Actually, I would tend to suggest the majority of polarization shift is simply due to the always changing attitude of the spacecraft with respect to the user.
The fact that all TVRO Satellite dishes worked perfectly well when switching back and forth from horizontal to vertical polarization when changing channels and once the dish was initially aligned then those vertical and horizontal polarizations remained accurate across the entire sky and across the many dozens of satellites, then would suggest th contribution due to faraday rotation was small (at C band anyway)...
Yes, there is some faraday rotations at HF and at extremely low elevations, but I think for one using an ARROW antenna, all of the polarity issues are simply due to the instantaneous orientation of the satellite. Not the ionosphere...
But, just my humble opinion...
Bob WB4APR
-----Original Message----- From: AMSAT-BB [mailto:[email protected]] On Behalf Of Bob Sent: Tuesday, May 31, 2016 1:52 PM To: Doug Andrews Cc: AMSAT-BB Subject: Re: [amsat-bb] Polarization
"The antennas I see in the photos of satellites we work are whips. Is the polarization becoming "circularized" as it re-enters earth's atmosphere or something?"
Yep, that is exactly what is happening. It is called Faraday Rotation, and as the signal from the satellite passes through the ionosphere, all sorts of polarity changes can and do happen. A linear polarized satellite antenna (horizontal or vertical) can appear to be the opposite or somewhere in between. That's why folks rotate their Arrow or Elk antennas -- trying to match the polarity.
Using a circular polarized antenna helps a bunch -- it doesn't matter what the polarity of the linear satellite antenna happens to be at any moment in time.
But there is no free lunch -- Even a circular polarized antenna might need to be switched from Right Hand Circular Polarization (the default) to LHCP from time to time depending on what nasty thing the ionosphere is doing at any given moment. Changing the polarity switch might bring a S0 signal up to S5, a 30 dB improvement. I had that happen to me during a recent ARISS contact.
73, Bob, WB4SON
On Mon, May 30, 2016 at 8:25 PM, Doug Andrews [email protected] wrote:
I too have wondered about this. I have not had much trouble hitting SO-50 and some success on AO-85 with a 5 watt handheld and arrow antenna without turning it. Worth a try. DougKG7UNU
Sent from my Sprint Samsung Galaxy® Note 4.
-------- Original message -------- From: Ken Alexander [email protected] Date: 5/30/16 4:41 PM (GMT-08:00) To: [email protected] Subject: [amsat-bb] Polarization
I clipped this from another message because I didn't want to drag the discussion off course. It's a question I've been wondering about since getting into this a few short weeks ago.
I've also read (but haven't tried yet) about the trick of rotating the antenna 90 degrees on transmit, once you've established the best receive orientation.
73 de Bill, KG5FQX
So far, with SO-20 I have rotated my Arrow antenna for best reception of the downlink and don't think I've had too much trouble being heard. At the same time I have wondered whether I should twist the antenna when transmitting to orient the 2m elements to give the same polarization as in receive. I don't know if this is a good idea or not, and frankly I have enough trouble remembering calls and grids, tracking the satellite, adjusting frequency and switching back to the correct VFO to worry about one more thing.
I've seen that some commercial OSCAR antennas use circular polarization. The antennas I see in the photos of satellites we work are whips. Is the polarization becoming "circularized" as it re-enters earth's atmosphere or something?
Comments and observations would be most welcome!
73,
Ken Alexander VE3HLS, FN03
Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
_______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
That is an excellent observation about the use of horizontal or vertical polarization for geostationary satellites -- obviously that works well in that case (much higher frequencies and zero relative motion).
Perhaps it is a combination of relative motion and signals passing through different parts of the ionosphere that causes the problem. When talking about VHF signals coming from the ISS in particular there isn't any random attitude change (unlike a tumbling/spinning satellite). Certainly there is a slow and predictable change in attitude due to relative motion. If that is all it was, then a single RHCP or LHCP antenna would do the trick 100% of the time.
The ARISS crew highly recommends the use of CP antennas (certainly understandable) AND polarity switches. I pushed back on that requirement due to cost and availability issues, but went ahead and installed the polarity switch on our CP antennas. With certainty I can tell you that at random times it made a huge difference in downlink signal level.
The effect usually happened close to the horizon -- perhaps the VHF signals are undergoing polarity changes due to tropospheric ducting. But the use of a CP polarity switch proved to be very useful. Sometimes it might not happen at all. And at least once it happened near the point of closest approach.
it is an interesting issue for sure, and not much literature on it.
73, Bob, WB4SON
I pushed back on that requirement due to cost and availability issues, but went ahead and installed the polarity switch on our CP antennas. With certainty I can tell you that at random times it made a huge difference in downlink signal level.
There was discussion on the ARISS OPs mailing list as to what to recommend to schools and what would give best "bang for buck" I am still not 100% convinced that a RHCP/LHCP switch is indeed necessary for ARISS contacts. The only way to confirm would be one station with both RHCP and LHCP both receiving at the exact same time and logging or recording the results. Most/all stations that have said the switch helped have not compared in real time not switching. Over time the ISS signal can vary wildly, in particular the solar panels can totally block the signal completely from time to time. There is a couple of stations with dual VHF circular systems (mostly ARISS tele-bridge ground stations) and I have asked them to help. Until we have real world comparisons at least in the case of the ISS, I am not sure a switch is needed. Can someone do tests to check?
