On Mon, 10 Aug 2009 11:44:52 -0700, Roy Lewallen
wrote:

I also did some experiments in the early '70s to see if CP would reduce
fading. I built a couple of types of omnidirectional CP antennas -- a
"skew planar", and a copy of a commercial FM BC antenna, for mobile use
with the local 450 MHz repeater.

A "halo" type of antenna? Some of the commercial broadcast FM
antennas are eliptical polarized. Most of the signal is horizontally
polarized, but there is a small vertical component in order to improve
performance in vehicles.

We tried several antennas at the repeater end. Something like this
one seemed to work best:
http://iris.nyit.edu/~sblank/VPFMfig5.gif
We had 4 elements with a coax cable phasing mess.

I soon discovered that as soon as I
placed the antenna over the top of the car, the polarization became
nearly linear. I've since learned that it's because of the nature of the
reflections from the ground plane, and it's easily seen with EZNEC+.
When I put the antenna far enough away from the car to minimize
reflections, the lowered gain offset any possible advantage. Overall,
they worked out worse than a conventional vertically polarized antenna.

I tried to use CP on both ends and eventually gave up. Thanks for the
explanation, but I have a different theory. The polarization changes
sense (direction) every times it's reflected. We standardized on RH
CP. When the RH CP signal hits the car, it is reflected as LH CP. If
the LH CP signal arrives at the repeater antenna, which is RH
polarized, they cancel. If it became linear, it would theoretically
only present a -3dB polarization loss, which is not huge.

It might have been interesting to try CP at the repeater, but that was
never done.

I can testify that it worked quite well for solving the specific
problem. We were trying to eliminate picket fencing (frequency
selective fading or Rayleigh fading). While there were some half
hearted experiments with various CP mobile antennas, the major effort
was at the repeater end. This was about 1971 so the technology used
was rather crude. One student was doing his senior project (reqd for
graduation) around this test. Several of us were enlisted to help.
When was in the land mobile radio biz many years later, I repeated the
tests with similar results.

We hung a thermal chart recorder onto the first limiter testpoint (on
a Motorola Sensicon T43 receiver) and plotted signal strength versus
time as a mobile drove through the problem area with the xmitter keyed
continuously. The test was repeated with various tower mounted
antennas. The linear antenna had more signal (gain) than CP, but also
had many more fades, what were far more pronounced. In all, I would
call it an improvement in quality, but not in range.

The problem with ground reflection ruining the circularity makes it very
difficult to achieve circular polarization for HF skip communication.

Well, I supplied several examples of commercial HF antennas that are
circularly polarized. I'm tempted to try building one, just to see
what works or breaks.

A second problem is that the majority of CP antennas, such as the
quadrature fed crossed dipole "turnstile", are circular only directly
broadside, and increasingly elliptical as you move away from that direction.

That's why high accuracy GPS antennas use choke rings at the antenna.
It widens the pattern so that it picks up more of the sky, but also
maintains some semblence of CP at the horizon.

Any interest in me scanning and posting the chapter on circular
polarization repeater antennas from the TAB book? 13 pages with some
low quality pictures.

Roy Lewallen, W7EL

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

On Mon, 10 Aug 2009 16:23:09 -0700 (PDT), Art Unwin
wrote:

On Aug 10, 5:21*pm, dave wrote:
Art Unwin wrote:
The majority of antennas used today are (commercial) circularly
polarized
Ham antennas remain in the linear domain (ala the Yagi and similar)
There are many reasons espoused in CP advantages in "point to point"
What is the main advantage hams hold over the more popular circular
polarized antennas in its "skip" type useage versus "point to point" ?

Is there a CPOL advantage for atmospherically propagated HF? *I can
think of some disadvantages.

I am all ears. Go for it

Applied CP made simple:

1. Circular polarization is like a drill. Instead of bouncing off
the ionosphere, it drills right through it. Therefore, little or no
skip with CP.

2. Circular polarization is sensitive to the direction of rotation.
If you're sending CP with your right hand, or using a microphone in
your right hand, then you can only hear right hand circular
polarization.

3. CP changes sense every times it bounces off something. Therefore,
you can only hear even numbered bounces with the same sense. Odd
bounces disappear meaning you can hear short skip signals (NVIS), and
long path, but nothing in between.

4. Right hand CP is the most common, because most tuning screws use a
right hand thread. If you want to use left hand CP, you'll need to
find some rather scarce left hand threaded coil forms and tuning
slugs.

I hope this helps.

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

You need to read up on it. It does have it's uses and I know hams that use
it. Usually by Satellite guys that talk to mobiles too. It gets used by
quite a few commercial broadcast FM stations. The theory is that a mobile
antenna will hear either the main path or reflected path but to a lesser
extent, both so multi-path distortion is minimized. Typically the
reflection cause a reversal of polarization. There is no substitute for
talking to the far station in whatever polarity is agreed on.

