Terry,
I tend to agree with all you mentioned, but there is much more:
1) First, what I wanted to say is that there is not ideal solution for all birds due to the different variables for each bird, such as spinning rate, direction of spinning and antenna placement, type of antenna, Tx power etc… If all parameters not known and without these information it is difficult to calculate the path budget and/or design the best setup for the ground station. 2) I also confirm multi-path is an issue and a good example to illustrate, it is an issue particularly at low angle, more over the sea and at higher frequency it is easily observed, this is well experienced here e.g. AO-92 on L-mode has quite deep fading on the uplink at low angle as compare with same situation on the U/V mode (we are in an small Island and surrounded by sea). 3) How I know that it is on the uplink? It is very simple, the fading is on my voice transmission fade deeply while at the same moment telemetry is received rock solid no difference in signal strength. This an example how observation may help to determine what is the real issue. This is why I advices to Burns to try and he will surely have a better understanding and may experience for himself. 4) There is many techniques well known to correct multi-path fading (keeping this issue as example) such as space diversity etc.. however this complicate the whole installation and not really worth for a HAM station/our purpose. More one can even analyze (with proper test equipment/software) the downlink signal and determine what would be the best polarization for uplink, automate the station, etc… I guess going that way will be a long technical debate beyond the scope of actual discussion. 5) I have take one example, the other variables mentioned such as reflection, obstacles such as trees/buildings etc.. also causing QSB, which are unique to each ground stations, there are also solutions, one should be aware of its own environment and adapt its station design to mitigate same.
In a nutshell/ my conclusion/ advice on the RHCP/LHCP issue: to operate the birds for HAM purpose the simplest way is just to have polarization relays on both uplink and downlink antennas, then manually switch polarization as required depending on the fading, which has quite different patterns depending on the bird and their trajectory, after some time an practice it becomes natural and you push the button without even thinking. I do so all the time during Sat. passes.
Same conclusion as yours.
Hope this help all.
73
Jean Marc
On Mar 20, 2018, at 7:34 AM, Wendy and Terry Osborne wandtosborne@gmail.com wrote:
Hi Burns, Jean Marc and Satellite Fans,
Here is my explanation of why Antenna Polarisation varies: If the satellite is transmitting random linear polarisation then a CP antenna should pick up most of the signal, however remember that a CP antenna uses phasing between the Vertical and Horizontal antennas to determine the sense (RHCP or LHCP) of the circular polarisation. If you consider the satellite to also have Vertical and Horizontal components to its polarisation then how these components change in phase when passing through the ionosphere determines whether RHCP or LHCP will work better.
In addition there is the problem of reflections (ground and adjacent objects) with the receiving antenna. You can see this by looking a plots of the Vertical radiation pattern of antennas at various heights. At the low heights of typical satellite antennas any Horizontal component of the received signal will vary depending on the elevation angle of the satellite. Any Vertical component will be pretty constant. As an example AO-91 seems to work better with Vertical polarisation at low elevation angles and changes to Horizontal at high elevation angles (height above ground 1.5 metres).
So a satellite receiving antenna needs to be able to vary the polarisation to be able to avoid fading at some elevation angles.
73, Terry Osborne ZL2BAC
-----Original Message----- From: Burns Fisher Sent: Tuesday, March 20, 2018 7:33 AM To: Jean Marc Momple Cc: amsat-bb@amsat.org Subject: Re: [amsat-bb] Antenna Polarization Question
I'd really like to understand this better, but it is still not making sense to me. I believe that if you are transmitting with a CP antenna, the E and M waves actually go through an entire circle in one carrier cycle and a CP antenna is able to "follow" that. Surely a satellite is not spinning at anywhere close to 145 or 450 million revs per second, so I don't get "spinning satellite" as an explanation for why an LHP or RHP antenna might work better at different times.
What I do get is that a CP antenna can receive linearly polarized waves at any angle equally. But this should be true whether the antenna is LHP or RHP, and I would not think which direction should matter if the signal is linear in the first place, even if the signal is spinning slowly.
That all said, I have definitely heard people say that they can get better reception by changing from LHP to RHP. I'm not saying this is not true. Just that I don't understand it.
73,
Burns WB1FJ
On Mon, Mar 19, 2018 at 1:57 PM, Jean Marc Momple < jean.marc.momple@gmail.com> wrote:
Guys,
Long debate for not much. I have followed this stream and believe that I need to intervene.
It is very simple, most HAM birds does spin (sometimes a lot) and there is no way one can anticipate as different from one bird to others. It is just operator skills (on the spot) to determine/switch from LHCP or RCCP based on what is the best received signal strength. It works both on the Uplink and Downlink. There is no miracle formulae and it is a just operator skills as mentioned before.
For commercial birds it is totally different game and should not be compared with our humble Ham birds, they have much more means to do things that we cannot afford to do, except if all HAM worldwide donate to AMSAT, say $10 we then may be able to match some of the features commercially available.
Just a suggestion and food for thought.
73
Jean Marc (3B8DU)
On Mar 19, 2018, at 8:54 PM, Franklin Antonio antonio@qti.qualcomm.com
wrote:
When you say a satellite "has" RHCP, we have to be careful that we're
communicating clearly about what "has" means. Every satellite "has" both, by virtue of he way antennas work.
I suspect that you mean that the satellite has an antenna which
transmits RHCP in its main lobe. It is important to realize that the signal from such an antenna is only RHCP in its main lobe. The sidelobes are gonna be LHCP. In between, you can get anything in between. So if a satellite is oriented so that its antenna points right at you, and they designed it to be RHCP, then that's what you're gonna get, but if it is pointing off to the side, then you get something else.
This means that there are situations in which you might get a stronger
received signal if you switch to LHCP, or maybe even to linear. In the commercial satellite biz, they design satellites so that their antennas point at the users. Hams build cheaper satellites, which typically don't have sophisticated attitude control, so sometimes they point away from you. Also, hams try to use the things even when conditions aren't the best. If that's your aim, then most folks have found that polarization switching sometimes helps.
From: AMSAT-BB amsat-bb-bounces@amsat.org on behalf of Jordan Trewitt
Sent: Sunday, March 18, 2018 2:43 PM To: Eduardo PY2RN Cc: amsat-bb@amsat.org Subject: Re: [amsat-bb] Antenna Polarization Question
Maybe I'm not understanding it, but why does one need to switch between both, unless a particular satellite has LHCP or RHCP? Jordan KF5COQ
On Sun, Mar 18, 2018, 16:34 Eduardo PY2RN py2rn@arrl.net wrote:
Hi Brian, Both. You are going to need to switch between RHCP and LHCP often
during a
sat pass. The same happens if using linear polarization (V/H) but in
this
case the switch between V and H will happen much more often than in CP. 73 Ed PY2RN
From: Brian via AMSAT-BB <amsat-bb@amsat.org>To: "amsat-bb@amsat.org" amsat-bb@amsat.org Sent: Sunday, March 18, 2018 6:21 PM Subject: [amsat-bb] Antenna Polarization Question
Should I be using LHCP or RHCP when setting up the 2 meter and 440
yagi's
to work the LEO's.
Thank you
Brian, KG8CO
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AMSAT-NA.
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AMSAT-NA.
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Sent via AMSAT-BB@amsat.org. 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