Hi

I want to find a place where I can learn about the effects the atmosphere
has on the polarization of signals from satellites. Does anyone know of
any web sites where that might be discussed?
Thanks
Jerry

On 28 Mar, 15:25, "Jerry Martes" wrote:
Hi

I want to find a place where I can learn about the effects the atmosphere
has on the polarization of signals from satellites. Does anyone know of
any web sites where that might be discussed?
Thanks
Jerry

W4rnl talks about satellite antennas, might be worth a look

On Mar 28, 3:25 pm, "Jerry Martes" wrote:
Hi

I want to find a place where I can learn about the effects the atmosphere
has on the polarization of signals from satellites. Does anyone know of
any web sites where that might be discussed?
Thanks
Jerry


Referred to as Faraday Rotation.. ARRL handbook talks about it in the
EME section. Rotation is less as frequency increases (so VHF is
affected a lot more than UHF is affected more than microwaves)
Amount of rotation is determined by the total electron content in the
path, which, of course, varies with everything you can imagine, but
primarily the amount of sunlight.
It's related to the difference (dispersion) in propagation speed with
frequency (or, index of refraction, if you want to work it that
way)..

More than 360 degrees of rotation isn't unusual at VHF.

What's really interesting is that it rotates the same way no matter
which direction you go, so if you had a planar reflector above the
ionosphere, and you launched a wave that's say polarized North-South,
and there's 45 degrees of rotation, when it came back down it would be
rotated 90 and be polarized East West...

This is one reason why GPS radiates on more than one frequency (you
measure the phase difference between the two, and that allows you to
estimate the TEC, which in turn allows you to estimate the change in
propagation velocity, which improves the accuracy of your fix.)

Most high accuracy orbiting radars that operate in P or L band do
something similar, especially if they are making polarimetric
measurements (handy if you want to do things like distinguish between
trees and grass, for instance)

How much detail do you need? I can probably give you URLs to cover
everything from simple estimates of magnitude to mind bendingly
detailed theoretical discussions.

wrote in message
ps.com...
On Mar 28, 3:25 pm, "Jerry Martes" wrote:
Hi

I want to find a place where I can learn about the effects the
atmosphere
has on the polarization of signals from satellites. Does anyone know of
any web sites where that might be discussed?
Thanks
Jerry


Referred to as Faraday Rotation.. ARRL handbook talks about it in the
EME section. Rotation is less as frequency increases (so VHF is
affected a lot more than UHF is affected more than microwaves)
Amount of rotation is determined by the total electron content in the
path, which, of course, varies with everything you can imagine, but
primarily the amount of sunlight.
It's related to the difference (dispersion) in propagation speed with
frequency (or, index of refraction, if you want to work it that
way)..

More than 360 degrees of rotation isn't unusual at VHF.

What's really interesting is that it rotates the same way no matter
which direction you go, so if you had a planar reflector above the
ionosphere, and you launched a wave that's say polarized North-South,
and there's 45 degrees of rotation, when it came back down it would be
rotated 90 and be polarized East West...

This is one reason why GPS radiates on more than one frequency (you
measure the phase difference between the two, and that allows you to
estimate the TEC, which in turn allows you to estimate the change in
propagation velocity, which improves the accuracy of your fix.)

Most high accuracy orbiting radars that operate in P or L band do
something similar, especially if they are making polarimetric
measurements (handy if you want to do things like distinguish between
trees and grass, for instance)

How much detail do you need? I can probably give you URLs to cover
everything from simple estimates of magnitude to mind bendingly
detailed theoretical discussions.


Hi Jim

I am considering the design of a horn to illuminate an 8 foot diameter
solid surface dish at 4 GHz, for reception of geosynchronous satellite
signals that are linearly polarized.

One rraa reader has informed me that the rotation is refered to as Faraday
rotation and gave some links to it. I see that the amount of rotation
may be small enough to be negligable for my application.

I would like to know how rapid the rotation changes with time.

Thanks
Jerry

Jerry Martes wrote:

wrote in message
ups.com...
On Mar 28, 3:25 pm, "Jerry Martes" wrote:
Hi

I want to find a place where I can learn about the effects the
atmosphere
has on the polarization of signals from satellites. Does anyone know of
any web sites where that might be discussed?
Thanks
Jerry


Referred to as Faraday Rotation.. ARRL handbook talks about it in the
EME section. Rotation is less as frequency increases (so VHF is
affected a lot more than UHF is affected more than microwaves)
Amount of rotation is determined by the total electron content in the
path, which, of course, varies with everything you can imagine, but
primarily the amount of sunlight.
It's related to the difference (dispersion) in propagation speed with
frequency (or, index of refraction, if you want to work it that
way)..

More than 360 degrees of rotation isn't unusual at VHF.

What's really interesting is that it rotates the same way no matter
which direction you go, so if you had a planar reflector above the
ionosphere, and you launched a wave that's say polarized North-South,
and there's 45 degrees of rotation, when it came back down it would be
rotated 90 and be polarized East West...

