On Mon, 28 Sep 2009 14:13:31 -0700 (PDT), wrote:

I have been looking for a decent CP design to try out on my
repeaters .
For many years I have been looking at the reasoning for FM
broadcasters having used CP pol with great success.

I also have a small indication that linear antennas (eg:linear
collinears)and non-linear antennas(eg:folded loop dipoles) by design
may have slightly different characteristics in the far field that
tend towards a greater degree of cross/CP from the mechanical design .

Methinks you'll be better off with eliptical polarization, where
perhaps 70% of the power is vertically polarized, and the rest is
horizontal. The idea is that most of the energy will be properly
recieved by the traditional vertically polarized mobile antenna. The
remainder will be to fill in the gaps, where the polarization changes
to something not so vertical.

Search Google patents:
http://www.google.com/patents
for "omnidirectional circular polarized antenna". Also, look at FM
broadcast xmit antennas. Some really strange stuff there.

Anyone had good success with installing a CP repeater antenna(2m or
70cm) to assist with the deap fade nulls in mobile uplink to the
repeater.

Yes. I can't find my previous rant on the subject, so I'll try again.
Around 1968 thru 1970, I was at Cal Poly Pomona doing some experiments
with CP UHF antennas. The problem was that the local repeater was
experiencing deep Raleigh fading as mobiles moved along the San
Bernardino Freeway. The ideas was that CP would reduce these fades so
that listening to the repeater didn't sound like it was accompanied by
a machine gun. It worked but with a price. The fades were
dramatically reduced, but so was the coverage area. Antenna gain was
down at least 3dB as was maximum range for the repeater. I thought it
was a good tradeoff, but not everyone agreed. The stations that were
marginal (usually because they were too lazy to install a proper
antenna) were gone. However, the reliability of the local mobiles was
greatly improved. Range won over fade reduction and the antenna was
replaced after a few months of testing.

There was an article on the subject in "The Practical Handbook of
Amateur Radio FM and Repeaters" by Bill Pasternack and Mike Morris.
1981 by Tab Books.
http://openlibrary.org/b/OL4421123M/practical_handbook_of_amateur_radio_FM___repeaters
http://www.alibris.com/booksearch?qsort=p&isbn=0830612122
Sheesh. I paid about $5 at a hamfest.

Since it's not online, I scanned the applicable chapter, chopped out
some irrelevant junk, and posted them to:
http://802.11junk.com/jeffl/circular-polarization/
I'll convert them to a single PDF after I figure out why Irfanview is
generating garbage. Bug me if you have trouble reading the JPG's.

I want to try rhp for TX and lhp for RX - anyone tried this
combination before?

Yep. I tried that to improve TX-RX isolation. I gave up. Mounted on
a single pole, with separate phasing harnesses, I never could get the
antennas anywhere near 50 ohms. There was also just too much coupling
between the RH and LH sections for that to work. Another explanation
might be that at the time, I didn't have a clue what I was doing, but
we won't go there. The rather high VSWR certainly didn't help with
the isolation as it detuned the tx and rx cavities (formerly a
duplexer). Some rather crude testing showed that we were better off
with a single antenna, the stock duplexer, and a single CP
polarization. There was also a problem with distant stations. They
wanted to use CP on the base station to reduce the effects of
multipath (usually off the infamous L.A. smog inversion layer). That
would have required two antennas for each base station. I suggest
that you stick with one circular polarization for now.

Incidentally, note the scanned photo of the Loop Mountain site
http://802.11junk.com/jeffl/circular-polarization/cp-02.jpg
and compare it with the previous intermod factory and post-storm
removal exercise at:
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn02.html
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn03.html
One of the reasons I'm current intact and still sane is that I don't
own any repeaters and find excuses not to do any tower work.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

"Roy Lewallen" wrote in message
...
christofire wrote:
. . .
* HP is less effective than VP for VHF communication with mobiles because
the ever-present ground reflection has reversed polarity (i.e. it's in
antiphase with the signal propagating over a direct path).
. . .

Reflection of a CP wave results in a reversed CP wave only when the
reflection is directly normal to the reflecting surface. That's not at all
the case for typical ground reflections. What you actually get from ground
reflections is an elliptically polarized wave, likely having the same
sense as the impinging wave. In experiments I ran about 35 years ago, I
found nearly linearly polarized waves resulting from reflection of a CP
wave from a vehicle top.


No doubt, but what I wrote above was about horizontal and vertical _linear_
polarisations, HP and VP. I did wonder after I sent it if 'polarity' would
be misread as polarisation.

Chris

christofire wrote:
"Roy Lewallen" wrote in message
...
christofire wrote:
. . .
* HP is less effective than VP for VHF communication with mobiles because
the ever-present ground reflection has reversed polarity (i.e. it's in
antiphase with the signal propagating over a direct path).
. . .
Reflection of a CP wave results in a reversed CP wave only when the
reflection is directly normal to the reflecting surface. That's not at all
the case for typical ground reflections. What you actually get from ground
reflections is an elliptically polarized wave, likely having the same
sense as the impinging wave. In experiments I ran about 35 years ago, I
found nearly linearly polarized waves resulting from reflection of a CP
wave from a vehicle top.


No doubt, but what I wrote above was about horizontal and vertical _linear_
polarisations, HP and VP. I did wonder after I sent it if 'polarity' would
be misread as polarisation.

Chris

My mistake, I apologize for misreading it. Yes, HP does reverse
polarity, exactly as you say, when reflecting from the ground.
Furthermore, the amplitude of the reflection is nearly the same as the
incident wave (that is, the reflection coefficient is nearly -1), even
when ground conductivity is relatively poor, especially at low angles.
So the sum of the incident and reflected waves is near zero.

