Hi
Yes those relay contacts are called BIFURCATED. They are - or today
better to say - were - used in telephone lines switching systems,
because the telephone companies were addressing the same problem -
switching of small signal. Some manufactures are making them still for
instance OMRON makes those under the name G6A-234P. Those relays are
usually excellent BUT - usable up to few hundred W on HF. Maybe they
would survive more but I have not tried.
One thing which helps in amateur conditions is to reverse the usage of
contacts as is done very often on VHF specifically EME i.e. to have the
NC contact on Transmit and NO contact on Receive since the force
developed by the solenoid is usually bigger than the reverse force of
some tiny spring.
What I wanted to mention was the DANGER of using huge high power relays
in some "on the tower" aplication where you can hardly reach it and test it.
73!
Jiri
OK1RI
dan hearn wrote:
> 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 app
> 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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