This station
accessory consists of two separate sections. One is the
switch selectable control box and power supply that sits
on the operating bench, and the other is the remotely
mounted relay switching box that mounts on the tower.
Four Jennings RJ1A-26 SPDT vacuum relays with
26 Volt coils are used 1. These provide reliable
legal limit antenna switching.
Power to
drive these relays is supplied using the antenna coax
center conductor. RF chokes and bypass capacitors keep
the RF isolated from the DC supply voltages. A home made
50 to 25-ohm balun transformer is used to match the 25
Ohm antenna impedance when both antennas are selected
at the same time 2. This balun consists of
14-trifilar turns of #14 wire wound on a T-200-6 core 3. I assembled this transformer inside a round
tin enclosure with a lid, and potted the transformer after
assembly and testing. This potting procedure or toroid
enclosure is not necessary.
Two one-kV
mica capacitors with unmarked values are shown on the
schematic of the toroid transformer. These are selected
and their values determined during testing. Choose values
that provide the lowest SWR using a test frequency of
30 MHZ from an SWR analyzer. These capacitors compensate
for stray inductance values of the toroid transformer.
Once these capacitor values are experimentally determined,
just solder them in place permanently. Expect values required
to be about 10 to 30 pF or so.
Clamping
diodes are used across three relays to suppress switching
transients caused by the relay coils. A Neon gas bulb
is used for the same purpose for the relay that has bipolar
DC voltages applied to it.
The station
control box switch selector has a regulated 26 Volt DC
power supply. Plus 26 Volts applied to the coax connector
selects the lowest antenna. Minus 26 Volts selects the
highest antenna, while no voltage supplied selects both
antennas.
A single
supply polarity is used. The isolated common of the power
supply is reversed with the supply voltage to provide
the necessary apparent minus voltage. This switching logic
is done by the control box front panel mounted antenna
selector switch. This is a two pole three position on-center
off-on toggle selector switch.
The relay
switching components are mounted in a die-cast aluminum
box that also resides inside a weather resistant enclosure.
The outside enclosure I used has a clear removable swing
away front door that provides access to the internal relay
components.
You may
use nearly any type of outside weather enclosure provided
it gives protection to the inner components. I got my
outside enclosure from a surplus source, and no more are
available there. These specific types of enclosures are
expensive if purchased new, so search through your junk
box, flea markets or on the Internet for an alternative
that will work for you.
This selectable
antenna stack match is easy to test without installing
it on the tower. Connect a SWR analyzer to the "transmitter"
coax connector. Connect 50 ohm PL259 terminators on each
of the "upper" and "lower" antenna
connectors. Test for a low SWR reading while selecting
all antenna combinations. Once you are satisfied with
this test, you are ready to install the remotely mounted
relay box on your tower.
Use
the same length of coax from each antenna is required.
This keeps the
antennas in proper phase when both are active at once.
Use the identical
type of coax preferably from the same reel.
The only
precaution during use is to remember to never hot switch
the relays while you are transmitting! Only change antennas
when you are listening. Following this simple precaution,
I have obtained good operation from this two antenna stack
match for six years while running legal limit power. No
problems have developed since installation. |
