Trying to resend this...for some reason it bounced :
Interesting...
I see from several relay manufacturers specs that they actually give a
minimum contact current
for reliable operation. This varies from microamps to milliamps depending
on
contact material.
This is for open frame or standard sealed relays. I was not sure if it
applied for vacum units as well.
The reason is apparently that you need to break down microscopic surface
contamination in order
to ensure a proper noise free contact. A fixed DC current of a few
milliamps would ensure this.
It could easily be done even in a high RF-power circuit using proper
decoupling. But take care that
your decoupling networks DO NOT show any resonances near any working
frequency! Otherwise
you might get beautiful fireworks :-) . Several circuits have been
published
over the years for DC signalling
and control via the station coax feeder. These are suitable in this
application as well if you terminate them with the
proper resistance value . You need only a few milliamps, so using a
low-inductance high value high voltage resistor as the coupling
element on both sides of the relay contact might be good. This will ease
the
need for decoupling and eliminate
most resonance problems. You only need to do this in the through path as
RF
power will take care of
the situation on the antenna side contact.
Peter LA7SL
> ----- Original Message -----
> From: "David Cutter" <d.cutter@ntlworld.com>
> To: "Jim Brown" <jim@audiosystemsgroup.com>; <amps@contesting.com>
> Sent: Monday, December 01, 2008 7:45 PM
> Subject: Re: [Amps] Vacuum Relay Issues
>
>
>> I've cured problems similar to this with mechanical switches by running a
>> small dc current thro the contacts from a 12V source. It would tricky to
>> do
>> this with rf involved, an isolated supply for a start, but it might be
>> worth
>> the effort.
>>
>> David
>> G3UNA
>>
>>
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