Ignored this thread first time around, but then purchased a Johnson Matchbox
with a post for a long wire. Sat,, for a camping trip or vacation, do you guys hace and tips as to height, lenghts to avoid, flat vs. sloping etc? I would like to test it here at home where I can put up about 220 feet straight. Also, since its going to the matchbox single wire terminal, I assume I do not have to drive an 8 foot ground ros just for this, although, I "Could" be wrong.
There are many factors to consider, depending on what bands you want to work, your
available supports, etc. But here are some general points that I've found from my
experience using long wires, especially for portable operation:
1) The same RF current flows into both your antenna wire and your ground system. If you
don't provide an intentional ground system then the outside of your coax, the chassis of
your rig and tuner, and any wiring for headphones, mic, battery cables, etc. will be your
ground system. That isn't always desirable. But an 8' ground rod is actually a pretty poor
RF ground (besides being inconvenient for transient operation) so isn't a good approach
anyway.
I've operated without an intentional ground system on several occasions, and sometimes
I've gotten by with it, especially at QRP. Other times I've had problems with RF feedback
causing quirky symptoms (keyer running amok, panel lights blinking, garbled audio, etc.)
Usually a quarter wave radial wire for each problem band attached to the back of the
tuner will solve such problems by giving the RF somewhere to go.
2) Using an antenna wire that is a multiple of a half wave makes a high impedance, so the
currents at the feedpoint aren't as large for the same output power. This makes ground
losses less of a problem. (The simple radials used in step 1 are not usually sufficient for
this purpose.) Personally I've had good results with wires around 130' - 135' for 80m
through 10m (at least the pre-WARC bands), though some people don't like using a high
impedance feedpoint.
3) At a minimum try for 3/8 wavelength of wire on your lowest band. More is better.
But I haven't been impressed the times I've used a really long wire, as the pattern tends
to get more directional on the higher bands, and this can be counterproductive. We tried
a 600' long wire one year on Field Day and it wasn't any better than a dipole (and often
worse.) Also the MatchBox will do better with higher impedance loads, and may not be
able to match a wire if it is too short.
4) Height is important for DX on the higher bands. But if you are mainly interested in
relatively local coverage (within 500 - 1000 miles) on 40m and 80m, then low height
isn't as much of a problem. In order to get as much of your wire as high as possible, I'd
suggest an inverted-"L" arrangement, where most of the wire is strung horizontally between
two supports, with the remainder serving as a lead-in dropping down to the rig. But
to a large extent the wire configuration will depend on what supports are available to
you. A long
vertical wire (over about 5/8 wavelength) may not be as effective as a shorter
one, though you can use up to 1 wavelength or so with adequate results.
5) Make sure your MatchBox has been modified to jumper around the receive relay:
they date from before transceivers were common, and are designed for a 600 ohm output
to a separate receiver when the relay isn't energerized.
6) The Matchbox does not provide a DC path to ground from the antenna terminal. I had
one situation where there was enough static charge picked up on the antenna wire that
the capacitors in my tuner were arcing over. A 100K 10W wirewound resistor from the
antenna to ground (I used a BBQ skewer - it doesn't need a low resistance) will help to
dissipate the charge. On the other hand, if the high static level is due to nearby
thunderstorm activity, quit operating and disconnect everything. You aren't going to
be able to create a ground system capable of handling a direct lightning hit in a temporary
portable station.
Probably the best performance I ever got from a long wire was on Field Day from southeast
Alaska with my old Ten-Tec Argonaut. I had a sloping long wire running from a tall tree
on the shore out over the tide flats to a post I had put up at low tide. When the tide came
in the sloping wire over salt water gave me an effective low-angle radiator (though I still only
made about 40 contacts.) Among other things, that requires the right combination of slope
angle and wire length in wavelengths.
