J.
The key to success in tuning an unknown LF antenna or any for that
matter is first finding where it is resonant.
I use a large stable inductor. I call mine the 'mother of all inductors'
and it is wound on a big pail and works out to be about 7.5mH. It's not
pretty or eff. but will resonant the system somewhere low and has lots
of taps all up and down it...
1) connect said inductor in series with the proposed antenna. Use all
the inductance...
2) attach a small TX WITHOUT any filtering or other frequency sensitive
networks on the output see:
http://www.mlecmn.net/~lyle/test-tx/test-tx.htm , use a signal generator
as the exciter with a sweepable dial knob or other suitable source as in
the link. The key being you need to SWEEP the frequency to find
resonance where ever it may be...
3) power her all up at LOW power and sweep the frequency and determine
where resonance by monitoring the DC input current. Start low and work
your way up until you get the first peak in current. Be careful, you're
looking for the first large peak, harmonics above the natural freq can
be misleading.
4) measure the inductance somehow (meter, scope, etc...) of the coil and
using the known frequency of resonance calculate the C of the antenna.
5) calculate the L the coil should be to resonant that antenna using the
C above and 137KHz.
6) build coil with variometer and install, Use scopematch to fine tune
the system for resonance and impedance...
7) Enjoy being QRO.
73 Scott
J. Allen wrote:
Alan, Steve and All,
Thanks for the feedback.
The antenna is basically an inverted-L antenna. Using approximate
dimensions, the vertical portion is a 110 foot tower, but the
attachment point for the horizontal is down at about 100 feet. The
horizontal is composed of two, 390 foot long wires that form a V with
the included angle of about 50 degrees. The ends of the L wires are
supported at about 70 feet and 63 feet and the wires are single span
and have bout 10 or 12 feet of sag for stress. The antenna is over a
set of about 30, #11 radials of various lengths from 30 to 1200 feet
in length. The soil is very dry sand in a flat in the bottom bowl
formed by mountains. The antenna is fed through about 250 feet of 50
Ohm hard line from the shack.
At the base of the antenna, the vertical portion terminates at the
input of a Variometer the other end of which ties to the radial ground
system. The variometer is currently link coupled to the coaxial
cable, but the problem was there when the grounded side of the
variometer was tapped up like an autotransformer, and also when the
antenna was fed through a very large roller inductor /
autotransformer. In fact there have been a number of different feed
methods tried and all failed.
If I am not mistaken, the antenna calculated at about 1500 pF.
Nothing I have tried to date has resulted in successful tuning. The
most power I have been able to sustain is about 30 Watts. Higher
power has always resulted in smoke.
The main limiting factor in all this work is the operator... I have
symptoms of early Alzheimer's Disease, and get confused about what I
am doing or where I am in a process. A written step by step which I
could check off as I go would be a great asset.
It is important to the Canadian and possible future US amateur plans
for a LF portion on 137 that I be successful here. We have great
resistance from the power companies with their Power Line Carrier
systems here. I am fortunate that I live in one of the two areas of
Canada where 137 kHz is used for PLC, and the central purpose of my
experiments is to get the station up to full power and show
compatibility of PLC and amateur radio.
I have had really wonderful support from Steve and Scott, and have
both of the UK LF books as guides but still cannot seem to get the
antenna tuned. I am familiar with the use of an oscilloscope and
maintained the PLC system which I am demonstrating non-interference with.
As soon as I can get the antenna tuned properly, I can run the power
level up and begin beaconing essentially 24/7 for the remaining months
of our experiments.
Has anyone produced a very detailed first time tuning guide for
beginners like me? I am about 1000 miles away from the closest
assistance (VE7SL/VE7TIL)
Thanks again,
JA
VY1JA
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