Directv Slimline arm for SWM LNB too short

Discussion in 'DIRECTV Support Forum' started by Farside, Aug 31, 2018.

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  1. Farside

    Farside Topic Starter New Member

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    To start out with, I live in Colorado and I am a Directv subscriber and have a big Slimline dish attached to the side of my house. Had it put there instead of on the roof so I can sweep the snow off of it during the winter. Anyway, I also have an RV and wanted to get a portable dish for the those trips away from home. Purchased a new Slimline with a tripod and a new LNB. Tested out setting it up on my deck, and was able to find the satellites with no problems. Then I tweaked the signals using a meter to get the best reception. Hooked the dish up to the receiver and found the signal strength on the portable to be lower than my regular mounted dish. Scratched my head and thought it might be the LNB or the cables being used. Checked all the cables and connectors and found no issue there. To eliminate the LNB, I swapped it out with the house one and the LNB on the portable dish. The House LNB on the tripod showed lower readings, while the new LNB on the house showed up great. Well, in my eyes, that eliminated the LNB as being the issue and got me into thinking it might be the dish itself. Measuring the dish and comparing all the parameters I found the arm on the portable was about a 1/2 inch shorter than the one on the house. So, I unlocked the LNB from the arm and gently pulled it out as far as I could, and still barely able to lock it in place. Lo and Behold the signal strength increased another 10-12 points on the meter scale. Now I am faced with a dilemma, if the focus on the LNB is that far out, what is my next step? Do I bend the dish a little this way and that (which isn't easy) to see if I can improve the focus, or do I lengthen the arm? Whichever, I figure both are going to cause issues trying to make this thing work. I'm also still trying to figure out how much money the manufacture saved by producing a shorter arm on the antenna assembly. Anyway, I'm just wondering what has anyone else done in this situation, and what suggestions do you have to tackle it?
     
  2. slice1900

    slice1900 SatelliteGuys Pro

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    Definitely do not try bending the dish. It would be impossible for you to bend it correctly, accurately and consistently. Whatever you do you'll end up making it worse and have to buy a new dish.

    Lengthening the arm sounds more reasonable, though it sounds weird that it should have a 'short arm'. Maybe the portable is designed that way to make it smaller and easier to store? People in RVs probably don't care as much about getting the best possible signal - the only difference it will make being 10-12 points lower is that it will take a little less rain to knock you out. The picture is the same whether the signal on the HD transponders is 95 or 25, digital is pretty much "all or nothing".
     
  3. Farside

    Farside Topic Starter New Member

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    The SlimLine dish on the house and on the tripod are the same dish, just has a shorter arm. With a shorter arm, that throws off the focal length. The mathematics for figuring the focal length of your LNB is pretty simple. Normally you would take the diameter of the dish and multiple if by itself (Diameter X Diameter = A) and then take the depth of the dish from top and bottom and multiple it by 16 (depth X 16 = B). After you have these two measurements, then you divide, A/B. That will give you the focal distance of the LNB. With my measurements from the new dish, the focal length should be around 20 inches from the center of the dish. However, I have found the arm brings the LNB too close, to around 19.5 inches. Playing with the numbers, bringing the top and bottom of the dish closer together by about a 1/4 inch would fix the problem, and spreading the top and bottom of the dish, or flattening it, will just increase the focal length and make things worse. I'm thinking possibly during storage or shipment the dish may have been flattened. Someone could have walked or stood on the box. Anyway, making an extension for the arm may be more problematic, and then securing the LNB to it might actually offset the centering focal point on the dish. I'm still thinking about this one.
     
  4. raoul5788

    raoul5788 Studebaker driver Staff Member HERE TO HELP YOU! Supporting Founder Lifetime Supporter

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    :welcome to Satelliteguys Farside!

    Interesting the difference in arm length, I've never heard that before. Definitely don't try bending anything, it will most likely make things much worse. Perhaps you should contact whoever you got the dish from and see about getting a replacement for it.
     
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  5. Jimbo

    Jimbo SatelliteGuys Master Lifetime Supporter

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    Did he buy another SWM dish or a specially made Portable dish ... didn't know such a thing existed.
    As far as I know, all the SWM dishes are the same reflector and arm (and LNB) wise.
     
  6. Jimbo

    Jimbo SatelliteGuys Master Lifetime Supporter

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    I think I would try a different dish set up before I'd make any changes to the set up, UNLESS, its a spare that you don't care about ... I have one like that, that I played around with awhile back to try to get an arm that would fold to make it easier to store under the bed in the MH.
     
