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
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 basic relay and enclosure were used as described in the
original WA4NJP article. (I found them both in stock at Mouser
Electronics.) However, I selected the 24 VDC relay model
Magnecraft W199X-13 DPDT so I could use it with the 28 VDC I had
available for switching coaxial relays in my ham shack. l I
used a hacksaw to cut 3/8" off the bottom of the
relay base so the relay would fit into the enclosure. I 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. I then
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.
So how does it work? I was quite surprised, actually! With
a newly calibrated MFJ-259B just returned from the factory, it 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. I would still recommend
using this in conjunction with an additional coaxial isolation relay
ahead of a preamplifier, however.
Page last revised on 18 November, 2010
