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Date: Wed, 26 Jun 2013 18:49:05 -0400
From: John Andrews <w1tag@charter.net>
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Subject: Re: LF: MF propagation
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Stefan,

> The feeder is 10mm diameter stainless steel. I guess it takes at least
> 300A before it melts :-) So the limit would be the withstand voltage of
> the ceramic isolator, even on MF :-)

Brings back an old memory - from about 40 years ago. My boss and I were 
doing some routine maintenance on the power divider for a 3-tower AM 5kw 
broadcast array on 580 kHz. The box (circa 1939) had a fairly large coil 
for power division, and a couple of networks for matching and a part of 
the phasing. The big coil therefore had a bunch of taps, including one 
to short out some turns at the "hot" end. The short was done with an old 
piece of tinned braid, and its position had to be shifted a couple of 
times a year.

My boss remarked that the jumper (about 15 cm) was looking sort of 
ratty, so I suggested that we replace it with a solid copper strap. He 
remarked that he had always wanted to try a nice shiny piece of 1.2 cm 
stainless-steel "Wraplock" as an RF conductor. We had several rolls of 
it, used in securing conduits and cables to towers. So he cut off a 
length, drilled the needed holes, and we put it in place. Turned the rig 
on, and the current into the divider was normal. Then it started to 
drop. And drop.

Killing the transmitter, we opened the cabinet, and found a nice black 
stainless steel jumper that was still very warm. We switched to copper, 
and all was fine. I'd guess that the current through the strap was 
around 10 Amps -- not sure after all these years. So, I'd suggest 
treating stainless-steel with caution as an RF power conductor!

John, W1TAG