A QRO 50 MHz COAXIAL RELAY/SWITCH

In the 2003 Proceedings of the Southeast VHF Society Conference, WA4NJP published an article showing how a Magnecraft W199AX-13 DPDT power relay could be used with a Bud CU-234 cast aluminum box to create a DPDT coaxial relay capable of handling high RF power.  If you do not already have a relay to switch the antenna feedline between a receiver and the transmitter, this type of relay is what you need to completely switch an amplifier and and out of the circuit for a transceiver.  This original article in .pdf format can be downloaded from my website HERE. 

My initial interest was to use his recommendations and construction techniques to build a SPDT coaxial relay that could be used as a main Transmit/Receive relay or an A-B switch to select between two different 6m antennas. 

The same above switch and enclosure were used as described in the original WA4NJP article.  (I found them both in stock at Mouser Electronics.)  I used a hacksaw to cut 3/8" off the bottom of the relay base so the relay would fit into the enclosure.  The photo to the right shows how I removed the coil and moving contact assembly from the relays to saw off the bottoms. 

 I then soldered copper straps across the pair of NC and NO terminals and extended them to either side of the box to 7/16 DIN connectors.  On this first SPDT relay, I disconnected the flexible wires from the screw terminals and connected them together at the Common connector with a copper yoke. All copper straps had their corners rounded and filed smooth to reduce arcing.



I use 28 VDC here to control all my coaxial relays, so I provided 28 VDC through a feedthough capacitor to a polarity protection diode, which then was again bypassed to ground with a .01 uFD capacitor.  The voltage was fed through 50 ohms (a pair of 100 ohm 1 w resistors from Radio Shack) and a 100 uH 2A RF choke (also from Radio Shack) to the relay coil.  The resistors were to drop the voltage down closer to the 24 VDC used by the relay coil.  The other side of the coil was attached to the chassis ground.  I had a thin sheet of teflon that I laid over the relay before screwing on the cover, to provide additional protection against RF arcing to the inside of the cover.

To the right is shown the same enclosure and relay built as a DPDT coaxial relay to switch the Harris solid state 6m amplifiers in and out of the circuit during transmit.  This is very similar to the original WA4NJP article.

Since these relays operate on 24 VDC, a special control box was constructed to provide 24 VDC on transmit.  The control box was designed to be triggered automatically by the RTS line of the serial cable from the computer, or manually by a footswitch.

The NC connections on relay connected the antenna to the transceiver.  When activated (during transmit), the transceiver is connected to the BNC connector, which matches the input connector on the Harris amplifier, and the antenna over to the amplifier output.  Except for the one BNC connector shown, all the connectors were teflon UHF connectors.

So how do they work?  I was quite surprised, actually!  With a newly calibrated MFJ-259B just returned from the factory, the SPST relay reads 1.0:1 SWR with X=0 all the way to 75 MHz!  Actually at 144 MHz, it still reads 1.2:1 SWR.   This is certainly better than some vacuum relays I have built for 6m! I had no way to measure isloation with this relay, but I suspect it to be superior to the vacuum relays, which seemed to have serious problems. 

The DPDT relay also exhibited 1:1 SWR over the 50 MHz band.  I would still recommend using this in conjunction with an additional coaxial isolation relay ahead of a preamplifier, however.




Page last revised on 14 May, 2012