when does the skew apply ?

Discussion in 'Free To Air (FTA) Discussion' started by vixens, Dec 8, 2012.

  1. vixens

    vixens Thread Starter Active SatelliteGuys Member
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    I know the screw applies for C Band.
    I know the screw does not matter for Ku circular, e.g. Dish and DTV
    * Does the scew apply for FTA Ku linear, also for Shaw Direct?

    Thanks
     
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  2. Anole

    Anole SatelliteGuys Guru

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    skew
    - C band: yes
    - circular: no, except if you have two LNBFs on a dish
    - FTA: yes
    - Shaw: don't know
     
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  3. ke4est

    ke4est Long Live FTA
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    Fixed your title
     
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  4. cyberham

    cyberham SatelliteGuys Family

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    For Shaw Direct: Yes. They use linear transponders just like FTA.
     
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  5. vixens

    vixens Thread Starter Active SatelliteGuys Member
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    why on two LBBFs ? but not on one ?
     
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  6. AcWxRadar

    AcWxRadar SatelliteGuys Family
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    Vixens,

    Just to elaborate (since your question has already been anwered). Here is a way to visualize what skew means for a LINEAR sateliite transmission...

    Picture a big old rainbow in the sky, this one hanging from the east to the west and to your south if in the northern hemisphere or to your north if you are in the southern hemisphere.
    Everyone has seen a rainbow, so it is a simple analogy to use here. This imaginary rainbow simulates the Clarke Belt where the geostationary communication satellites reside.

    Picture that rainbow with + signs staggered all across it. The + signs are the satellites and each + sign must have the vertical bar "|" perpendicular to the arc of the rainbow and the horizontal bar, the "--", tangent to the rainbows arc.

    If you were to look due south at one of these "+" signs on the rainbow, it would appear to be perfectly vertical and level. No skew is required here. But, think about looking to the western end of the rainbow or the eastern end. The "+" sign has to lean to the one side on the west and the opposite sid on the east - from YOUR point of view or from your geographic location. Regardless of which hemisphere you are in, if the arc leans to the east, the satellite "appears" to lean to the east. If the arc leans to the west, the satellite "appears" to lean to the west.

    Your linear LNBF has two probes fixed inside at the bottom of the waveguide throat which are set in a "+" sign arrangement. These pickup probes must align to the satellite's "+" antenna array layout to capture the maximum signal. So, you must tilt or "skew" your LNBF to match the perceived angle of the satellite in the sky. So rotate the LNBF so that the top of it follows the direction of the arc of the imaginary rainbow to align the "+" signs or the polarity. If the satellite is to your right, lean the "+" sign to the right or rotate the LNBF to the right (remember to think of this when standing BEHIND the dish and not in front of it). Or, think of it as you would if you were aiming a rifle with a scope and crosshairs. Tilt the crosshairs to the direction of the arc. You would be standing BEHIND the rifle (I certainly hope). A little humor there.

    I must point out that if you are rotating one LNBF to align the polarity, it is referred to as "POLARIZATION", not "SKEW". Skew comes into play when you have multiple LNBFs on ONE eliptical dish. Here, you may have to twist the dish's reflector so that all the individual satellite signals bounce off the reflector dish and converge on their one intended target, which is JUST ONE of the multiple LNBFs attached out on the arm. So, multiple LNBFs - adjust the SKEW of the DISH. ONE LNBF - adjust the POLARITY or rotation of the LNBF.

    RADAR
     
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  7. SatelliteAV

    SatelliteAV SatelliteGuys Family
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    Radar,

    Great reply!

    Will add a few notes on the terms Skew and Polarity. The term "skew" applies to both single and multiple LNBF types.

    Having been involved in the satellite industry for almost 30 years, I have heard the term "skew" from the first day on the job from hundreds of dispatcher/schedulers used for single LNB systems. The term "Polarization" or to select the polarity typically refers to the switching or setting between Horizontal and Vertical. The amount of feed rotation past the true vertical/horizontal calculated for the service location is the amount of skew applied to the polarity. Example: The site requires a skew of +23 degrees to receive the vertical polarity.
     
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  8. Anole

    Anole SatelliteGuys Guru

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    Circular LNBFs can be rotated to any angle; it doesn't matter.
    But when you have two on a dish, they each have to see their respective satellite.
    Skewing the dish aligns each LNBF with its bird.
    One LNBF will be higher than the other.

    On Hawaii, the DishNet 110 & 119 LNBFs are almost one above the other. (extreme example).
    That's because the satellites are so close to the horizon.
    When both satellites are near your true south, the LNBFs will be side by side.
     
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    Last edited: Dec 8, 2012
  9. Anole

    Anole SatelliteGuys Guru

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    One addition to the comments of AcWxRadar, above:

    If you have an elliptical dish and single LNBF, the feedhorn is matched to the shape of the dish.
    You must skew the dish for linear reception!
    Only if you are using a round dish can you get away with not skewing it.

    As for which way to twist an elliptical dish (or LNBF) ...
    - I like to stand behind the dish, but it doesn't matter :)
    - if the satellite you're aiming at is to the right of your TS, push the right side of the dish down toward the ground (clockwise)
    - if the satellite you're aiming at is to the left of your TS, push the left side of the dish down toward the ground (counterclockwise)
     
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  10. Cham

    Cham Professional Hobbyist
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    I always liked the idea of turning the whole dish assembly to line up multiple LNBFs with the focal points. Much easier to deal with than the standard FTA dishes where you have to set up a bracket by bending and contorting it so everything lines up.
     
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  11. turbosat

    turbosat SatelliteGuys Family

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    I wanted to say something about the screw and the pay companies here, but I shall not!
     
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  12. olliec420

    olliec420 Active SatelliteGuys Member

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    I never thought about HI, does any one have a pic of that lol, I'd like to see that
     
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  13. primestar31

    primestar31 SatelliteGuys Family

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    Here you go: hawaii_ref_2001.jpg

    :hula: lol
     
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    Last edited: Dec 9, 2012
  14. Anole

    Anole SatelliteGuys Guru

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    Sure. This is a picture I found on SatGuys years ago.
    It is their Dish 500 (gets 110 & 119, I guess)
    No idea what they use today.
    Maybe they use a wing dish for 129/HD ...?
     

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  15. AcWxRadar

    AcWxRadar SatelliteGuys Family
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    I like to put it simply as "Stand behind the dish and drive it like the steering wheel on a car! If the satellite is to the right, turn right."

    If there are TWO satellites, then you have to know where the centerline between them is, if that centerline is to the left of true south, then you have to steer left.

    RADAR
     
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  16. AcWxRadar

    AcWxRadar SatelliteGuys Family
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    That's a cool dish setup. Reminds me of some of the strategies implemented to pull in Hispasat and other extreme sats (when you run out of mechanical elevation adjustment on the dish).

    RADAR
     
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