monoblock LNB.

PaulR

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The Visiosat BiSat is a special case as it has been designed for multiple LNBs. I suspect that for each LNB it's effectively around a 60 - 70cm dish.

You can get away with small dishes and monoblocks it's large dishes that I think there will be problems with. Have you actually put a monoblock onto a 1m or bigger dish?
 

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A monoblock with fixed feedhorns will be pointing towards the focal point of an 80cm dish.
Not quite correct - each feedhorn will be situated at different "focal" points roughly the same distance from the dish face but horizontally apart due to the reflection of the satellites' positions.
 

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Mmm, maybe but I don't think so. I still think they should look through the same focal point whether on the surface of the dish or at a position just in front I'm not sure. But they won't be different surely as whole point of having separation between the feedhorns (whether monoblock or separate LNBs) is that this is reflected off the dish to point to different satellites.

This only applies to standard shape dishes of course. Special shapes such as toroidal are a completely different case.
 

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That looks as though it's adjustable for angle but not separation so would also be looking at a focal point in front of the dish. The accompanying description doesn't what size dish it's optimised for so I could be mistaken. But, equally, it doesn't specifically say it fits all sizes of dish.
The one I have is adjustable for separation (the two parts slide apart). If this model is the same as the one, I have it is designed for an 80cm dish - so 4 degrees on an 80cm should be near enough 3 degrees on a 1m.
 

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But they won't be different surely as whole point of having separation between the feedhorns (whether monoblock or separate LNBs) is that this is reflected off the dish to point to different satellites.
No the focal point of each satellite is different because of the reflection at the dish. Think of a parabolic car headlamp reflector, Main beam uses a filament at the focal point and dipped beam uses an offset filament to give an offset beam - the filaments are the lnbs and the light beams go to the satellites.
 

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LNB's put in an antenna of 80cm separated 6º, they are the heads to 1.5cm, in an antenna of 100cm this to 3cm, mount 2 antenas so for some friends.

The double LNB (same box), if they are together, the polarization angle, is not problem, when you have to stretch it to the maximum, does not allow the correct adjustment of the polarization.
I leave a photo as an example of the signal reflected in an antenna and where the signal is, so it is better with loose loose LNB for its correct adjustment .
----------------
LNB's puesto en una antena de 80cm separados 6º, estan las cabezas a 1.5cm, en una antena de 100cm esta a 3cm, monte 2 antenas asi para unos amigos.

El LNB doble (misma caja), si estan juntos, el angulo de polarizacion, no es problema, cuando lo tienes que estirar al maximo, no permite el correcto ajuste de la polarizacion.
Dejo una foto como ejemplo de la señal reflejada en una antena y donde esta la señal, por eso es mejor con LNB sueltos separados para su correcto ajuste.

Malaga ==> 130cm seguramente, Surely.
Better separate LNB's/Mejor LNB's separados.

Monoblock_80_100cm.jpg
 

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No the focal point of each satellite is different because of the reflection at the dish. Think of a parabolic car headlamp reflector, Main beam uses a filament at the focal point and dipped beam uses an offset filament to give an offset beam - the filaments are the lnbs and the light beams go to the satellites.
It's more complicated on a headlamp because of the masking to control the beam pattern but if anything I think that analogy actually proves my point.
This picture shows exactly what I mean about a fixed monoblock on the "wrong" size dishes.
 

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Mmm, maybe but I don't think so. I still think they should look through the same focal point whether on the surface of the dish or at a position just in front I'm not sure.
If that were the case then it would not be so critical on the lnb's distance from the dish. Signal level peaks when the focal point of the dish is at the "sensor" within the lnb.
 

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It's more complicated on a headlamp because of the masking to control the beam pattern but if anything I think that analogy actually proves my point.
Hardly - the main beam filament is at the focal point and the dipped offset to one side (and NOT looking at the focal point. I am talking here about very early basic headlamps without the modern beam shaping/masking modifications.
 

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If that were the case then it would not be so critical on the lnb's distance from the dish. Signal level peaks when the focal point of the dish is at the "sensor" within the lnb.
Agreed that it's not critical for distance between LNB and dish for this particular aspect but it is important for the correct distance so that the LNB "illuminates" the dish correctly for its f/D ratio.
 

