"Art Unwin" wrote in message
...
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?)

I hae not noticed it, but will not say that it can not hapen.

I have worked Texas from NC on two meters and lots of times way to the north
on 2 and 432. I was not working the sats while that was going on.

On Tue, 11 Aug 2009 13:13:38 -0700, Roy Lewallen
wrote:

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.

Sorry, my original reply to this comment was screwed up thanks to me
talking on the phone while writing.

Yeah, the problem with turnstiles CP is at the horizon. At the
horizon, one element of the turnstile would be roughly perpendicular
to me, thus acting as a simple linear dipole. The other crossed
element would be seen from the end, resulting in no radiation in my
direction. So, at the horizon, a turnstile is mostly linear
polarization.

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.

If you really want a turnstile GPS antenna:
www.arrl.org/tis/info/pdf/0210036.pdf

As for the choke ring, you've probably seen them in the center of
C-band DBS/TVRO dish antennas.

Links:
http://www.javad.com/jns/index.html?/jns/technology/Choke%20Ring%20Theory.html
http://www.trimble.com/infrastructure/gnss-choke-ring-antenna.aspx?dtID=overview
http://www.gpsworld.com/survey/news/trimble-choke-ring-antenna-uses-dorne-and-margolin-dipole-3620

More detail:
www.novatel.com/Documents/Papers/3D_choke_ring.pdf
If you remove the radome in the center, it's a "pinwheel" antenna,
with which I'm totally unfamiliar. Note the comments on "low
elevation tracking", which is what I was mumbling about for improving
the performance at the horizon. The above article don't show it, but
the choke ring does maintain some semblance of CP near the horizon.

Patent on the dual frequency (L1 and L2 for GPS) choke ring:
http://www.google.com/patents/about?id=EiwIAAAAEBAJ&dq=6278407

On Aug 11, 9:00*pm, "Ralph Mowery" wrote:
"Art Unwin" wrote in message

...

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?)

I hae not noticed it, but will not say that it can not hapen.

I have worked Texas from NC on two meters and lots of times way to the north
on 2 and 432. *I was not working the sats while that was going on.

I used to have a longwire that really liked to work into Louisanna
from NC. One of the lobes must have been going right into Sldell.

Jimmie

On Aug 11, 5:10*pm, Art Unwin wrote:
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- Hide quoted text -

- Show quoted text -

Sounds like the same old Art to me, wants opinions and help on an
antenna and he is not willing to disclose what he is really talking
about.
I would Ploink him if I could.

Jimmie

JIMMIE wrote:
On Aug 11, 5:10 pm, Art Unwin wrote:
On Aug 11, 3:28 pm, JIMMIE wrote:
NO- Hide quoted text -

- Show quoted text -

Sounds like the same old Art to me, wants opinions and help on an
antenna and he is not willing to disclose what he is really talking
about.
I would Ploink him if I could.

Jimmie


No, don't even think of it. He is the best entertainment on the net
when he is off his meds.

tom
K0TAR

Jeff Liebermann wrote:

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.

So the ionosphere knows the difference between CP and linear?

Interesting, since horizontal and vertical both reflect, and CP is a
combination of both.

Could you please explain what's happening with CP?

Thanks.

tom
K0TAR

On Tue, 11 Aug 2009 23:01:49 -0500, tom wrote:

Jeff Liebermann wrote:

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.

So the ionosphere knows the difference between CP and linear?

I suspect the decision making abilities of the ionosphere approaches
zero. Attributing intelligence to inanimate objects is generally a
bad idea.

Interesting, since horizontal and vertical both reflect, and CP is a
combination of both.

Could you please explain what's happening with CP?

Ummm.... no, I can't.
Please read my other points and see if they pass a sanity check.

Incidentally #2 should read:
"If you're sending CW (not CP) with your right hand..."
Sorry(tm).

I just hate it when someone takes me seriously.

--
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 Tue, 11 Aug 2009 13:13:38 -0700, Roy Lewallen
wrote:

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.