Having the RHCP/LHCP switch is better than not having it since it gives you one more ace in the hole. Sure, it might only provide true value for a few seconds due to all the other variables, but when it occurs, it is nice to have for such high visibility events (especially if someone else is buying the hardware)... but for me personally, I'd probably not bother with the complexity for just a few more seconds of null avoidance...
Just an opinion.
Bob, Wb4aPR
-----Original Message----- From: AMSAT-BB [mailto:[email protected]] On Behalf Of Daniel Cussen Sent: Wednesday, June 01, 2016 5:00 AM To: AMSAT-BB Subject: Re: [amsat-bb] Polarization (attitude mostly)
I pushed back on that requirement due to cost and availability issues, but went ahead and installed the polarity switch on our CP antennas. With certainty I can tell you that at random times it made a huge difference in downlink signal level.
There was discussion on the ARISS OPs mailing list as to what to recommend to schools and what would give best "bang for buck" I am still not 100% convinced that a RHCP/LHCP switch is indeed necessary for ARISS contacts. The only way to confirm would be one station with both RHCP and LHCP both receiving at the exact same time and logging or recording the results. Most/all stations that have said the switch helped have not compared in real time not switching. Over time the ISS signal can vary wildly, in particular the solar panels can totally block the signal completely from time to time. There is a couple of stations with dual VHF circular systems (mostly ARISS tele-bridge ground stations) and I have asked them to help. Until we have real world comparisons at least in the case of the ISS, I am not sure a switch is needed. Can someone do tests to check? _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
For the ARISS events that have recently concluded in the UK with Tim Peake, we used the following setup for the bulk of the contacts.
1x Wimo X Quad with vertical, horizontal, crossed left & right polarisation switching. http://wimo.de/xquad-antennas_e.html
1x Wimo Crossed yagi, 2x10 element http://wimo.de/yagi-antennas-wimo_e.html Model WX220with fixed right hand polarisation.
Both antennas had mast head preamps, were fed via (separate) 50m lengths of Ecoflex 15+ coax cables to the radio (TS2000X) through a common RF path of a PA and Bird wattmeter.
Whilst I don't have precise numbers on the times we switched polarisation, there were far too many occasions on all contacts where the XQuad, albeit shorter/smaller gain and with switched polarisation etc outperformed the larger, higher gain, fixed polarisation crossed yagi during the ARISS contacts. The crossed yagi however, appeared to be better when the ISS was on the horizons.
You cannot prescribe a setup that will be 100% perfect in all circumstances - as we say here "pays your money, takes your choice". It is about what the operator is prepared to invest in terms of equipment vs the expected return.
For us here in the UK, we prefer to maintain the flexibility on the XQuad but have the crossed yagi for safety. Besides who else has tried transporting a 2x10 element crossed yagi from site to site! Oh, and one other point that was important for us, was the difference between having six space craft docked to the station and not having six craft docked. That made a big difference from TCA to LOS.
73
Ciaran M0XTD
-----Original Message----- From: AMSAT-BB [mailto:[email protected]] On Behalf Of Robert Bruninga Sent: Wednesday, June 01, 2016 5:11 PM To: [email protected]; AMSAT-BB [email protected] Subject: Re: [amsat-bb] Polarization (attitude mostly)
Having the RHCP/LHCP switch is better than not having it since it gives you one more ace in the hole. Sure, it might only provide true value for a few seconds due to all the other variables, but when it occurs, it is nice to have for such high visibility events (especially if someone else is buying the hardware)... but for me personally, I'd probably not bother with the complexity for just a few more seconds of null avoidance...
Just an opinion.
Bob, Wb4aPR
-----Original Message----- From: AMSAT-BB [mailto:[email protected]] On Behalf Of Daniel Cussen Sent: Wednesday, June 01, 2016 5:00 AM To: AMSAT-BB Subject: Re: [amsat-bb] Polarization (attitude mostly)
I pushed back on that requirement due to cost and availability issues, but went ahead and installed the polarity switch on our CP antennas. With certainty I can tell you that at random times it made a huge difference in downlink signal
level.
There was discussion on the ARISS OPs mailing list as to what to recommend to schools and what would give best "bang for buck" I am still not 100% convinced that a RHCP/LHCP switch is indeed necessary for ARISS contacts. The only way to confirm would be one station with both RHCP and LHCP both receiving at the exact same time and logging or recording the results. Most/all stations that have said the switch helped have not compared in real time not switching. Over time the ISS signal can vary wildly, in particular the solar panels can totally block the signal completely from time to time. There is a couple of stations with dual VHF circular systems (mostly ARISS tele-bridge ground stations) and I have asked them to help. Until we have real world comparisons at least in the case of the ISS, I am not sure a switch is needed. Can someone do tests to check? _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
participants (4)
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Bob -
Ciaran Morgan -
Daniel Cussen -
Robert Bruninga