Polarization

[YO9ICT]

I am working toward building 2 yagis that will eventually be moved on azimuth/elevation for satellite work. I don't know which type of polarization the antennas should be. From my experiments, seems that 95% of satellites downlinks work with RHCP. Please help me with an advice for the uplinks.

If any info is required just tell me.

[AI3V]

You want the polarization to match the receive antenna on the spacecraft.

WITH ONE EXCEPTION:

If the Squint Angle of the spacecraft is such that the antenna is no longer pointed at you, but you are instead on a Sidelobe of the spacecraft antenna, the "hand" of the polarization will be reversed for each successive sidelobe.

A RHCP antenna will have LHCP first sidelobes, Then RHCP 2nd sidelobes, LHCP 3rd sidelobes and so on.

Rege

[W7GIB]

I am in no way an expert. But if the satelites are RHCP(Right Hand Circular Polarization), would you not be recieving Random or UnPolarized by the time you recieve the signal? And in Electronics, Circular Polarization is as seen from the Tranmsitter to the source? While in Radio Astronomy it is as seen from the Source to the Receiver?

I hope somebody with a greater understanding steps in.

[AI3V]

If the satellite runs LINEAR polarization, then , depending on the path the signal takes thru the ionosphere, you will see a effect called Faraday Rotation

The signal will still be LINEARLY polarized, but will rotate to a different angle with respect to the 2 antennas.

This angle will remain constant, as long as the satellite EDIT: and the ionosphere,/EDIT remains stationary.

The only other way to change to random polarization is for the LINEAR polarized signal to reflect of another object, and satellite signals are generally far to weak to copy a reflection.

A Circular Polarized signal will reflect back at the opposite hand, and each reflection will reverse the sense.

Some RADAR's use the fact that a reflected signal will have a random polarization by using a cross polarized antenna, that is the RADAR transmits a linear polarized signal, and listens for the echo on a second, Orthogonally Polarized antenna, always 90 degrees from the transmit antenna.

This provides 20 db or so of additional TX/RX isolation in the RADAR Duplexer.

Rege

[W7GIB]

AI3V Thank you very much for adding that.

[YO9ICT]

Ok, thanks. Understand the theory.
So far with my 70cm x-Yagi RHCP I can copy AO-27, SO-50, AO-51, HO-68. There is also SO-67 but I presume I will be able to hear it as well.
1st question:With this x-Yagi RHCP I will be able to hit AO-7, VO-52?
2nd question:I will also need to make a x-Yagi for 2 meters for uplink to AO-27, AO-51, etc. and downlink from AO-7 and VO-52. This will still be RHCP or.. ?

[KC4UMO]

Ok, thanks. Understand the theory.
So far with my 70cm x-Yagi RHCP I can copy AO-27, SO-50, AO-51, HO-68. There is also SO-67 but I presume I will be able to hear it as well.
1st question:With this x-Yagi RHCP I will be able to hit AO-7, VO-52?
2nd question:I will also need to make a x-Yagi for 2 meters for uplink to AO-27, AO-51, etc. and downlink from AO-7 and VO-52. This will still be RHCP or.. ?

Both my yagis are RHCP with no switching circuit installed. I work bother AO-7 and VO-52 with no problems and get great reports. Both of my yagi's are homebrew. Hre is the info on my 70cm:
http://www.gokarters.com/vbforum/showthread.php?t=1749

What type are you using and what rigs?

[YO9ICT]

I am using yagis based on the design of Guy ON6MU, two of them mounted on the same boom with RHCP phasing. So far, the 70cm performance is good and the 2m version is not yet constructed. Rig ft-857d

[AI3V]

One other thing about circular polarization.

To phase 2 Yagi antennas to develop CP, it is VERY easy to make a mistake, and instead of CP, you will end up with a degree of "elliptical polarization", or, if you really make a mistake, you will end up with simple linear polarization.

Whatever the antenna, I recommend that you listen to a local signal that is transmitting linear polarization, vertical or horizontal makes no difference, and note the received signal strength as you rotate your CP antenna along its boom axis.

A true CP antenna will not show any difference in signal strength,And the signal strength will be the same whether the antenna is set for RHCP or LHCP, while a antenna with EP or LP will see a difference in signal strength as you turn it.

I know of some commercial antennas that are very poor CP.

Since many spacecraft do not have a very steady downlink signal, the poor circularity in the earth station antenna goes unnoticed.

Rege