LNB Skew on Cassegrain

1st Official Post

    Hi.


    I calculated the needed dish setup as such:


    https://satlex.us/en/azel_calc…de=nl&diam_w=60&diam_h=60


    But I am confused in which direction I should rotate my LNB for optimal reception of the linear satellite down-link.


    My satellite dish is of the cassegrain type, where I put in the LNB on the backside.


    If I would stand behind my dish, would I rotate 15.60 degrees clockwise, or counter clockwise?


    And I also assume that without LNB rotation (or elevation) the octogon F-connectors are pointing to the ground right?


    Your help is very appreciated :)


    '73

    René

    PC7X

    Quote from DD0KP

    that is correct.. Because you want to receive H and V there is no rotation of the lnb from normal 0deg position needed.

    Not needed, well the satellite is hanging 25 degrees to the east over africa.


    When these radio waves hit my country, since we live on a sphere, I would think that indeed as calculated, for optimal reception, you would want to rotate your LNB 15.60 degrees.


    And since I only have a pretty small antenna, I would like to get the maximum out of it.

    You can twist it for optimal signal or for minimum signal on the opposite polarization. Not on the beacon signals because they are RHCP.

    I am not a fan of those "calculate the number of degrees" solutions (also for the direction) because it is so difficult to then set the thing to exactly that number of degrees... in my case the dish is on a rotor so I set the LNB vertical when the dish is pointing south (satellite position 5e) and the only skew I use is that caused by the rotation around the tilted axis used by the rotor to track the geo belt.

    I think that is not completely optimal, but the signal loss is not much. If anything, there would be more crosstalk from the other polarization.

    Hi Rene,


    Nice to see you here too. Besides that I'm also a fan of 'tweaking' the LNB for max signal, your question is indeed a bit mind boggling.


    Assuming that the satellite is East from your longitude:


    For a 'normal' (offset) dish, standing in front of it, with your back to the satellite you've to turn clockwise.


    For a Cassegrain, standing behind the dish and looking at the satellite you've to turn it anti-clockwise.


    In other words it's the same, depending upon what perspective you're looking.

    Quote from Remco

    For a Cassegrain, standing behind the dish and looking at the satellite you've to turn it anti-clockwise.

    Hello Remco,

    I don´t think so. Cassegrain is a two mirror system. The signal is upside down and reversed. But, the angle from the signal is still the same.

    OK, the last sentence in my previous post may be/is confusing.


    What I wanted to say is that the skew angle is identical in both cases.
    So, if you've e.g. -15 degrees and are behind the dish, thus looking at the satellite, the LNB has to be turned anti-clockwise, no matter if you've a 'normal'/offset dish or a Cassegrain.

    Hello,


    is it really the same angle ?

    Each reflector acts as a mirror. When you take porarisation: in a "normal" dish you have a LHCP feed and get (because auf the mirror-effect) RHCP polarisation.

    The skew angle is "part" of the polarisation. If you are standing behind a normal dish with view to the satellite, the LNB has to be turned counter clock wise for all east positioned satellites. But in yor case you have an additional mirror and thus I believe your LNB has to be turned clockwise (seen from the same postion as described above).

    in my QTH (JN58xx) it is a bit more then 13 degree for 26 E. But you can also take the empiric way and tune to a TV transponder on BADR4 (e.g. 11996 SR 27500) - it's the strongest one) and see the change in quality of the signal.


    (For all TV-Sat DXers - Astra is not a "normal" satellite in this way, because SES "tilted" the satellites a bit to have a skwe angle of near to zero in DL - the main viewers).


    73 de Johannes, DL5RDI

    I think so. Do not confuse circular and linear polarization! Linear polarization is a momentary segment of circular polarization. The turning of the polarization is of course inverted by the mirror, with two mirrors inverted twice. However, considering only the instantaneous moment of a linear section, the angles in a two-mirror system are identical.

    Quote from DL5RDI

    If you are standing behind a normal dish with view to the satellite, the LNB has to be turned counter clock wise for all east positioned satellites. But in yor case you have an additional mirror and thus I believe your LNB has to be turned clockwise (seen from the same postion as described above).


