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chriswlan
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Offset antennas "depolarization"..... HOW?
15th Nov, 2017 at 7:55pm
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I spent many hours on line trying to find out how that happens, but all I can find is papers on how to mitigate it... Anybody here knows a link that explains HOW an offset reflector can slightly "crosspol" an incident signal?

I don't want a heavy math explanation, but rather some graphic explanation. Also: if the incident vector is parallel to the "symetry plane" of the offset antenna, is there no depolarization? Is it different if the signal is "horizontal" polarity? Slanted?

If a two-way dish uses crosspol, can the "depolarization" be diffferent on the 2 signals???? Depends on the skew???

Thank you
  
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Nimrod Kapon - Oasis Networks
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Re: Offset antennas "depolarization"..... HOW?
Reply #1 - 15th Nov, 2017 at 7:59pm
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Hi Chris,

There are a few reasons for bad polarity isolation.

To make it as simple as possible, the best way will be to start with optimize the pointing of the dish, first with Azimuth, then elevation.

And YES, then optimize the skew.

Good luck,
Nimrod
  
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Re: Offset antennas "depolarization"..... HOW?
Reply #2 - 17th Nov, 2017 at 2:14pm
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Slight depolarisation and slight pointing offset occurs with front-fed offset reflectors with the F/D ratio less than about 1.

To fix it you need to use a mode-matched feed (like some higher quality Ku band Prodelin feeds) or to add a subreflector (like some older Ka band dishes) or use a longer focal length dish. e.g F/D=1.  Many dishes have F/D=0.6

If you use a mode matched feed the throat is not circular but had three hidden slots inside a fat lump on one side of the feed throat. The 3 mode generating slots must be aligned with the  feed support arm and the entire dish and feed must be turned, as one item, to adjust linear polarisation. This needs a big circular polarisation scale behind the dish on a wedge shaped box.

As to a non-mathematical graphic explanation I am not sure I am right but it makes sense to me. Here goes:

Imagine two antennas.

One is circular front centre fed and has good cross-pol. It is represented by ring of wire forming a multi-sided polygon. Each part of the wire produces a beam whose content is predominately the linear polarisation of the direction of the wire. All the elements of the multi-sided polygon add up and make a perfect beam.

The other antenna is offset. There are five of these shown in the drawing. Each offset antenna is represented by a single straight wire.  This will generate a beam with a predominant polarisation, aligned with the wire and thus not what was wanted. If you have a circular polarisation feed the result will be a linear polarisation beam.

If some genius from antenna design dept or university reads this please help with better explanation. Thanks.
  

cross-pol-offset-dish.png (Attachment deleted | 39 Downloads )

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chriswlan
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Re: Offset antennas "depolarization"..... HOW?
Reply #3 - 22nd Nov, 2017 at 1:12am
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[quote author=21242D292E400 link=1510775747/2#2 date=1510928068]Slight depolarisation and slight pointing offset occurs with front-fed offset reflectors with the F/D ratio less than about 1.

>>>I seem to read that the longer the f/d, the better.... which kinda means the less the feed is away from the bore axis, the better -ie center fed being perfect... Also: minimizing the angle difference between the feed axis and the antenna bore axis...


If you use a mode matched feed the throat is not circular but had three hidden slots inside a fat lump on one side of the feed throat.
>>>I've seen Ku throats that have 2 longitudinal slots at 180 degrees, that must be kept registered with the feed copol linerar pol plane... similar idea perhaps.... trying to compensate for the offset-ness of the whole antenna...

The 3 mode generating           sation. This needs a big circular polarisation scale behind the dish on a wedge shaped box.
>
>>>>[is this for copol only?] This is similar to the one online item I found that implies that "things" have to be parallel to the symetry plane... But then that leads to another question: if depolarisation is reduced that way, what happens when using a crosspol feed: is ONE pol optimized, while the other is made the WORST???

>>>>I'm on the equator, skew at 90degrees, elevation is "too low", big scintillation on Rx....could the fading be EVEN worse on the TX pol that is at 90 deg to the plane of symetry??? Ie: the Tx pol is Hz, while the reflector symetry plane remains vertical, as "normal" -no spinning of the 8ft reflector.

I REALLY REALLY WOULD LIKE TO KNOW THAT ONE!

In which case, if I was to spin the entire reflector 90 deg, (while retaining feed attitude to match the satellite) on its bore axis, I'd favorise the polarity that presently is way too weak? Or get a transponder with the opposite crosspol ...

Put another way: is depolarization minimized for BOTH Tx n Rx, IF using COPOL, and aligning/spinning the entire antenna so its plane of symetry is parallel to that single satellite pol????


Wonder why it is so hard to find anything online on WHY/HOW offset reflectors distort polarities!

Chris
  
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Re: Offset antennas "depolarization"..... HOW?
Reply #4 - 27th Nov, 2017 at 6:04pm
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The cross-pol caused by front fed offset antennas, without mode matched feed is small and unlikely to be noticed. Cross pol migyt be 20 dB instead of 30 dB and this has negligible effect on an overall C/N of 10-12 dB.

You observations about cross-pol and low elevation  scintillation are very interesting and I don't know the answer.

Scintillation is often caused by refraction at the boundary of large flat layers, each with different temperature and humidity.  The boundary is approximately horizontal but has humps and hollows like gentle slow waves on the sea, but hundreds of yards across. The humps or hollows act like lenses and amplify or reduce signals.

I would like to know if these approximately horizontal boundaries preferentially act on a particular polarisation.

What polarisation is used for terrestrial radio relay links and why ?

Read here: http://www.cablefree.net/wirelesstechnology/microwave/

Quote "Polarization diversity: Multiple versions of a signal are transmitted and received via antennas with different polarization. A diversity combining technique is applied on the receiver side."

This document also looks interesting:
http://www.hep.manchester.ac.uk/u/xiaguo/waveoptics/Polarisation_supplement.pdf
Not sure what it means for linear polarisation passing through or reflecting from horizontal air/air boundaries.


  

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