I'm so lost.. Dish size, lnb type, location & signal: Madeira

I may have missed this (long thread) - but 1.8m on the roof is surely asking for trouble, unless it's a flat reinforced concrete roof. What are the winds like in Madeira?
 
jeallen01 said:
FWIW, and unless this has been mentioned before, :
- the larger the dish is, the narrower is the beam width that produces decent signal levels
- thus most single focus dishes (unless very small and in strong signal areas) don't work at all well with multiple LNBs intended to receive signals from satellites spread over an arc of more than about 12 degrees of arc (and even that is "pushing your luck" in many instances).
Click to expand...

Well, this is new to me. I always thought the two effects were not connected.

- I know the larger the dish, the narrower the beam width, and the more important to place the LNB exactly in the focal concentration of the signals.
- I know with multifeed reception the more angle-difference of the byrider-LNBs from the one and only focal point of the dish, the weaker the signal.

But is this latter effect also dependant on dish size, as you say? So that the bigger the dish, the more dB loss for the same angular difference to the central LNB?
Usually I've seen advised: for multifeed reception, take a big dish, so that you can counteract the loss of dBs for the outer LNBs.

I cannot figure out why it would work the way you say. Can you explain, please?

Greetz,
A33
 
a33 said:
So... there is no explanation?

Greetz,
A33
Click to expand...
Without being an expert on wave-front propagation in radio frequency antenna apparatus, I would dare speculate that the answer lies in the ratio between the wave-length and dish size. The smaller the dish, the larger a dispersion (or "un-focusedness") wrsp to a specific wavelength, say, 11 GHz.

When you move to C-band (~4 GHz), you seem to get away with a much more relaxed positioning of LNBs.
In Ka-band (22 GHz), the mounting position of the LNB is ultra-critical to reception. LNB fixtures for Ka-band dishes seems much more rigorous than the Ku cousins.

I think (and I am still speculating), that the larger the dish, the faster the signal quality (and probably also strength) drops as you move away from the centre-position.
Therefore, the conjecture would be that multi-LNB arrangements on large, say, 3m dishes does not scale up directly from multi-LNB on, say, a 60 cm dish.
Unless you also scale up the wavelength... (which is harder)

I repeat: This is all speculation and conjecture on my behalf - I am happy to be proven wrong (or right!)
(But discussion like that is what makes this forum worthwhile!)

My guess is that as you move beyond 1.2-1.5 metres, the effectiveness of a multi-LNB arrangement starts to fade.
Instead, you would want to build a toroidal dish to cater for multi-LNB reception.
To the best of my knowledge, the largest mass-manufactured consumer toroidal was the SMW OA-1600.
The currently largest is the Wave Frontier T-90.
Then there are the built-to-order very-large ones primarily from GD SatCom, but that's generally for the budget of government intelligence agencies and large corporations such as Microsoft.

So for multi-sat operation, such as what OP wanted, I still believe the best bet is to have a T90, or even two T90s (like our Norwegian Friend here Dual T90 setup ), and point them at the stronger/relevant sats in the sky. This would give thousands upon thousands of tv channels instantly switchable - more than a decent sized hotel audience could be satisfied with in a normal evening's watching.

But I think that OP has determined that a 1.8m motorised Laminas will do the job, and has now gone off to pursue some other challenge, like multi-level pool excavation planning, with due vigour and a get-results-fast-approach, so my solution becomes academic at this point... :)
 
@st1 : Thanks for your input!

Indeed, discussion and dialogue is what makes this forum (or any forum) instructive. :)

However, following your reasoning here,
st1 said:
I think (and I am still speculating), that the larger the dish, the faster the signal quality (and probably also strength) drops as you move away from the centre-position.
Click to expand...
I would arrive at the complete opposite of your conclusion, which was:
st1 said:
Therefore, the conjecture would be that multi-LNB arrangements on large, say, 3m dishes does not scale up directly from multi-LNB on, say, a 60 cm dish.
Click to expand...

My conclusion would be (but I'm no expert, as you) that with multifeed reception on bigger dishes you have much less interference from neighbouring satellites and so better reception, precisely because the reason you gave: because the signal on the central/neigbouring LNB drops more quickly as you move aside from that LNB, than with a smaller dish.
So the signal drop you mention would only be to the rays received from parallel to the dish main axis.
The rays that come in from a small angle (from the 'by-rider satellite') have their own 'focal point', for which a bigger dish would still give a better reception, I would say.

The statement/reasoning of @jeallen01 I still cannot follow; I cannot remember I've seen it before...
But thank you for trying to give me an explanation.

Indeed, as I wrote before, I've usually seen the opposite advice: For multifeed reception, take a big dish, so that you can counteract the loss of dBs for the outer LNBs.


greetz,
A33
 
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