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Rule of Thumb for coax chokes
On Sun, 13 Aug 2006 12:34:35 GMT, Cecil Moore wrote:
Walter Maxwell wrote: Cecil, I suggest you re-measure the self-resonant frequency of the coil by itself, and if you have the means to do it, also measure the series resistance at both the self-resonant frequency and at 4.0 MHz. I'm betting there will be a large difference in the resistances, and that the self-resonant frequency will be much greater than 6.6 MHz. I'm not sure how to measure the 1/4WL self-resonant frequency with an MFJ-259B without a ground plane. I suppose it could be done using a 1/4WL counterpoise in free space. Let me just report what the MFJ-259B readings are with the isolated 75m Texas Bugcatcher coil across the MFJ-259B terminals. The first dip in impedance is at 14.7 MHz where the MFJ reads 620+j0 ohms. The second dip in impedance is at 24.4 MHz where the MFJ reads 88+j0 ohms. Is the first dip the 1/4WL self- resonant point and the second dip the 1/2WL self-resonant point? I want to make it clear that the previously reported 6.6 MHz self-resonant measurement was made with the base-loaded coil mounted a few inches away from my GMC pickup ground plane. The ground plane was no doubt in the field of the coil at the bottom end so the coil was certainly not isolated as it is in the above reported measurements. Cecil, I measured the self-resonant frequency of the loading coils with a Measurements 59 grid dip osc with the coil mounted away from all metallic objects. The Webster KW-80 coil that measured 14 MHz for the self-resonant frequency, and 8 ohms resistance at 4.0 MHz, as I remember it from several years ago, was about 3" in diameter and around 7 to 8" long. I don't recall now how I measured the Q, but it was close to 400. I measured the resistance with a GR-1606A RF bridge. If you have a GDO I suggest you remeasure the self-resonant frequency, and then measure the resistance at that frequency with the MFJ 259, and then again at 4.0 MHz. From that data you'll be able to determine the actual Q. Seems like it should be around 500, as Reg calculated. But like I said earlier, I believe the self-resonant frequency of your 6" bugcatcher will be greater than 9 MHz. Walt, W2DU |
Rule of Thumb for coax chokes
"Cecil Moore" wrote So what would you call the frequency at which a coil alone is resonant when mounted as a base-loading coil over a ground plane? ================================= Cec, I would call it the frequency at which the coil alone is resonant when mounted as a base-loading coil over a ground plane. It would depend on whether the ground plane was a bicycle or the deck of a super-tanker. ----- Reg. |
Rule of Thumb for coax chokes
"Walter Maxwell" wrote Reg, I had never given much thought to the series relationship of the capacitance between turns. I had always considered them as being in parallel, thus the honeycomb, or the basket-weave configurations to minimize the interturn capacitance. Have I misconstrued the purpose of those configurations? Do I also understand you correctly that with a specified length of the solenoid, and a given diameter, the total interturn capacitance is independent of the number of turns, because the capacitance between turns adds in series to the same value regardless of the number of turns? ==================================== Walt, As I said, I was referring only to the solenoid form. Below the self-resonant frequency and for some way above it, the distributed self-capacitance is equivalent to a lumped capacitor across the ends of the coil. Coi Because capacitances between adjacent turns are in series with each other, the capacitance between turns only matters when there are only one or two turns. So, for ordinary proportioned coils, when there are more than a few turns, the self-capacitance tends to become independent of the number of turns, wire diameter and wire spacing. The wire turns can be considered to form the outside of a Faraday cage. To calculate self capacitance, consider wire spacing to be zero. When isolated in space we have the capacitance between the two fat halves of a dipole. Which is calculable from length and diameter of the coil, and is equivalent to a lumped capacitance between its ends, which may be used to calculate the self-resonant frequency. Or the self-resonant frequency can be calculated directly from dimensions and number of turns. In the past I have measured the self-resonant frequency of coils of all sorts of dimensions. From antenna loading coils, coax choke coils, to 6 feet long, 1 inch diameter, 1000 turns, 160-meter helical antennas. In all cases measurement results agree with the calculating formula within the uncertainties of the measured input data. ---- Reg. |
Rule of Thumb for coax chokes
On Sun, 13 Aug 2006 22:15:38 +0100, "Reg Edwards"
wrote: "Walter Maxwell" wrote Reg, I had never given much thought to the series relationship of the capacitance between turns. I had always considered them as being in parallel, thus the honeycomb, or the basket-weave configurations to minimize the interturn capacitance. Have I misconstrued the purpose of those configurations? Do I also understand you correctly that with a specified length of the solenoid, and a given diameter, the total interturn capacitance is independent of the number of turns, because the capacitance between turns adds in series to the same value regardless of the number of turns? ==================================== Walt, As I said, I was referring only to the solenoid form. Below the self-resonant frequency and for some way above it, the distributed self-capacitance is equivalent to a lumped capacitor across the ends of the coil. Coi Because capacitances between adjacent turns are in series with each other, the capacitance between turns only matters when there are only one or two turns. So, for ordinary proportioned coils, when there are more than a few turns, the self-capacitance tends to become independent of the number of turns, wire diameter and wire spacing. The wire turns can be considered to form the outside of a Faraday cage. To calculate self capacitance, consider wire spacing to be zero. When isolated in space we have the capacitance between the two fat halves of a dipole. Which is calculable from length and diameter of the coil, and is equivalent to a lumped capacitance between its ends, which may be used to calculate the self-resonant frequency. Or the self-resonant frequency can be calculated directly from dimensions and number of turns. In the past I have measured the self-resonant frequency of coils of all sorts of dimensions. From antenna loading coils, coax choke coils, to 6 feet long, 1 inch diameter, 1000 turns, 160-meter helical antennas. In all cases measurement results agree with the calculating formula within the uncertainties of the measured input data. ---- Reg. Thanks, Reg, for the valuable insight. It does pay to read the posts made by one G4FGQ. Walt |
Rule of Thumb for coax chokes
Is Cecil still beating that same old dead horse that only he rides?
