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#11
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On Apr 2, 8:33*am, Cecil Moore wrote:
wrote: I was wondering if the self shielding properties of the toroid would have contributed to this conclusion, and because of these properties, the toroid not have any electrical degrees, so to speak, so when it come to the radiating element it would need to be slightly longer to see the electrical degrees for the wavelength or resonance frequency injected into it. thanks again for the info. I suspect that the VF of the toroidal loading coil is much higher than the VF of an air-core loading coil, i.e. the toroidal loading coil occupies fewer electrical degrees of the antenna. It makes sense that if the toroidal loading coil occupies fewer electrical degrees of the antenna that those degrees must be furnished somewhere else. The toroidal loading coil seems to be closer to the lumped circuit model than is the large air-core loading coil which generally requires analysis using distributed network techniques. http://www.ttr.com/TELSIKS2001-MASTER-1.pdf There are two things happening with a base loading coil. The loading coil occupies a certain number of degrees, e.g. ~36 degrees for a 75m bugcatcher coil. The stinger occupies maybe ~11 degrees for a total of ~47 degrees. The other ~43 degrees comes from the phase shift at the impedance discontinuity between the coil and the stinger. With a center loading coil, a few degrees are lost at the impedance discontinuity between the base section and the coil. That's why a larger coil is needed for a center-loaded mobile antenna. -- 73, Cecil, IEEE, OOTC, *http://www.w5dxp.com I am a little confused, you say about the coil and the stinger occupying a portion of the total 90 degrees, (I can follow this not a problem), however I thought the purpose of the coil was to add inductive reactance, due to the shortened length of the radiator, less than 1/4 wave, therefore having a capacitive reactance overhaul. Let the shortened vertical be 34-j234 ohms. My understanding is that depending on on what the reactance is at some freq, I need to offset this negative reactance with an equal positive reactance. I wasn't looking at from a degrees point for view. Or is it just 6 of one, half dozen of the other, both be equal just expressed differently, I like the degrees point of view for a couple of other arrays I am experimenting with. (long wires) I think it will simplify things a whole lot. thanks again for the info and the links, very valuable info. 73 brett |
#13
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On Apr 2, 7:17*pm, Cecil Moore wrote:
It depends upon which math model is being used and whether the math model is valid under the existing conditions. We can neutralize capacitive reactance with a lumped inductance or with a stub or with a helical coil. The results are approximately the same but there are differences. If the current amplitude changes significantly between the bottom and top of the loading coil, the lumped circuit model is not valid because the coil is an appreciable percentage of a wavelength more akin to a transmission line than a lumped circuit. -- 73, Cecil, IEEE, OOTC, *http://www.w5dxp.com Now it is clearer to me. I understand why there is a difference using lumped circuit analysis as opposed to the the actual coil or stub analysis. You have provided much valuable info on this subject. Thanks again. I will load some math program on my pc any do some of the analzsis of my design and compare it to what I have measured now with the ones I have built The reason this came about to me was to use some very short but extremely strong 1.5" insulators that I have.In order to get the required inductance with the length of insulators that I have the losses in the coil would be large, that is why I chose the toroid. Did you ever try to model a resonator where the coil required is actually longer than the insulator, so that the vertical part of the resonator is actually inside the coil , would it somewhat have a larger inductance then the expected air wound value, similar to a coil with a ferite inside it (tuning slug) or would the radiator not even see the amount of the coil that it covered therefore have a lower inductance. I was thinking, instead of spreading the turns on the coil to adjust amount of inductance simply move the vertical element in and out of the coil to adjust it. There discussions before were for a center loading coil which will be constant, but this last part of discussion I would like to try for base loaded coil and make it variable but not tap it conventionally . Hope I described well enough for you see what I have in mind, Again thanks for the info.73 brett |
#14
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#15
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wrote:
Did you ever try to model a resonator where the coil required is actually longer than the insulator, so that the vertical part of the resonator is actually inside the coil , ... Never modeled it but have seen its effects during 75m mobile shootouts. One fellow had a hamstick with the stinger bottom extending down into the hamstick loading coil. It was a very lossy condition which improved when he hack-sawed the excess stinger off. Do you have EZNEC? If so, I can modify a helical loading coil to extend the stinger into the middle of the coil and send the file to you. Conductors within the loading coil field lower the effective coil Q. However, I have had good luck with a 1.25" fiberglass pipe running through the center of the coil providing mechanical support. Perhaps you could consider swapping your insulator for a piece of fiberglass pipe or rod. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
#16
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Richard Clark wrote:
