Distance LNB to Dish

[boxerman23]

Does anyone know the equation used to measure the distance from dish center to lnb, I have being offered a 1.5m prime focus dish, however it has no arms to hold the lnb, although this is easy to construct i need to know at what distance to mount lnb from center of dish.

Thx in advance

 

[AM556]

Much easier than with an offset dish.
In the absence of calculations you need to make a concave mirror out of the dish. To do this, make up a weak (but not dish water!) solution of wallpaper paste, stick aluminium foil to the dish surface as smooth as you can. Point the sish at the sun...DO NOT LOOK AT THE FOCAL POINT..take a piece of paper 6" square attached to a stick and place it approx where the LNB should be, you will clearly see a bright spot at the focal centre. NOTE..you MUST have the dish pointed square at the sun! To do this, superglue a small nail in the dead centre of the dish and perfectly square (measure from the rim to the point of the nail, equal all round is perfect). When you point it at the sun, you are spot-on when the nail doesn't create a shaddow.
I have done this twice and it works a treat :thum:

 

[bonou2]

look at here ------ http://www.satlex.de/en/fdratio-params.html?diam=150&depth=15

 

[[email protected]]

f=D*D/16*d where f-focal distance,D- diameter of the dish, d- depth of the dish.

 

[boxerman23]

f=D*D/16*d where f-focal distance,D- diameter of the dish, d- depth of the dish.

Perfect, it works i have tried this calculation, I am not good at mathematics so just for people like me - diamater squared, divided by 16 times the depth of the dish. this will give you the focal length to the collar of the lnb. (ps, it is only the diameter of the parabolic that is measured, some dishes have an outer collar for strength, this extra bit should not be included in the calculation)

best