At 18.24 07/03/99 +1100, Steve Olney wrote:
The questions I have are:
1. How do you measure the capacitance to ground to be able to calculate
the inductance of the loading coil? I doubt you can just attach a
capacitance meter between the base of the antenna and ground. I am
guessing that you just make an estimate and find the required inductance by
trial-and-error.
My personal experience is that many capacitance meter are fooled by the
RF and noise pick up by the antenna. I solved the problem connecting the
vertical in parallel to a fixed C and then measuring the difference. The fixed
one was a couple of nF.
Then I built an RF impedance bridge, and calibrated it on the bench, to be
able to measure R(loss) and C(aerial). I had the same reading as with the
capacitance meter.
2. Is it possible to connect a smaller *variable* inductance (about 10% of
the loading coil inductance) between the base of the antenna and the
grounded elevated radials to effect a convenient tuning arrangement?
It should be possible. I have a different arrangement, having a variometer
at the base of the vertical and no inductance at the top, so no direct
experience,
but I see no particular problems to have, let say, 90% of the L on top and
the remaining at the base. The current distribution should not vary too much.
3. Given the arrangement outlined in 2. above, is it possible to tap into
that smaller inductance at the base to effect matching to the 50 ohm line
or don't you get the transformer effect required because the major portion
of the inductance is elevated and not inductively coupled?
Again, I see no problems. Personally I forgot SWR and coupling ... my system
has an input Z of about 35 Ohm (dry WX) to 50 Ohm (rain). The coax line is
too short in terms of lambda to worry about SWR ! I directly go to the TX
output.
On what frequency do you expect to trasmit, and what ERP ... ?
73 de IK1ODO Marco
Rivalta, ITALY JN35SA (N 45 01' 25.6", E 7 31' 09.4")
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