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Hardly - the main beam filament is at the focal point and the dipped offset to one side (and NOT looking at the focal point. I am talking here about very early basic headlamps without the modern beam shaping/masking modifications.
So you're talking about the old style of pre-asymmetric headlamps. But I still don't see why you think it proves your case more than mine.
 

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But I still don't see why you think it proves your case more than mine.

Your case being, if I understand correctly:
a. the aiming point/aiming angle of an LNB is quintessential for good reception
b. in a monoblock the angles of the two feedhorns are therefore not parallel, but at an angle
c. that therefore (due to the used angles) monoblocks are only usable at the specified dish size, mostly 80cm.

As I wrote earlier (referring to point a.), aiming angle isn't that important. Not an LNB is the primary aiming object, as you seem to suggest, but the dish itself is. The LNB 'only' has to be in the right spot (the focal point). Ideally the LNB is aimed at the G-spot, but if it is not, reception is usually still very good. That is probably why very many LNBs aren't aimed at the G-spot, without problems...

With multifeed (off-focus) reception, aiming point is equally secondary to exact location of the LNB.
Best aiming point would be the "horizontal G-spot", as I called it in a topic in the section 'Dish Setup Guides, Information threads and FAQs'.
However, even with separate LNBs on a multifeed bracket, people use all kinds of aiming points. So it isn't really critical.

By the way: you seem to use the term 'focal point' in quite a different way as I (and @davemurgtroyd ) do. The focal point is 'the right spot' I mentioned above. A normal paraboloid dish just has one focal point; a multifeed dish can have a 'focal line'.
The aiming point for multifeed LNBs should be the 'horizontal G-spot'; so that is not called the focal point.

ad b.: I don't think that is really the case, I have never noticed it. Also the drawing of @Tururu shows parallel feedhorns, it seems. Have you measured the angle in various monoblocks?

ad c.: I still don't share that conclusion.

greetz,
A33




.
 

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Agreed that it's not critical for distance between LNB and dish for this particular aspect but it is important for the correct distance so that the LNB "illuminates" the dish correctly for its f/D ratio.
You are misreading my post. The lnb being at the focal point is far more critical than an lnb illuminating the dish. Ifeally the lnb should be at the focal point AND be chosen such that its "reception angle" illuminates a maximum area of the dish (ideally not beyond it which may increase noise from the background behind the dish).
 

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c. that therefore (due to the used angles) monoblocks are only usable at the specified dish size, mostly 80cm.
Not strictly true. Results will only be optimal and view the correct satellite separation on the correct dish size they are designed for. However experiments would show that monoblocks for 80 cm dishes can be used on dishes from at least 60cm to 1m albeit with a different angle of separation to its "specification" on any but weak satellites. I believe the "acceptance" angle of the wave guide should cope with receiving most of the signal.
 

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So many points to reply to.

First of all, yes I am misusing the term focal point. That is where the LNB is positioned, not what I was saying. I'm not sure that there is a term for what I mean. Perhaps focusing point? Bit on the dish that the LNB is pointing at? The best position for further LNBs to point at is the same position that the primary, central LNB points at. If a secondary LNB is not also pointing at this primary position then, yes, it can be made to work but it will inevitably reduce the performance a bit.

What I am trying to say is that a monoblock doesn't have the facility to point at this primary position for both feedhorns at all sizes of dish. An adjustable monoblock may have the best feedhorn angles for 80cm but it won't have the best angles for any other size dish. As I said already, it's unnoticeable on a smaller 60cm dish as the satellites wanted are stronger. In addition, as the aperture angle is wider on a smaller dish, you can probably even use a non adjustable monoblock and get signals for both wanted satellites.

In summary, I'm not saying that an adjustable LNB won't work on a larger dish just that it won't perform at its best and, whereas, it can theoretically pull in comparable signals on its designed-for-size dish compared to a multi LNB setup it won't be the same for a larger dish.

I hope that clears it up.
 

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Not strictly true. Results will only be optimal and view the correct satellite separation on the correct dish size they are designed for.