    1) Counter clock wise because it's east from my location, or clockwise with a negative angle, still meaning counter clockwise.

    2) Then, inverted again: clock wise because of second mirror

    3) And then again: counter clock wise because the LNB IS on the back of the dish


    Conclusion: I have to turn it counter clock wise while standing behind my cassegrain dish, facing the satellite.


    And indeed Remco, if you describe your position in relation to the dish when you're about to rotate the LNB, as "Facing the satellite", then it doesn't matter if you're dish has one or two mirrors.


    My sanity check:


    1) Two mirrors would cancel each other out, when it comes to any flipping effect no matter if it's up-side-down or left-to-right


    2) So I can stand behind my dish, and just imagine how the polarisation from a satellite hanging over Africa (from the south-south-east in relation to my location) would look like:


    [Blocked Image: https://pc7x.net/ccw.png]


    Quote

    In a linearly polarized system, a polarization misalignment of 45 degrees will degrade the signal up to 3 dB. Polarization misalignment near 90 degrees can result in signal degradation greater than 20 dB. Accurate measurements of signal strength at polarization near 0 degrees and from 80 degrees to 90 degrees require careful control of the positioner and signal strength meter.


    So it should matter something like near 1dB I estimate, and something like double that, If I rotate in the wrong direction :)


    Thanks everyone for the insight you provided.

    This is our LNB and if I look at it in the mirror the horizontal polarization remains horizontal and the vertical polarization remains vertical. I can look at it through two mirrors but it does not change anything. The vertical remains vertical and the horizontal moves from right to left, but always horizontal remains.

    The speech for circular polarization is different.

    For us, who have the satellite to the east, the rotation is counter-clockwise looking from behind the antenna, towards the satellite. Yes, the connectors must point downwards, to the right, always looking at the antenna from behind.

    Quote from I0LYL

    This is our LNB and if I look at it in the mirror the horizontal polarization remains horizontal and the vertical polarization remains vertical. I can look at it through two mirrors but it does not change anything.


    Theoretically, that is only true if your location is either on the equator or at 25 degrees longitude.


    This picture: https://pc7x.net/ccw.png


    Shows it from my location without rotation of the LNB,

    you see that the red lines and the blue lines are not in the same orientation.


    That's why you want to rotate your LNB accordingly.

    Hello Rene,


    you are right with the LNB even in the cassegrain antenna counterclockwise... I disregarded that you also turn the LNB 180° (looking towards the dish in "normal" configuration, but turned 180° around its vertical axis and then pointing towards the "sky" in cassegrin antenna.)

    Thus the polarisation is of course "mirrored" by every reflector, but the LNB orienttation is in both cases counter clockwise because of the additional "mirroring" of the LNB.


    73 de Johannes, DL5RDI

    Rene,


    As you already quoted and I also wrote before: the main effect is not signal strength of the wanted signal, but the attenuation of the opposite polarization.

    Some satellites have different transponders on H and V on the same frequency, and then it is somewhat important that you do not receive the V transponder when you have selected H polarization.

    So you look for the "null" in reception in H setting of a very strong V signal.

    But the strength of the wanted signal does not really vary much when you are off by like 15 degrees.

    The amateur transponder operates on non-overlapping frequencies for H and V, so this will not be very important.

    Of course by the time the skew is 65 degrees as in Brazil, it does become important.

    • Official Post
    Quote from pe1chl

    The amateur transponder operates on non-overlapping frequencies for H and V, so this will not be very important.

    We would still recommend users to use a proper skew setup to avoid wideband noise from the WB transponder. If the WB transponder is driven hard (which we do not suggest and do not hope!), the IM3 products may end up in the NB downlink range. That is why we designed the two transponders to have separate polarisation planes.

    Quote from pe1chl

    Ok... but still it will be more important to "null" the crossed polarization than to find the maximum for the desired polarization.


    You mean, because some ASTRA satellites are transmitting with a pre-skewed setup from pretty much the same longitude, because they target west-europe?


    But even in these cases, rotating towards Es`hail-2 would do both.. right?