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Re: LF: MF propagation

To: [email protected]
Subject: Re: LF: MF propagation
From: Stefan Schäfer <[email protected]>
Date: Thu, 27 Jun 2013 22:07:53 +0200
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Hi Marco and John,

For some reason i didn't receive John's answer on the topic, odd.

John, thanks for the story. Well, it wasn't my idea to use a steel wire. It was rather a construction engineer who was working for the university. They hung up the wire for me. However, a copper wire wouldn't be the best idea as well here. The length is 70m and the wind blasts can be quite high. Furthermore the wire had to be dimensioned to withstand a direct lightning strike. Not my idea, too. But not bad :-) All in all i know the wire will take any natural wind load (isolators take 60 kN !) and snow or a few 100 birds (automatic variometer will would compensate the additional C, i.e. will match the birds :-) ) and it's loss resistance will be in the range of a typical amateur radio copper wire, due to the high circumference. Ah yes, the higher diameter will prevent partial discharges due to the high electrical fields. I guess the benefit from a parallel RF litz wire (Decca) would be minimal and the litz wire would suffer from environmental erosion (water, UV radiation...)
Marco,
Yes, we should have a big fieldday some time! I would like to visit the UK for that. Guess we can use the field of Eddie's neighbour :-)
73, Stefan/DK7FC


Am 27.06.2013 21:36, schrieb Marco Cadeddu:
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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