LNB vs LNB with horn antenna cut off

  • For the NB transponder I use a POTY, a very practical single dish solution.


    For the WB transponder I noticed that a bare LNB in the focal point (not via the waveguide/pipe of the POTY) gives me 3db extra SNR (MER of the beacon goes from 3 to 6db on a 90cm dish).


    Also on the NB I receive 3db more signal on an unmodified LNB.


    I also tried with only the waveguide (copper pipe) (without the POTY) on an LNB with the horn cut off: also 3 db less. With or with out lenses, with multiple LNBs, with varying position within the focal point, on a 90 and a 120cm dish.... a bare LNB seems always 3db better.


    Also I see that quite a lot DATV WB stations use 2 dishes...is this the reason?


    I can imagine the horn antenne that is cutt off results in a loss, but isnt a lense to resolve that, or only partially?


    PS I need those 3 db extra for DATV reception, without I only decode the beacon at max 3db


    Thanks

    Henk, PA3GUO

  • PA3GUO

    Changed the title of the thread from “LNB vs LNB with horn cut off” to “LNB vs LNB with horn antenna cut off”.
  • You need to have the right sort of lens in just the right position for it to work well. Too far in or out and it will no longer be at the focus. You should not be losing as much as 3 dB with a rocket LNB lens, perhaps a dB. Phase noise is a problem too. Was your bare LNB unmodified and the POTY one modified for an external reference?

  • I just had a thought - what sort of dish is it and which lens are you using? You might also find one LNB polarisation works better than the other. That's because they are not really designed for 10.5GHz and the probes are not ideally placed with reference to the back-short.

  • G0MJW yes, all 4 LNBs are unmodified. I started with 2 identical ones, and later added 2 others from a different brand/design, as I wanted to make sure it wasnt a difference in design. The 2 new ones performed identically, and both also when mounted in the dish 3db better as the previous ones mounted in the POTY. Then I removed from one the horn antenna and placed that in the centerpoint of the dish, with a POTY lens, both via a copper wave guide (from the POTY kit) and directly: 3db lost.


    All using same receiver and cables&connector only swapping LNBs.


    Dish 1 Triax 90cm offset

    Dish 2 120cm offset

  • Then it's not working for you. If you are not happy with the POTY then try something else. With an ex-rocket LNB lens it works fine for me and many others but some people appear to have difficulty, at 10 GHz it's usually from an inappropriate lens. I only recommend the ex-rocket lens or the HB9PZK rexolite lens.

  • Hallo,

    I did a new Dualfeed-System for a friend with some new ideas. Mill a brass tube. There is no need to reduce the outer diameter of the rocket but removing the lense flange. Clean it with sand paper. The brass tube has a new flange to carry the lense. All three parts can be sticked together. No loss in LNB because all is original.



    Solder the patch and the reflector disk to the tube. One can mill 3mm (plus copper thickness) shoulder to the tube to get the right distance between the patch and reflector. Don't forget to drill the "feed hole" when both parts are fixed flat together with a srew before soldering. In this case, the reflector disk hole has a reduced diameter to stay on the shoulder.




    This is the ready LNB. It gives a compact unit and can be mounted with the most dish equipment.



    If the upper bracket gives no room for the S-Band-Cable, take a metal bracket. The picture shows my own feed with heavy Cellflex cable. The transition to SMA-Surplus-Cable is fixed with an alu bracket


    By the way, rockets with 25 MHz TCXOs are extremly stable. No fast changes in the final frequency can be observed like it was with 27MHz from Connor Whinfiled mounted in Octagons.

    There is only little change with outside temperature which can be easily regulated with SDR Console.


    Many Thanks to Simon in this way ! He did a great job all over the year. We appreciate this very much.


    Finally, the feed is a compact unit. It can easily be changed and compared with other antennas, probably a helix.

    73s, have fun
    Happy New Year to all "Helians"

    Andreas

  • PA3GUO Hello Henk, my test setup shown here is certainly not new and shown by others as well. It is VERY easy to construct by drilling a 22mm hole in to the plastic LNB cap and sliding in the waveguide tube. My quick NB test last year showed in comparison a more or less equal SNR performance between the LNB placed in the 60cm dish feedpoint and the combination LNB - Patch antenna with Venton rocketlens. The lens is just taped quickly to the patch for the test.

    The LNB's used are Megasat Diavolo Twin PLL 25MHz that work very well for me. Highly stable without modification and perfect NB reception using SDR-Console with Beacon-Lock.


    73 Ed


  • PA3GUO Dag Henk, to my surprise the difference between the "bare" LNB in the upper picture and the patch/LNB/rocket-lens combination in the lower picture, was close to zero!. I checked the QO-100 beacon SNR via SDRuno (SDRPlay RSP2pro) on both systems with short interval and found very much the same SNR (there is always a kind of QSB...).

    I probably was lucky to find at first try the optimum focal point position for the patch/lens, so you might have to play with this as mentioned by others as well.


    Note: I used two identical LNB units in this test. Both are unmodified (just one having the 22mm hole in the front cap) and from a batch of six pieces they all had very close SNR performance.

    As a general remark on the 22mm hole drilling: try to keep it centered very well to get the patch-waveguide tube inserted centered with the LNB waveguide. With the drilled hole offset, the LNB will be mounted at an angle, looking at least ugly...


    73 Ed

  • Unfortunately the CW beacon SNR is not a very good test because with a large dish you are mostly going to see the transponder noise floor. The SNR is dominated by the uplink signal to noise ratio for big dishes, you could use Bochum and not see much improvement. A much better test to aid in optimising your setup is to measure the height of the transponder noise floor above the background noise floor. Maximising this should result in the best performance. The Wideband beacon MER is a much more sensitive test but even then once you get above 10 dB improvements are very small. Of course, if you are seeing the transponder noise floor at a good level it could be argued there is little point in optimising further. Better to use the time by getting on the air.


    Mike