[Boatanchors] HOMEBREWED Antenna Changeover relay.

[Philip B Atchley]

Hi All,
In preparing to interface the Viking Ranger (picking it up early next
week) and Hallicrafters SX-71 receiver I decided to build my own TX/RX
interface unit (I'm an old hand at homebrewing). When I used the Ranger
and Hammarlund HQ-110 before, I used a borrowed Dowkey "clone" antenna
relay (at least I presume it was a clone, it didn't say Dow). I found it
rather clumsy to work with as the owner has it mounted on a foot high
piece of wood and jury rigged connectors for the Ranger etc. Nor did it
short the receive antenna connection during transmit without doing a
lash-up with the external contacts. 
In the "true" spirit of ham radio I decided to build my own antenna
changeover relay, preferably at no or little cost. The goals I came up
with were:
1. Switch antenna from RX to TX (naturally).
2. Short the receiver antenna connection during TX (no need for full
QSK).
3. Be able to handle at least 100 Watts (Ranger runs 40W AM).
4. Automatic operation, I.E. change over without having to flip an
additional switch.
5. Have additional relay contacts for receiver muting.
6. Have internal control for Axillary receiver RF gain control.
7. Be housed in a compact, sturdy, well shielded enclosure.
8. Be "universal" in operation. I.E. Operate on 120 VAC (Ranger) or 12
VDC (other rigs) without extensive modification.
At first thought the last item was expected to be the toughest, not so!
Here's what I came up with. As it turned out, I was able to build it
entirely with parts from the junk box at no monetary outlay whatsoever.
1. Housing: I used the heavy gauge aluminum housing of a scrapped LPF (it
had a cutoff of about 15MHz). At 6 x 2.25 x 2.25 inches with a SO-239 on
each end it was ideal! 
2. Relay: I used a 4P2T relay (Gold Contacts) with a 12 Volt coil. This
provided all the contacts I needed for the various functions.
3. Relay Power supply: A very small 19V Center tapped transformer
(9.5/9.5VAC) with attached circuit board/rectifier provided 12 Volts to
the relay when filtered with a small capacitor. The transformer was from
a scrapped "boom box", I keep those kind of items. This allows 120VAC use
with the Ranger, wires easily swapped for direct 12 VDC operation of the
relay, OR, plug the unit into the wall and break the 12 Volt line with an
outboard switch for rigs with no relay control.
4. Connectors: I used the two original SO-239 connectors for Transmitter
and Antenna connections and mounted a small Chassis Mount RCA jack for
the Receiver connection. A Terminal strip with 6 pairs of screws allow
connections for receiver muting/RF Gain Control.
5. RF Gain Control: A 5K wirewound pot was mounted in the box for
Auxillary Receiver Gain Control for monitoring the sent CW.
6. Power connection to TX (Ranger). A wire was run directly from the PC
board on the Transformer out a grommet in the box. The case of a scrapped
Xtal was fitted to this wire to plug into the Relay Socket (A ceramic
Xtal Socket) of the Ranger.
How does it work? Well, I don't have the Ranger here yet, but testing
with 120VAC all the functions switch OK. I don't anticipate any problems
using this unit and it will easily mount under the overhead shelf of the
operating table allowing easy access to the Receiver auxiliary RF gain
control.
This just shows that one doesn't need to spend BIG BUX to make simple (or
not so simple) interface units. I often wonder how MFJ and others make so
much money on relatively simple devices (Like a TNC/Mic switch box etc)??


73 from the "Beaconeers Lair", 
Phil, KO6BB   Merced, Central California.

All Real Radios "Glow In The Dark".
RECEIVER: Hallicrafters SX-71, Grundig YB-300

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