"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

.... CAUTION: I drive just like you do!

"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

On Mon, 07 Mar 2005 08:29:20 GMT, "Jerry Martes" wrote:


"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



Time travel? What, do you have a TARDIS? Or a vortex manipulator?

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

A*s*i*m*o*v 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.
________________

A planar, circular loop of diameter ~0.1 lambda is linearly polarized, with
peak gain* in the plane of the loop. This loop will have vertical
polarization when in the vertical position, and horizontal polarization when
in the horizontal position.

Lacking ~equal h-pol & v-pol gain at the same time, such a loop is not
circularly polarized -- even though the physical configuration of the loop
is a circle.

*theoretically about 1.74dBi (far field, free space)

RF

Richard

Do you know the approximate input impedance of a 0.1 lambda diameter
single turn loop?

Jerry


"Richard Fry" wrote in message
...
JM Do you have an actual application for this circularly
polarized directional UHF antenna with the "loop" included?

A*s*i*m*o*v 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.
________________

A planar, circular loop of diameter ~0.1 lambda is linearly polarized,
with peak gain* in the plane of the loop. This loop will have vertical
polarization when in the vertical position, and horizontal polarization
when in the horizontal position.

Lacking ~equal h-pol & v-pol gain at the same time, such a loop is not
circularly polarized -- even though the physical configuration of the loop
is a circle.

*theoretically about 1.74dBi (far field, free space)

RF

"Jerry Martes"
Do you know the approximate input impedance of
a 0.1 lambda diameter single turn loop?
__________

About 2.5 ohms.

RF

"Richard Fry" wrote in message
...
"Jerry Martes"
Do you know the approximate input impedance of
a 0.1 lambda diameter single turn loop?
__________

About 2.5 ohms.

RF

Richard

Can you guide me to a web site where that loop input impedance is given in
R+jX? I will set up and measure some loop input impedance at 137 MHz
within the next few days. But, I'd like to have a better "feel" for what
I'm doing before I take time to build the loop and set up the test
equipment.

Thanks
Jerry

"Jerry Martes"
Can you guide me to a web site where that loop
input impedance is given in R+jX?
_____________

I don't know of one off-hand. Maybe Google, or you could download free
EZNEC to experiment with.

If it will do, a quick NEC-2 model of a ~0.1 lambda diameter loop I just did
shows 2.20831E+0, -1.31989E+03. This R is a little lower than the first
number I posted -- which I had looked up in Kraus, 3rd edition.

RF

Richard Fry wrote:
"This R is a little lower than the first number I posted--which I looked
up in Kraus, 3rd edition."

An excellent source for everything about antennas, I think. I chose
Arnold B. Bailey who has a special affinity for loops, I think. In "TV
and Other Receiving Antennas" on page 403 Bailey says:
"The small loop has a very low resistance (not much over 0.5 ohm for a
circumference of 0.25 wavelength) and a very high positive Q indicating
inductive reactance. The Q will depend on the thickness of the cross
section of the conductor (P factor, as previously used for rod
antennas). For 200-Mc balanced circular loops, where L=0.25 wavelength
(coil diameter=0.25 wavelength/pi), the following Q`s are
representative:
1. For 1-inch diameter rods, QA = 175
2. For 1/4-inch diameter rods, QA = 280
3. For No. 10 wire, QA = 400."

Bandwidth is about equal to 2/QA.

Q is about X/R, or X is about RQ.

The loop radiation resistance of 0.5 ohm is so small that loss
resistance likely may affect the actual loop resistance total.

I`ll leave the reactance (RQ) for the user to calculate for whatever
frequency and Q may interest him.

Best regards, Richard Harrison, KB5WZI

On Mon, 07 Mar 2005 19:13:14 GMT, "Jerry Martes"
wrote:


"Richard Fry" wrote in message
...
"Jerry Martes"
Do you know the approximate input impedance of
a 0.1 lambda diameter single turn loop?
__________

About 2.5 ohms.

RF

Richard

Can you guide me to a web site where that loop input impedance is given in
R+jX? I will set up and measure some loop input impedance at 137 MHz
within the next few days. But, I'd like to have a better "feel" for what
I'm doing before I take time to build the loop and set up the test
equipment.

Thanks
Jerry


Hi Jerry,

The free version of EZNEC should cope with this simple problem. To
answer your question it reveals:
Impedance = 4.887 + J 853 ohms

73's
Richard Clark, KB7QHC