ouistion about prime focus dish serfice and refleting signal

[mosllam]

hello every one

I have notice some thing for many years

that the small circle close to dish center in reflecting the same degree of signal as the bigger circle close to dish edge

but the big circle is olmost three time the area of the small circle

-- I have this low gain - bad local made 160 cm prime focus

-- and a china made lnb with wave guide like the invacom feed

-1- I but three pices of metal close to the center

-2- and the same three cose to the edge

-- s/n ratio was 59% with no metal pices

-- when the metal is close o the center signal drop to 43%

-- but when moving it to the edge it drop only to 54%

the hole dish area is about 2.08 m

-- the small circle is about .57 m - and reflect about 28% of s/n

-- the big circule is about 1.51 m - with about 30% of s/n

-- is that normal and does it happen with any prime focus dish ?

-- if not what cold be the resons for what happend with me ?

 

[Vipersan]

Probably not the best and most accurate dish you have ..but your experiment although a bit vague sort of makes sense if you think about it in terms of surface area of the dish divided in and out of the circles ..
If the dish are within the inner red circle is the most accurate and therefore focussing a higher concentrate of signal then the three pieces of metal are reducing the focus potential by a greater proportion of surface area ..than if they are placed outside the red circle ignoring the area inside the circle..
Your test doesn't really tell us anything other than the dish is either badly made and inaccuracies increase outside the red circle ..
OR
That the dish is incorrectly illuminated by the feed ...
Possibly adjusting the concentric ring lnb feed disc to better match the dish ..and thus improve illumination will yield results ..
rgds
VS

 

[mosllam]

Probably not the best and most accurate dish you have ..but your experiment although a bit vague sort of makes sense if you think about it in terms of surface area of the dish divided in and out of the circles ..
If the dish are within the inner red circle is the most accurate and therefore focussing a higher concentrate of signal then the three pieces of metal are reducing the focus potential by a greater proportion of surface area ..than if they are placed outside the red circle ignoring the area inside the circle..
Your test doesn't really tell us anything other than the dish is either badly made and inaccuracies increase outside the red circle ..
OR
That the dish is incorrectly illuminated by the feed ...
Possibly adjusting the concentric ring lnb feed disc to better match the dish ..and thus improve illumination will yield results ..
rgds
VS

thank you for the explanation

if I start working on dish surface and adjusting lnb - what result should I wait before stopping

-- should I continue until the outer area give twice or more signal than the inner circle as the outer circle is about three times in area

 

[Vipersan]

thank you for the explanation

if I start working on dish surface and adjusting lnb - what result should I wait before stopping

-- should I continue until the outer area give twice or more signal than the inner circle as the outer circle is about three times in area

Impossible to say ..
a) because we dont 'see' the same section of the clarke belt..
b) because it not a commercial spec dish ..
Basically you are aiming for even distribution of signal focussed accross the dish surface..
this might not be possible if the dish is not made to high spec..
Just do the best you can..and good luck.
;)
rgds
VS