Antenna relay switch design

Question

I want to build a generic multi-antenna switch using relays. Many projects on the web have done this but the relays have silver contacts. This would be fine if I were to transmit to all antennas so the slowly forming silver oxide would be removed but this is not possible. I know I could use gold contacts or even a weird phasing scheme with two relays (gold and silver) so the gold contacts engage after the silver for "make" and before the silver for "break". In practice, how is this problem solved? This device will be in an attic so humidity and heat will be a factor. The desired frequency range is 3 to 30MHz.

Silver contact relay examples: RT314F12 in relay switch (https://www.hamradioworkbench.com/4-port-coax-relay.html) or G2R-1-E-T130 DC12 (http://www.ad5x.com/images/Articles/AntSwitch.pdf).

A QEX project (http://www.arrl.org/files/file/QEX_Next_Issue/2014/Mar-Apr_2014/Dzado.pdf) uses G6RN-1 relays that are gold plating on silver but I imagine the gold plating will not hold up to 1500W (I'm happy to be wrong about this because these relays are cheap.)

Answer 1

The high temperature and humidity conditions of an attic are dwarfed by the environmental requirements of automotive applications. Picking relays that are rated for automotive use is therefore one way to overcome this design constraint. Many of the industrial relays are now supplied in a sealed form factor that will also sidestep any problems related to humidity.

If your application is for receive and low power transmit applications then pick relays for which the contacts are not excessively rated. Remember that relays are typically never switched while transmit power is applied so the extinguishing characteristics of the contacts are rarely a concern. With modern, small signal relays, contact corrosion is rarely reported.

If you have a particularly sensitive receive application (e.g. very low radiation resistance antennas), you can include a second relay for each primary antenna relay that periodically passes the minimum required current and voltage through the antenna relay contacts to ensure proper contact cleansing. Several cycles of the relay are typically necessary for sufficient cleansing. Make certain to exclude the attached radios from this cleansing current, for example, through capacitive isolation.

If your relays must handle higher transmit power, then size relays according to the best practices in the articles you referenced. The relatively high current and voltage from the transmitter will typically be sufficient to clean the contacts.