Re: Dish


a_van_belle
 

--- In MSG-1@yahoogroups.com, johnrigsec@a... wrote:
Question.
Is the modern LNB specifically designed (geometrically) to work
with an (elliptical) offset dish?
Or, can you also place it at the focal point of a prime focus dish?
John, Offset dishes may look elliptical but seen from the sat (or
from the LNB) they are ROUND !

Modern LNB's are only suited for offset dishes. The feedhorn is
different to get a smaller beamwidth, compared to most prime focus
dishes with F/D=0.5 (offsets have F/D of 0.6 to 0.7).

You can put a modern LNB in the focus of a prime focus dish, but you
will only use 40 to 60% of its surface, giving low signal level.

With Eumetcast on the low band (11096 Mhz) you may be able to find a
used "old type single band" LNB with a scalar feed ring assembly and
H/V servo mechanism or Ferrit H/V switcher.
But you will be shocked by the noise figure of these oldies, 1.3 to
1.8 was excellent these days !
Nowadays you have 0.6 to 0.7 dB for 1/5 of the price !

Another drawback of Prime focus is that they do gather snow !
At 1.7 Ghz this is no problem but 2 cm's of snow in the dish can make
your a 11096 MHz signal disappear completely.

Greetings,
Arne van Belle

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