Hi John,
thanks for sharing this experience with us!
It should be nice to share similar topics, maybe around a fire drinking
either an old fashioned bourbon (or bear depending on personal
preferences..)
Sometimes we give compromise to ruggerness or aesthetic but RF continues to
have its own habits.
73, Marco IK1HSS
----- Original Message -----
From: "John Andrews" <[email protected]>
To: <[email protected]>
Sent: Wednesday, June 26, 2013 10:49 PM
Subject: Re: LF: MF propagation
> 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
>
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