2.4 meter Prime Focus high quality dish performs worse than a 90 cm Chinese offset dish on Ku

Discussion in 'Free To Air (FTA) Discussion' started by enb141, Sep 3, 2018.

  1. RimaNTSS

    RimaNTSS SatelliteGuys Pro

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    Edge to edge is not important at all, you need to know the size of the working area. And you also need to check the geometry of the dish by performing the string test. And third thing..... what is the leaning angle of the face of the dish when you have it pointed to satellite?
     
  2. enb141

    enb141 Topic Starter SatelliteGuys Pro
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    When I measured the blue line, I did it only taking in count the inner part of the dish

    What's the string test?

    how to measure the leaning angle of the face of the dish?
     
  3. Brct203

    Brct203 SatelliteGuys Pro

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    yes we all agree that the dish needs a prime-focus type feedhorn, the suggestion to use the other LNBF was just to get a quick diagnostic by eliminating one variable.
     
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  4. enb141

    enb141 Topic Starter SatelliteGuys Pro
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    I did and I got about the same results so now I have to check for another LNB that designed for prime focus dishes for real, what I still don't know is why invacom claims their LNB is designed for prime focus dishes which is clearly false

    https://www.gionlineshop.com/Downloads/ADJUSTfeedHORN.pdf
     
  5. Titanium

    Titanium AI6US
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    Sorry. I highly disagree with your assertion that Invacom claims are false!

    The Invacom feedhorn IS designed and adjustable for the lower FD ratio prime focus reflectors (a feedhorn is not a LNB). It works exactly as described when mounted properly on a low FD ratio reflector that is a good parabola that is appropriate for the 10.7-12.75 frequency range, not warped and the feed is placed in the correct convergence and focal point. In my usage and testing, the Invacom adjustable scalar/feedhorn has always out performed a standard fixed scalar / high FD type LNBFs by 20% and more on an proper prime focus reflector.

    Obviously, in reading this thread, the feedhorn was simply mounted without calculating the FD and FL or optimizing the scalar placement or FD setting. Maybe you are expecting a miracle to happen without calculating and optimizing the feed? It is easy to blame a quality product for our own misuse and misunderstandings.... :(

    1. A string test is performed by placing 3 or more strings across the face of the reflector (edge to edge) and several angles around the dish. Observe where the strings intersect. Do the strings lightly touch or is there a gap between the strings? Gaps indicate that the reflector parabola is not accurate and is warped. This causes reflected signals to be directed away from the feedhorn opening and lost. A small gap may be acceptable (but makes the dish inefficient). A large gap means much of the reflected satellite signals are not entering the feedhorn resulting in poor performance and efficiency losses.

    2. The dish angle is measured by placing a straight edge on the 12 / 6 o'clock positions on the reflector edge. If the angle does not equal the calculated angle for the received satellite, the feedhorn is not centered on the reflector and some signal is being lost because it is not focussing into the feedhorn.
     
  6. Brct203

    Brct203 SatelliteGuys Pro

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    I'm not sure that I understand why you're saying it's not made for Prime Focus dishes. In my opinion the fact that the other LNBF gave you similar results tends to mean that there might be something wrong with the dish, either a geometry problem (but it seems that so far that looks fine) or maybe that it's just not so good at Ku - which is in contradiction with the claims by the manufacturer. I think that at this point none of the tests have yielded a clear conclusion.

    You were asking earlier about the string test. The idea is to make sure that the disk is not warped. The way to do this is to take 2 strings, and something to attach the strings to the edges of the dish (binder clips might be a good option), and set both strings across the dish, perpendicular to each other, for example one vertical and one horizontal. The should barely touch where they intersect (the should touch but not push each other more than by the thickness of the string, so at most a couple of mm). if they don't touch or one deviates the other by more than 2-3 mm, then the dish is warped. That could be caused by a less than perfect assembly of the dish, or by accident (especially on metal dishes), or by mediocre manufacturing (unlikely on a Prodelin)
     
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  7. Brct203

    Brct203 SatelliteGuys Pro

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    ah, Brian beat me to it :) I need to type faster :)
     
  8. enb141

    enb141 Topic Starter SatelliteGuys Pro
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    About the feedhorn + the LNB invacom, I did lots of tests with different positioning of the feedhorn threading (from 0 to the LNB to 0 mm to the dish). In pretty much all tests it gave me the same results, the only way I was able to get a little bit better results was to putting further away the LNB+feedhorn to the dish, in this case scenario I was getting better results but still worse than a 90 cm offset dish with an offset LNB. So to me that LNB is not working as it should for prime focus dishes, at least the one I have.

    Thanks for explaining the string test, I'm gonna check that tomorrow.

    I'm also going to check the elevation to see if matches the calculated one.
     
  9. Titanium

    Titanium AI6US
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    Changing the threaded position is only changing the FD ratio adjusting the amount of reflector illumination or over illumination and side lobe null.

    Proper placement and optimization is a result of the combination of both setting the FD ratio (threaded adjustment) and placing the feedhorn opening at the correct FL (distance) from the reflector.

