[TowerTalk] Re: [Amps] Relays for RF switching

[Tom Rauch]

Please don't take this as argumentative, but I disagree with most of this.

> Contacts are often just staked to the arms, this makes for poor RF
> contact. A bit of silver solder works well, probably regular solder is
> OK also.

I've never seen a problem from contact staking, and can't imagine one
occuring. After all, the relay depends on a small contact patch for the
connection with minimal pressure. Staking actually displaces or flows the
metal and the connection pressures are enormous.

Soldering will make things worse. It will remove temper from the material
and you run the risk of spattering rosen or solder on the contacts.

> The arms are often made of stainless; I have no idea what alloys are
> used but these will heat at RF, increasing as the frequency goes up.

The contact bars are generally berylium copper. I don't think I have ever
seen stainless in the hundreds of relays I have looked at.

> The flexible leads to the solder terminals are often not copper and are
> spot welded to the stainless arm. I replace with very flexible RG-141
> braid.

Actually woven braid is the worse possible thing to use. The best would be
something like thin wide copper foil, and a close second would be flat
parallel strands that are NOT twisted. The worse case situation would be a
loose-woven conductor. The reason is skin effect.

IMO you are better off just getting a relay that works.

99% of the problems I find are poor insulation, use of cadmium or tin
plating on contact supports, and long woven leads.

I've sampled and tested literally hundreds of relays, and I picked the
particular relays used in many products based on hundreds of hours of
testing. There are some relays I would NOT use also, but that make it into
products. People often pay more money for and copy the worse choices!!

For example the BRAND of relay used in samples of a popular antenna tuner I
bought actually runs the RF right through the magnet pole and frame of the
relay coil, with only a few thousands of inch of plastic as an insulation!!!
That relay fails at maybe 200-500 volts of RF at 30MHz from internal heating
of the coil! For half the price, a relay is available that does NOT run the
RF through the frame and the magnet pole and that relay handles 5kV of RF at
30MHz between the pass contacts and the relay coil.

There is no way to patch around a bad relay. Just buy a good one.

73 Tom