Newbie QO-100 POTY - LNB questions

  • I am bringing an old 1.8 metre prime focus spun aluminium alloy dish out of storage and want to use it to access the QO-100 satellite. Initially for RX and TX of SSB stuff here in the Midlands UK. But I want to future proof the whole installation with a view to adding wide band ATV as an option. I may have quite a few questions I'm afraid, , but already joining this forum has been a huge help. I intend to initially buy a Pluto SDR, and have it in a weatherproof box by the dish, with it's own power supplies and feed the Windows 7 64 bit dedicated PC in the shack via a hard wired LAN. Is there any potential issue with doing this to reduce co-ax feeder lengths? For TX I am thinking of one of the Bulgarian 20W amps with a pre amp between it and the Pluto with a Bias T Am I right in thinking that I can pay a premium for an LNB already configured for a GPS locked reference signal?

    Now this was a quality dish in its day , but it's now old... The LNB was used for general satellite stuff some years ago at my old house. I am initially looking for thoughts and recommendations for which LNB to buy that's currently available. Ideally I would like to lock the LNB to GPS. I have Trimble Thunderbolt but it's 10MHz. I also have a Ublox NEO-M8T-0-10 GPS unit which will output user defined frequencies, GPS locked.

    I am not averse to buying a Leo Bodnar GPS however. I also want to mount the POTY antenna for TX on the same dish, at the same time. Rather than try and explain the current LNB mounting verbally I felt it much better to upload a zip file of some photos to my server for viewing. I have a small machine shop as I am in the motor race game, so I could make some simple constructions for mounting the new LNB / POTY.

    Looking forward to any advice as I want to put the 4 inch tube in for the dish and get it mounted before the wife changes her mind ;) As I said, I am in the UK as regards buying an LNB. Thanks for reading and for any replies. 2E0ILY

    The photos are at

  • Hi,

    from what i see on your pictures and learned from your post, i think you can remove the original feedhorn and make a clamp or something for the poty to put it in front of the disc that held the original feed. Next you might buy a cheap lnb, cut off the horn and push it in the tube of the poty. That is what i did, my poty is homemade with copper tube from the hardware store. After some work with sandpaper, it fits nicely. You may want to maximize SNR . To do this, replace the 3 bolts in the disc with longer ones and extra nuts. This way you can move the disc together with the poty futher out. In your dish, the poty will sit about 5-10mm closer to the dish than the original feed, you need to compensate for that until you have max. SNR. You now should be pretty close to best position for the TX part of the poty, too. 1.8m is a killer anyway, i have 1.5m.

    LNBs - at least here in DL - are as cheap as 5€ . So if you destroy one , it's not the end of the world. Known working LNBs i use , are:….html?&trstct=pos_0&nbc=1…tml?&trstct=vrt_pdn&nbc=1

    Now that you no longer are in the EU, you may find similar offers from a local. I feed the lnbs with 25Mhz over coax. The single lnb via one coax where i inject the reference, the quad lnb over one of the four f-sockets. With four way lnb you can use both transponders same time.



  • Another question has come to mind I am afraid. If I use the tripod plate as in the photo I hope to attach, it is of 170 mm OD, made of steel. My POTY will sit in front of it and I will turn an Acetal plastic adapter to sit in the plate in place of the annular rings LNB feed and the current old LNB, with the 22mm copper tube going through the plastic ring to a new LNB behind the steel plate. With the steel tripod plate being a larger diameter than the POTY reflector, do I need to have it a certain distance behind the reflector plate to stop unwanted interaction please?

    I believe the 22 mm copper tube forms a wave guide to the LNB, is its length important?

  • Hi,

    I don't think the plate will interact with the poty other than prevent it from being in the correct distance. It will shade the downlink a bit more than the poty alone does. The copper tube is any length, as long as it is not a multiple of a quarter wave @10.4GHz, but you'll want it as short as mechanically useful. So go ahead , make a long tube and put everything in place. When you found the best distance for the poty , You can cut off the excess length. With a dish that size you will have no problem other than finding the satellite :-)

    A different approach is to use the steel plate as the reflector for a helix. You migt even keep the original annular rings. They are there for a reason.

    73, Martin

  • Thanks again Martin. I turned up an Acetal adaptor boss to mount the POTY 22mm tube concentrically in the plate today, and am awaiting the LNB to arrive. I will start with extending the tripod legs to bring the aperture of the 22mm tube to the same distance from the centre of the dish as the standard LNB and annular rings.

    The POTY instructions say to not use the supplied plastic "lens" if fitting it to a prime focus dish like mine, and to leave the tube open. I am concerned about water and insects going down the open end of the 22mm copper tube into the LNB. Can I cap it in any way that doesn't do "bad things"? The more I get into all this the more it becomes apparent that my microwave knowledge is very lacking! :(

  • Don't worry. My microwave knowledge was zero before i bumped into this.

    As a cap i use the plastic cap that came with the copper tube from the hardware store. There was one at either end to protect it from dents, so i tried one for the poty. With or without the cap, the signal difference is marginal. You can use any material that is not heating up when you cook it in a microwave oven.

    Or try the original lens for now and replace it later. It will work, just not optimal. You will only notice this on the wideband transponder where every fraction of a dB counts, but not on NB transponder.

    If you don't mind destroying the original lens, you may cut off the excess material just leaving the length of the OD long enough to sit on and seal the end of the tube . Cut off the tapering material but leave enough of the ID so it has enough grip . Maybe apply a little glue you can easily remove if you go for a different approach later.

    73, Martin