Walter Maxwell wrote: On Sat, 12 Aug 2006 14:30:34 GMT, Cecil Moore wrote: Reg Edwards wrote: I agree, a coil at sufficiently high frequencies begins to behave something like a transmission line with a very low velocity factor. Just below its self-resonant frequency, it behaves somewhat like a transmission line of less than 90 degrees. snip But maybe one hoot. :-) My 75m bugcatcher coil is operated relatively close to its measured self-resonant frequency of 6.6 MHz. If I wound a bugcatcher coil to be self-resonant on 4 MHz and then used 2/3 of that coil for a loading coil on 4 MHz, its VF would not change and its electrical length would be 60 degrees accompanied by the appropriate 60 degree delay through the coil. Hi Cecil & Reg Sometime during the '70s I measured the self-resonant frequency of the 80m Hustler loading coil, 6MHz. The series resistance of that coil was 31 ohms at 4 MHz. That is why they claimed 'lower swr than with othe brands'. What a fraud. On the other hand, I also measured the Webster KW-80, self-resonant at 14.0 MHz, with a series resistance of 8 ohms at 4 MHz. I reported this on Page 6-12 in Reflections. So I ask you, Cecil, why would you want a bugcatcher self-resonant at 4 MHz for operation at 4.0 MHz, even if you used only 2/3 of it as a loading coil. Looking just to heat the coil instead of radiating the energy into space? Walt, W2DU |
Rule of Thumb for coax chokes
"Walter Maxwell" wrote Reg, I had never given much thought to the series relationship of the capacitance between turns. I had always considered them as being in parallel, thus the honeycomb, or the basket-weave configurations to minimize the interturn capacitance. Have I misconstrued the purpose of those configurations? Do I also understand you correctly that with a specified length of the solenoid, and a given diameter, the total interturn capacitance is independent of the number of turns, because the capacitance between turns adds in series to the same value regardless of the number of turns? ==================================== Walt, As I said, I was referring only to the solenoid form. Below the self-resonant frequency and for some way above it, the distributed self-capacitance is equivalent to a lumped capacitor across the ends of the coil. Because capacitances between adjacent turns are in series with each other, the capacitance between turns only matters when there are only one or two turns. So, for ordinary proportioned coils, when there are more than a few turns, the self-capacitance tends to become independent of the number of turns, wire diameter and wire spacing. The wire turns can be considered to form the outside of a Faraday cage. To calculate self capacitance, consider wire spacing to be zero. When isolated in space we have the capacitance between the two fat halves of a dipole. Which is calculable from length and diameter of the coil, and is equivalent to a lumped capacitance between its ends, which may be used to calculate the self-resonant frequency. Or the self-resonant frequency can be calculated directly from dimensions and number of turns. In the past I have measured the self-resonant frequency of coils of all sorts of dimensions. From antenna loading coils, coax choke coils, to 6 feet long, 1 inch diameter, 1000 turns, 160-meter helical antennas. In all cases measurement results agree with the calculating formula within the uncertainties of the measured input data. ---- Reg. ========================================= Thanks, Reg, for the valuable insight. It does pay to read the posts made by one G4FGQ. Walt ========================================= Walt, Yes, there is only one G4FGQ. Although I confess I don't spend much time on the air these days. Poor health! When it comes to antennas, one reason why I don't publicise the source code of my programs is that they are full of proven little tricks like the foregoing which give answers in the right american ball-park. There is always the danger that unjustified, unqualified criticism would spoil the confidence and integrity to be placed in them by novices. If for any reason you don't like a program you can always have your money back! One day I might list my 60 years of engineering experience but it may be construed as bragging. And a compliment from you, Walt, is a compliment indeed! ---- Reg. |
Rule of Thumb for coax chokes
On Mon, 14 Aug 2006 15:05:50 +0100, "Reg Edwards"
wrote: "Walter Maxwell" wrote ========================================= Thanks, Reg, for the valuable insight. It does pay to read the posts made by one G4FGQ. Walt = snip ======================================== Walt, Yes, there is only one G4FGQ. Although I confess I don't spend much time on the air these days. Poor health! When it comes to antennas, one reason why I don't publicise the source code of my programs is that they are full of proven little tricks like the foregoing which give answers in the right american ball-park. There is always the danger that unjustified, unqualified criticism would spoil the confidence and integrity to be placed in them by novices. If for any reason you don't like a program you can always have your money back! One day I might list my 60 years of engineering experience but it may be construed as bragging. And a compliment from you, Walt, is a compliment indeed! ---- Reg. Reg, with your experience, along with your wonderful assistance to others through your myriad of useful programs made available at no cost to others, but with much cost to you in terms of time spent creating them, you've earned your bragging rights many times over. Apparently, we have a mutual complimentary relationship. Walt |
Rule of Thumb for coax chokes
Reg Edwards wrote:
I would call it the frequency at which the coil alone is resonant when mounted as a base-loading coil over a ground plane. That's the self-resonant frequency "in situ". -- 73, Cecil http://www.qsl.net/w5dxp |
Rule of Thumb for coax chokes
Walt,
I'm a selfish person just like everybody else. I do it purely for self-satisfaction. ---- Reg. |
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