[Amps] Relais for an antenna crossover switch

[Tom W8JI]

Guido,

Relays not rated for RF are a very complicated matter. 
First, they are rated to hot switch at the current listed. 
But at radio frequencies there are often standing waves when 
hot switched or transformer action in transmission lines and 
this can mean very high switching voltages. So while the 
current is OK, the voltage may sustain an arc and burn 
contacts or insulation.

Some relays also have thin stand heavily woven very flexible 
wires that have very high RF resistance even though they are 
good at DC or low AC. So the wires can limit RF current.

So far as contact area, the contact area touching in the 
contact  is very tiny. Because the contact area that touches 
is so very small and the contact so large, the contact and 
the contact carrier do not need to be derated for steady 
current. Only when the contact carrier is very thin and very 
long and the contact very small, like in a vacuum relay, is 
derating an issue when the relay is not hot switched. Of 
course at VHF other things come into play, but you can 
consider RF below 30 MHz like DC in most large standard 
relays. The exception is often just the armature wires.

30 amp relays can be a problem with reliability because you 
cannot have wiping or cleaning current during switching. 
This may make the receive drop out at times when first 
switched. This problem comes from air pollution 
contaminating contacts, building a very thin layer of film. 
Plating does not help. Lower current relays are often more 
reliable for receiving. Bumping the relay with power will 
almost always clean the contacts since the insulating film 
is only a few molecules thick.

The key to good relays are short wide leads to contacts, low 
capacitance, and good RF path insulation.

I cannot open any of your links so I cannot look at what you 
do or the relay, but grounding can sometimes make things 
better and sometimes make things worse. Many of the relay 
systems I've tested for isolation actually have very poor 
isolation because they ground the relay where they should 
not. It is really all about the layout and routing of paths 
and NOT so much about if you ground unused ports.

73 Tom