[Moon-Net] Prime-focus vs. Offset-dish

charlie at sucklingfamily.free-online.co.uk charlie at sucklingfamily.free-online.co.uk
Wed Nov 23 10:55:43 CET 2016


Hi Dom

As another point of reference, I get 10dB sun noise reliably with my
"portable" 1.2m prime focus dish on 10GHz, which has a few dents in.

I think it is all a matter of choosing the correct feed.  I now use a
linear polarised version of the SM6FHZ 0.692wl feed  on both  my 3m prime
focus  dish (0.3 f/D) and the 1.2m (0.35 f/D).  This is a very well
optimised design, and many thanks to Ingolf for all his work!

Going from the Andrews  feed that came with the 3m to the SM6FHZ brought
my moon noise from 1.9dB at best to 3.1dB at best.  These correspond to
pretty good aperture efficiencies.

Personal opinion is that very often folks don't pay the same attention to
feeding prime focus dishes as they do with offsets, and this may be one of
the reasons for the view that offset dishes work better.

73

Charlie G3WDG




> Hi Manfred ,
>
> I am interested to know how much sun/cs U get on 10Ghz with your system.
> On my "tropo" system , consisting of a 100cm offset , all WG and a
> sligthly worse preamp than yours , I get up to 10dB sun noise.
> I can't came close to this on the 120cm prime focus I use for small
> pistol EME. I agree with U that shadowing is nbot a problem , at least
> for reasonnably sized PF. It is a question on illumination efficiency
> and lower noise pickup.
> I have no shadowing on the small eme system as I'm using a rear mounted
> tvter and a shepperd crook.
> 73
> Dom/F6DRO
>
>
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