Why arn't dishes round ?

[Mikey11]

can somebody explain to me why ku-band offset dishes are not round?

what i mean is, when i buy a 36 or 39 inch dish it is not 36 or 39 inches ALL the way around, the dish is higher than it is wider, like an oval, it might be 36 or 39 inches high but the width is skinnier, why dont they make them round?

i have bought many different brand name dishes and they all seem to be this way,

 

[guacharaca]

The reason is geometry Watson. When you view your dish from the eye of the lnb or from the satellite in the sky, the dish appears to be round. The offset dish needs to be oval to achieve the round appearance from each of those two angles even though it is not round when viewed between those two angles. If you draw an oval on a piece of white cardboard and then tilt that cardboard backwards, that oval will appear to be round to your eye.

Also a 36" dish will fit in my hatch while a 39" dish does not, so all 39" offset dishes had to be made oval so they could fit in a 2006 eclipse.

 

[jazzba]

Maybe you are trying to narrow the signal laterally

 

[brex2001]

Offset dishes are a section of a paraboloid surface so the collection & focus are offset from one another. Imagine a parabola spun around its central axis then sliced with a plane at an angle to this axis. Sliced perpendicular to the axis would be a circle. Increase the angle of offset from the perpendicular to about 24 degrees to make an offset reflector. The dishes are elliptical because it would be impractical to tool a stamping die in steel all of the section of the paraboloid surface section. And it's a good approximation.

 

[harshness]

Perhaps they're also chasing after the idea that a "skinny" dish has a lower torsional bending moment in a gusty wind.

 

[brex2001]

Perhaps they're also chasing after the idea that a "skinny" dish has a lower torsional bending moment in a gusty wind.

Good point. If the whole 'upper' surface of the parabolic section were included it would make a great sail!

 

[PopcornNMore]

Also a 36" dish will fit in my hatch while a 39" dish does not, so all 39" offset dishes had to be made oval so they could fit in a 2006 eclipse.

Too funny!

 

[Hermitman]

Here's a small experiment that might help. Point a flashlight directly at a wall and you'll see a circle. Now, point the flashlight about 25 degrees upward at the wall and the beam becomes an ellipse, just about like the shape of an offset antenna. The shape of the dish helps reflect the satellite signal directly to the lnbf opening. Good luck.

 

[Anole]

I agree

Pretty much says it all:

When you view your dish from the eye of the lnb or from the satellite in the sky, the dish appears to be round.
The offset dish needs to be oval to achieve the round appearance from each of those two angles even though it is not round when viewed between those two angles.

How to measure a Ku dish:
http://www.satelliteguys.us/free-ai...p-how-measure-satellite-dish.html#post2089236

 

[jazzba]

It could be round...but the LNB would be in the centre, blocking some signal. so it is offset....no blockage.

 

[Mr Tony]

Also a 36" dish will fit in my hatch while a 39" dish does not, so all 39" offset dishes had to be made oval so they could fit in a 2006 eclipse.

If you have a truck you can haul a 10 footer and not worry