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Satellite "beam" width

Discussion in 'General Satellite Discussion' started by wiz561, Dec 30, 2010.

  1. wiz561

    wiz561 Mentor

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    Dec 7, 2010
    Hi!

    I have a feeling that this is a pretty common question about satellite. From what I've read, the signal beams down from the transponders to the satellite dish. The dish then reflects it into the LNB which then carries it to the cable and into the house.

    I was wondering how large of a "beam" is beamed to the dish? I can't imagine it being kind of like a narrow "laser" beam. Is it a beam the size of a satellite dish (~18")?

    Part of me is just curious, and the other part wonders if the house they are building next to me will affect my signal. The house is about 100' away from the dish and is almost inline with the one outer edge of the dish. If you had to draw an imaginary pipeline from the dish to the sky, it would maybe (if that) just touch the outer edge. So far, I have no interference, but I'm just wondering at what point it will fade out.


    Thanks!
     
  2. tcusta00

    tcusta00 Active Member

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    If the house is 100' away you're probably fine. Your 3 or 5 LNB dish is actually "pointed" at multiple satellites, so the size and shape of the dish are designed to catch them all.

    This drawing shows you that the actual angle that the dish is "pointed" is higher than it appears:

    [​IMG]
     
  3. wiz561

    wiz561 Mentor

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    Dec 7, 2010
    thanks for the response. I was looking around more and also found this pretty good image that explains it...

    http://www.satellite-calculations.com/Satellite/Antenna/offset_dish.gif


    glad to know that if it's 50 to 100' away, it probably won't affect it. Nonetheless, it's interesting to note how vertical the beam is.

    Going back to the original question, how wide is the beam? 18"?


    Thanks!
     
  4. tcusta00

    tcusta00 Active Member

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    I think you're thinking about it incorrectly. The "beams" cover the entire continental US, or, in the case of the spot beams for locals, your DMA area. The satellite is at a fixed point in the sky. For instance, one of the birds is at 101°, so your dish is pointed at that. If you were just pointing at that one satellite you wouldn't need an 18" dish, you could probably get away with a much smaller, round dish, as they did in the past. Now you're pointing at multiple satellites to get all the programming they put out, and they're at different spots in the sky, so your dish needs to be shaped larger and differently (in DIRECTV's case, an oval) to catch all the birds.

    There is a small margin of error with each satellite, which is why it takes some practice to point one accurately.
     
  5. BattleZone

    BattleZone Hall Of Fame

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    [​IMG]
    Most satellites have a wide beam. The shape of the beam is usually determined on the ground before the sat is launched, based on where in the sky it is intended to go and what is needs to cover on the ground. Sats may later get moved to another location, so that will often create less-optimal footprints on the ground.

    [​IMG]


    [​IMG]
    Some specialized "spot beam" satellites reuse the same frequencies over and over in different, narrower beams, usually to provide local content, which varies from place to place. This is the spotbeam map for DirecTV's SD locals from the 101 satellite. Most spotbeams from the HD sats are smaller.

    The numbers on the legend represent the transponder numbers, so you can see how the transponders are re-used in different locations. The same transponder frequency can be reused multiple times with spot beams, whereas if you used CONUS beams, you'd need more satellites and precious (and rare, expensive) FCC licenses.
     
  6. Laxguy

    Laxguy Honi Soit Qui Mal Y Pense.

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    Thanks for posting the question, and what an informative answer; thanks also.
     
  7. makaiguy

    makaiguy Icon DBSTalk Club

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    Aiken, SC
    You seem to be asking as though the satellite were sending a beam specifically directed to, and focused on, your dish. This is not the case at all.

    The beam from the satellite is much, much broader, ranging from perhaps one or two hundred miles in width for regional spot beams to "CONUS" beams that cover the entire CONtinental US. If you are located anywhere within the coverage area, you can receive the beam.

    Think of a floodlight mounted in your ceiling that illuminates the whole room. You can read your book anywhere within this broad beam, as could anybody else in the room. That would be equivalent to a CONUS beam.

