Where am I going wrong?

moordown66 said:
Am getting a picture off BBC HD on Astra 2 with occasional break up. Quality is 40%, not strength.
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Well if you are getting a picture at all then the alignment/adjustment can't be that far out at all - as you know, precision is vital for DSAT reception and if 28E was way out you just would not get a peep.

So as well as refining the alignment, do consider the issue of skew, as previously mentioned :)
 
Tivù said:
Well if you are getting a picture at all then the alignment/adjustment can't be that far out at all
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With an offset LNB on a big dish that's not so true. For example an 80cm dish and a 9º offset LNB might see the same signal level as a 50cm dish with the LNB central, but with the 80cm dish and offset LNB the RF cloud from each satellite will be much bigger due to the offset signal not being focused properly to one point.
 
Good point, amigo.
 
moordown66 said:
What do you mean by RF cloud? The patch on the dish that the satellite will hit?
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A satellite dish is an optical system, it functions exactly the same as a Newtonian telescope. I.e. there is a mirror that collects the EM radiation and focuses it at one point (the focal point). The observer views the mirror from that point and everything appears in focus. If the observer is not at the focal point a point of light will appear out of focus. In focus means that an infinitely small object in free space (a star for example) will still appear infinitely small to the observer after passing through the optical system however much magnification is applied, whereas an out of focus object will appear as a sphere of light that has size. In the case of a dish with an offset LNB the LNB is not at the focal point of the optical system hence the RF arriving from the satellite, rather than being focused to one single point, forms a cloud of semi-focused RF into which the feedhorn of the LNB is placed. But, even though the RF is a cloud, rather than an infinitely small point, the aim is still to try to position the LNB at the point where the rf cloud is most concentrated, i.e. collect the maximum amount of signal.
 
Nice result, yes well worth the time doing it again.
 
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