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Off-set V Prime focus Dish

trigger

Specialist Contributor
Messages
676
Likes
50
Location
Puerto del Carmen, Lanzarote
My Satellite Setup
30W, 27.5W, 7W, 0.1W
4.8E, 9E, 13E, 19E, 23.5E, 26E, 28E
Technomate 5402HD super
Bein Sports on 25.5E
FTE Mini S2T HD Analyser
Horizon HD S2 sat meter
My Location
Lanzarote, Canary Islands
#1
Can any one offer any technical info on which is the better type of dish, or is it simlpy up to reflector size. I'm not looking for manufacturer specific info so assume they were both manufactured in the same material etc - which one would perform better?

Thanks
 

Llew

cerca trova...
Staff member
Messages
14,318
Likes
3,282
My Satellite Setup
Triple Dragon, Dreambox 8000, Echostar AD3000ip, TBS6522,6925,6983 PCie cards.
Gibertini 1.25m motorised dish driven by the AD3000, with either Inverto BU Quad or Norsat / XMW Ka LNBs . SMW 1.05m + 3 other dishes. Speccy: Promax HD Ranger+
My Location
The Flatlands of East Anglia
#2
Smaller dishes (say less than 1M) are usually off-set as the Lnb/holder doesn't shadow the dish so much, hence a greater efficiency in signal pickup.

Llew
 
Messages
97
Likes
0
My Satellite Setup
3.0 m OffSet dish - Astra II
1.2 m Offset dish - Astra & Hot Bird
0.60 mts Hispasat
0.60 mts Nilesat

Box Kaon 570 & Pace 2600
My Location
Tenerife - Canary Islands
#3
Talking always about the same size....just an example 2 meters dish...always off-set preform better the the prime focus.....

the reason is the lnb in the prime focus, this part make a shadow in the dish, so this part is useles..... but this factor you will see it better in fringe areas...
 

bignick

Specialist Contributor
Messages
336
Likes
0
My Satellite Setup
Back in the UK and missing Cyprus!!!
My Location
Aylesbury
#4
I know the quoted statement SHOULD be right, but frankly out here in Cyprus it doesn't seem to be! Offset dishes have a finite size where they stop being effective of about 2.8m but I think after 1.8m a prime focus is the better option. They are less hassle to set up and more robust in use. The loss of the area caused by the shadow of the LNB is also made up by the fact that Prime focus dishes allow the use of C120 LNBs and feed horns better than Offsets.


With a bit of luck that put the cat amongst the pigeons ;)

Cheers

Nick


enoc said:
Talking always about the same size....just an example 2 meters dish...always off-set preform better the the prime focus.....

the reason is the lnb in the prime focus, this part make a shadow in the dish, so this part is useles..... but this factor you will see it better in fringe areas...
 
Messages
1,514
Likes
1
My Satellite Setup
Pace 2200 Sky digibox with ftv card, Comag SL65 FTA sat receiver, 40cm Sky minidish, Setpal terrestrial receiver (for free uk tv only!).
My Location
Midlands
#5
It's simple mechanics. Offset lnb doesn't "shadow" the dish, as correctly described above, and is always preferable, but above a certain dish size the antena arm gets too long, and can't guarantee the "millimeter accuracy" required for signal reception, hence 3 prong support instead.

It's also more difficult to manufacture the correct shape for large offset dishes, a parabola is easier to make!

But for performance, bigger is better, since you're collecting more signal.