I'm having a problem which has me stumped. I'm going to describe my
complete antenna and feed system because something in it is misbehaving
and I don't know what!
My system works on all bands from 40 to 10 (or it should).
The antenna is a full-size 20m rotary dipole. It is all aluminum tubing,
no traps or stubs. Just a dipole. I am feeding it with about 30 feet of
"true ladder line," which is open wire line made of #16 insulated wire
spaced about 3-1/4" with black PVC spacers every 18" or so, except near
the antenna and the rotor where I've added extra ones so that the
spacing doesn't change when the antenna rotates.
The line comes into the shack and is connected to a static drain, which
is a box with two 10-megohm high voltage resistors to ground and a
couple of spark gaps. Then a piece of 450-ohm window line about 3 feet
long connects it to a pair of large air variable capacitors in series
with each leg which knock out some of the reactance on 40m to make it
possible to tune more easily. Then a very short piece of window line
connects to a big 5kW DX Engineering 4:1 balun, spec'ed for tuner
service, and finally via a piece of RG-213 18" long, to a T-network tuner.
My K3 drives a TL922 amp and I have an SWR meter in line.
Now here is my problem: it works OK on all bands except 40 meters. On
40, it tunes up fine with low power, but when I run more than a couple
of hundred watts, after perhaps 10 seconds of key-down, the SWR starts
to climb. I have watched it go to 4:1 before I stop sending for fear of
destroying something.
The SWR rises both on the meter in the tuner and the extra one I have in
line.
Classic symptoms of something heating up. But what?
- The tuner components are all cold.
- The coax to the balun and its connectors are cold.
- The balun itself is just barely perceptibly warmer (I have to touch
the core to tell).
- The window line, the static drain resistors, the air capacitors and
all the connections in the shack are cold.
I know the SWR is astronomical on 40 meters, so currents and voltages
are high. But nothing in the shack seems to be heating up. Any more
ideas of where to look?
--
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/
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