[AMPS] Comments please

[Peter Chadwick]

Paul asked :

>I'd be pleased to hear any comments on the specification
> for a reed relay for the RF output switch of an HF amp.
>.. (into 50 ohms please)


I've been on holiday for a week, so only just read the e-mails....and yes,
thanks for asking, it was fine and the autotuning on my antenna tuner is now
finished and working!

I tried reed relays some 30 years ago for antenna switching in a marine tx. The
standard reed is of course, made of magnetic material: this means that the skin
resistance is multiplied by 1/sq.rt of mu, the permeability. This can lead to
the interesting phenomena of the RF current heating the reed up until the
material passes its Curie point, whereupon the reed opens. Then it cools down,
and the cycling carries on every two or three seconds. Reeds for RF need to be
pretty heavily plated with a good conductor. I believe some manufacturers make
reeds especially for this application.

A similar  problem appeared when I went to Racal.  The project I was in charge
of had been running for some time: one difficulty was that the receiver
preselector selectivity was too low, because of low tuned circuit Q....this
screwed up the losses, too. The cause was switching the coils with reed
relays........substituting a Ledex driving conventional wafer switches took the
tuned circuit Q from around 30 to over 120.


73

Peter G3RZP


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