Trouble with Two LNBs

[diapason]

Having trouble with trying to receive two satellites on the one dish. I bought an arm in Glasgow, cost £19 (!) and it is curved with a slot for adjustment. Tried to mount two LNBs on my 80 cm dish and each LNB is working well. The only trouble is that I can get either 13E or 19.2E but not both without moving the dish. Now I know that 13E requires the dish to be a little higher and I tried to compensate by mounting this arm at an angle but still I have to move the dish.

I've also bought a DisEq switching box but thought that I should ensure that all LNBs were receiving signals first before even attempting to wire this switch box up - and how does the receiver know when to switch?

I have attached two photos to illustrate. Thoe first shows my monstrous TV aerial and VHF radio one, as well as my two dishes. On the bigger one, I fitted the arm which holds two LNBs, but the trouble is that I can fix one to receive, as the attached pictures show, 13E (Hotbird group) and that is the one which is wired at present on my 80 cm dish. The second LNB does not seem to be able to receive anything from 19.2E unless I move my dish. In so many pictures I have seen, two LNBs receive two satellites on the one dish. There is a degree of adjustment in the form of a curved slot but I've tried all positions and cannot receive two "birds" on the one dish. The smaller 65 cm Fortec Dish up above is definitely pointed to 28.2E and there's no problem with that. It seems I have the choice of having both LNBs to the right of centre or one on each side of centre and that is the first position I have tried, but to no avail so far.

So I have not had the chance yet to wire up the DisEq switching device as I thought it better to be able to receive signals first before trying electronic switching. I still have only the one down wire to the house, so it's BBC World Service and Italian stuff tonight as I've had enough for one day!!

[nanochickin]

The two lnbs look to far away from each other to me. You have a similar set up to me. My 2 lnbs are about 3/4" away from each other. A lot closer than your look. I get 13e and 19.2 east no problems.

Hope this help

[jontydog]

Yep they do look a little far apart on the pics m8

[Jimdefruit]

Hi diapason,

Your mounting bar is the wrong way around. The lnbs need to be pointing at the centre of the dish.

The approximate spacing of the lnbs, centre to centre can be calculated by taking the tan of the angle difference between the sats and multiplying the result by the distance between the lnb and the centre of the dish.

e.g. 19.2 - 13 = 6.2
tan 6.2 = 0.1086
lnb spacing centre to centre = 0.1086 x distance lnb to dish centre

Fix the 13E lnb and get a good signal, then fit the 19.2E lnb the required distance to the left of the 13E lnb when looking from the front of the dish.

[diapason]

Many thanks, all, for that information. I did wonder about which way round the bar should go, as the markings on it seemed to suggest the way I have it, but the LNBs are not pointing into the centre of the dish. I'll turn it round and then maybe have more luck. The bar also comes with fixing holes at one end but that would mean the LNBs are away over to the left of centre as you look facing the dish - or should they be? Forgive my blissful ignorance!!

[Jimdefruit]

Not sure about the fixing holes on your bar but I would guess it is so you can fix two bars on the arm if you have a lot of lnbs.

Going back to your earlier question about the diseqc switching, you set the lnb switching in your receiver. You tell the receiver all Hotbird channels are on lnb 1, Astra channels on lnb 2 etc.

[BGonaSTICK]

Nice spot Jim. Interesting and useful calculation too.

The LNBs also look a little close to the dish face. I take it the dish originally had a plastic insert in the end to hold the LNB, and so you've lost an inch or two in boom arm length?

Not much you can do about it, except make sure the LNBs are pushed right back in the new clamps to compensate a bit.

Good luck!

[diapason]

These solutions get better and better. There was a plastic insert which used to fit on the end of the boom but I think the LNBs are about the same distance from the dish as was the single one which I had. What I did have to do was raise the angle of the LNBs up a bit by putting a wedge at the front end, as I thought the LNB was too low and the signal wan't too high. This increased the signal so I'll make it permanent, but there's still room for improvement. Pouring with rain today in Paisley so it'll all have to wait for another day and we're still receiveing Hotbirds, but thanks, everyone - I might be needing more help yet - if I succeed then I'll know a bit more and happy to help others - I'll advise progress.

[jontydog]

nice one diapason this is the way the human race learns by asking questions and listening to the answers.

[diapason]

Hey everybody, I have had a tremendous success today and all thanks to all your collective expertise!!.

I now have my Fortec 60 cm dish receiving 28.2E, and my old 80 cm dish receiving BOTH Hotbird on 13.2E and Astra on 19.2E in excellent quality. Special thanks to JimdeFruit for pointing out the error of my ways and the fact that the arm on which my LNBs were mounted was the wrong way round, AND for those calculations. I followed them to the letter and the distance from the arm to the centre of the dish was 60 cm which meant that the LNBs should have been 6.5 cm apart and the one reciving Astra 5 cm above the other one locked on to Hotbird. Thanks, also, STICK as I did lose a little length but the LNBs seem to be in roughly the same position as the old single one was. And to Nanochicken, the LNBs are almost touching each other but I seem to have the best signals from each satellite.


I've only got the one downwire at this stage leading into the house, but on locking everything into position and moving the downlead, after having made sure that I got the best signal on Hotbird, lo and behold, everything on 19.2 came into view, loud and clear.

One or two tricks - I did have to make a wooden wedge to put under the arm, as the LNBs were pointing along the line of the boom and not high enough. I also used two blue plastic Rawlplugs under the LNB pointing to 19.2 to raise it up the required 5 cm. They also acted as rollers to allow me to move the LNB sideways to find the right position.

My next and last stage, will be to wire the DiscEq switch box to control everything from within the house, but that's for another time. I've now moved the downwire to receive an evening's programmes about Joyce Grenfel from BBC Four.

All the best to all of you and once I've finally finished all of this, I'll send a photo. - from happy diapason!!