[Antennas] A lot of good info...Fat rugged dipole?

Thu Jul 28 02:15:32 EDT 2016

A lot of responses, some privately, most to the list, I'll try to address
them here...
Some provided some calculations with varying results. Some suggested
different elements, the folded dipole, good idea and possible for this
project if I can do it with materials in the 1-1.5" or thicker range, which
still might not be strong enough. I have a 37P-4 glide slope antenna on a
vehicle for RFI work, basically one half of a folded dipole around 327 MHz.
Another suggested the spherical or bi-conical elements used in radar and
ECM. I have some of those and dug up one I built in the mid-90's.
A brief run-down on the project. I'm not the only one involved, some are
State and Federal agencies/oversight groups, I'm just an independent monitor
that would report to some of them under certain conditions. Not knowing ALL
the rules, I'll have to be somewhat vague about their equipment, no so about
mine.
There is a device operating out in the field. Depending on conditions, it
radiates energy of a few tens of Watts or more. Most of this is monitored
around 312 MHz, their chosen frequency, not mine, but so my results are
comparable, I'll monitor there as well.
The idea is simple, monitor this frequency, see when and to what extent
radiation is present and get a baseline when there is (rarely, if ever),
none.
I can get as close as 45' without interfering with the equipment.
There's a lot of ways, radio-wise to monitor this and usually I just mill
about with an AM receiver and a magnetic loop for an antenna. This takes up
a lot of time I have less and less of.
I had this brilliant idea. I need a way to detect this energy, send it back
to my shop where I can then record/document and quantify it. I need
something as simple as possible to do this, maintain low power requirement,
prevent noise in the system and minimize the potential for altering the
measurements due to internal noise and minimize the chance of a system
failure.
Since the emissions are in the many-Watts range and I can be so close, I
came up with the idea of a simple diode detector with a dipole or comparable
for the antenna. This is placed close to the source, within the field,
radiation is converted to a voltage. With a little luck it's strong enough
to transmit elsewhere over a microwave link with a ruggedized patch antenna,
and I can avoid having to place an amplifier and potential source of noise
in the circuit.
There's a problem at the site. Not really a problem, but a potential for a
problem. Things in the field attract idiots. The chance of it being shot
is close enough to non-existent to not be a concern, but I have a source of
bullet-proof cabinets if it should become an issue.
The biggest concern is two things. One, people walking up to the device and
just pulling on stuff to see what it is, two, by far the biggest problem,
wild horses. It is not unusual to have several herds and up to 45 horses
come through and even winter over where this would be mounted. There's even
a horse trail within a few tens of feet.
Horses are some of the silliest animals I've ever seen. If you move
something, they gotta run over and see what changed and sniff and paw at it
for an hour. This attracts more of the darned things, next thing you know
you have a half-dozen or more fiddling about with stuff. If they can play
with it, they will. I've seen them toss boards into the air with their
teeth all day long, just to watch it fall. I've had them wrapped up in guy
wires and try to climb into vehicles with me. Nothing freakier than being 
in
the back of a truck late at night and a stallion trying to get in the back
door to see what you're doing. In the winter they wait until I leave, then
move to where the truck was warming and will curl up together three or four
at a time on the warm spot under the truck when I leave. God save you if
you wear a shirt with bright patterns on it, they'll run up and try to eat
it off of you.
Anyway... I need a device that's as horse-proof as possible. I can mount
it on a heavy post sunk several feet in the ground with several hundred
pounds of concrete around it. I can come up with a cabinet they can't bend.
The problem is, they love to rub their back-sides on things. I have a
camera at one site and have gotten hundred of shots of horse butt. As you
can imagine, the antenna(s) will be the first to go.
I'm leaning toward the ECM (electronic counter-measures) type conical or
spherical elements, the ones I have that are comparable have a bandwidth of
500-600 MHz and the lower end is around 300 if I recall correctly, but I
would have to double check. With a little luck, I'll be snagging a 6" lathe
soon, if not, I can hire out.
I'm thinking a 6x6 square post sunk three to four feet, a quarter-ton of
concrete a couple of elements turned out of solid aluminum with a hardened
3/4" rod or bolts holding them to acetal insulators 1.5-2" thick and bolted
all around to the post.
Sounds like overkill for a crystal radio...
Kurt