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David G. Nagel · September 11th 08, 06:52 AM posted to rec.radio.amateur.antenna

wrote:
On Sep 10, 10:29 pm, Art Unwin wrote:
On Sep 10, 9:23 pm, Art Unwin wrote:

On Sep 10, 8:45 pm, wrote:
Art Unwin wrote:
What is the main factor that prevents HF radiation from focussing
for extra gain?
Money.
If you can afford to build a 20m parabola about 2,000 feet in diameter
and the place to mount it, you'll get lots of gain.
--
Jim Pennino
Remove .spam.sux to reply.
Then are you saying it is the antenna size that is the main factor?.
So my antenna which is physically small
can be focussed on a dish which would provide straight line radiation
or a radiation beam?
Working on a single element on the ground with a optimizer instead of
a half sphere I got a
straight vertical line at the sides which suggested a gun barrel
radiation with a perfect earth as the reflector.
Gain was around 8db vertical which is why the question regarding
focussing! If it was properly focussed the gain should be more.
2000 foot dish seems somewhat odd, probably based on a "straight"
wavelength and not a small volume in equilibriumas the directer
right?
Art
Let me ask the question another way. Whether it is believed or not,
if a 80 Metre antenna was compressed to the size of a couple of shoe
boxes
would the dish be reduced in size accordingly?
Regagards
Art- Hide quoted text -

- Show quoted text -

No. The shoebox size antenna would approximate an isotropic if it did
radiate. It would still have to be placed at the focal point of a very
large parabola due to the size of the wave length. Such an antenna, I
believe, on the island of Puerto Rico (the SETI antenna) although it
is currently used primarily as a receiving antenna. That parabola is
positioned to have a very high radiation angle and might not be be
that good for terrestrial DX.

Actually that is the Arecibo Puerto Rico Radio Astronomy Telescope. The
SETI project used it for a while but now has it's own antenna farm in
the US Southwest.

Dave N