Multi-lnb losses off prime focus

D

Dggrr

Member
My Satellite Setup
SF8008, TD88, Gibertini 1.25, T-55
My Location
SE
I have an 88cm motorised dish but have found that I am missing many lower power feeds in the SE UK.

So I have set up a 1.2m dish with a prime focus of 10 East and a second LNB on 7 East. This seems to be working well.

Surprising there seems to be little information on the losses of an off-prime focus LNB for orbital separation.

The only factual reference I have found is Multiple Satellite Feed Systems – SSE who quote a 1db loss at 3 degrees. However, this is for C Band and centre fed dish, not Ku band and offset.

Most multi-LNB setups seem to address commercial broadcasts and not feed hunting, for good reason of course.

Any thoughts on this? For example, if I added a 3rd LNB to my 1.2m dish for 16 East with the prime focus at 10 East, would I see any benefit over the motorised 88cm dish which is always at primary focus?

For the sake of argument, let's assume that any LNB placement at focal length and skew are perfect :)
 
A few years ago on a Channel Master 1.2m aimed at 10e I added an offset 7e LNB. The offset 7e signal was worse than a Channel Master 90cm aimed at 7e.
 
Most (if not all) prime focus dishes have a circular focal point,(if it is a round dish) when you get off to one side of centre the focal point will start to go oblong for any satellite that is not on centre, you can counter this a bit by offsetting the dish to a point in the middle of the two satellites.

You will lose signal strength on the prime satellite but it will work a bit better, for the 10 E and 7E combo you can offset by 1.5 degrees. (to 8.5 degrees) However some dishes do not have a very wide receive beam width, so the offset may or may not work too good.

An elliptical dish would work better at this then a prime - they have a wider beamwidth.
 
Losses are expected compared to aiming directly, more farther apart more loss, even worse if TPs are weak (feeds).
An elliptical dish would maybe work better, but it also depends.

People that normally setup multi LNBs at the same dish is because they got enough signal to deal with, either dish large enough or sats strong enough, then those setups offer better usability compared to motorized in regards of having multi sats available at same time (multi tuner recordings/live tv)
 
Dggrr said:
Any thoughts on this? For example, if I added a 3rd LNB to my 1.2m dish for 16 East with the prime focus at 10 East, would I see any benefit over the motorised 88cm dish which is always at primary focus?
Click to expand...

Well, the reception surface of a 1.2 m dish is 1.9 times as big as a 0.88 m dish, so I would certainly try to put a byrider LNB at 16E!
In fact, the TD88 would be just about 80 cm wide, IIRC? So you could still afford more than 3 db loss, with 16E on the CM1.2...

Even better would be the CM1.2 motorized..... :cool:

BTW, you do have two offset dishes, I presume? The term 'primary focus' is somewhat confusing, as it seems to indicate a prime focus dish.

Greetz,
A33

Edit: So, finally putting your plan to action? Great!

2 (or more) lnbs on a CM 1.2? - SatsUK

I'm wondering if anyone has a good solution for this. I currently only have Triax 88 on a diseqc motor and it misses a lot of feeds on both 7 & 10 East. I'm thinking of putting my CM 1.2 backup as fixed but would like to get both 7E and 10E. My thoughts are that this would require some...
www.satellites.co.uk
 
@all, Many thanks for the replies

The terms prime/primary focus seem to get used confusingly around the Internet when it comes to prime focus/centre fed dishes and multi-lnb on offset setups. I've been using "prime focus" here to refer to an offset dish that is pointed at 10E and all other LNBs are offset from this, i.e. not pointed halfway between 10E and 7E. This was based on the observation that 10E feeds are often very weak.

@a33 - thanks for the reminder on my earlier post which I had completely forgotten about! In my defence, it took me nearly four years to run a cable to the 75mm post for the CM 1.2 that I had put up at the the top of the garden - shortly after the 2016 post, I went to work in France for a couple of years. I'm now "economically inactive" and have lots of time.

I did try a few efforts with the CM1.2 at multi-LNB, but like ozumo did not have much success. I recently got an OP125L which is a dream for multi-LNB. Although both about 1.2M , the CM and the Gibertini are vastly different, as shown by their focal distances, CM=820mm, Gib=975mm.

I'll give the 16E LNB a go sometime.....
 
Back
Top