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Does reactance of dipole depend on diameter ??
Oops, I made a couple of mistakes the
Dave wrote: I wish to know if the reactance of a dipole that is physically 0.5000 wavelengths in length depends on the diameter of the wire or not. I know a dipole 0.5 wavelength long is not resonate, but inductive so you need to shorten it a few percent to bring it to resonance. I know the length at resonance depends on wire diameter. But I'm interested if the reactance does very with wire diameter when the antenna is physically 0.5 wavelengths long, which means it will be somewhat inductive. A book published by the ARRL by the late Dr. Laswon (W2PV) Lawson J. L., “Yagi Antenna Design”, (1986), The American Radio Relay League. ISBN 0-87259-041-0 has a table of reactance vs the ratio K (K=lambda/a, where a is the radius) for antennas of 0.45 and 0.50 wavelengths in length. I've reproduced that table below. The first column (K) is lambda/a The second column (X05) is the reactance of a dipole 0.5 wavelengths in length. The third column X045 is the reactance for a dipole 0.45 wavelengths in length. K X05 X045 ------------------------- 10 34.2 23.1 30 36.7 6.4 100 38.2 -14.1 300 39 -33.6 1000 39.6 -55.5 3000 40 -75.7 10000 40.4 -98.1 30000 40.6 -118.6 100000 40.8 -141.1 300000 41.0 -161.8 1000000 41.1 -184.4 What one notices is: 1) Reactance for 0.45 lambda is very sensitive to radius, varying by more than 200 Ohms as K changes from 10 (fat elements) to 1000000 (thin elements). 2) The value for a dipole 0.5 lambda in length changes much less (only 6 Ohms), but it *does* change. 3) For infinitely thin elements (K very large), the reactance of a dipole 0.5 lambda in length looks as though it is never going to go much above 41.2 Ohms. Certainly not as high as 42 Ohms. Now I compare that to a professional book I have: Balanis C. A., “Antenna Theory – Analysis and Design”, (1982), Harper and Row. ISBN 0-06-0404458-2 There is a formula in Balanis' book for reactance of a dipole of arbitrary radius and length, in terms of sine and cosine integrals. It's hard to write out, but the best I can do gives: Define: eta=120 Pi k=2/lambda k = 2 Pi / lambda, not 2 / lambda. You can possibly see that when the length is 0.5 lambda, the sine term in there is always zero, so the radius 'a' has no effect on the reactance. What is interesting about that is that for a length of 0.5 lambda, the reactance does not depend on wavelength at all - it is fixed at 42.5445 Ohms. So two different books give two quite different results. Sorry, I mean the reactance does not depend on radius when the dipole is 0.5 wavelengths in length. |
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