horizontal polarization of a satellite signal

[napst3r]

Hi All,

I would appreciate if anyone could help me out with this question.

A horizontally polarized wave is transmitted from a geostationary satellite and
is received at an earth station which is west of the satellite and in the northern
hemisphere. Will the wave received at the earth station be horizontally
polarized?

 

[davenn]

it maybe but it could end up changed. Its not unusual for polarisation to be flipped by its trip through the ionosphere

Dave

 

[duke37]

Horizontal at the satellite will not be horizontal at the ground unless the receiver is due north of the satellite?
A nice bit of 3D geometry which is beyond me. Prehaps if it is 30 deg west, the signal will be tilted 30 deg?

 

[(*steve*)]

As a clue, think of the man in the moon and how he looks in the southern hemisphere.

 

[Raven Luni]

Interesting - I knew nothing about RF polarisation until seeing this thread and looking up a couple of articles. I always thought of a signal as a time varying spherical phenomenon originating from a point source. What intrigues me more is the fact that the level is determined by the cosine of the angle between the planes. This is exactly how diffuse lighting is calculated in computer graphics - and that DOES deal with point sources.

So is 'polarisation' really just a measure of how well the receiver is facing the source?

 

[Raven Luni]

And as for these diagrams that show perpendicular electric field and magnetic field in a straight line - well that would be fine if it was a laser like phenomenon, but you cant realise something like that in 3 dimensions. How would you map this to a sphere (or any other omni-directional volume). You cant. You'd certainly need more than 3 dimentions.

 

[davenn]

...........
So is 'polarisation' really just a measure of how well the receiver is facing the source?

no, polarisation is determined by the transmitting antenna as to whether its horizontal, vertical or circularly polarised.
For maximum received signal strength, you receive antenna will have the same polarisation as the transmitting antenna. I cant give you the maths for all this ... its not my forte
Suffice it to say ... there is a significant difference in signal strength if your antennas have opposite polarity, ~ 28 dB. This is very easily demonstratable particularly on our amateur radio VHF and up bands, where over a short range, say up to several hunderd km, line of sight paths where there is no skywave ( ionospheric reflection) present as part of the propagation process.

Once the radiated (transmitted) signal interacts with the ionosphere, all bets are off, as this can result in multiple polarisation changes and is one of the factors that produce signal fading that can be heard on the Short Wave (HF) band, 1 - 30MHz signals.


cheers
Dave

 

[napst3r]

Thanks for all the replies and interest, as far as my knowledge goes polarization with regards to satellite signals is "horizontal polarization"= parallel to the equatorial plane
"vertical polarization" = parallel to the earth axis

i think that should clarify the visualization.

so i think its safe to say that the signal revived on the earth could be depolarized from its original form due to the ionosphere , and its irrespective of where the earth station is situated

 

[(*steve*)]

napst3r, read my clue.

Imagine there's a big green arrow on the moon's surface.

Does that arrow appear to point the same way for all people no matter where they are?

Assume it points "north" on the moon (and assume that moon north is the same as earth north).

Which way does the arrow point to a person in the US when the moon is (a) rising in the east, (b) setting in the west, (c) at its zenith?

And how does this same arrow appear at the same locations for a person in (say) Argentina?

 

[Merlin3189]

Since the geostationary sat is on the equator and the polarisation is in the equatorial plane, I'd have thought that at any point in the radio field, the polarisation would be parallel to the equatorial plane and all observers would see it locally as polarised parallel to their local plane parallel to the equatorial plane. Whether this is what they call horizontal, I don't know. In this plane a line which is perpendicular to the direction of the satellite will point towards the ground (not vertically), except when the satellite is due south. When the satellite is 90 deg away, I think this line would be vertical, though I don't think the change in angle is linear.
But I don't think we use many satellites more than about 15 deg from south, do we? *I think Cos 15 deg is about 96%, so we are not losing much if we stick to horizontal. Even Cos 30 deg is 86%. Since people align their satellite aerials for azimuth by trial and error with a meter, presumably the pefectionists could do the same with the rotation.
If there is bending of the waves through the atmosphere (which I think is not generally the case with these frequencies) then I suppose the wave might change its plane of polarisation?

*I thought someone had already written that misaligned aerials receive a signal proportional to the Cos of the angle between them. So that it becomes 0 at 90 deg.