"Asimov" wrote in message
...
"Jerry Martes" bravely wrote to "All" (06 Mar 05 23:31:40)
--- on the heady topic of " Circular Parasitic"

JM From: "Jerry Martes"
JM Xref: aeinews rec.radio.amateur.antenna:26527


JM Do you have an actual application for this circularly polarized
JM directional UHF antenna with the "loop" included?

Just curious about the UHF loop antenna as it is said to have a gain
of +1dB in the vertical position and -3dB in the horizontal position.


JM A simple "Turnstile" will give good CP. The satellite guys use them
JM with reflectors.

IIRC satellite folk use double or quad counter-rotating wire
corkscrews sticking out of a reflector don't they? A turnstile seems
pretty much omnidirectional but doesn't it need to be phased somehow?


JM I can give some information about simple crossed dipole antennas
JM that give pretty good CP recpiton at VHF.

This would be the same phasing techniques as with the turnstile? I've
seen a large 6 element array near my area and that impressed me but I
think the neighbours would be forming a posse if they saw one here.

BTW I made a portable antenna for low vhf using a 3/4 in. dia. plastic
curtain rod as a long coil form. The conductor is wound around the 6
ft long plastic rod with about 1/4 inch spacing. It works okay but it
is quite sensitive to proximity effects due to the spread inductance
and the 200pF cap used to series resonnate it. It is far shorter than
the wavelengths it receives but does the job. It isn't classy.

A*s*i*m*o*v

Asimov

At first I was thinking that you were curious about building a UHF antenna
with a loop as the driven element then try to include some parasitic dipoles
to produce some directivity. I couldnt offer any thoughts on that. But,
when you mentioned circular polarization and directivity, I wondered why you
wouldnt consider a helix for UHF.

On loopgain --
I wouldnt want to argue about the gain of a loop being anything above or
below that of a dipole. I would expect a loop to have very nearly the same
gain (directivity) as the dipole. I even thought that small loops have
exactly the same pattern shape as a short dipole. That would make their
gain the same if their efficiency is good.


On turnstile --
The turnstile is two crossed dipoles in the same plane fed 90 degrees out
of phase. It produces RHCP in one direction and LHCP in the other
direction.
The antenna I use is two crossed dipoles spaced 1/4 wave apart and fed
in phase. That gives good RHCP (or LHCP, whichever you want) toward both
directions.

Jerry