Sounds reasonable. Incidentally, the FM broadcast "cloverleaf"
antenna was invented by Philip Smith, the inventor of the Smith Chart:
http://www.ieeeghn.org/wiki/index.php/Philip_H._Smith_Oral_History#FM_Broadcasting_and_t he_Cloverleaf_Antenna

I blundered cross this page on a 6/2 meter CP antenna design. Looks
workable but very narrow band:
http://www.wa7x.com/cycloid_info.html

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.

With my limited abilities, it will probably take more than a "short
while" with an antenna modeling program. I've never done any
polarization studies. I'll take your word for this, but it would seem
that there would be a gradual transition from total reversal in sense
from a perpendicular reflector, to conversion to linear polarization
with shallow reflection angles. I'll see if I can find some research
on this. (I'm curious).

I once did some crude experimentation on the degree of isolation
provided by a reflective sense reversal. I just happen to have two
big 2.4Ghz helical antennas, both RH CP. I separated them by about 15
meters and measured the received signal level. I then placed an
obstruction (corner of building) along the line of sight, and supplied
a flat plate reflector. I didn't think to try varying angles of
incidence and reflection and just ran it at 45 degrees from the plane
of the flat plate reflector. The signal dropped about -15dB which I
guess is about all I could expect in an uncontrolled environment. The
loss would indicate that the signal was still substantially circularly
polarized at 45 degrees. I still have the helix antennas and can
repeat the test if necessary (and if I can find the time).

Thanks for the clarification.

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

Jeff Liebermann wrote:

Ummm.... no, I can't.
Please read my other points and see if they pass a sanity check.

Incidentally #2 should read:
"If you're sending CW (not CP) with your right hand..."
Sorry(tm).

I just hate it when someone takes me seriously.


Ahh. I see.

Should have read on rather than just reading the first and responding.

tom
K0TAR

Roy Lewallen wrote:

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


For high performance GPS receivers and measurements, it's important that
the apparent position of the antenna be be independent of the look angle
to the various satellites. Partly this is by making antennas with a
phase center that is look angle independent, partly this is by making
sure you're not receiving a combination of direct and reflected waves.

Remember that for precision GPS, what you're looking at is essentially
the carrier phase within a single chip time (about 100ns). The carrier
phase outside the correlator's time window doesn't contribute to the
observable measurement (because it's got a random 180 degree phase shift
superimposed on it).

So what you're really worried about is interference that causes an
apparent change in phase of the carrier (at 1.5GHz.. call it 20cm
wavelength). In precision GPS, you're talking millimeter scale
measurements, or, say, better than 1 degree of apparent phase shift. A
reflected signal that is 35 dB down is enough to get this sort of error.


The multipath from "distant" reflectors is fairly easily dealt with by
putting the antenna on a pole. Distant, here, means a few meters away..
farther than that, and the multipath signal's code phase is far enough
away that it doesn't contribute to the measurement. The chip length is
about 100 ns, or 30 meters. So, putting the antenna on a pole a few
meters up, with a plate underneath it that cuts off the "view" of the
ground closer than a few meters away guarantees that you won't see any
reflections from something closer than 20 meters path length different.

You also choose an antenna that has very little gain below several
degrees above the horizon.

But, just any old flat plate won't work, because you can have a creeping
wave propagate across the surface AND you don't want the plate itself to
reflect a signal. Solution: make a plate that is a RF "black hole" at
the frequency of interest.. it's a series of grooves that are carefully
designed to attenuate the reflected and evanescent wave propagating
across the surface (just like in a corrugated horn).

The most common design is by Dorne and Margolin, and I guarantee you've
seen these if you've seen surveyors doing GPS measurements. They're
about 2' in diameter with several wide grooves around a small conical or
hemispherical radome covering the actual antenna. Sometimes, the whole
thing is covered by a hemispherical radome.

http://facility.unavco.org/project_s.../antennas.html

Is a photo of a typical geodetic installation (sub-mm accuracies)

http://www.trimble.com/infrastructur...?dtID=overview

is the actual antenna.