Page 1 of 1

Dish wars

Posted: Fri May 17, 2019 8:36 pm
by radiogareth
Whilst I have been having excellent results with a TRIAX 1.1x1.05 offset dish - once I shimmed the LNB arm with a piece of gaffa tape I was getting the Q100 beacon at 8.5--8.7dB. Using a dual port Octagon I found that one port was better than the other, by at least 0.1dB - if you believe your meters! Thanks to whoever suggested this :-)
So when a larger dish popped up on the local Facebook for sale ads for £25 I bought it. With 1.2x1.15 and twin stabilizing arms and a decent 'tilt' mechanism (unlike the Triax) it looked good. So I duly fitted it and despite my finest tweaking, I can't get it over 7.8dB. The adjustment is noticeably finer but I was using my phone to remote access my desktop so could see the tiniest tweak making a difference. I wasn't expecting much improvement (as a percentage extra catchment area I'd hope for SOMETHING) but not a decline.

So maybe not all dishes are created equal.

Picking up another surplus dish next weekend, this time a 1.5M. I really WOULD hope for an improvement with this one!


Gareth G4XAT

Re: Dish wars

Posted: Fri May 17, 2019 8:58 pm
by g0mjw
There are at least two limitations on the maximum MER, signal to noise ratio and phase noise. At some point one will dominate. The signal to noise depends on the up-link and down-link. If it is up-link limited a better receiving dish will follow a law of diminishing returns. That won't be 1.2m but might start to be an issue above that - once you can see the transponder noise floor that's probably enough.

Then there phase noise. With the same dish and feed I get 10 dB MER on BBC Arabic with a GPS locked PLL LNB vs 13 dB on a DRO LNB. That's a wideband signal, but illustrates the point that the far out phase noise of these locked PLLs is not all that good. Close in they are much better than DROs but still not perfect. So if you get good results on narrow signals but not on the beacon, its probably phase noise.


Re: Dish wars

Posted: Sat May 18, 2019 6:29 am
by radiogareth
Thanks Mike, so tweaking for best MER isn't necessarily the route, although presumably useful for peaking up in az/el? Looking at the beacon signals on NB might be better? Still puzzled why a bigger dish with the same LNB didn't provide some increase, unless the dish isn't as 'good' - they are both pressed steel.
How do I know I'm looking at the transponder noise floor? On my SDR I get a raised 'HUMP' in which I see the edge beacons and other ops signals. I'm assuming that's it, so bigger isn't better, although I'm assuming it would reduce required uplink power?
Still learning......

Re: Dish wars

Posted: Sat May 18, 2019 8:29 am
by g0mjw
I don't know about the dish - but some are better than others.
  • Pressed steel not so good as aluminium unless it has been copper plated, or solver plated - unlikey for the cheap ones.

    Channel master solid dishes are not really solid, they are actually mesh dishes encapsulated in fiberglass. Still good but not if the mesh has corroded over time. If it has lived in a damp climate it might not be as good. Tin foil might fix but probably its scrap.

    Mesh dishes in general are likely to be a bit down on solid ones - and remember wind loading for a mesh dish is not reduced as you might hope above a windspeed threshold.

    The focal length and position of a suitable feed are very important, you can easily lose many dB by not being at the focus and the efficiency will be poor if the feed does not match the focal length.

    There is often a good reason something is cheap.
I would try to maximise the carrier to noise ratio on the narrow band beacon, or even the 10.6 GHz beacons, using a good SDR. That will eliminate uncertainty over the phase noise.