Multi LNB on a Big Dish query

D

DishDick

Regular Member
My Satellite Setup
.
My Location
Gran Canaria
Hi,

Could someone tell me if the distance between centre points of multiple LNBs varies with dish size or if the distance remains constant irrespective of dish size? I.e to watch 2 satelites on a 1meter dish (offset) would the linear distance betwen the 2 lnbs be the same as 2 lnbs geting the two satellites on a 3 meter dish (offset )(my trig is not that good).

Finally the important question, taking into account the size of a lnb head, what would be the closest degree satellite I could get to the prime satellite on a 3 meter dish. I.e.if lnbs touching what degree of separation would that equate to?

Thanks in dvance.
 
It changes - the lnb distance apart for any two particular satellite slots will decrease as the dish size increases.
 
It changes - the lnb distance apart for any two particular satellite slots will decrease as the dish size increases.
Click to expand...

No, in general the LNB distance apart will INCREASE as dish size increases.



A very rough calculation can be used to work out the spacings of the LNBs.

1. measure the distance from the dish centre to the focal LNB- Call this F

2. The anglular separation of the satellite at the focus and the offset one. Call this A.

3. Let the distance between the LNBs be P

Then, Tan(A)= P/F thus P= F x Tan(A)


Or in words, take the Tan of the angular separation and times by the distance between the LNB and the dish.


It will give a ballpark figure, a bit of jiggling around to get the best signal will be needed.


Generally the bigger the dish, the longer the focal length and thus the further apart the LNBs will be. BUT, this is not always the case. If the larger dish is a prime focus for example, the focal length, most likely will be fairly short compared to the diameter. So it is possible to have the same separation on a 1m offset dish as a 2m prime focus (for example).


To work out the closest sats you can receive with your dish, first measure how close you can get the LNBs (P1) (measure between centres), and the distance away from the dish (F).

The closest sat you can receive is the roughly:-

InverseTan (P1/F) separation.
 
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