Topband: Gold flashed relay contacts useful for antenna relays?

[k3ky at radioprism.com]

> Can anyone tell me if gold flashed relay contacts
> solve the problem of using the same relay for both
> transmit and receive?  In theory there should be no
> minimum current needed, but you wonder if putting
> 5+ amps through the contacts will wear away the
> gold flash.  There are also issues of mechanical
> wear, and contamination accumulating on the surface
> of the gold.
> 
> Rick N6RK
> 

Gold plated relay contacts will only keep the gold flash intact
if they are run 'dry circuit', i.e. switching *no* current. In
any case, gold flash is very thin, and will wear away mechanically
sooner or later. Probably sooner.

Even quite small switching currents are sufficient to 'burn away'
the gold flash quickly- we're talking just a few milliamps here.
This is considered normal, not a defect. Unless you are running
dry circuit, think of the gold flash as simply a 'preservative'
which keeps the contacts fresh until they are first put to work.

Once the gold is gone, it's up to the base metal in the contacts
to do the job. Contact resistance will rise, but may not be a
problem in your application. In general, relays which have a
bit of 'overtravel' after making contact can provide a good
wiping action to keep that contact resistance manageably low.

In your relay, the gold may remain intact for a while on the
receive side of the contacts, but it will eventually wear away.
Then it comes down to how the relay is designed, and whether or
not the contact base metal can keep itself sufficiently clean
to avoid excessively rising contact resistance.

Remember that hot switching on the transmit side can quickly
eat away those contacts and reduce them to junk. It is
essential to make sure things are properly sequenced. If your
TX is producing output while the contacts close or open, you
will not get very good life out of those contacts. This is not
a trivial matter to set up properly, as your TX can produce
output a lot sooner than the time needed for the contacts to
move, make contact, and stop bouncing. 

In practice, gold is only truly needed in a tiny number of
small signal switching applications. For most others, it is
sufficient to keep the contact resistance reasonably low, also
stable over the long term. This is acomplished by choosing the
right relay for the job and making sure you are always running
within the rated contact switching limits. Relay design and
specification is a whole sub-field in electronics design. There
are a number of 'gotchas' lurking in wait to snare the unwary.
It's always best to research and find the right type which is
specified for the intended use. A good shortcut- find out who
is manufacturing HR antenna switching gear and ask them what
they use- and why. Think of gold flash as a 'relay contact
shelf life extender'.

That being said, there is usually no harm in using gold flash.
Just don't count on it to last. What matters is the CR 
(contact resistance) of the base metals in a set of contacts,
plus whether the relay in general is able to hold up under
the RF curents and voltages present, and the timing constraints
of the application. For higher HF and up into VHF, VSWR effects
also become critical.