup. Where was ground? -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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Cecil Moore wrote:
Tom Donaly wrote: Did you make such a coil and measure its self-resonant frequency? The reason I ask is that I put the dimensions of an old coil I had (D=155mm,length=140mm,wire diameter=1.3mm,N=27 turns) into ON4AA's calculator and got a self resonant number of 7.4137 Mhz. When I measured it, though, it was 8.93 Mhz. Where did I go wrong? Maybe I entered the numbers incorrectly. Please describe your test setup. Where was ground? I'll tell you what, Cecil. If you'll test your coil, and describe how you did it, I'll tell you what I did. Fair? I know you're champing at the bit to claim my test setup was all wrong and that I used standing wave current when I should have been using traveling wave current, and all that, but you'll just have to wait. I reported this in order to get you keyboard theorists off your fundaments and start dealing with the real world. If you don't, you'll forever be plagued by the nagging thought that I might be right. Horrors! 73, Tom Donaly, KA6RUH (P.S. If you don't know how to test for a coil's self-resonant frequency, you should go back to Texas A&M and ask for your money back.) |
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Tom,
You posted earlier "Why would anyone use MoM if there were a set of symbolic equations that would work just as well?" We now have a situation where MoM (EZNEC) and a set of symbolic equations (Corum method) are predicting very similar SRFs. "Verbal sparring" about measurement methods aside, I think it's important to try to understand what's causing the differences between the EZNEC/Corum value and the measured value. Of course it's possible that EZNEC is in error, but it seems odd to me that the Corum value (derived by a completely unrelated technique) would also produce the same erroneous value. My EZNEC model had the coil 12" above Real Mininec ground, and connected to ground via a 12" vertical wire with the source in it. I simply looked for the frequency at which the source impedance was purely resistive. The SRF was relatively insensitive to the height of the coil above ground, and changing the ground type made no difference. I wonder if that model is anything like your own test set-up, and if not whether the differences could explain the different SRFs we're seeing. 73, Steve G3TXQ |
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Tom Donaly wrote:
I know you're champing at the bit to claim my test setup was all wrong and that I used standing wave current when I should have been using traveling wave current, and all that, but you'll just have to wait. That's not true at all, Tom. It's a trivial procedure to use standing waves to determine the approximate 1/4WL self-resonant frequency. But as with 1/4WL monopoles, there is *NO 1/4WL self-resonant frequency* without a ground plane or counterpoise and the quality of the ground-plane/counterpoise has an effect on the self- resonant frequency. Sorry, I don't have anything approaching a perfect ground plane for 4 MHz. I suppose one could use two identical coils and turn it into a dipole but I don't have another 75m Texas Bugcatcher loading coil. Let me say once again: A 1/4WL monopole, all by itself in free space is *NOT resonant*. After all, making guy wire segments 1/4WL long is one way of breaking up their resonance. We are not looking for super high accuracy/precision/resolution here. Almost everything is an approximation because we don't share exactly the same test environments. All I am after is the technical truth - there's nothing personal involved. Neither is a 1/4WL coil self-resonant all by itself in free space. I don't know what Reg was thinking if he advised hanging the coil from the ceiling without a counterpoise. A traveling wave can be used to determine 1/4WL self-resonance but it is a little more complicated than using an MFJ-259B with standing waves. A load resistor minimizes reflections while current probes are used to measure the phase shift through the coil. When the phase shift is 90 degrees, that's the 1/4WL self-resonant frequency. I'll tell you what, Cecil. If you'll test your coil, and describe how you did it, I'll tell you what I did. Fair? 1. I connected my 75m Texas Bugcatcher coil through about a foot of wire to the bumper mount on my GMC pickup and connected my MFJ-259B to the coax connector under the bumper. I tuned for lowest impedance above 4 MHz. The 1/4WL self-resonant frequency was ~6.6 MHz. These measurements were done and reported in March, 2006. 2. I used a tri-mag mount sitting on the hood of my GMC pickup fed through ~6' of coax. The 1/4WL self-resonant frequency was ~6.9 MHz. 3. I used a traveling wave on top of a wooden bench and found the frequency at which the phase shift through the coil was ~90 degrees. That frequency was ~7.2 MHz. 4. I modeled the coil with EZNEC and got some segmentation length warnings. EZNEC reported the 1/4WL self-resonant frequency to be 7.96 MHz. The spread in the above frequency figures is about +/-8%. Again Tom, the only thing I am after is the technical truth. If that is also what you are after, we should have no personal conflicts. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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wrote:
... and changing the ground type made no difference. But Tom hung his coil from the ceiling making ground about five feet away through empty space. What if you raise your system five feet above ground and don't run a wire to ground? i.e. no counterpoise at all. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
Dual-Z0 Stubs
Cecil Moore wrote:
4. I modeled the coil with EZNEC and got some segmentation length warnings. EZNEC reported the 1/4WL self-resonant frequency to be 7.96 MHz. The above modeling was done using a traveling wave, i.e. with the coil terminated in its Z0. 5. Using standing waves, i.e. no termniation, EZNEC said the 1/4WL self-resonant frequency was 7.724 MHz. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
Dual-Z0 Stubs
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Dual-Z0 Stubs
Tom Donaly wrote:
The presence of anything at all near the coil should lower its resonant frequency. That's my experience. Conversely, the farther away from the coil the ground plane is located, the higher the self-resonant frequency. The location of the ground plane has a significant effect on the self-resonant frequency of the coil. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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