[Amps] Blue Disc Ceramic caps ??

[peter chadwick]

When I was at Marconi's, back in the 1960s, there were marine tranmsitters being made that ran a pair of 4-400 at about 1 kW out. Covered the marine bands from  416 kHz. Obviously, with the antenna load being something like 350pF in series with 5 ohms, there were some fair size inductors and in fact a variometer for the PA Tuning. This was all in a framework of synthetic resin bonded paper (SRBP) and carefully held together with brass screws. Somebody in production decided that a significant cost reduction could be made by substituting cadmium plated and passivated  steel screws.
Which was fine until the test people fired it up on 500kHz. Of course, around the variometer and so on, there was a pretty intense magnetic field, and the result was that the SRBP burst into flames.......
When GBR, Rugby Radio was built in the very early 1920s, operating on 16 kHz and around 300kW, the attic of the tx hall had the aerial tuning inductor in it. True Litz wire, in a cable about 2 inches in diameter, and the coil about 4 feet in diameter. The roof consisted of tiles nailed down with steel nails - and it caught fire, too. They changed to wooden pegs.
Steel and tank circuits don't go that well together.
73
Peter G3RZP


========================================
Message Received: Aug 18 2012, 12:41 AM
From: "Rob Atkinson" 
To: "Bill, W6WRT" 
Cc: "AMPS" 
Subject: Re: [Amps] Blue Disc Ceramic caps ??

>
> What's wrong with steel? As long as you don't put it INSIDE the coil, of course.
> 73, Bill W6WRT

some steel alloys heat up when conducting RF. Not all do but it is
difficult to tell which do, and do not visually, so it is preferable
to avoid steel as a series RF conductor. Non-ferrous alloys or
elements such as brass, copper, and aluminum are usually employed.
I've used brass washers in some situations in which the steel fastener
is merely performing a mechanical not electrical function.

This is why you are apt to find brass hardware in RF tank circuits.

73

Rob
K5UJ
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