[CQ-Contest] Re: [TowerTalk] 80m Dipole ssb/cw switching

[Jim Breakall]

At the K3CR contest station here in State College, PA we used to raise and lower antennas
for 80M that were up at 180 feet between several towers.  We had end lengths of wire
permanently installed and just used jumpers with Burndy wire clamps to connect and disconnect
these extra lengths for CW and SSB.  It was really a pain to do, especially in November every
week it seemed.

I modeled and designed a system that we use now on all of our 80M antennas that uses only
one stub made of #10 alumoweld wire with homemade spreaders that is attached to one
side of the dipole right at the center at the balun.  It hangs vertically downward and is attached to 
the coax cable that is coming down from the balun to the ground.  We use a very hefty latching vacuum
relay to short or open this stub (shorted for SSB and open for CW).  The vacuum relays
are available from Allen Bond at Max-Gain systems (www.mgs4u.com) and are very
reasonably priced.  Look on his President's page to find them.

One of the issues is that the relay coils (one for opening and one for closing the contacts) take
about 2 to 3 amps of current in a short pulse.  There can be quite a voltage drop if you try
to do this over a long run of control cable (in our case about 600 feet).

We came up with a solution where we put capacitors up at the relay box that charge up through
very small gauge control wire and then there is another relay up there to switch the charged
capacitor to the relay coil.  This works fantastic and we built a small control box with pushbuttons
on it for all 80M antennas and a toggle switch for CW/SSB for each.  You only have to press the
button to switch it and that is it until the next time you press the pushbutton.

I was thinking of maybe writing this up for the Antenna Compendium to let others see the circuit
we came up with.  If you want further info on this before I try to write it up, please contact me and
I could send a schematic and some parts info.

I hope this helps and it really saves us a huge hassle compared to the way we used to do this.

I am sure that others have done something similar.  I like the latching vacuum relays since they
stay switched once you pulse them and they can handle lots of current and also are sealed from the
elements.

73  Jim Breakall  WA3FET (K3CR, WP3R, KC3R)


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