Advice Needed why offset dish has fixed mount that's not adjustable?

[nadal liu]

I was asked the question: why with prime focus dish we need to adjust lnb position to find the focal point, but with offset dish there is fixed mount that is not adjustable?
This is always a known fact and I never gave it a thought and I took it granted.
The friend asked me that he said the reason could be prime focus dish can be used with ku and c band and that the focal point could be different for the two, but I doubt it.

My guess is that offset dish is usually smaller and even if the lnbf position is off from the focal point, the distance off is smaller.

Please advise.

 

[Lazarus]

I understand what you're saying, but a lot of use do try to optimise the mount on Offset dishes: Two simple ways are manipulating the LNB arm slightly (I use matchsticks to shim out the elbow on the arm of my TD110) and use of an LNB with a long neck (eg Inverto Black Ultra) in order to be able to move the LNB nearer to, or further from, the dish.

 

[nadal liu]

Thanks for your reply.
Also, a second question he asked was "it is true that use of ku band lnbf for offset dish on a prime focus dish is not optimized, and based on what theory?"
Please advise.

 

[RimaNTSS]

On the offset dish it is almost impossible (OK, very very hard) to find proper LNB position by experiments only. Producers made their dishes and made LNB holders for those dishes pretending everything is perfect, but in reality nothing is perfect and nothing will be perfect. Even producers of branded dishes are not perfect. I would say, it is always make seance to check geometry of dish, also check whether LNB is located in antenna's focus and pointing to the right point on the surface of the dish.

 

[BM300]

The friend asked me that he said the reason could be prime focus dish can be used with ku and c band and that the focal point could be different for the two, but I doubt it.

My guess is that offset dish is usually smaller and even if the lnbf position is off from the focal point, the distance off is smaller.

Please advise.

About the focal point being diffrent between C and Ku, it is not the case my friend.
Otherwise we need to move the feedhorn everytime we change from C to Ku satellite.
Remember that there satellites that have both C & Ku bands.

See the pictures of the Seavey Feadhorn that shows using 2 C and 2 Ku LNBs on the same feedhorn.

 

[battenfan]

Also, a second question he asked was "it is true that use of ku band lnbf for offset dish on a prime focus dish is not optimized, and based on what theory?".

Lnbs for prime focus dishes often has wider opening angle than ones for offset dishes.
See illustrations in this pdf: http://www.qsl.net/n1bwt/chap4.pdf

 

[nadal liu]

Thanks.
I see your point.