Hello Vic,
It sure behaves like a ferrite core heating up! After that, you must check
everything up at the antenna. One thing you might try is to see how long it
takes for the VSWR to drop to normal after a long key down session. A long
time indicates something with some mass to it that holds heat. If it takes
awhile to cool down, I would not suspect hot wires or an overheated joint.
Dave K1WHS
----- Original Message -----
From: "Vic Rosenthal 4X6GP/K2VCO" <[email protected]>
To: "Elecraft Reflector" <[email protected]>
Sent: Tuesday, January 12, 2016 9:24 PM
Subject: [Elecraft] OT antenna question
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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