    If there were instead a spotlight mounted in your ceiling, it might throw a beam 4 or 5 feet wide. You could read your book anywhere within this beam, as could anybody else close enough to you to still be in the beam, but nobody outside the beam in the rest of the room could read. This would be equivalent to a spot beam.

    In neither case is there a book-size beam aimed directly at your book.
     
  8. FTA Michael

    FTA Michael Hall Of Fame

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    Although the other posters have very effectively explained how really, really wide the satellite's beam is, forget all that for a moment.

    Dear wiz561, what matters to you is how much of that wide, wide beam gets collected by your dish and concentrated onto your LNB. So effectively, the beam portion that you need to care about is the size of your dish.

    Imagine a very long cone (effectively a cylinder) stretching from your dish to the satellite. Any obstruction in that cone/cylinder will block that much of the total signal your dish would otherwise receive.

    For example, there is a thin (~ 1/2-inch) power line that runs across my dish's "cone". Because of that, I'm only getting about 95% of the signal I would get with an unobstructed view. Good thing that seems to be more than enough. :)

    Enough blockage, if not enough to prevent you from using the dish, will reduce signal strength to the point where it's susceptible to rain fade. Winter installs that are lightly blocked by bare tree branches can become unusable when the leaves return.

    All of which probably has nothing to do with you. You can use trigonometry to verify that a distant obstruction isn't tall enough to be in the way, but it sounds like you'll be fine. Have fun!
     
  9. wiz561

    wiz561 Mentor

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    Dec 7, 2010
    Thanks all for the great information. I'm glad my question was answered and others have been helped too!

    I especially liked all the pictures. They helped explain a lot. I always thought that transponders correspond to different areas of the country, and that is why I might get 95% on one and 30-some % on another. So, thanks for confirming this.

    I guess I'm still a little confused about the beam. OK, it sounds like, going with the previous posters book analogy, the beam (light) covers the CONUS. If this was the case, then why can't you just set your dish up and get a signal? Why do you have to point it towards the satellite, if the beam covers the entire country? Is it just because you need the strongest signal, and by pointing it at the dish, you get the strongest?

    But again, thanks for taking the time and explaining this and posting the pictures. They helped a lot and helped me (and others) understand how the signal gets down to us.
     
  10. James Long

    James Long Ready for Uplink! Staff Member Super Moderator DBSTalk Club

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    You can also use http://www.dishpointer.com/ ...

    Enter the dish address and select the satellite you want to check. Go!
    Move the center pointer to where the dish is/will be.

    Then check the "show obstacle" box and move the marker to the object you're concerned about. It will show range and height of where the signal to your dish will pass.

    For example, the signal feeding my 61.5 dish passes 97 feet above ground at the treeline 133 ft away. As long as those trees don't exceed 97 feet I'm good. (Although a tree growing 22 ft from my dish had a branch 16ft above ground that was in the way before I got out the saw ... :))

    For multiple satellite dishes the pointer can be aimed at any satellite. That's where the tool is really helpful in blocking one of the paths needed.

    Note that it is only an estimate. The images do not always match the exact GPS coordinates ... but it should give you a good idea.
     
  11. James Long

    James Long Ready for Uplink! Staff Member Super Moderator DBSTalk Club

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    There are too many satellites using the same frequencies ... and your dish helps boost the signal from the chosen satellite by focusing the signal from the reflector into the feedhorn on the LNB. Think about holding a magnifying glass to focus the ceiling light. Line the glass up with the light and the page and you could burn a hole in the book (if the light was bright enough). But turn that glass a little to the side and you focus the energy elsewhere. Getting the most signal into that feedhorn is why you use a dish ... and don't just aim the LNB at the sky.
     
  12. makaiguy

    makaiguy Icon DBSTalk Club

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    Each transponder on the satellite emits only several hundred watts, essentially the power of a large light bulb, from 22,000 mile out in space. When you spread this over the whole beam area on the earth's surface, the resulting signal at any location is very weak.

