Relay contacts

[KA3EKH]

I was thinking of building up a remote box to put at the base of the tower that would allow me to select different antennas for different radios that I have in the shack. This remote switch will allow me to switch between the antennas for 20, 40 and 60 meters for use with the 100 watt radios that I have. The 375 watt RCA on 160 has its own dedicated antenna along with the VHF radios. Now I have a patch panel and most of the radios and antennas come to that but its getting crowded so the idea is to have the common from the tower mounted switch run back to a signal patch on the panel also this way when the power is off no antenna is routed to any radio.
I want to use three Struthers Dunn relays that are about fifty years old, They were originally installed in something called a optical multiplexer built by RCA that controlled 16 MM film projectors and slide projectors to feed a TK-27 telecinie system that I disassembled back in the early eighties. Some of the relays ended up in the remote control interface for controlling two analog television transmitters and the transfer interlock that I built back around 84 and was in service at a PBS station up until the analog sign off. So these little open frame relays are way dependable and are also kind of a thing of beauty, at least for a relay. The question I have is the relays are all 12 volt DC coils but the contacts are rated at 115 VDC 0.5 amps. There are three contacts on each relay and was thinking I can double up two and use the third for signaling but just don’t see what makes a relay contact AC or DC? And if there would be an issue using a set of contacts for RF that is DC rated? These relays have huge arms, springs and are way more beefy then most of the piss poor sets of contacts I have seen used in many transceivers, but thought I would ask before I start building to see if there is something I am missing.
 

[wa3dsp]

From the way you describe these relays 115V @ 500ma for the contacts seems like an awfully low rating. Are you sure that is correct? That almost sounds like the coil current. I have mini relays with far smaller contacts than that rated at 15 amps. I would wire one up and try it on RF.

For RF in a 50 ohm matched circuit you would seldom have more than 200V and 3A.

One problem you run into is the contact material. If it is not gold and there is no current flow (wetting) through the contact as in an RX circuit you could sometimes have intermittent problems. One option is to isolate with RF chokes and blocking capacitors and run some amount of current (10-20ma or so) through the contact. Some relays get around this by a self wiping action.
 

[KK4YY]

I had to read it twice... 115 VDC (not VAC) at 0.5 amp contacts. But whatever the rating, that was 50 years ago when they were new and unused. Check for pitting and give them a cleaning. Also consider the environment you'll be using them in... outside were there's moisture and condensation? A sealed relay might be better there.

Relay contacts will pass AC, DC, RF, whatever. How much is the question. Unless you plan to hot-switch antennas you won't have to worry about arcing when contacts make or break so I don't think voltage is a big issue here as long as you're using coax. Current is another story. Calculating the current won't help because you still don't know if the relay will handle it. If you have a dummy load that you can dump power into for a while (10 minutes or so) go ahead and test one of the relays in the shack. That's the acid test. Bring power up in stages and see if it gets hot. If it stays cool you're good to go. 100 watts will probably be no problem but testing is knowing.

Take time to calculate voltage drop on the wire runs out to the remote DC coils. It can be a lot if you have long runs of thin wire. Use a wire table chart to find the resistance of the total wire run (out and back). You can overcome the drop by feeding a higher voltage from the shack (which can be calculated as well).

"...a thing of beauty, at least for a relay." Strangely, I know what you mean.

Don