Jim's method is very good. It is simple and very effective. With a little work
the DC voltage to fire the
relays can be run on the coax feedline as well.
This exact design was used by Jim Lawson, W2PV (sk) at his New York multi multi
station.
Another efficient method is feeding the 1/2 wave dipole with open wire and
using a Johnson Matchbox. This
gives maximum frequency flexibility.
73,
Tim K3LR
Jim Breakall wrote:
> 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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