I believe that one of the most important factors involved in small
signal switching has not been addressed. That is the mechanical nature
of the relay contact assembly. Some relays are designed so that the
contacts merely open and close. Others are designed so that the contacts
slide or "wipe" as they close. You can see this action if you slowly
close the armature with your finger and watch the contacts. Large
contacts do not solve the problem, in fact the relays used in
transceivers usually have small contacts. They are frequently in the
form of small bars operating at right angles to each other. These are
surprisingly reliable. These designs have fairly large PSI force to push
thru the oxides or other contaminents on the contacts.
I was at a trade show once where a manufacturer of precious metal
alloys for contacts was exhibiting. There was a book on contact design
on their table. The salesman said it was written for internal use by the
company"s designers. I talked him out of a copy and took it with me. The
book talked about problems with small signal switching reliability as
well as power switching, etc. They discussed Gold, Rhodium and other
metal contacts. Even Gold was not totally reliable in this application.
In fact there was no material that would guarantee total small signal
reliability.They talked about bleeding a small current thru the contact
via a large resistor or RFC to help punch thru the contaminent film.
When I retired, I did not take the book with me and, sadly, I have
forgotten the name of the company. I have used several types of relays
in 4 sq arrays and other phased arrays with very few problems except for
lightening induced damage. I usually parallel 2 sets of contacts and
look for a design that wipes the contacts.
73, Dan, N5AR
Logan Dietz wrote:
>
> I have used all kinds of relays from Radio Shack to surplus. If you use
> DPDT at least 10 amp contacts and parallel the contacts, I have not noticed
> any deterioration. Occasionally there will be a problem, but keying down
> with the amp (NOT hot switching) will fix it. I envision that there is a
> bug or spider web that is evaporated by the power...
>
> By the way, using 24v on 12v relays helps too. They pull in better.
>
> Chuck, W5PR
>
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