I took the suggestions offered here, and recommended the "modified" AS2259 crossed dipole. The problem now is that the team thinks that the antenna footprint is HUGE. I tried explaining to them how HF radio worked, and all I got was "that footprint is the size of our field hospital".
So back to the drawing board. Is there a way to make a vertical work for what we are trying to do? They talked about being able to mount a whip to the Mobile Command Center, which is similar to a FEMA trailer. There should be plenty of steel to ground to, but I don't know how effective a whip would be for communicating over only a couple of hundred miles (ie NVIS). I have already explained what will be lost by not using a proper antenna, and now they are talking about ditching the ham equipment all together if I can't make it work in a small foot print.
What other ideas do you guys have?
Thanks so much for all of the assistance.
Mike
There are other options at the expense of efficiency. In the end it all comes down
to how often and in what circumstances you expect to need to use HF. (And that
applies to the rest of the ham gear as well.) If you have adequate VHF/UHF coverage
in most areas, you might not need it at all. Our Search and Rescue team often set
up a base camp in the bottom of a canyon because that was the trailhead into the
wilderness area: sometimes we could hit a Fire repeater from there, other times we
had to put a manual relay station up on a ridge - especially to communicate with
the field units, who were out of radio contact by the time they got a few miles down
the trail.
Generally HF wouldn't be used for mobile communications because VHF is more
convenient over relatively short distances. In difficult terrain you probably can
get 10+ miles on 80m with vertical antennas using ground wave, though antenna
efficiency becomes more of a limiting factor (attenuation is relatively high, so
radiated power has a direct effect on usable distance.) Otherwise it is more suited
to communications from a base camp or command post. Whether that meets the
needs of your team will depend on how they organize a search: if the command
post is expected to be mobile, it might not be useful at all. That's a decision you
need to make based on your team.
But there other antenna options. For NVIS propagation you need a horizontal
antenna. One common approach is to make a dipole using two mobile antennas
and stick it on top of a mast. This gives you a dipole 16' long that doesn't need
to be guyed off to external points, commonly referred to as a "Hamstick Dipole",
even if it uses mobile antennas that don't carry the "Hamstick" label. Our ARES
unit and several individual members bought commercial systems of this type: they
are usable on 40m (though noticeably worse than a wire dipole), but we've never
been able to get any sort of reliable results on 80m. Bandwidth is very narrow - we
could watch the SWR varying due to a tree branch waving in the wind 5' away from
the antenna. Tuning is touchy, and efficiency is perhaps a few percent. I have the
team kit in my barn right now to see if I can make it usable by extending the elements
with some aluminum tubing.
The "Buddipole" (
http://www.buddipole.com/ ) is similar, but uses larger diameter
coils which can improve efficiency over mobile antennas using a fiberglass tube with
a top whip, and includes a lot of other accessories. But even at that, efficiency
will still be rather low and the operating bandwidth narrow.
Another option would be a loop of some type: for example, hanging a wire loop off
the top of a mast and making something like this:
http://www.ycares.org/index.php?option=com_phocadownload&view=category&download=29:medium-tuned-loop-for-160-meters&id=15:antennaslikely with an autotuner at the feedpoint (if it will handle the high voltage.) It
should work reasonably well on 80m and 40m with a smaller loop.
Since you have a large metal mass available for a ground plane, you can use
various sorts of mobile or similar vertical antennas from it if you mount them
horizontally off one side. I've seen mobile setups where the mobile whip
is mounted on the back of the roof angle angled 15 to 30 degrees from horizontal
sticking off the back of the vehicle. A conventional ball mount should provide
the necessary range of adjustment. The longer you can make the antenna,
especially between the feedpoint and the loading coil, the higher the efficiency.
Wire antennas will also work fed against the metal frame, either a full quarter
wave, using a loading coil or trap, or a random length fed with an autotuner.
If you really want to use a vertical whip for 100-mile communications, then
the digital WinLINK system might be the best choice: it allows you to connect
to a digital station at a longer distance using any of a number of different bands
and send email messages via the internet from there.
In my experience, HF is an option that might be useful under particular
circumstances depending on terrain and the communications needs of your
group. Or it might not. It takes a lot more work to set up antennas and maintain
contacts than on VHF/UHF. Unless you have an existing HF network using
commercial equipment, generally I'd leave the HF link to the "Communications
Auxiliary" (radio geeks) and let the SAR folks focus on VHF/UHF for more tactical
communications. But that is a decision that you have to make based on your
specific team, resources, etc.
