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Oct 2nd, 2010 at 8:22pm
1. If you point the antenna really high up so that the beam is definitely clear of the trees you will get a low reading. As you bring the antenna down, when the beam meets the trees the level will rise slightly, the same as if you put your hands over the feed window. Your hands or trees are warm (approx +293 deg K) relative to absolute zero temperature (0 deg K or -273 deg C). Compare with the cold outer space which is near 6 deg K (assuming clear sky). Torrential rain is +283 deg K. The sun about 10,000 deg K. Your system noise temp (LNB=20K + antenna=60K) is about 80 deg K so the increase of 290K when the beam hits the trees should be noticible as an increase. This sort of test may be applied to an LNB to see if it is working. Hold it either with its waveguide facing upwards to the sky or with your hand over the LNB input waveguide.
2. If you put your eye behind and just below the lower edge of the dish and next beside the feed support strut and look just over the top of the circular rim of the feed horn aperture then that is approx the line of the lower edge of the beam. It should be well clear of the trees.
If you are trying to point at a very low elevation, under 5 or 10 deg you may have problems making the elevation low enough as the lower back edge of the dish may strike the pole. Read the antenna assembly instructions/specification. Only if this is the exact problem, the cure is to turn the dish upside down so the feed arm is at the top.
Best regards, Eric.
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