I'm still on the hunt for a decent set of relays for a remote matching network.
My goal? 1.5KW remote switching box. I went to a local surplus emporium in
Minneapolis to scour the junque boxes for decent relays. Eureka! A boat load
of 12v relays! 10A, breakdown voltage of 2KV between contacts, and 4KV between
contacts and coils. 12v switching. DPDT and DPST styles. Visual inspection
shows a good 0.050" to 0.060" air gap on the contacts. I selected an Aromat
JC2aF-DC12V.
Tonight, I decided to examine them in more detail. Aromat spec sheet says 0.1
ohm on the contacts. Contacts are silver alloy. Maximum switching power
2,500VA. MTBF 10^5. Kewel! They look perfect. They are cheap $4. They are
small open frame styles with short RF paths of copper strap material.
I soldered RG174 cables to the contacts to switch both shield and center. One
cable went to a 50 ohm load, the other to the Autek VA1. Open! Switch the
relays about 50 times and then they start to work. I can see that the contacts
are closing alright. But no RF gets through. So I took a stray JA QSL card
laying on the desk and ran it through the relay contacts. Then they start to
work. SWR reads 1.12 :1. Close enough. But unreliable. They don't always
connect--especially on the Normally Closed side of the relay. Repeating it
with another relay yields identical results.
Theory 1: These are designed to be hot switched. The tiny arch from a hot load
fuses the contact. Stop worrying about it. Push some decent power through
them. They will fuse and be just fine.
Theory 2: My gut tells me that a robust design would include a self cleaning
wiping action on the contacts, like on a band switch. I will not get reliable
switching using traditional relays.
So how does Ameritron get reliable connections? And Com-Tek with their phased
arrays? Are relays unsuitable for this application? Should the contacts be
gold? What am I missing here?
Any help the engineer types can provide would be appreciated.
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