    Calculate the correct FD ratio and FL using the reflector diameter and depth. For a description of proper feedhorn install and optimization with calculations, see this guide distributed with Titanium Satellite LNBFs: www.titaniumsatellite.com/downloads/Guides/LNBF%20Install%20Guide.pdf
     
    #29 Titanium, Oct 5, 2018
    Last edited: Oct 5, 2018
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  10. N6BY

    N6BY Geek
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    100% agree. I also have an Invacom for my prime focus Birdview 8.5' dish and it works very well on KU band. The dish has an F/D ratio around 0.4.

    As mentioned earlier, KU band requires far more surface accuracy than C-Band. In general, the surface inaccuracies should not exceed about 1/6th of a wavelength at the operating frequency. At 12 GHz, the wavelength is 2.5 cm, so the dish has to be accurate to within 0.4 cm (4 mm). Significant deviation from that results in parts of the dish canceling out the signal from other parts of the dish.

    As for the placement, I found that the distance from the dish to the feedhorn was critical. It had to be within a few millimeters and it took a lot of trial and error to find the optimum distance.
     
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  11. enb141

    enb141 Topic Starter SatelliteGuys Pro
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    Thaynks for the advice N6BY and Titanium, after doing the string test, the dish seems to be flawless, doesn't seems to be a noticeable gap between the strings, so now back to do some tests about what should be the exact position of the focal point, but I'm still wondering why the performance is so bad, I mean even with bad positioning of the focal point the reception should be way better than a 90 cm offset dish, just my two cents.

    Another question

    Do the focal point distance should be measured from the end of the feedhorn or from where exactly?
     
  12. martin-f

    martin-f SatelliteGuys Family

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    Personally i would start again, first job would be to get the dish on the correct king-post and secured before it fly's off, then make sure your pole cap is plumb then get your angles correct using a digital inclinometer and don't forget to make sure LNB skew is correct and try again.
     
  13. enb141

    enb141 Topic Starter SatelliteGuys Pro
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    After doing more tests about putting the LNB lower or higher distances from the dish-feedhorn and I got better results with the offset Ku LNB than the Invacom + feedhorn.

    With the offset I got -44db.
    With the invacom + feedhorn I got -51db.

    With the Chinese 90 cm metal dish offset dish I got -35db.

    So I think the invacom is broken because I don't know why it is performing so bad with this antenna.
     
  14. Titanium

    Titanium AI6US
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    Using the formulas for feed and scalar placement, what are the calculations for the FD and FL for your dish?

    Did you verify centering by measuring the edge to the center of the feedhorn throat in 3 or 4 points around the reflector to verify the feed is perfectly centered?

    Did you measure the vertical elevation angle from the 12 o'clock to the 6 o'clock positions to verify that it matches the calculated satellite elevation angle for your location?

    Did you use a 90 degree builder square or a laser level to verify the scalar and/or feed is parallel and aimed to the exact center of the reflector?

    If your performance finding is different than experienced hobbyists have found for the past 13 years, where might you think that the problem is?
     
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  15. enb141

    enb141 Topic Starter SatelliteGuys Pro
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    I tried different "angles" of the LNB, in other words I moved slightly and tried to match the distance to the center of the dish by "eye", also with a caliper but I think I'm gonna need a laser device to measure the distances, I was using a flexometer but is soo difficult to do measurements with that thing so I'm gonna buy a laser device for doing the right measurements.

    I adjusted the scalar but I'm not sure if is from outside or from the inside that it should be measured

    Thanks by your tips Titanium, next steps will be angles and laser measured distances
     
  16. RimaNTSS

    RimaNTSS SatelliteGuys Pro

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    To measure distances from Phase center of the LNB to different parts of the antenna you can make very simple but useful device, It is 40mm in diameter and has metal tip which represents the position of the Phase center. To the end of that metal there is piece of thread of wire tightened. So, you place the wooden part instead of your LNB and measure distances to the edges of the dish and also to the very center of the dish. Simple as that IMG_20180613_163848 (Large).jpg
    Add more pictures: This is how it works on smaller offset dish. No difference on bigger PFA. IMG_20180502_164022 (Large).jpg IMG_20180502_164046 (Large).jpg
     
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  17. enb141

    enb141 Topic Starter SatelliteGuys Pro
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    Cool idea, but what should be the length of the metal part, does it matter or doesn't?

    That's gonna help to measure the distance from the center of the LNB to the edges of the dish and it should be the same distance in the whole dish (prime focus) right?
     
  18. RimaNTSS

    RimaNTSS SatelliteGuys Pro

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    I make metal part 20mm, but then you also can move the wooden part inside the holder. How to place the wooden part..... depend on the LNB you use. For example this LNB has Phase center somewhere inside it, approximately where I made red circle. Wooden part must be made out of oak :shh just kidding.... any type ow wood, plastic or any other material.
    You are right for PFA all distances from the tip of metal to any side should be exactly the same. IMG_20181008_161727 (Large).jpg
     

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