    The actual signal reception is done by the LNB at the end of the arm on the dish. But if all you had out there was the LNB, the amount of the beam hitting the LNB would be far too weak to be useable because the LNB is so small. By using a parabolic dish, the portion of the beam hitting the entire surface of the dish is captured, increasing the amount of signal captured by perhaps several hundred times. Everything hitting the dish is all reflected to fall on the LNB. But only signals from a very small portion of the sky will be reflected onto the LNB -- signals from outside this small area will be reflected also, but they'll miss the LNB. Consequently the dish must be aimed fairly precisely at the satellite's location in the sky.
     
  13. gomezma1

    gomezma1 Godfather

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    What is the difference between circular and linear polarity on LNB's? How can you identify what type lnb is needed for FTA satellites?
     
  14. P Smith

    P Smith Mr. FixAnything

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    - usually only 50-60% of reaching a reflector of the dish energy coming into feed-horn.
     
  15. Davenlr

    Davenlr Geek til I die

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    Commercial satellites (FTA) use linear (horizontal/vertical) polarity. Direct Broadcast satellites use circular polarity.

    The difference is inside the feedhorn, with the way the feedhorn focuses the signal onto the antenna. Circular is like a corkscrew pattern, and linear is either 90/270 degrees or 180/0 degrees. In Linear, you have to concern yourself with SKEW, or lining up the horizontal antenna to match the satellites view of horizontal. Circular has no such alignment.
     
  16. gomezma1

    gomezma1 Godfather

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    so to view some fta birds that have linear polarity the lnb will have to have some skew adjustment? if you don't apply this adjustment you will no get a signal?
     
  17. P Smith

    P Smith Mr. FixAnything

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    If you will check your dish parameters with www.dishpointer.com you'll know all the answers ;)
     
  18. BattleZone

    BattleZone Hall Of Fame

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    Originally, all TV satellites used linear polarity. A simple explanation is that they could use the same set of frequencies twice, from the same satellite, by using one vertical antenna and sending vertically-polarized waves, and another horizontal antenna, sending different information on the same frequencies but polarized horizontally, and because the receiving antenna (in this case, inside the LNB) could only "see" either the horizontal or vertical signals, it would "ignore" the other.

    [​IMG]

    But, this system required that the LNB be rotated to match, almost exactly, the orientation of the transmitting antenna. And because your angle to the satellite is different depending on where you are on earth, this meant that folks 100 miles away from you needed a different setting than you did. It also made it impractical to pick up more than one satellite on a single dish.

    DBS satellites were different from earlier designs in many ways. They use different (higher) frequencies, much more transmitting power (allowing smaller dishes), and use CIRCULAR polarity. Instead of being polarized in a horizontal or vertical pattern, they are polarized in either a left-hand-twist or right-hand-twist corkscrew.

    [​IMG]

    Using circular polarity, you no longer need to be concerned about the rotational alignment (skew or tilt) of an individual LNB, which is why a DirecTV 1-LNB dish has no skew adjustment. And it also allows a single dish to pick up signals from multiple satellites without having to design a complicated system to individually align each LNB. You still need to align the dish to the arc of satellites if you're picking several on the same dish, but you don't do anything to the individual LNBs.

    99% of "Free-To-Air" content is on either C-band (big dish) or Ku-Band "Fixed Service Satellites" (FSS), which are only medium powered and are linearly polarized.
     
  19. P Smith

    P Smith Mr. FixAnything

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    that means Ku actually.
     
  20. FTA Michael

    FTA Michael Hall Of Fame

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    And to add to BattleZone's thorough explanation of signal polarities, here's one more tidbit to answer an unasked question: Why do satellites transmit two, opposite polarities?

    Satellites transmit in opposite polarities to maximize the number of transponders they can use without interfering with each other. Adjacent transponders always use opposite polarities, so when their signal ranges overlap, the LNB can easily pick out which signal it's trying to receive. Transponder two away from each other have enough frequency "distance" from each other to prevent interference or bleedover.
     

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