Polarization Losses on QO-100 Uplink

1st Official Post

    I have tried to make a circular feed based on the idea of DC2TH with a 2x 4el. PCB yagis from LZ5HP. I found that one yagi gives me good result so I decided to try this feed. Used 100cm offset dish.


    I make similar construction like this one:



    But I have make a small experiement with 1 yagi only. If the yagi was horizontal (above or bellow LNB), the signal was noticable stronger than I try vertical (left or right). The difference was abt. 4dB.


    Question - it is due that the feed is not absolutely in the focus point or due that the offset dish push the energy not consequent?

    Did anybody observe similar results?

    Your uplink should be circularly polarised so if you are using a linear antenna it's orientation would not matter though 3 dB down.


    The phase centre of the feed needs to be at the focus. The phase centre of two vertical antennas will be in the centre of that feed, the phase centre of just one will be where the antenna is, offset to the left or right. This means your dish pointing will be skewed. You can work out how much with simple trigonometry but why does this not impact the horizontal antenna? The same should apply in the vertical plane. I don't know. All I can think of is perhaps your dish is well aligned in azimuth but not well aligned in elevation?


    What about putting a single yagi in the middle and seeing what happens as it is rotated?


    Mike

    Quote from om0aao

    But I have make a small experiement with 1 yagi only. If the yagi was horizontal (above or bellow LNB), the signal was noticable stronger than I try vertical (left or right). The difference was abt. 4dB.

    I have the same experience with you, I’m using a linear polarized uplink antenna for QA100 and I found that some orientation between vertical and horizontal are better than others. The mystery is that the best orientation for my QTH changed few months ago and my question is, “Is QA100 receiving antenna real RHPA?”


    73s Alex

    Quote from DB2OS

    Yes, QO-100 Uplink on 2.4 GHz is right hand circular polarisation RHCP.

    In that case what happened to the satellite antenna few weeks now and I had to rearrange the orientation of my yagi to return to the previous max downlink of my signal??? It had dropped 3 DB and then returned to my usual max after correction of 15 deg .


    73 Alex

    Alex,


    this is not related to the satellite.


    If your uplink antenna (feed) is really linear polarized then you should not see a difference in the downlink signal strength when turning your feed.


    However make sure that you are not turing the uplink feed and downlink feed at the same time as this will influence the downlink strength.


    It might be better for such a test to use the Web-SDR to watch the downlink signal when optimizing the uplink antenna (feed).


    The QO-100 transponder has not changed.


    Kind regards


    Matthias


    http://www.dd1us.de

    Dear OM Rasto,


    Thank you for the nice paper I learn a lot. Yes POTY is 3 BD better that linear polarized antennas and Helix is ever better. My observation -and I spoke to 2 more Amateurs that they had the same experience- is If you have a linear antenna and rotate 90 dec the downlink signal from QO100 varies. My setup is 90cm dish and yagi at the focus, for receiving I have another dish 110cm, octagon FunCube plus SDR and SDR console. One thought I have is, that the geometry of the dish is favorable to the Horizontal polarization. But I had a change of my signal -digressed 3 db- few weeks now and I had to realigned few deg the yagi to gain again my max signal. All these measurements with the same receiving setup and always comparing with the Digital Beacon of the QA100 which had never change significantly. Either the propagation path changed (that’s impossible) or the behavior of the satellite antenna changed… any comments please from the sattelite gurus?


    73s Alex

    • Official Post
    Quote from sv8qg

    Either the propagation path changed (that’s impossible)

    Very much possible. See TEC (total electron content) and Faraday rotation (which is negligible at 11GHz, but present at 2.4GHz). Check out NASA Jet Propulsion Lab's excellent documentation: https://descanso.jpl.nasa.gov/propagation/1108/1108.html


    The QO-100 S-band RX antenna is good in terms of circularity but not perfect. So you will see it as shown by Rasto in his excellent paper when you rotate a linear uplink antenna. As the uplink propagation path changes with time, the angle of maximum/minimum signal will move as well. You could do extensive measurements here and write a very interesting paper about S-band Faraday rotation.

    I do not think it applies here but the attenuation in rain is different between Horizontal and Vertical. This is because falling raindrops are not spherical but oblate spheroids due to the air resistance. They have a larger dimension horizontally vs vertically which results in a greater extinction cross section and increased scattering and absorption. The actual orientation and shape depends on the drop size and air currents, so besides sitting out with an anemometer and disdrometer (or a tray of flour) to measure these parameters it's best to use the statistical models from ITU-R.


    So why does the rotation of a 2.4 GHz linear antenna result in a difference? Perhaps the gain of that antenna varies a bit as you rotate the feed yagi? Maybe your Balun is not entirely effective and the Yagi has some squint? 3 dB seems a lot of variation to see.


