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#11
November 30th 07, 08:48 PM
posted to rec.radio.amateur.antenna
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Loading Coils; was : Vincent antenna
On 30 Nov, 12:25, K7ITM wrote:
On Nov 30, 10:59 am, Jim Kelley wrote: K7ITM wrote: On Nov 29, 9:11 am, Jim Kelley wrote: ... Over the range of a few octaves, propagation delay on the other hand does not vary to any significant extent as a function of frequency. Ostensibly, it should be equal to sqrt(LC) series L, shunt C. Actually, Jim, I do expect it to have considerable frequency dependence. I think you can find info about this in books that address the design of travelling-wave tubes. I can't think of an example of an active (or reactive) device which doesn't have frequency dependent characteristics. To the extent that indices of refraction are frequency dependent, propagation velocity does in fact vary with frequency. If it didn't, we wouldn't see rainbows. Dielectric constants do indeed have a frequency dependence. But to first order, at radio frequencies, in amateur applications, for the purposes of this discussion, and in my opinion, the effect is less than considerable - particularly if we assume the L and C in sqrt(LC) are correct at the frequency of interest. ;-) 73, Jim AC6XG OK, that leaves us with a difference of opinion, or a difference in what we are describing. There was an article in "RF Design" maybe 15 years ago now by John Mezak, K2RDX, describing a helical transmission line model for coils. At the time, he offered free software to execute the calculations (which also, to me, offered a very practical way to calculate coil parameters like inductance, effective shunt capacitance, and first parallel and series self resonances). He later charged a nominal fee for an improved version of the software, which I have. For the "100 turn, 10 inch long, 2 inch diameter" coil wound with 15AWG copper wire, using John's program, I see a variation of about 2:1 in propagation velocity between 1MHz and 20MHz. Since the first parallel self-resonant frequency is predicted to be around 8MHz, it's perhaps not fair to look as high as 20MHz, but even between 1MHz and 4MHz, I see about 25% change in predicted propagation velocity. You may say that perhaps John messed all that up terribly, but I don't think so...and there are other places you can find similar results. There's an excellent inductance calculator on-line athttp://hamwaves.com/antennas/inductance.html, and though the absolute value of its prediction of propagation velocity is about 5% different than Mezak's, they both show very nearly the same percentage change with frequency. It might be worth having a bit closer look at, Jim. Perhaps it's just that you're thinking of a different effect than what these two programs (and the theory behind them) are modelling. Cheers, Tom- Hide quoted text - - Show quoted text - Where can I obtain a copy of Johns program? TIA Art |
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