Huh? I thought this topic is about using a monoblock for a specific arc-difference (e.g. 6 degees, for 13-19E) on a specific dish size (e.g. 80 cm) on another dish size for another arc-difference.
That's what this discussion is about, I think.
From the drawing of @Tururu one could imagine that a 6 degree monoblock for 13-19E on a 80 cm dish could also be used for receiving 13-16E (3 degree arc difference) but then on a 160 cm dish (if that has the same f/D).

By the way, naming a monoblock (as manufacturers do) after arc-difference is misleading, because not the arc difference is important, but the angle-difference (that equals about the azimuth, but not quite, by the way) on the site on earth you are at. As everybody knows, I guess, the angledifference (azimuthdifference) of certain satellites varies on various places on earth.

Greetz,
A33
 

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The best position for further LNBs to point at is the same position that the primary, central LNB points at. If a secondary LNB is not also pointing at this primary position .....


Well, I guess you still think the pointing of the LNB is quintessential, and that you haven't read the thread about the 'horizontal G-spot'? Alas..... :(:(:( .
And have you checked pictures of the Visiosat BiSat G3D-bracket with 4 LNBs? What do you say about that, then?


Can you confirm that you have measured the angle between the feedhorns in various monoblocks and that they are not parallel, and also different for monoblocks for 80 cm dishes and 64 cm dishes?
Maybe that needs clearing up first. Now we are talking along non-touching lines.


Maybe for your dish size you could try a 4.3 degree monoblock or a 6 degree monoblock for a 80cm dish.
Though they are designed for a different dish size, the distance between the two LNBs inside might be approximately what you need on your 1-1.3 meter dish.
But it really is 'might'. Separate LNBs give much more fine-adjustment possibility :).

@Tom W H : The reason I said 'might' is that the range 100cm-130cm is pretty much, so you would need to be more precise there.
Furthermore we don't know, if that dish has the same f/D as a 'normal' 80 cm dish.
And thirdly, a bigger dish has a narrower beamwidth, so the LNB feedhorns in the monoblock have to be more precise in their right spot; a bigger dish is 'less forgiving' for that.
So if a monoblock for 3 degrees would be exactly fitting for a 80 cm dish on your location, then a 4,3 degree monoblock for a 80 cm dish would be fitting exactly on a ( 80 x 4,3 / 3 = ) 114,666 cm dish (with the same f/D) on your location for the 3 degrees separation.
However if there would be a misfit with the 3 degree monoblock on your location on a 80 cm dish, but the dish is 'forgiving', then you could still have reception on the 80 cm dish, but you would multiply (with the above kind of calculation; edit A33) the error for another dish size, with possible dramatic results (no reception).
You would then have to calculate the LNB distance from the effective focal distance of your dish and the angle difference between the two wanted satellites at your location, to see if there is a monoblock with about that calculated internal feedhorn-distance

A problem of a monoblock is also, that after tilting the monoblock to its proper angle for the both satellites, you cannot adjust the skew of the LNBs anymore. Some monoblocks are preskewed, but the information about that is difficult to find. That is a problem not linked to dish sizes, though. But if you have room for normal LNBs on the bigger dish, with seperate LNBs you have much more adjustment possibilities.

greetz,
A33
 
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Huh? I thought this topic is about using a monoblock for a specific arc-difference (e.g. 6 degees, for 13-19E) on a specific dish size (e.g. 80 cm) on another dish size for another arc-difference.
That's what this discussion is about, I think.
From the drawing of @Tururu one could imagine that a 6 degree monoblock for 13-19E on a 80 cm dish could also be used for receiving 13-16E (3 degree arc difference) but then on a 160 cm dish (if that has the same f/D).

By the way, naming a monoblock (as manufacturers do) after arc-difference is misleading, because not the arc difference is important, but the angle-difference (that equals about the azimuth, but not quite, by the way) on the site on earth you are at. As everybody knows, I guess, the angledifference (azimuthdifference) of certain satellites varies on various places on earth.

Greetz,
A33
Note I said "optimal" performance. I assume (have not actually measured it) that monoblecs are designed witha slight "toe in" of the wave guides - appropriate for the dish size and satellite separation). By "toe in" I mean that the centres of the wave guides are closer together at the nearest point to the dish than at the farthest on fixed separation monoblocs. Obviously this toe in is set for a particular dish size and separation. I believe the "signal acceptance angle" of these waveguides will allow their use on the "incorrect" size dishes. From memory of my adjustable the body of the lnb that they slide apart on is not prefectly straight but slightly curved to increase this "toe in" with increased separation.
 