Jeff Liebermann wrote:
On Mon, 10 Aug 2009 11:44:52 -0700, Roy Lewallen
wrote:

I also did some experiments in the early '70s to see if CP would reduce
fading. I built a couple of types of omnidirectional CP antennas -- a
"skew planar", and a copy of a commercial FM BC antenna, for mobile use
with the local 450 MHz repeater.

A "halo" type of antenna? Some of the commercial broadcast FM
antennas are eliptical polarized. Most of the signal is horizontally
polarized, but there is a small vertical component in order to improve
performance in vehicles.

The "skew planar" looked like a cloverleaf antenna with each "petal"
rotated 45 degrees. The other was a copy of a broadcast antenna
advertised to be circularly polarized. I used a simple hand held dipole
and field strength meter to judge polarization. I know now it was
subject to a number of shortcomings, but I feel it did a reasonable job
of indicating circularity. Both antennas were reasonably circular.

. . .

I tried to use CP on both ends and eventually gave up. Thanks for the
explanation, but I have a different theory. The polarization changes
sense (direction) every times it's reflected. We standardized on RH
CP. When the RH CP signal hits the car, it is reflected as LH CP. If
the LH CP signal arrives at the repeater antenna, which is RH
polarized, they cancel. If it became linear, it would theoretically
only present a -3dB polarization loss, which is not huge.

No, that's a common misconception. A circularly polarized wave produces
a circularly polarized wave of the opposite handedness only when
reflected from a plane normal to its direction of propagation. That's
seldom the case in a communication environment. When reflected from
surfaces at other angles, the result is a change in circularity, from
elliptical to nearly linear depending on the angle of reflection and the
reflection coefficients of the surface. A short while with the modeling
program of your choice will confirm this.

. . .

A second problem is that the majority of CP antennas, such as the
quadrature fed crossed dipole "turnstile", are circular only directly
broadside, and increasingly elliptical as you move away from that direction.

That's why high accuracy GPS antennas use choke rings at the antenna.
It widens the pattern so that it picks up more of the sky, but also
maintains some semblence of CP at the horizon.

I wasn't aware of any GPS receivers using crossed dipole "turnstile"
type antennas. All the ones I've seen use either quadrifilar helix or
patch antennas. Can you point to a reference or two regarding the choke
rings -- I don't know what these are or what they do, and would like to
learn.

. . .

Roy Lewallen, W7EL

On Aug 11, 12:30*pm, "JB" wrote:
You need to read up on it. *It does have it's uses and I know hams that use
it. *Usually by Satellite guys that talk to mobiles too. *It gets used by
quite a few commercial broadcast FM stations. *The theory is that a mobile
antenna will hear either the main path or reflected path but to a lesser
extent, both so multi-path distortion is minimized. *Typically the
reflection cause a reversal of polarization. *There is no substitute for
talking to the far station in whatever polarity is agreed on.

You bet that they have to read up on it.
For those who consider themselves experts and others idiots.
There is some sort of federation for antenna builders and the
President of that group states we will have a disaster on our hands if
we do not come up with a new technology for what is required in 5
years. Apparently cell phones will be divided into three bands and the
big boys want access to all. This means that they need three separate
antennas on the cell phone to cater for all (his words). They don't
like that idea because it means three antennas in close proximation to
each other. The response to that challenge is to group together the
research facilities to find a "new" technology if ever there is one. A
lot of money at stake and it is for any of you that are knowledgable
in the field to apply for.
Those who are really "knowledgable" already know there is no "new"
technology only the one that they use and they all have researched it
to death such that all the answers have been unfolded to them. It is
also too late to ask God to be a bit more fair and provides some
different options to what we already have.
All I ask of them is to share with the rest of us exactly what
happens to CP when it collides with anything in transit to a receiver.

On Aug 10, 11:11*am, Art Unwin wrote:
On Aug 10, 9:04*am, "christofire" wrote:





"Art Unwin" wrote in message

....

The majority of antennas used today are (commercial) circularly polarized

That probably isn't true for VHF/UHF. *Take a look at the website of a major
supplier of professional antennas such ashttp://www.amphenol-jaybeam.com/base-station-antennas-search.php. *Entering
CP in their search engine for base-station antennas yielded 2 results wheras
entering VP yielded 365!

One of the reasons for greater use of linear polarisations in professional
applications is frequency re-use on the orthogonal polarisation some
distance away - i.e. the value of the limited VHF/UHF spectrum. *That's
certainly true in Europe.

Ham antennas remain in the linear domain (ala the Yagi and similar)
There are many reasons espoused in CP advantages in "point to point"
What is the main advantage hams hold over the more popular circular
polarized antennas in its "skip" type useage versus "point to point" ?

