Topband: Relays in RX array

W0UN -- John Brosnahan shr at
Wed Mar 22 08:06:10 EST 2006

>Pete didn't indicate in his regular query as to whether this was a 
>Rx array or some sort of Tx antenna.  So rather than rambling on 
>with guesses, maybe Pete can describe his application.

Actually, Ford, the SUBJECT line of Pete's message says it all!  Look at the
subject in the email you just sent!    ;-)

           Subject: Topband: Relays in RX array

RX array it is.   QRO needs no additional current to keep the contacts clean on
the TX side.  But you have to be a bit more careful on the RX side of 
a TR switch
relay about oxidation.  But here in the TR relay any phase shift will not make
any difference since any beam-forming in a phased array has already been
done.  The only issue then is attenuation through the oxidized contacts.

My experience is only with the Dow-Key style "T" shaped coax relays when
run in an RX mode.  The long, moveable arm does not have the geometry to get
good wiping action and even the gold-plated contacts will oxidize and poor
RX performance will occur within weeks.  This is not always demonstrated
by weak RX performance but rather by phase shift through the 
relay.  The contacts
are insulated by a very thin (molecular thin) layer of oxidation.  There is now
enough capacitance to carry a signal (albeit, attenuated somewhat) but the
signal is phase shifted by the reactance of the series capacitance of the poor

In HF radio astronomy we went to a mercury wetted relay for consistent
performance ultimately, but we found that the power for the preamps was
enough to keep the Dow-Key relays performance good.  The relays were
used in phase shifters as part of an HF interferometer and the DC power for
the preamps were injected into the coax after the relays originally.  We just
moved the injection point to before the relays so that they switched the DC
when they were adding or subtracting delay lines.

It has been my belief that the little 14-pin DIP relays that are completely
sealed would probably be OK for RX applications without any DC voltage being
switched but I have never made the measurements to prove this to my
own satisfaction.

BTW  The measurements that we made were all done with an HP-8405
Vector Voltmeter and this was done 30 years ago at the University of
Colorado Radio Astronomy Observatory.  Unfortunately I don't recall the
current requirements for the preamps so I can't comment on how much
current we were using with the Dow-Key style relays, but I am sure it
was some 10s of mA to maybe 100 mA.   Probably more than was
necessary to keep the contacts from oxidizing.

--John  W0UN

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