    Mike

    Quote from DH2VA

    Very much possible. See TEC (total electron content) and Faraday rotation (which is negligible at 11GHz, but present at 2.4GHz). Check out NASA Jet Propulsion Lab's excellent documentation: https://descanso.jpl.nasa.gov/propagation/1108/1108.html


    The QO-100 S-band RX antenna is good in terms of circularity but not perfect. So you will see it as shown by Rasto in his excellent paper when you rotate a linear uplink antenna. As the uplink propagation path changes with time, the angle of maximum/minimum signal will move as well. You could do extensive measurements here and write a very interesting paper about S-band Faraday rotation.

    Faraday rotation is certainly important at 2.4 GHz but it doesn't cause 3 dB of loss unless QO-100 antenna axial ratio os very poor, which I am sure is not the case. Could this be caused by multipath?

    Quote from sv8qg

    Either the propagation path changed (that’s impossible) or the behavior of the satellite antenna changed… any comments please from the sattelite gurus?

    The satellite is stabilized and I don't think anything can change.

    What can change, in my opinion, is the path of your uplink signal due to propagation (rotation) ... but not only.

    Your antenna is metal, exposed to the sun. The metal exposed to heat deforms. Slightly but deformed. It would be interesting if you have the opportunity to repeat the tests at night, and in broad daylight to see if there are variations.

    P.S.

    in any case 3 dB are many ...

    Dear Alex,


    As Achim and Mike wrote, the problems of signal propagation from the ground to satellite and vice versa is quite complex. In general, there are three types of effects on the signal path.


    1. The atmospheric effect (causes depolarization, attenuation, rain, snow, fog etc)


    2. The ionosphere effect (scintillation – causes short term attenuation)


    3. The depolarization effect (Faraday rotation – magnetosphere + ionosphere effect)


    More about this topic you can find also here:


    https://www.itu.int/dms_pubrec…31-12-201309-S!!PDF-E.pdf


    If you want to measure polarization properties of your signal path, you must be sure, that your Tx antenna has the same pattern for E and H plane, when you change polarization. Also, you should be able to switch between polarization in short time. Unfortunately, I'm afraid that the Yagi antenna as a feed is not very suitable for this measurement. I guess, that a waveguide feed (coffee can), fitted two probes (perpendicular to each other) would be better solution. If you want to measure circular polarization, a septum feed should be the best option.


    73, Rasto


    Hi Achim,


    Your statement: “The QO-100 S-band RX antenna is good in terms of circularity but not perfect.” then calmed me down. I think that my efforts to improve the circular polarization of my duo-band feed (with round patch disks) according to the "cut and try" principle were probably not in vain (see the attached pictures).


    73! Peter, DJ7GP


    Peter,


    Experimenting is always a good idea. A patch antenna with a single feed that is perfectly circular can't produce circular polarisation. There needs to be something else. I settled on the dual resonance method. The original POTY (it was not called that, that was Remco's idea) used an ellipse but that's hard to make, so I tried an approximation to an ellipse. A circular patch with notches, circular with tabs and the square with cut outs. The square was good enough and is easiest to make with simple tools, so that's what I ended up with. Other iterations had directors, choke rings and so on. Another way to do this dual resonance trick is with a feed and a tuning capacitor, but I found that really hard to get just right. G3RUH managed it but not with the waveguide. Achim did manage it but I don't know exactly how.


    What wasn't obvious early on is were the simulations accurate? I was taking huge liberties with the meshing in the simulation and with the full version of CST we could probably design a more forgiving, broader band patch. In the end, its only when something is reproduced in hardware that you can be sure it works, or not. So, keep on cutting and trying because if it works in hardware, even without simulation, it works.


    Mike

    • Official Post
    Quote from G0MJW

    Achim did manage it but I don't know exactly how.

    If you refer to the QO100 antenna: none of my business.. For me up and until last year, my trusty G3RUH patch was the gold standard. Even most helix antennas are not 100% circular (had a nice chat with Kent WA5VJB about that). And all measurements of DJ7GP show that actually measuring it is far from trivial (as any precise measurement).

    Quote from DH2VA

    If you refer to the QO100 antenna: none of my business.. For me up and until last year, my trusty G3RUH patch was the gold standard. Even most helix antennas are not 100% circular (had a nice chat with Kent WA5VJB about that). And all measurements of DJ7GP show that actually measuring it is far from trivial (as any precise measurement).

    Actually I think I meant Peter DJ7GP. Your names and callsigns are so similar... not. Sorry, I'm not good at remembering names.

    Quote from DH2VA

    The QO-100 S-band RX antenna is good in terms of circularity but not perfect.


    Good morning


    Thank you Achim, that's the point. Also thanks all of you for your comments, Rasto for the pdf, I had a lot of reading to do.


    I must apologize for my mistake about 3 DB lost in my downlink signal must be “3 DB less SNR as SDR console reports” I just remember it I updating to the new version 3.0.23 .


    73s Alex