  7. Claude Greiner

    Claude Greiner SatelliteGuys Master Supporting Founder

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    Where did you get the Dish from?

    The reason why I ask is that there are 4 different versions of slimline dishes out there.

    Could be possible an installer grabbed the wrong parts from a different type of dish.

    They come in single and 4 packs.

    The 4 packs are a mess once you open them trying to build a dish.

    I actually pay more money and order them boxed separately as it’s easier to grab 1 box and put it in the truck then grabbing a loose dish.

    I have tried pre-assembling then at the store, but then we eventually go through them and we are back to square 1
     
  8. slice1900

    slice1900 SatelliteGuys Pro

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    But if they have the same reflector size/curvature shouldn't all four versions have the same length of LNB arm? Or do the different types have LNB arms with different methods of attachment, therefore changing their effective length if you 'mix and match'?
     
  9. Jimbo

    Jimbo SatelliteGuys Master Lifetime Supporter

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    I would think they would all have to be the same length as you can use multiple different LNBs with them.
     
  10. slice1900

    slice1900 SatelliteGuys Pro

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    What I mean is, if they attach slightly differently then using an LNB arm designed for a different dish could cause it to be the wrong length. i.e. imagine if there was a little 1" stub sticking out on one that you connected to, but on another one there was no stub, then an arm designed for a dish with the stub used on a dish without the stub would not be at the correct focal length.
     
  11. Jimbo

    Jimbo SatelliteGuys Master Lifetime Supporter

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    I haven't looked lately but iirc, they are spot welded ...
    I suppose different brands could be different.
     
  12. Inclined Orbit

    Inclined Orbit SatelliteGuys Pro

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    The mathematics for figuring out the focal point of an offset dish with a feed looking at multiple orbital slots would be very difficult, if not impossible for most of us and determining it by trial and error will end up in error. For a dish like a Slimline operating at 20GHz and above the exact focal point would be critical to probably less than a cm in all directions. Ideally a feed horn is within 1/4 wavelength or so of phase center of the focal point and at 20GHz that's about 3.75mm. I used to know the Slimline specs but don't remember those fine details. It sounds to me like there is a specific LNBF that fits this dish with a short arm.

    The original AT9 Ka/Ku dish had slightly different arms with mating LNBFs from different mfrs and they were not interchangeable. With the Slimline series they standardized the arm and LNBF focal point and all mfrs must meet the same dimensional specs so LNBFs from all mfrs are interchangeable.

     
  13. Farside

    Farside Topic Starter New Member

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    OK, back again with a final follow-up. The kit I purchased from Amazon was the cheapest I could find, and this one came out of China. It took awhile to get here, but I figured it was a back-up for the RV, and I could wait for it. With the poor performance it was giving me, I immediately went into discovery mode. As you read before, I did all types of comparisons with a good working antenna system, length, width, depth and such. Eventually I set the dish aside and just got another Slimline dish off Craig's List for free. Surprisingly, the new dish brought in the signals at 99s, even using the LNB that came with the bad dish. Looking at that bad dish, I scratched my head for awhile, and let it sit in my garage for a weekend. The only other thing that I noticed about the dish, comparing it to the other two, was that the grey coloring was slightly off, and that brought back some memories of when I was working on large dishes in the Air Force, and one day I came in and we had completely lost all of our signals. Over the weekend a contractor had repainted the dome and had used a lead based paint. So, just for a final check, I scraped some of the paint off the dish and gave it to my neighbor to have it analyzed. Sure enough, they had painted the dish with a lead based paint. I guess it coming out of China, they don't have strict rules on using lead based paint. I notified Amazon, and I also have noticed the dish kit is no longer available. I was looking for a cheap dish, and that is what I got.
     
    #13 Farside, Sep 18, 2018
    Last edited: Sep 18, 2018
  14. raoul5788

    raoul5788 Studebaker driver Staff Member HERE TO HELP YOU! Supporting Founder Lifetime Supporter

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    So the new dish is working correctly, you have signal strengths in the 90s on all the sats?
     
  15. Farside

    Farside Topic Starter New Member

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    Yes, I have been testing out the new dish on my deck for the past week, with above 95 signal strengths on all the transponders on 101, and have the skew set up to catch 103. Signals are a little lower on 103, but not much. I'll stow the antenna away later this week in the RV. I have a trip down to Texas that I will be making next month, and set it up down there. Elevation will change about 10 degrees, and the skew will be different, but I'm expecting that it should receive the birds just as well.
     
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  16. Jimbo

    Jimbo SatelliteGuys Master Lifetime Supporter

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    Glad to hear you got it all going ...

    Where abouts in Texas are you heading ?
     
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