This is one reason why GPS radiates on more than one frequency (you
measure the phase difference between the two, and that allows you to
estimate the TEC, which in turn allows you to estimate the change in
propagation velocity, which improves the accuracy of your fix.)

Most high accuracy orbiting radars that operate in P or L band do
something similar, especially if they are making polarimetric
measurements (handy if you want to do things like distinguish between
trees and grass, for instance)

How much detail do you need? I can probably give you URLs to cover
everything from simple estimates of magnitude to mind bendingly
detailed theoretical discussions.


Hi Jim

I am considering the design of a horn to illuminate an 8 foot diameter
solid surface dish at 4 GHz, for reception of geosynchronous satellite
signals that are linearly polarized.

One rraa reader has informed me that the rotation is refered to as Faraday
rotation and gave some links to it. I see that the amount of rotation
may be small enough to be negligable for my application.

I would like to know how rapid the rotation changes with time.

That depends on geomagnetic activity, because Faraday rotation is also a
function of the earth's magnetic field.

With a signal that started out linearly polarized, there are three
sources of change:

1. Simple geometry - if a geostationary satellite orbiting above the
0deg meridian has an antenna that is horizontally polarized when viewed
from a ground location on the same meridian, that same antenna will
appear vertically polarized when viewed from a location 90deg west. This
effect is easily calculated and compensated.

2. Faraday rotation, as above.

3. Dispersion ("smearing") of the linear polarization, at times of high
geomagnetic activity. From experience with a 432MHz array that had fully
rotatable polarization, polarization could range from accurately linear
at geomagnetically quiet times (with a very deep null at 90deg offset),
all the way to dispersion of the signal around the whole 360 degrees. A
completely dispersed signal will have a 3dB loss compared with
accurately aligned linear polarization, so you have to include that
possibility in your path loss budget. (At lower frequencies, dispersion
is also accompanied by ionospheric absorption, but at 4GHz you may not
need to worry about that.)

Bottom line: if you have enough signal/noise ratio to handle a permanent
3dB loss, then it will be much more convenient to ignore the fact that
the signal started out linearly polarized. Instead, use a
circularly-polarized feedhorn which will be insensitive to the
polarization of the arriving signal.


--

73 from Ian GM3SEK

"Jerry Martes" wrote in
news:iyJOh.23960$FD1.9394@trnddc05:

....
Hi Jim

I am considering the design of a horn to illuminate an 8 foot
diameter
solid surface dish at 4 GHz, for reception of geosynchronous satellite
signals that are linearly polarized.

One rraa reader has informed me that the rotation is refered to as
Faraday
rotation and gave some links to it. I see that the amount of
rotation may be small enough to be negligable for my application.

I would like to know how rapid the rotation changes with time.

Jerry, I am a little surprised that a geostationary satellite would use
linear polarisation on a 4GHz feed, but that might just express a lack of
experience.

A long time ago, I worked with the Intelsat series, and they were
circular polarisation. Earth stations had no means of adjusting the
orientation of feeds, they were RH or LH circular, the uplink was
opposite to the downlink IIRC.

More recently, I worked on the design of a bird that used polarisation
diversity. It used LH and RH circular, and reused the same frequency band
on both polarisations.

If your bird is truly linear, you could use a circular antenna with a
slight reduction in G/T, but with the flexibility of eliminating the
orientation variable and the mechanical aspects of an antenna with
adjustable orientation (remembering that the feed orientation will vary
with position of the earth station).

Notwithstanding that transmission might be circular, the received signal
might not be perfectly circular as a result of some of the effects you
have described.

Owen

"Jerry Martes" wrote in message
news:iyJOh.23960$FD1.9394@trnddc05...

wrote in message
ps.com...
On Mar 28, 3:25 pm, "Jerry Martes" wrote:
Hi

I want to find a place where I can learn about the effects the
atmosphere
has on the polarization of signals from satellites. Does anyone know
of
any web sites where that might be discussed?
Thanks
Jerry


Referred to as Faraday Rotation.. ARRL handbook talks about it in the
EME section. Rotation is less as frequency increases (so VHF is
affected a lot more than UHF is affected more than microwaves)
Amount of rotation is determined by the total electron content in the
path, which, of course, varies with everything you can imagine, but
primarily the amount of sunlight.
It's related to the difference (dispersion) in propagation speed with
frequency (or, index of refraction, if you want to work it that
way)..

More than 360 degrees of rotation isn't unusual at VHF.

What's really interesting is that it rotates the same way no matter
which direction you go, so if you had a planar reflector above the
ionosphere, and you launched a wave that's say polarized North-South,
and there's 45 degrees of rotation, when it came back down it would be
rotated 90 and be polarized East West...

This is one reason why GPS radiates on more than one frequency (you
measure the phase difference between the two, and that allows you to
estimate the TEC, which in turn allows you to estimate the change in
propagation velocity, which improves the accuracy of your fix.)

