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Re: [Amps] Relais for an antenna crossover switch

To: "Guido Tedeschi" <>
Subject: Re: [Amps] Relais for an antenna crossover switch
From: "Tom W8JI" <>
Date: Wed, 27 Jun 2007 10:10:22 -0400
List-post: <>

First, I have never seen in single case in all the hundreds 
of relays I have tested at high power RF where the contact 
RF behavior is an issue. Sometimes the contact support, 
which can be steel or an alloy with high resistance or 
exaggerated skin effect is an issue with heating, very often 
the flexible wire leads are an issue with heating, but I've 
yet to see a contact heat issue if the contact is OK at low 
frequency and the same current. I have tested many relays!!

On your switch, my feeling is you are going through far too 
many relays and the connection lengths will be too long. I 
understand what you are doing to make it safe, but I'm 
afraid with so many relays and so much connection length 
isolation and SWR will not be so good.

You could compensate for this with capacitors (like the 47pF 
you show) and with selection of layout and materials, but 
large numbers of large relays in the path makes isolation 
and SWR more difficult above a few MHz. If you find that not 
a problem, then all is well.

Watch your groundplane! It carries as much current as the 
center conductors do, and is just as responsible for 

73 Tom

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