Extensive use is made of 'mixed polarisation' *for transmitting VHF FM
broadcast services in Europe. *It may not be pure circular but it contains
significant vertically- and horizontally-polarised components. *The mobile
and portable receiving antennas have whatever polarisation they end up with,
more or less by accident, and fixed rooftop antennas are usually linearly
polarised. *DAB and terrestrial television are transmitted using V or H
linear polarisation.

Chris

Hmm * * *A bit too technical for me! *When I model my antennas it
shows *gain for cp being 3 db above the gains of vertical and
horizontal for the same antenna. Is that what you call "mixed
polarization? *In other words, it picks up all polarizations with a
max deviation in signal strength of only 3 db.
I find it hard to make any sort of comparison when using reflective
waves at HF because it is not clear to me exactly what sort of
rotations *occur at the reflections on earth and of its layers
together with possible rotation in transit in between in the absence
of true comparison experiments.- Hide quoted text -

- Show quoted text -

Describe the circular antenna you are modeling? With the gain you are
claiming it doesnt seem to be an apples to apples comparison. I
suspect you are comparing a fullwave loop to a halfwave dipole. Show
the data you are using for your model or be prepared for most to
suspect you of the usual handwaving.

Jimmie.

On Aug 11, 3:28*pm, JIMMIE wrote:
On Aug 10, 11:11*am, Art Unwin wrote:



On Aug 10, 9:04*am, "christofire" wrote:

"Art Unwin" wrote in message

....

The majority of antennas used today are (commercial) circularly polarized

That probably isn't true for VHF/UHF. *Take a look at the website of a major
supplier of professional antennas such ashttp://www.amphenol-jaybeam.com/base-station-antennas-search.php. *Entering
CP in their search engine for base-station antennas yielded 2 results wheras
entering VP yielded 365!

One of the reasons for greater use of linear polarisations in professional
applications is frequency re-use on the orthogonal polarisation some
distance away - i.e. the value of the limited VHF/UHF spectrum. *That's
certainly true in Europe.

Ham antennas remain in the linear domain (ala the Yagi and similar)
There are many reasons espoused in CP advantages in "point to point"
What is the main advantage hams hold over the more popular circular
polarized antennas in its "skip" type useage versus "point to point" ?

Extensive use is made of 'mixed polarisation' *for transmitting VHF FM
broadcast services in Europe. *It may not be pure circular but it contains
significant vertically- and horizontally-polarised components. *The mobile
and portable receiving antennas have whatever polarisation they end up with,
more or less by accident, and fixed rooftop antennas are usually linearly
polarised. *DAB and terrestrial television are transmitted using V or H
linear polarisation.

Chris

Hmm * * *A bit too technical for me! *When I model my antennas it
shows *gain for cp being 3 db above the gains of vertical and
horizontal for the same antenna. Is that what you call "mixed
polarization? *In other words, it picks up all polarizations with a
max deviation in signal strength of only 3 db.
I find it hard to make any sort of comparison when using reflective
waves at HF because it is not clear to me exactly what sort of
rotations *occur at the reflections on earth and of its layers
together with possible rotation in transit in between in the absence
of true comparison experiments.- Hide quoted text -

- Show quoted text -

Describe the circular antenna you are modeling? With the gain you are
claiming it doesnt seem to be an apples to apples comparison. I
suspect you are comparing a fullwave loop to a halfwave dipole. Show
the data you are using for your model or be prepared for most to
suspect you of the usual handwaving.

Jimmie.

NO

Roy/Jeff

This is all interesting stuff that I have always wanted to experiment on
some more. I note the comment (was it Roy?) about the positioning of a
CP antenna over a vehicle roof and the effect that had on the total
signal polarization. I slapped my forehead on that one! (ie I should
have known and tested for it)

I also used a skew planar loop. It was I guess maybe 400mm above the
vehicle roof. It had 4 loops in phase. The halo/3 leaf HP (clover) was
the same basic construction. At the time the results of H vs V really
astounded me. In my case OTH UHF performance was maybe 12db "better"
mainly from the reduction in flutter.

No, that's a common misconception. A circularly polarized wave produces
a circularly polarized wave of the opposite handedness only when
reflected from a plane normal to its direction of propagation. That's
seldom the case in a communication environment. When reflected from
surfaces at other angles, the result is a change in circularity, from
elliptical to nearly linear depending on the angle of reflection and the
reflection coefficients of the surface. A short while with the modeling
program of your choice will confirm this.

Yes of course! (slaps forehead again!)

