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1.2m Ku Offset Dish Declination Update

Posted: Fri Jul 17, 2020 4:38 am
by tek2000
All,

If any of you purchased our 1.2m Ku offset dish and you wish to track the satellite arc from horizon to horizon (e.g. 15w - 129w), you will need longer declination bolts. :doh The shorter declination bolts will only allow you to track a small section of the arc (e.g. 87w - 125w). We now include the longer bolts with our 1.2m antenna or 1.2m antenna kit. Please see the fix required in the diagram below.

kudishfix.png

To understand why you need to add declination to a prime focus dish to track the satellite arc, please study Fig. 1 and Fig. 2 shown below. Essentially, we add declination so the dish bends down a bit and can see the the satellite arc, rather than looking into outer space after setting the elevation angle corresponding to the setting latitude. With a prime focus dish, the declination for your latitude can be found from a table like the one found on this forum (click on TVRO Guides >> Declination Table).

The same principle applies to an offset ku band dish. However, you must remember that an offset parabolic reflector is made from a section of a prime focus reflector. For example, if you use only the upper half of a prime focus dish, you would essentially have an offset dish. However, the mount would no longer attach to the center of the original parabola, but would be offset. This offset changes the geometry a bit and requires more declination (bending down) of an offset dish than for a prime focus dish. For the 1.2m offset dish we sell, it turns out that you need to add an extra 25 degrees of declination on top of the declination indicated in the TVRO Guides >> Declination Table.

Some of you got around the short declination bolts by adding extra elevation. Yes, this will work and allow you to track a smaller part of the ku band arc. However, if you want to track the entire arc as accurately as possible, you will need longer bolts and the extra 25 degrees of declination.

My apologies for not catching this earlier. I have never used the 1.2m dish before. I track both the C and Ku band arcs on a single 12' prime focus dish. Special thanks to "John" for testing this fix and confirming that it works! :beer

kudish_northpole_1.jpg
kudish_northpole_2.jpg