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Originally Posted by gopoc
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While moving the LNB straight out for about 3.5 to 4.5 inches following the arm direction, the quality level jumped to 80-90. WOW. I measured the LNB arm length and I got 25 inches. The focus length on Fortec Star 120 cm dish is 720 mm (28.34 inches) and when I measured from the center of the dish to the center of the LNB connected to its arm I got 645 mm (26.32 inches).......
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This "may" make things seem worse, however you do NOT measure focal length of an offset dish to the "center of the dish" as you say. There is no absolute way of measuring it, however the closest thing to being a way to measure it is to measure from the front of the LNBF to the near edge of the dish. In most cases, this would give you even a smaller measurement, however. But one of the members here recently found that his LNBF support arm was bent, placing it nearly up where a prime focus feed would be. It almost sounds to me like your lnbf arm might be bent too.
If you place a straight-edge across the front of the dish from top to bottom, then place a big carpenter's square on this straight-edge so that it aligns with the lnbf..... where on the dish does the lnbf hit? Ie close to the bottom edge, or near the center of the dish??? I'm almost guessing that yours is bent up, so the lnbf is closer to the center of the dish, when looking perpindicular to the straight edge across the dish.