Topband: Antenna relays

[Tom W8JI]

> With silver-cadmium relay contacts I use on my ladder line switching, I
> find I have to occasionally transmit a dit to get receive to work again,
> especially if I haven't been on the radio in hours or days. My mental 
> model
> is that I have to "blow through" the surface contamination. Would
> definitely never use the silver-cadmium contacts in a receive-only path.
> (Even though I have been known to make some Q's while transmitting on my
> receive antennas, it was purely by accident!)

That's exactly what you are doing, blowing through a very thin surface 
contamination.

There are a few common forms of high resistance or open contacts. The very 
things that allow hot switching go contrary to receiving. For best results 
contact overlay has to be gold, it has to be cold switched (or switched with 
low voltage and current to not burn off the plating), and the contact needs 
to have a small surface area. Some relays are split into multiple small 
areas to increase contact area pressure and parallel areas.

The worse thing is a large area, because pressure across the area decreases 
for a given spring pressure.

A second issue, now more prevalent with everything being made offshore, are 
contaminants inside relays. The plastics can leech contaminants, or air 
sealed inside just might be dirty. This usually cures itself over time.

Low frequency performance, tests, and specs don't mean much. Skin effect 
seriously aggravates heating in some wires and contact current carriers.