Alex - You asked:
>I have a halo that I could use on 6 meters, but I wonder how high it
needs to be off the roof. I know for the turnstiles it needs to >be a
half wave or greater in order not to heat just the clouds. Does the
same go for a halo?
Yes, the same goes for a halo. You will notice a big difference in
performance when it is more than a halfwave above the nearest
conducting surface. Below about 3/8 wavelength the maximum radiation
is straight up. The straight up component of the radiation doesn't
significantly get reduced until you are a half wave above the nearest
metal.
In some ways it is worse for the halo than for the turnstile, which
are made of dipoles. The halo radiation resistance is lower than the
turnstile, so ground losses reduce the antenna efficiency more than
with the dipole. And the closer to ground the more the ground losses
are. It is not so bad with a metal roof as it is with soil though.
>What if the 6m halo is just 2 feet or so from the roof? Would that
be a total disaster? I really would appreciate your thought on >this?
At 2 feet above the roof, which is 0.1 wavelengths on 6M, you will
have lots of radiation straight up. Ground losses start to become
significant at 0.1 wavelengths. Your efficiency will start to suffer.
You will not do well on troposcatter. Sporadic E is generally a low
loss propagation mode, and you have enough radiation at low enough
angles to make some sporadic E contacts when the band is open with
strong signals. You will miss the weak early openings and late
openings though.
Anything you can do to increase that distance above 2 feet will help.
If you can get the 2M turnstile up 40 inches, why can't you get the 6M
halo up there? That would be a big improvement over the 2 ft, but
still far from good. If you are worried about getting all that on one
mast, consider getting another mast and mag mount. If you are worried
about it staying put on the mag mount, use some temporary guys.
Having said all that, any antenna, even a poor one, is better than no
antenna, so if you have no other choice, go for it. Just don't expect
too much. You will work some sporadic E, but the troposcatter stuff
will be very limited.
When I first started roving I asked around about mag mounts. I got a
great answer from a guy in Iowa. "They work great, mine don't come off
more than once or twice a contest weekend."
You might consider a mast just for the 6M halo. It is pretty easy to
put one on a trailer hitch. I used EMT mast, but the 10 ft Radio Shack
TV mast would work just as well. There are a variety of ways to mount
it to a trailer hitch, I bolted a pipe cap with a large bolt in it to
the bottom of the mast, put that through fender washers in the trailer
hitch and attached a nut and a lockwasher. I braced mine with an angle
iron on the roof rack, but you could guy pretty easily if you don't
have a roof rack.
Just some ideas. The effort spent in getting that halo up high will
payoff. - Duffey
--
KK6MC
James Duffey
Cedar Crest NM
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