Friday, June 27, 2008

Fixed Antennas for Contests?

Recently the VHF Contesting E-Mail List has had some interesting discussions about "compact beams" for indoor attic installations. While I think the Directive Systems "Rover" beams are a great solution, some other interesting ideas have also been kicked around.

Even if you're not interested in "contesting", this list is still a great place to learn a lot about VHF in general. Here's the information:

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Today, there was a post from James Duffey, KK6MC which offered a number of suggestions that might be useful for those contemplating operation with indoor antennas. With his permission, I'm going to reprint that post below.

One of the ideas that he mentions is to install longer, fixed antennas pointed towards population centers to supplement shorter antennas on rotors. This really intrigues me! My attic had only two locations that were suitable for the installation of rotors and antennas. The largest of these contains my "stack", and I've just about maxed out the length of antennas that can fit that space.
I went so far as to have an architect help me decide where I could safely move structural supports to give me more space. But the attic contains other voids which could offer very long booms a home, only they couldn't rotate. Hmmm.... That really has the wheels turning.

As compared to amplifiers and hardline, antenna gain is the cheapest way to get a bigger signal. Especially on 2 Meters and 432, I could really benefit from longer antennas. The trick now is to figure out which directions and available, and which would do me the most good.

My goal wouldn't be higher contest scores, but more grids.

Here are Jame's suggestions, which I think all have a lot of merit.

A Moxon rectangle as a simple to build with proven performance 6M
beam. It can be built simply from materials obtained at your local
hardware or building supply store:

< http://www.n2mh.net/moxon.htm >

If you have room to swing the Cushcraft A270-10s you will have room
for this.

For 144 MHz, 222MHz, and 432 MHz, the WA5VJB Cheap Yagis are easy to
build, have good patterns and gain for their length.

< http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf >

To see how you can put two of these on a single beam, look here:

< http://www.wa5vjb.com/references/Cheap%20Antennas-LEOs.pdf >

There are various sizes so that you can pick the one you that will fit
in your attic.

These beams can all be stacked. You can go lower in stacking distance
than is usually suggested. If you don't have much space, two feet is
OK on 2M above the 6M beam and a foot above that for 432. The pattern
will start to deteriorate, but the SWR and gain will not change much.
You will have to accept compromises.

You don't need to stack the beams so that they are all parallel. You
can put the boom of the 2M and 432 MHz beams parallel to the elements
of the 6M beam for instance. This is less than optimum in terms of
passing stations from one band to another, but it can get you a bit
more space.

You also don't need to swing the beams a full 360 either. You can make
the Moxon so that it is reversible; see Cebik's page for details on
this. You can also build beams for the higher bands this way, put the
beams back to back with a common reflector and then switch the feeders
from one driven element to another to change the direction.

Depending on the shape of your attic, You may also consider multiple
antennas pointing in different directions. For contests, it may be
usseful to have long boom antennas pointing towards population
centers, even if they can't be rotated.

Some thoughts, I hope that you find them useful. Go ahead and try
things, it is better to get on the air with a sib optimal antenna than
waiting until you get the "best" solution. With an antenna up and
installed, you can operate and see where your problem areas are, then
pay attention to improving those. - Duffey
--
KK6MC
James Duffey
Cedar Crest NM

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