Effective dish size calculation for an offset LNB

W

wallycharlo

Regular Member
My Satellite Setup
DM800 / 1m Gibertini XP
My Location
Western Austria
Howdy all,

So, I have an 85 cm dish, and wish to have it focused on Astra 2 @ 28E.

I then wish to put on a 2nd LNB for Astra 1 @ 19E.

I would like to calculate how much effective dish area which this 2nd LNB will see.

Anyone know where I can find a formula for this? I'm guessing it will be the area of an ellipse somehow, but am not sure how to get the properties of the ellipse, based on the angular seperation from the prime focus (i.e. 9 degrees in this case).

cheers ... Wally
 
Couldn't tell you a formula I'm afraid - I'm not even sure there is one!

But in practical terms if you can get all the channels you want on Astra 1 on a 60cm, or even 50cm, then you'll be OK.
 
Yeah, should be able to pick it up with a 60cm no bother ... just curious about who to work this out, must be possible somehow !
 
(Most) parabolic antennas will conform to a flat plane reduction when viewed at an angle, up to around twenty degrees or so, so you can still use projective geometry to determine the basic illumination area.

_http://jwilson.coe.uga.edu/emt669/Student.Folders/Alvis.Dwayne/instruct/iu.html

However, once you have calculated the effective area seen by the LNB then you will have to start using the formulae of beamwidth and gain at individual frequencies supplied by the manufacturer of your dish (plus the feed horn characteristics of the LNB), which I doubt you are ever likely to get hold of.
 
Channel Hopper said:
However, once you have calculated the effective area seen by the LNB then you will have to start using the formulae of beamwidth and gain at individual frequencies supplied by the manufacturer of your dish (plus the feed horn characteristics of the LNB), which I doubt you are ever likely to get hold of.
Click to expand...

True, I'm aware that there are many factors at play, though my thinking is that the effective dish area will give a pretty good first order approximation.

Thaks for the link channel hopper, I'm hoping now that some smart chap somehwere has a nice little applet which I can enter the details into and get what I want out the other side ... which I guess will be based on those calcualtions you sent on ...

I would like to get a feeling for how far I can push it ... i.e. the relationship between how far away I am from my primary focus, and the effective dish size I am seeing ; for 28E and 19E it's pretty much a given that it will work fine ; but how about, say, 28E and 7E? ... the effective dish size seen by the 7E would be much smaller I would imagine ...

hmmmm
 
wallycharlo said:
Thaks for the link channel hopper, I'm hoping now that some smart chap somehwere has a nice little applet which I can enter the details into and get what I want out the other side ... which I guess will be based on those calcualtions you sent on ...
Click to expand...

How would anyone designing applets know the area of the dish you are using ?

What about your skills at simple trigonometry ?
 
Channel Hopper said:
How would anyone designing applets know the area of the dish you are using ?
Click to expand...

My assumption would be that the dish size would be a variable which one defines.
 
hello
the 80 cm dish should be ok.
the second lnb on 19 degrees should be like a 60 cm dish and it should be about 3 db less.
you will need a box that has disec tone and an disec switch as well.
if you are using an german box that has disec tone than you will get the free channels on 28 as well as the free german channels on 19.
ralph
 
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