One inductance value (however it is wound) is not equivalent everywhere - hence the proposition of a coil having an equivalent length is rather preposterous. Nobody said one inductance value is equivalent everywhere. In fact, just the opposite is true. The equivalent length for different coils with the same inductance depends upon their physical configurations. For instance, w8ji's loading coil (100t, 10tpi, 2"dia) calculates to have a VF of 0.0328 while my 75m Texas Bugcatcher coil with approximately the same inductance calculates to have a VF of 0.0198, a 66% difference proving your above premise to be false. ************************************************** ***** THERE IS NO REQUIREMENT THAT TWO COILS WITH THE SAME INDUCTANCE OCCUPY THE SAME NUMBER OF DEGREES OF ANTENNA. ************************************************** ***** To understand the details, just compare the following 1/4WL resonant stubs: ---25.3 deg Z0=600 ohm---+---10 deg Z0=50 ohm---open ---70.6 deg Z0=100 ohm---+---10 deg Z0=50 ohm---open Both stubs are electrically 1/4WL = 90 degrees long. How can 35.3 physical degrees of stub perform a 90 degree stub function? Hint: There is a 54.7 degree phase shift at the '+' junction. Why does the second example require 80.6 physical degrees of feedline? Hint: There is only a 9.4 degree phase shift at the '+' junction. Why does it take 70.6 degrees of Z0=100 ohm feedline to perform exactly the same function as 25.3 degrees of Z0=600 ohm feedline? Both stub segments, Z0=600 and Z0=100, are providing the same inductance at the '+' junction point, yet one is 2.8 times longer than the other. Is "the proposition of a piece of transmission line having an equivalent length rather preposterous"? -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
#17
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Cecil Moore wrote:
wrote: Did you ever try to model a resonator where the coil required is actually longer than the insulator, so that the vertical part of the resonator is actually inside the coil , ... Never modeled it but have seen its effects during 75m mobile shootouts. One fellow had a hamstick with the stinger bottom extending down into the hamstick loading coil. It was a very lossy condition which improved when he hack-sawed the excess stinger off. some times a hack saw *can* be a fine tuning tool. Do you have EZNEC? If so, I can modify a helical loading coil to extend the stinger into the middle of the coil and send the file to you. Conductors within the loading coil field lower the effective coil Q. However, I have had good luck with a 1.25" fiberglass pipe running through the center of the coil providing mechanical support. Perhaps you could consider swapping your insulator for a piece of fiberglass pipe or rod. Ahh, I had the idea of doing such a thing (add vertical travel) for my bugcatcher to fine tune swr if it needs tweaked a little. But it looks like I'll be hopping outside the car a lot after all. For 75/80 meters, I have like 4 separate taps. - 73 de Mike N3LI - |
#18
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On Apr 2, 10:34*pm, Richard Clark wrote:
On Thu, 2 Apr 2009 16:42:32 -0700 (PDT), wrote: Did you ever try to model a resonator where the coil required is actually longer than the insulator, so that the vertical part of the resonator is actually inside the coil Hi Brett, After following Cecil's pet theory, and your statement of finding it useful, your question above becomes a remarkably perplexing application of that knowledge. The theory should have answered your question before hand. * Thanks very much for the information you provided to my posting. The theory to the question was answered, I was asking the question once again to the group to see if others have input on the subject I posted. Knowing me I will probably ask many more questions that I have already have the answer to, just to raise conversation and debate to get other views on the subject. Thanks again for the info on the subject, much appreciated. 73, vo1bbn Brett |
#19
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On Apr 2, 10:34*pm, Richard Clark wrote:
On Thu, 2 Apr 2009 16:42:32 -0700 (PDT), wrote: Did you ever try to model a resonator where the coil required is actually longer than the insulator, so that the vertical part of the resonator is actually inside the coil Hi Brett, After following Cecil's pet theory, and your statement of finding it useful, your question above becomes a remarkably perplexing application of that knowledge. The theory should have answered your question before hand. * Thanks very much for the information you provided to my posting. The theory to the question was answered, I was asking the question once again to the group to see if others have input on the subject I posted. Knowing me I will probably ask many more questions that I have already have the answer to, just to raise conversation and debate to get other views on the subject. Thanks again for the info on the subject, much appreciated. 73, vo1bbn Brett |
#20
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On Fri, 03 Apr 2009 10:39:37 -0800, Richard Clark
wrote: On Fri, 03 Apr 2009 07:34:20 -0500, Cecil Moore wrote: Richard Clark wrote: One inductance value (however it is wound) is not equivalent everywhere - Nobody said one inductance value is equivalent everywhere. Nobody is not very many people which should make this a short list. But I don't see any names - strange. In fact, just the opposite is true. which must mean Nobody said one inductance value is NOT equivalent everywhere. or, perhaps: EVERYBODY said one inductance value is equivalent everywhere. or, perhaps: EVERYBODY said one inductance value is NOT equivalent everywhere. or, perhaps: Nobody said one inductance value is equivalent NO where. or, perhaps: Nobody said NO inductance value is equivalent everywhere. or, perhaps: EVERBODY said NO inductance value is equivalent NO where. ... proving your above premise to be false. Which apparently nobody (somebody, everbody?) said. The power of persuasion (Cecil's Degenerative form of the Sub-optimal Hypothesis of Information Transformation) at its inventive best to be able to wrestle two contradictory claims together and prove them both true and false simultaneously! Cecil must have been posed some very difficult advice as a little nipper: "Nobody jumped off the roof because everone did - wouldn't you?" 73's Richard Clark, KB7QHC This reminds me of a quote from Yogi Berra (as I recall) about a retaurant in New York City: "The place is so crowded, nobody goes there anymore". W0BF |
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