Unfortunately, the sum of a vertically polarized field and its
reflection from finite conductivity ground is also nearly zero at low
elevation angles, for different reasons. There is a component that
propagates via surface wave, but it attenuates to essentially zero in a
very short distance at VHF.

Roy Lewallen, W7EL

Ok all noted on the many ideas.

Here is one for you boffins:

After having a look at the circular turnstile type antenna with a
groundplane ,I sent a request to the main manufactureers regarding
using this in an inverted state for what I wanted to do on the
repeater.Still yet to get a reply and after a few months I guess they
are still thinking about it.

If they advertised it so great for horizontal to near vertical
satellite work then the same principle should apply if inverted.

Style of eggbeater looking at .
http://pagesperso-orange.fr/on6wg/Do...Part1-Full.pdf

What do the Boffins suggest.

wrote in message
...
Ok all noted on the many ideas.

Here is one for you boffins:

After having a look at the circular turnstile type antenna with a
groundplane ,I sent a request to the main manufactureers regarding
using this in an inverted state for what I wanted to do on the
repeater.Still yet to get a reply and after a few months I guess they
are still thinking about it.

If they advertised it so great for horizontal to near vertical
satellite work then the same principle should apply if inverted.

Style of eggbeater looking at .
http://pagesperso-orange.fr/on6wg/Do...Part1-Full.pdf

What do the Boffins suggest.


1. How are you thinking of mounting it whilst upside down? Even if you
were to use a dielectric swan neck of some kind, you'd still have the feed
line passing through the volume into which the antenna is intentionally
radiating, so the radiation pattern would be upset to some extent. Of
course, if you were willing to modify the antenna you could probably pass a
co-axial feed line through an axial tube - but if you were going to that
much trouble you might as well build the thing yourself.

2. Their statement that: 'Each of the two loops forming the antenna has an
impedance of 100 ohms, and when coupled in parallel, they offer an ideal 50
ohms impedance. We will use the properties of a quarter wavelength coax line
to achieve a 90 degree phase difference between the two loops in order to
obtain circular polarisation.' is a bit suspect. This would be true for a
half wavelength of line, indeed that is the basis of a very common form of
balun, but a quarter wavelength of transmission line will cause an impedance
transformation. If they use 50 ohm cable then a quarter wavelength will
transform 100 ohms down to 25 ohms, and when this is connected in parallel
with another 100 ohms the result will be 20 ohms which is presented to the
feed line. Not a particularly good match!

My suggestion is: if you intend to purchase, proceed with caution and keep
asking the proprietor the questions you want answered.

Chris

"christofire" wrote in message
...

wrote in message
...
Ok all noted on the many ideas.

Here is one for you boffins:

After having a look at the circular turnstile type antenna with a
groundplane ,I sent a request to the main manufactureers regarding
using this in an inverted state for what I wanted to do on the
repeater.Still yet to get a reply and after a few months I guess they
are still thinking about it.

If they advertised it so great for horizontal to near vertical
satellite work then the same principle should apply if inverted.

Style of eggbeater looking at .
http://pagesperso-orange.fr/on6wg/Do...Part1-Full.pdf

What do the Boffins suggest.


1. How are you thinking of mounting it whilst upside down? Even if you
were to use a dielectric swan neck of some kind, you'd still have the feed
line passing through the volume into which the antenna is intentionally
radiating, so the radiation pattern would be upset to some extent. Of
course, if you were willing to modify the antenna you could probably pass
a co-axial feed line through an axial tube - but if you were going to that
much trouble you might as well build the thing yourself.

2. Their statement that: 'Each of the two loops forming the antenna has
an impedance of 100 ohms, and when coupled in parallel, they offer an
ideal 50 ohms impedance. We will use the properties of a quarter
wavelength coax line to achieve a 90 degree phase difference between the
two loops in order to obtain circular polarisation.' is a bit suspect.
This would be true for a half wavelength of line, indeed that is the basis
of a very common form of balun, but a quarter wavelength of transmission
line will cause an impedance transformation. If they use 50 ohm cable
then a quarter wavelength will transform 100 ohms down to 25 ohms, and
when this is connected in parallel with another 100 ohms the result will
be 20 ohms which is presented to the feed line. Not a particularly good
match!


OK - I've just noticed they specify RG62 for the quarter wavelength of line,
and that has 93 ohms characteristic impedance. It would transmform 100 ohms
down to 86.5 ohms which, in parallel with the other 100 ohms, would yield
46.4 ohms. Better.

Chris

Regarding the mounting , running the feeder out from a concrete
microwave tower one can run the feeder from the top or bottom of the
antenna.

wrote in message
...
Regarding the mounting , running the feeder out from a concrete
microwave tower one can run the feeder from the top or bottom of the
antenna.

Hi Brad

How is the eggbeater performance superior to that of a Lindenblad?

Jerry KD6JDJ

Hi Jerry

Very good question ,and I do not have any information regarding their
comparison.

On this tower due to the size and isolation I would have to split the
feeder and have a minumum of two or more antennas around the tower .
(Toer is 10m in diameter with steel reinforcing - high front to back
isolation)

For this specific case , a Lindenbald would perhaps have too much
horizontal space to mount a number of them around the tower.

Unless you are assuming that one uses one antenna split over the 4 ,
equally spaced around the tower.
Again this is a good question as under normal circumstances one does
not have the high isolation between the elements .

I could use a 1:4 wilkoson and mount folded dipoles at skewed angles ,
so at any one time a distant station should see at least two of the
4 .

This is a lot to ponder about .