I'll double down on the explanation I wrote below, because I believe it addresses the issue you're having trouble with. However, I've figured out another way to say the same thing using different perhaps more familiar language. Here goes...
Suppose we have a satellite that transmits (so they say) a Right Hand Circular Polarized signal. That means the electric vector spins around in a counterclockwise direction. Ah, but clockwise is an interesting concept. Lets think about a clock.
Suppose I launched a giant clock into orbit. It is big enough so that you can see it with a telescope, and it has a transparent face. What you see in the telescope are the giant clock hands.
As you look thru the telescope, fred asks you "Which way do the clock hands rotate? Clockwise or counterclockwise?" Now it happens that at this moment the clock is oriented so that you're looking at it from the side. You respond "I don't know. I don't see any rotation. They seem to be going just up and down!"
Then after awhile the clock's orientation has changed a bit, and now you can see rotation. You say "hey, now I see rotation, and it's clockwise!" You are now looking at the front of the clock.
The clock itself is slowly rotating so after awhile you say "Hey, its just going up and down again!" This means that the clock has now rotated so that you're seeing it edge-on again, from the other side.
After another wait you say "Wow, now I see rotation again, but now it's counterclockwise! It changed! How can that be?" This just means you're now seeing the clock from the back instead of the front. When you look from the back side, the clockwise rotation of the clock looks like counterclockwise rotation.
How the signal from an antenna on a spacecraft looks to you works the same way, except we use different names. Clockwise rotation is LHCP. Counterclockwise rotation is RHCP. Up-and-down movement is linear polarization.
If you ask a clock designer which way he built the clock to go, he'll think you're an idiot, and he'll say "clockwise of course". But keep in mind he's speaking under the assumption that you view his clock from the front. He doesn't say this, but he means it nevertheless.
If you ask a designer of an antenna which polarization he designed the antenna to generate, he may say "RHCP", but keep in mind he's speaking under the assumption that you view his antenna from the front (ie in the main beam). If you're viewing from the side or the back, the answer is different.
If you view a transmitting antenna or a clock from a different direction, then you see a different rotation or polarization.
You can write a bunch of complicated equations to describe all this, but at its heart its just simple geometry.
________________________________ From: Burns Fisher burns@fisher.cc Sent: Monday, March 19, 2018 11:33 AM To: Jean Marc Momple Cc: Franklin Antonio; 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.commailto: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.commailto: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.orgmailto:amsat-bb-bounces@amsat.org> on behalf of Jordan Trewitt <jmtrewitt@gmail.commailto:jmtrewitt@gmail.com> Sent: Sunday, March 18, 2018 2:43 PM To: Eduardo PY2RN Cc: amsat-bb@amsat.orgmailto: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.netmailto: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<mailto:amsat-bb@amsat.org>>To: "amsat-bb@amsat.orgmailto:amsat-bb@amsat.org" <amsat-bb@amsat.orgmailto: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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Sent via AMSAT-BB@amsat.orgmailto: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
Sent via AMSAT-BB@amsat.orgmailto: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 _______________________________________________ Sent via AMSAT-BB@amsat.orgmailto: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
_______________________________________________ Sent via AMSAT-BB@amsat.orgmailto: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