There are two things that always seem to surprise me when changing from
136kHz to 73kHz: the huge amount of extra inductance needed to resonate my
small antenna; and the enormous voltages generated at this frequency. Both
of these caused me trouble when setting up on Friday for the 73kHz Final
Fling.
At first I got my usual 1A antenna current from the 200W BK amp, then the
current dropped to a very low level. Panic! Had I blown up the amplifier
(fixable in a couple of hours) or had the elevated coil flashed over
(fixable in a day after recruiting assistance to drop the pole)? A check
with my Decca on 136kHz showed that the fault was also on that band, so it
wasn't the Tx. I could hear fizzing so I suspected something was flashing
over. I couldn't trace it so waited until dark. All was revealed then in a
pyrotechnic display from inside the tube carrying the antenna through the
house wall. I pulled out the cable and found it badly burned. Replacing this
with some UR67 fixed the problem. I went to bed happy that all was well - a
mistake!
Early Saturday morning, I did a final tune-up and had an antenna current of
just over 1A. However I could hear more fizzing noises. To make a long story
short, after two hours I had moved the bottom loading coil outside as even
the UR67 was glowing like a purple florescent light. Moving the coil had the
advantage of reducing the voltage passing through the wall and this stopped
all of the flashing over. However, because it was now further up the
antenna, its effect was less, and the resonant frequency went up to 75kHz!
Experience told me that I would have to add a surprising amount of
inductance to bring this down to 72kHz, and I already had my three big coils
in circuit. Experience also told me the quick fix - tape some ferrite pieces
to a plastic pipe and move it inside one of my coils (note that not all
ferrite is suitable - some gets very hot). At last I had a working system,
and the best current ever at 1.3A.
Result: three CW QSOs, G3LDO, G3AQC and M0BMU.
I will beacon for some of today, using CW and QRSS3. Please E-mail me for
skeds or requests for longer QRSS.
Will also be active Sunday. And Monday, too, right up to 2359UTC.
Mike, G3XDV
http://www.lf.thersgb.net
====================
|
