Dear Werner, LF Group,
At 19:52 25/01/2003 +0100, you wrote:
At first sight it work FB
Loop perimeter is 85 meters and 12 meters above ground
Series coil is 1.5 mH series cap is home made butterfly 280 pf
plate space is 20mm, current in the circuit is 3.5 Amp
Cap and coil are in the shack , and two wires are going
out through the wall widely spaced from the wall, and so
straight to the loop .
In the shack it smells like ozone Hi
You didn't say exactly how the tuning components and TX are connected to
the loop - but it seems to me that if there is a significant voltage
between the loop wire and ground, the antenna could actually be operating
as a vertical tuned against ground. Obviously the antenna radiates quite
well, but it is not clear that it is actually radiating in a horizontal
loop mode - 3.5A seems a low current for a loop antenna to radiate much
signal. The highest possible radiation resistance would occur if the loop
were circular - in that case the area would be 575m^2, giving Rrad of
0.44milliohms, and a very optimistic estimate of ERP of maybe 10mW,
assuming none is dissipated in the ground or environment.
I expect the loop will have an inductance between 100 - 200uH; If both the
1.5mH coil and 280p capacitor are connected in series with one terminal of
the loop, and the other terminal is near ground potential, the loop wire
will be at a low potential (with 3.5A , some 100's of volts maximum), and
there will be little radiation in "vertical" mode. But if this is the case,
with total L of 1.7mH, and C of 280p, resonant frequency will be about
230kHz or slightly more. If, on the other hand, the inductor is connected
to one terminal of the loop, and the capacitor to the other, the whole loop
wire will be at high potential, and there will be a large "displacement
current" flowing to ground through the antenna capacitance, so one would
expect the antenna to be operating primarily as a vertical. In this
condition, the capacitance to ground of the loop wire would effectively be
in parallel with the 280p tuning capacitance. Estimating this to be 6pF/m,
85m of wire gives 510pF, so 790pF total capacitance. With 1.5mH
inductance, this gives a resonant frequency of 146kHz, much nearer
reality! There will be additional capacitance due to the connections to the
shack, which would bring the resonant frequency to 136k. In this condition,
the voltage on the loop wire would be roughly 4.5kV, and the antenna would
be predominantly acting as a vertical, giving roughly 300mW ERP, again
assuming no environmental losses.
Cheers, Jim Moritz
73 de M0BMU
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