[Elecraft] OT antenna question

[Vic Rosenthal 4X6GP/K2VCO]

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/