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And have you checked pictures of the Visiosat BiSat G3D-bracket with 4 LNBs? What do you say about that, then?
Well...
The Visiosat BiSat is a special case as it has been designed for multiple LNBs. I suspect that for each LNB it's effectively around a 60 - 70cm dish.

Can you confirm that you have measured the angle between the feedhorns in various monoblocks and that they are not parallel, and also different for monoblocks for 80 cm dishes and 64 cm dishes?
Maybe that needs clearing up first. Now we are talking along non-touching lines.
No I haven't. I have to hope and assume that the angle is designed in.

Has anyone actually measured it? Maybe this whole conversation is based on an a false supposition!

Furthermore we don't know, if that dish has the same f/D as a 'normal' 80 cm dish.
I did mention somewhere above that this is one of the basic suppositions.

And thirdly, a bigger dish has a narrower beamwidth, so the LNB feedhorns in the monoblock have to be more precise in their right spot; a bigger dish is 'less forgiving' for that.
Again, something I mentioned earlier. And supposed that would help a monoblock to work without any practical problem on a 60cm dish.

And thirdly, a bigger dish has a narrower beamwidth, so the LNB feedhorns in the monoblock have to be more precise in their right spot; a bigger dish is 'less forgiving' for that.
So if a monoblock for 3 degrees would be exactly fitting for a 80 cm dish on your location, then a 4,3 degree monoblock for a 80 cm dish would be fitting exactly on a ( 80 x 4,3 / 3 = ) 114,666 cm dish (with the same f/D) on your location for the 3 degrees separation.
However if there would be a misfit with the 3 degree monoblock on your location on a 80 cm dish, but the dish is 'forgiving', then you could still have reception on the 80 cm dish, but you would multiply (with the above kind of calculation; edit A33) the error for another dish size, with possible dramatic results (no reception).
You would then have to calculate the LNB distance from the effective focal distance of your dish and the angle difference between the two wanted satellites at your location, to see if there is a monoblock with about that calculated internal feedhorn-distance
I think that is agreeing with what I've said.

A problem of a monoblock is also, that after tilting the monoblock to its proper angle for the both satellites, you cannot adjust the skew of the LNBs anymore. Some monoblocks are preskewed, but the information about that is difficult to find.
Also something that I have wondering about. Complicated by the fact that Astra have pre-skewed their transmissions on 19E and28E.
 

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I think that is agreeing with what I've said.

So, if you are agreeing with me, then feedhorn 'aiming' within a monoblock is not very important, but feedhorn distance is. Glad that is solved :) .

The two monoblocks I have are in storage right now,
But if you look here, under 'specifications', you see that the tubes between feedhorns and 'body' of the monoblock are perfectly parallel: Inverto - IDLB-SINM62-MNOO3-8PP -
In the feedhorn themselves I never noticed an angle, but I never watched with much attention I must say. So I assume they are also straight-angled and parallel.

By the way. The f/D ratio of a dish itself is not really important in this matter; important is only the effective focal distance of the dish, see for instance: Adjusting dish to Hotbird using Sky mini dish (though I now know that Delta Azimuth isn't the exact needed figure for the needed distance calculation, but the 'angle difference' -- a similar calculation but with a slightly different outcome).

f/D is only important for the comparison of manufacturer's monoblock information, as manufacturers don't usually mention the relevant focal distance, and/or the azimuth/angle difference the monoblock is meant for. That is a pity, because these would be the real relevant parameters.
Giving the exact distance between the centres of the feedhorns would give the same information; luckily on some webpages (such as the one above) you can try and measure/deduct the distance yourself from the drawings.

Greetz,
A33

Addendum: Link to a post/picture of the squinting LNBs of the Viosiosat BiSat G3B bracket: visiosat bisat G4; 1 of 2 smalle 'alps' koppen? - Schotelpark - Sat4all . They are rreeaallllyy squinting, way out from the 'horizontal G-spot'....
The squinting is needed, because else the distance between the LNBs would be much too big. Nevertheless it works quite satisfactory ;) .
 
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