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#11
December 8th 03, 08:09 PM
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In article , Bill Turner
writes: On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote: Write on the whiteboard 100 times: Inductance does not change with frequency...reactance changes with frequency. _________________________________________________ ________ Not true. Inductance and reactance are related by the formula XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and vice versa. How could you possibly define it otherwise? Bill, I can get down to first principles if necessary, but that isn't necessary, is it? INDUCTANCE doesn't change over frequency...even above the "self-resonance" due to distributed capacity between windings. That's very basic and applies up into the region where the frequency is so high the whole "coil" structure starts behaving like a distributed-constant conglomeration of equivalent parts. But, that's a specialty area and far above any practical application of home-made coils for RF. Reactance is a function of frequency and inductance. The reactance of an inductor DOES change over frequency. That's also very basic. For _practical_ home-made coils, the only major concern is the distributed capacity of the coil structure. Distributed capacity is the _equivalent_ of a fixed, parallel capacitor across the pure inductor part of the coil. That L and C will determine the "self resonance" of the structure. To find the distributed capacity of an inductor (the equivalent of a fixed parallel capacity connected across the inductor), the method described in the "Reference Data for Radio Engineers," fourth edition, 1956, ITT (aka "Green Bible"), chapter 10, pp 268- 269 can be used as follows: Using a Q Meter or other instrument with a calibrated variable capacitor, resonate the parallel L-C with the capacitor at two frequencies exactly an octave apart (1:2 ratio). Take the difference of the two variable capacitor resonating values as "deltac." Let "freqsq" be the _square_ of the highest of the two frequencies used. For uHy, pFd, and MHz: L = (19,000) / (freqsq x deltac) Inductance L is the "true" inductance of the coil, separated from the distributed capacity. The constant of "19,000" is a simple approximation considering that 1956 was the age of slide rules and electromechanical four- function calculators. If the parallel resonating capacitor is well- calibrated, the "true inductance" formula works out well. If the parallel resonating capacitor is not calibrated, forget the whole thing; there are several C-meters on the market that can allow rather precise +/- 0.1 pFd resolution calibration if anyone is into home metrology. Anyone wishing to play with simple algebra can figure out the formula from basic resonance equation at two frequencies exactly an octave apart. That will result in the true mathematical value of the constant given in the Green Bible. :-) Len Anderson retired (from regular hours) electronic engineer person |
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