Multi LNB and dish usage?

[marty]

I'll try and keep this as simple as possible :)
On a "normal" setup (single LNB) you use the most optimal part of the dish in theory. Manufacturers claim upwards of 70% efficiency.
Now on a multi LNB setup the LNB's are offset so I assume you use less than the optimal focus? and the more off centre you go the more gain you loose?

My question is, by how much? e.g. is using a 1m dish with an LNB offset the equivalent of using an 80cm with the LNB central?

[Rob van der Does]

This will obviously depend on how much the LNB's (i.e. the satellites) are apart: the closer the better.

But much better would be to use a multi satellite dish: the are oval and don't have one focus-point, but a 'focus-line'.

[Rob van der Does]

And besides: the bigger the dish, the smaller the opening-angle.
Meaning that a LNB being out of focus (as will be the case in your suggested line up) has a bigger effect on a large dish then it has on a small dish. In other words: the bigger the dish, the bigger the gain, but the bigger the loss when out of focus.

[marty]

OK, to put it another way :)

I want to get 28°/19° and 13° on one dish and stick a motor on the other.
Would I be better off using an 80cm dish or 1.1m dish? (you know where I live ;) )

I would like 28° on a fixed dish for instant switching to UK TV. I was thinking of setting 28° central the 19°/13° offset as ASTRA 1 and HOTBIRD don't require a big dish here.

Open to suggestions (but won't buy yet another dish :p two is plenty)

[Rob van der Does]

Would I be better off using an 80cm dish or 1.1m dish?
I prefer to await Huevos to answer that one.



I would like 28° on a fixed dish for instant switching to UK TV.
Why not combine the fixed dish and the rotor via one 4/1 switch? If you use twin LNB's you can have one tuner getting the combined, and the other tuner the fixed dish only. That's what I use: the best of two worlds (i.e. fast switching for the most used sats on two tuners and still all sats available on one tuner).



I was thinking of setting 28° central the 19°/13° offset as ASTRA 1 and HOTBIRD don't require a big dish here
That doesn't seem logical, as 13 will then be very much out of focus. I would put 19.2 in the centre, and 13 and 28.2 on both sides of it.

[marty]

That doesn't seem logical, as 13 will then be very much out of focus. I would put 19.2 in the centre, and 13 and 28.2 on both sides of it.


Depends on 2E really, try and see I suppose.

[Rob van der Does]

Yeah, easy enough to change back if SNR is too low.

[marty]

Busy afternoon :)
Fitted the motor to the 1.1m dish (again) after fixing the backlash :mad:

My big surprise was the multi-feed set on an old 80cm dish I had.
Doesn't look pretty but works (still need to wire it up and fit the 4/1 switch)
I'm getting 94% SNR on BBC FTA on 28°, 98% SNR on 19.2° and 84° on HOTBIRD.
Better than expected :eek:
Had to do a bit of bending to get the best results :p

32833

32834

Isn't my boy handsome :p

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[Rob van der Does]

So your handsome boy did the bending :)