Re: Offset dishes on 24 ghz EME

Dominique Dehays
 

Hi all ,

offset superiority is no longer to demonstrate. Here I very often run sun noise measurements and I have often done the comparison between an offset and a prime focus of the same size ( BTW size is the small dimension of the oval). Most of the time sun noise is 2db better with the offset.

The prime focus we use are very often low f/d dishes , then , feeding them with good efficiency is difficult  but feeding a 0.6 to 0.8 f/d offset is easy.

73

Dom/F6DRO

Le 19/10/2019 à 15:17, Chris Bartram G4DGU a écrit :
Hello Keith,

Offset dishes are probably a better bet for 24G EME than a so-called 'prime-focus' reflector. You are likely to get significantly higher efficiency. Most amateur objections seem to come from a lack of knowledge of the geometry, and a lack of suitable, simple to make, feed designs. You can use an offset dish on the horizon without any penalty, but the mount has to be capable of tilting the dish 'forward' so that the plane of the dish rim is inclined at the offset angle of the reflector. As I resume my own 24GHz EME project (after over a decade's break!) I am planning to use a standard 1.5m TV offset dish. That will replace my old 2.4m offset - which I was then using  quite successfully on 10G EME. For the 2.4m antenna -which had 0.8f/D - I designed a feed horn for 24 based initially on the Skobelev paper referenced in W1GHZ's book. I optimised that design and designed the circular to rectangular WG20 transition using a an EM modelling program.

Dishes/feeds used for point-to-point comms at 24GHz will be optimised for different criteria than those which are necessary for good performance over the EME path, and the feed supplied with the dish may not be particularly ideal for our use.

My old 24GHz feed has been used with a number of typical 0.65f/D 'TV dishes', and it works well. However due to underillumination it doesn't _quite_ get the performance of which those dishes are capable. I will be designing a 0.65 f/D feed once I've finally completed my new dish mount - but don't hold your breath!

73

Chris

G4DGU



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