Most high accuracy orbiting radars that operate in P or L band do
something similar, especially if they are making polarimetric
measurements (handy if you want to do things like distinguish between
trees and grass, for instance)

How much detail do you need? I can probably give you URLs to cover
everything from simple estimates of magnitude to mind bendingly
detailed theoretical discussions.


Hi Jim

I am considering the design of a horn to illuminate an 8 foot diameter
solid surface dish at 4 GHz, for reception of geosynchronous satellite
signals that are linearly polarized.

One rraa reader has informed me that the rotation is refered to as
Faraday rotation and gave some links to it. I see that the amount of
rotation may be small enough to be negligable for my application.

I would like to know how rapid the rotation changes with time.

Thanks
Jerry


Any particulat sat you are interested in. I you to have a downlink with GOES
east and west and you wouldnt believe how simple of an antenna it took A
pair of dipoles connected for circular polarity. LNA and downconverer were
at he antenna..

Jimmie

Owen Duffy wrote in
:

"Jerry Martes" wrote in
news:iyJOh.23960$FD1.9394@trnddc05:

.
A long time ago, I worked with the Intelsat series, and they were
circular polarisation. Earth stations had no means of adjusting the
orientation of feeds, they were RH or LH circular, the uplink was
opposite to the downlink IIRC.

More recently, I worked on the design of a bird that used polarisation
diversity. It used LH and RH circular, and reused the same frequency
band on both polarisations.

If your bird is truly linear, you could use a circular antenna with a
slight reduction in G/T, but with the flexibility of eliminating the
orientation variable and the mechanical aspects of an antenna with
adjustable orientation (remembering that the feed orientation will
vary with position of the earth station).

Notwithstanding that transmission might be circular, the received
signal might not be perfectly circular as a result of some of the
effects you have described.

Owen

Hi:
We just had a member that works in the satellite uplink/downlink business
at Penn State University give a talk on the subject. The satellites use
polarization as part of the frequency sharing system in geostationary
satellites. When they buy time on a satellite they are given a frequency
and a polarization to use. As there is a limited band of frequencies they
use polarization to help share frequencies with some working horizontal
and some vertical. The feeds on the dishes they use for uplinks and
downlinks have motorized polarization feeds and they adjust them to the
requested polarization.


John Passaneau W3JXP

John Passaneau wrote in
:

....
Hi:
We just had a member that works in the satellite uplink/downlink
business at Penn State University give a talk on the subject. The
satellites use polarization as part of the frequency sharing system in
geostationary satellites. When they buy time on a satellite they are
given a frequency and a polarization to use. As there is a limited
band of frequencies they use polarization to help share frequencies
with some working horizontal and some vertical. The feeds on the
dishes they use for uplinks and downlinks have motorized polarization
feeds and they adjust them to the requested polarization.

Yes John, I talked about that and I incorrectly used the term
"diversity", but it is frequency reuse as I noted using polarisation.

This can be done with circular (LH and RH) polarisation without a need to
rotate the feed unit, which for many installations will mean another
rotary waveguide joint which is to be avoided.

Launching circular polarisation in a feed horn is not difficult, and
developing autotrack error signals is relatively easy. An interesting
feed for amateur applications is the septum feed which gives access to
both circular polarisations from a fairly simple feed unit.

Owen

Owen Duffy wrote in
:

John Passaneau wrote in
:

...
Hi:
We just had a member that works in the satellite uplink/downlink
business at Penn State University give a talk on the subject. The
satellites use polarization as part of the frequency sharing system
in geostationary satellites. When they buy time on a satellite they
are given a frequency and a polarization to use. As there is a
limited band of frequencies they use polarization to help share
frequencies with some working horizontal and some vertical. The feeds
on the dishes they use for uplinks and downlinks have motorized
polarization feeds and they adjust them to the requested
polarization.

Yes John, I talked about that and I incorrectly used the term
"diversity", but it is frequency reuse as I noted using polarisation.

This can be done with circular (LH and RH) polarisation without a need
to rotate the feed unit, which for many installations will mean
another rotary waveguide joint which is to be avoided.

Launching circular polarisation in a feed horn is not difficult, and
developing autotrack error signals is relatively easy. An interesting
feed for amateur applications is the septum feed which gives access to
both circular polarisations from a fairly simple feed unit.

Owen

Hi Owen:

In this case the feed point has for lack of the right name a "pick up
loop" that is motorized and that is what turns for the differnt
polarisation. It is linear, not circular polarisation, at least on the
geostationary satellites they use. I would guess that linear is easier
for them to change. The way it works is they call a satatellite broker
and buy time, he gives them a satellite, a frequency and a polarisation
to use. Before the requested time the satellite transmits a test signal.
They tune to that frequency and tune their antenna for max signal, then
they transmit a test signal to the satellite, the satellite operator
verfies that their signal meets spec.'s and then the channel is theirs
for how every long they have paid for. It's more of a hands on operation
than I would have though. Their main busness is sports as they up load
all the sports events here on campus and for many of the non sports
events like news events and some classes that are held via satellite.

John Passaneau W3JXP