On Thu, 20 Mar 2008 05:44:50 GMT, "usmcbrat"
wrote:

Hi Folks:

I'm looking to make a directional antenna in the 215 to 218 MHz band (215 if
that's too broad), used for location of dementia patients who have wandered
away from their caregivers. I'm a volunteer with an organization that
locates these people. They wear a tiny transmitter which emits a chirp that
can be heard from 1-2 miles on the ground and 8-10 miles via helicopter.

I'm somewhat new to the program, and I'd like to do some investigation on my
own regarding antenna types that would be useful, and I hope you'll all
grant me a few questions.

My needs are this.... the antenna must be as directional as possible. It
must be most efficient in the 215 to 218 MHz band 215 is what our patients
are on right now, though after we fill up that range we have licensed
through 218 MHz. It must be easy for an individual to carry, and portable
enough to carry through any type of terrain. It must be shaped in such a
way that it's relatively safe and easy to use in an enclosed area, either in
a helicopter or a car. Currently the agency is using a 4 element Yagi,
which works well, but has the drawback of being a real eye-poker when used
in close quarters, and in fact the California Highway Patrol will not allow
them to be used in their helicopters because of that reason.

One of our Search & Rescue guys built his own 4 element Quad Yagi out of
some 1/2" PVC pipe and other parts that are readily available at any Home
Depot. It has greater sensitivity than our stock antenna, and the CHP has
said that it's OK to use in their helicopters. Rather than 4 horizontal
straight beams it has 4 squarish beams, with no points to poke people's eyes
out. I'm still waiting to get the plans from him for that antenna, and I
though that in the meantime I'd ask in here if any of you had some
interesting leads for me to follow.

Many thanks!

I just did a quick simulation on EZNEC of your proposed Quad. At 215
mhz the element spacing on the boom is about 1 foot. That means a 4
element yagi or quad should come out to 3 feet long. Similarly, the
element loops will be about 1 foot square.

With no matching attempted 2:1 Swr is easily achieved.

If you are so inclined you could model this antenna with the Demo
version of EZNEC. You could also prototype one or more on the kitchen
table with masking tape and sticks!

I would think the additional gain of the Quad would make it desirable
to drop back to three elements to reduce the size.

John Ferrell W8CCW
Beware of the dopeler effect (pronounced dope-ler).
That's where bad ideas seem good if they come at you
fast enough.