This is something that has come out recently in some experiments I have
been doing with mobile data comms on VHF SSB. I have a badly written and
incomplete page;

http://pages.suddenlink.net/vk2yqa

Before getting into the data side of it I had noted that a lot of
intelligence could be gleaned by looking at the Doppler effect from the
moving vehicle. Only a tiny 30Hz or so was noted on 144MHz but it showed
to my mind that a lot of signal comes from multiplathing and even some
comes from double reflections. The level of shift over the Doppler
"bandwidth" also showed that it was fairly evenly distributed. ie
reflections off plane objects like oncoming vehicles were not really any
stronger than from other directions. Such things as large oil tanks
though are quite obvious when I correlate the trip timing with nearby
objects. This also bought sense to the experiments I did back in the
80's where I tried a 3 el quad on the vehicle with dissappointing results.

So where is this going? I wonder what the result would be Doppler
"bandpass" wise if I used a CP antenna on the vehicle and base now? (The
above mentioned results are all VP) Given the sense change reflections I
wonder if I would get a "null" in the received bandpass due to movement.
I am trying to visualize the result of this;

http://pages.suddenlink.net/vk2yqa/img1.png

during that test. Would I get a series of bands parallel to the
envelope edges that would further indicate direction of travel relative
to the base. I realize that the direct path already gives that info,
just trying to get the likely scenbario in my head.

Apologies to Art for taking his post so far OT.

Cheers Bob VK2YQA

On Aug 10, 11:37*am, "Ralph Mowery"
wrote:
"P.R.Humphreys" wrote in message

...

Off subject I know but as a matter of interest what happens to a TX CP
antenna with a clockwise *twist transmitting to a RX CP antenna with an
anticlockwise twist, over short to medium vhf/uhf paths, I would have
thought a reduction in signal ?

Pedr GW6YMS

About the same as going from a horizontal to a vertical antenna. * You loose
about 20 db of signal.
One thing a CP signal will do that a horizontal or vertical signal won't *is
to reverse when it bounces off an object. *That is if you are doing moon
bounce *and send up a signal using right hand CP , your receiving antenna
will need to be left hand CP or you loose lots of db of signal.

Also on the satellites it is helpful to be able to change the CP *from left
to right as the signal sometimes reverses depending on the angle the
satellite antenna is pointing as it passes over.

Do you ever get to the point that your signal bounces off the upper
layers instead of passing thru? I note that some users of the vhf have
seen ducting effects (temp inversions?)

On Aug 11, 4:47*pm, Bob Bob wrote:
Roy/Jeff

This is all interesting stuff that I have always wanted to experiment on
some more. I note the comment (was it Roy?) about the positioning of a
CP antenna over a vehicle roof and the effect that had on the total
signal polarization. I slapped my forehead on that one! (ie I should
have known and tested for it)

I also used a skew planar loop. It was I guess maybe 400mm above the
vehicle roof. It had 4 loops in phase. The halo/3 leaf HP (clover) was
the same basic construction. At the time the results of H vs V really
astounded me. In my case OTH UHF performance was maybe 12db "better"
mainly from the reduction in flutter.

No, that's a common misconception. A circularly polarized wave produces
a circularly polarized wave of the opposite handedness only when
reflected from a plane normal to its direction of propagation. That's
seldom the case in a communication environment. When reflected from
surfaces at other angles, the result is a change in circularity, from
elliptical to nearly linear depending on the angle of reflection and the
reflection coefficients of the surface. A short while with the modeling
program of your choice will confirm this.

Yes of course! (slaps forehead again!)

This is something that has come out recently in some experiments I have
been doing with mobile data comms on VHF SSB. I have a badly written and
incomplete page;

http://pages.suddenlink.net/vk2yqa

Before getting into the data side of it I had noted that a lot of
intelligence could be gleaned by looking at the Doppler effect from the
moving vehicle. Only a tiny 30Hz or so was noted on 144MHz but it showed
to my mind that a lot of signal comes from multiplathing and even some
comes from double reflections. The level of shift over the Doppler
"bandwidth" also showed that it was fairly evenly distributed. ie
reflections off plane objects like oncoming vehicles were not really any
stronger than from other directions. Such things as large oil tanks
though are quite obvious when I correlate the trip timing with nearby
objects. This also bought sense to the experiments I did back in the
80's where I tried a 3 el quad on the vehicle with dissappointing results..

So where is this going? I wonder what the result would be Doppler
"bandpass" wise if I used a CP antenna on the vehicle and base now? (The
above mentioned results are all VP) Given the sense change reflections I
wonder if I would get a "null" in the received bandpass due to movement.
*I am trying to visualize the result of this;

http://pages.suddenlink.net/vk2yqa/img1.png

during that test. Would I get a *series of bands parallel to the
envelope edges that would further indicate direction of travel relative
to the base. I realize that the direct path already gives that info,
just trying to get the likely scenbario in my head.

Apologies to Art for taking his post so far OT.

Cheers Bob VK2YQA

I don't see it as OT, if you get a response run with it
cheers