Hi from Downunder. Query on very flat Ku offset dish (1 Viewer)

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Wahroonga Farm

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Jun 15, 2006
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Gloucester, Australia
Hi guys,

I've a query regarding a suitable lnb config for a very flat 1.2m offset dish. It has an f/D of around 0.9 requiring a matching Ku lnb with a look angle of only 60 degrees (rather than the typical 0.6, 90 degree). The dish is a high quality ex VSAT internet unit made by Gilat.

I will use the existing feedhorn (30mm cast alloy lnb mounting) via the C120 flange to mate to an Invacom C120 lnb for the primary focal point.

But what about additional suitable lnbs to match the high f/D dish as I would prefer to use this in a multi-lnb config?

Any thoughts/solutions greatly appreciated.
 
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mikekohl

Prehistoric Satellite Guru
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Jun 4, 2004
764
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Montfort, Wisconsin
0.9 f/d is extremely flat. There are no consumer LNBFs outside of the 0.6 to 0.7 f/d ratio, but since this has not prevented hobbyists from trying them on round prime focus antennas with 0.4 f/d ratios, I do not see why you would not get better than acceptable to average results trying a consumer LNBF on your dish.
I used to build spherical C-band antennas in the early 1980s; some were 1.25 f/d ratio, and my earliest ones used a 1.50. There is something to be said for an extremely WIDE capture area east and west of center with an f/d ratio of 1.0 or better. It was no problem with a larger C-band grid to see 50 degrees of sky
with little reduction in signal. Go for it!
 

Wahroonga Farm

Thread Starter
Member
Jun 15, 2006
8
0
Gloucester, Australia
Thanks Mike. That is what I suspected. Problem is, it's ok to use a higher f/d lnb on a lower f/d dish as effectively you are simply not 'looking' at all of the dish and since the dish is large you'll get good results.

However when a lower f/d lnb is used on a higher f/d dish the lnb 'overlooks' the dish. And what does it see? .... an awful lot of ground noise which swamps the signal from the dish. If you do the maths on 'look area' you'll see what I'm getting at.

BTW love your website and info here. Thanks again
 

mikekohl

Prehistoric Satellite Guru
Supporting Founder
Jun 4, 2004
764
157
Montfort, Wisconsin
In experiments that I did in 1982, an 18 foot spherical antenna (18 x 18 foot rectangular antenna) was tuned while pretending that the antenna was actually 20 feet in diameter. Do the math---the outer edges of each corner were located slightly more than 20 feet from center, with the flat middle parts of the edges not doing any signal contribution. Your observation was proven by a problem that I had with reception---until I cut down a pine tree off to the side of the dish a foot or two. Its removal improved the signal significantly, reducing the noise picked up by the long spherical feedhorn from past the edge of the antenna. End result was at least 1.5 dB more signal than by tuning the shape of this device as if it was only 18 feet, and allowing the outer corners to act as a shroud rather than to contribute possible extra signal.
 
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