calculating focal point C/Ku

[bubby-joe66]

I have a 6ft Andrews prime focas all aluminum dish on an Orbitron polar mount the 6 foot dish came with a dedicated Ku scale ring and for years the cband scale ring and bsc-621 was just fine and about 2 weeks ago it all fell apart, Close as I can figure most programs went to mpeg 4 none of the boxes I have will do mpeg 4 not visible to mpeg 2 boxes, new box will fix that.
Now to my question when figuring the focal distance for a prime focus dish is there any difference for differences between c-band and Ku band signals I have f/d figured at 27 inches for the dish and use a rifle bore sighter to centre the lnbf to the dish centre. fast and reliable most accurate aiming method I've found. Used the following to arrive at 27 inch f/d
D=72 so 72X72=5184
d=12 so 16X12=192
and finally 5184 divide by 192= 27 inch f/d for the dishes f/d

 

[Magic Static]

And finally divide the focal length (27) by the diameter (72) equal the Focal Ratio ( .37). Which determines how far through the scalar you want the feed horn to protrude. Then you can set the scalar with feed horn to the right focal length.

 

[Radioguy41]

See if this helps.

 

[bubby-joe66]

My question had more to do with the differences between the C-band collector and the Andrews dedicated Ku collector ring and would the focal length change for the same dish, My calculations to get to the focal length were Diameter 72 inch squared over 16 times depth 12 inch equalling 27 inch focal length that part is correct. Its far to early to load pictures being a long time away from here old post count is long gone. the Chaparrell scale ring has a diameter of 6 5/8 in. and 3 rings spaced 1/2 inch between the rings. The Andrews Ku scale ring is only 4 7/8 in. diameter with 11 rings 1/8 in ring spacing and is concave the opposite of the dish with 1/2 inch from the outside ring to the inside ring in depth the lnbf goes in from behind and collects signal similar the lnbf stacked on the rear of a bsc-621 with nothing protruding out front. My not being able to post pictures of both scale rings side by side may leave some not being able to see the differences to know what i'm getting at. But soon 2 posts down

 

[Radioguy41]

The focal length is the focal length, it doesn't change. Irregardless of what ring and/or LNB you use the point to align them to doesn't change because it's based solely on the dimensions of the dish.

 

[Magic Static]

Prime focus dishes use a flat scalar for C or KU band. The conical scalar is designed for an offset dish. It will not perform well on a prime focus dish

 

[bubby-joe66]

The focal length is the focal length, it doesn't change. Irregardless of what ring and/or LNB you use the point to align them to doesn't change because it's based solely on the dimensions of the dish.

Ok finally headway pure math wins

 

[bubby-joe66]

Prime focus dishes use a flat scalar for C or KU band. The conical scalar is designed for an offset dish. It will not perform well on a prime focus dish

the Ku scaler ring i'm holding in my hand has a 1/2inch deep concave surface with the 1/8 inch spaced rings turned into it, this is a commercial Ku head for a prime focus dish made by Andrews, it originally came off the dish i'm trying to re-install onto as the 4 original legs were damaged and thats why I went Cband long enough ago to pay for all my equipment going true FTA and thanks all when I accumulate enough post I'll post pictures of the Ku prime focus scaler ring beside a c-band ring