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From: David Jefferies
Date: 24 Nov 2001
Time: 13:48:29
Remote Name: 195.92.168.169
Dear Forum,
I am told that measurements on a two-turn loop antenna of copper tube, loop diameter 1 metre, and tube diameter 1cm, show what appears at first sight to be paradoxical results.
Normally we expect the inductance of a tightly coupled two-turn loop to be just four times that of a single turn. If the coupling is reduced, e.g. by separating the turns, then we might expect the inductance to drop.
My informant tells me that if one of the turns is rotated a little bit out of the plane of the other turn, then the inductance (as estimated from the resonant frequency of the total loop when tuned by a parallel capacitor) actually increases by several percent.
Can anyone repeat this observation?
My feeling is that the stray capacitance between the 1cm diameter copper tubes carries some of the current that would otherwise flow through the loop turns. This capacitance decreases as one turn is rotated out of the plane of the other, and so more current is left travelling round the turns to provide magnetic field and inductance.
It would be very interesting to test this with some field measurements.
I was not told the frequency of observation, but assume that this is a "small loop" or compact antenna and therefore much less than a wavelength in diameter.
Incidentally, the stray shunt capacitance carries displacement current. If the displacement current generates magnetic field......
(your starter for a discussion).
regards
David Jefferies.