Why is this LNB setup working?

[archive10]

Hello,

Todays little challenge:

I don't quite understand why the LNB mounting with my Viasat-supplied Telesystem PE57 works. The LNB is a Sharp BS1R8EL400A that came with the dish.

As can be seen from the pictures:

1) the dish is not circular, and nothing on the package or internet suggests that the feedhorn is adapted to the oval shape (as e.g. LNBs for Sky minidishes are).

2) the LNB is clearly not pointing to he centre of the dish. At all.

There are no adjustment means for the LNB holder - it can only go on in that angle.

With a "normal" round and direct view of an offset dish, the LNB in this configuration seems to be covering quite a lot of non-dish, and probably missing other parts of the dish (and/or getting a good chunk of LNB holder as well).

Either the dish is incredibly badly designed (unlikely), the LNB is highly customised for this odd-shaped dish (more likely), or there is something I have missed entirely on how offset dishes work (also likely)...

Googling the Sharp LNB seems to show that it is standard quad-LNB, no special letters or markings as with LNBs for Sky minidishes. And you can buy the dish without an LNB, with no special instructions for choosing one. Which does not support the idea that this is a specialised LNB (e.g. with special feedhorn).

Unfortunately I have not had sufficient opportunity to take the feedhorn cover off and physcially check, as the LNB and dish is in more or less constant use

It should be noted that it works remarkably well, I get something like 74 dbuV and C/N of 11-12 from main channels on 1W. And it feeds to separate boxes over individual 30-35 meters of cable with no problem.

What do ye gurus think?

 

[Analoguesat]

For perfect matching the scalar rings inside the lnb feedhorn cover would be oval shaped (like they are on Sky UK dishes).

LNB's with round scalar rings would not illuminate the dish correctly and would see "behind" the dish - potentially allowing noise into the sysytem.

However modern satellites are so high powered mismatches are less critical than they used to be.

I use a Sky UK lnb offset on a 1m circular dish aimed at 16E to give me 23.5E and it works very well.

 

[Robbo]

Looking at your photos, it seems that the LNB is centred on a point above the centre of the dish. I would guess that where the LNB illuminates area that is not the dish,it is looking at the sky, but the bottom of the dish is properly illuminated. The sky is less noisy than the ground, so maybe why performance is OK.

 

[rolfw]

It is probably not critical where the LNB appears to be pointing, the main thing is that the dish focusses the signal at an area within the feedhorn, sitting the LNB at an angle may effectively modify the feed horn shape to match the dish shape.

 

[archive10]

For perfect matching the scalar rings inside the lnb feedhorn cover would be oval shaped (like they are on Sky UK dishes).

LNB's with round scalar rings would not illuminate the dish correctly and would see "behind" the dish - potentially allowing noise into the sysytem.

However modern satellites are so high powered mismatches are less critical than they used to be.

I use a Sky UK lnb offset on a 1m circular dish aimed at 16E to give me 23.5E and it works very well.

Ah! well spoken indeed. I think will try and see if I can find a moment during the upcoming weekend to take of the white cap and see what's inside the LNB. But your answer confirms my suspicion that there probably isn't much special about the LNB... We shall see.

 

[archive10]

Looking at your photos, it seems that the LNB is centred on a point above the centre of the dish. I would guess that where the LNB illuminates area that is not the dish,it is looking at the sky, but the bottom of the dish is properly illuminated. The sky is less noisy than the ground, so maybe why performance is OK.

Another good suggestion indeed. If you look carfully really though, it's pointing at the underside of my roof. Not sure if that's any quieter than the sky, but I suppose that the noise fears are less relevant as far as most of the dish is illuminated. Interestingly, the website of Telesystems says F/D is about 0.65, so a run-o-the-mill 0.6 LNB would see MORE than the dish (if my mind-math does not betray me) regardless of where it was pointing...

 

[archive10]

It is probably not critical where the LNB appears to be pointing, the main thing is that the dish focusses the signal at an area within the feedhorn, sitting the LNB at an angle may effectively modify the feed horn shape to match the dish shape.

Hadn't thought of that! Intriguing idea... get the focus point right, then tilt the LNB. But that would still leave the design open to noise from the non-dish area that the LNB can still see. But perhaps the noise-issue is less these days with high-power satellites as analoguesat writes.

 

[archive10]

Experimental update:

While having the screwdriver and the meter out the other week, I checked if simply tilting the LNB to point at the centre would make a difference, and if changing the LNB would make a difference.

In short, tilting the LNB to point at the dish centre gave me consistantly 1.7 dB more signal but varying degrees of better MER. Swapping the Sharp for an Inverto Black Ultra Single gave me more strength, but similar MER. Weird that it's not pointing there from the start; I can see no mechanical reason for it.

Still haven't ripped the plastic lid of the Sharp (in constant use by SWMBO - so no room for screwups), but it seems very likely that it is a round feedhorn in there...

 

[ralphmagno]

hello,
seen the pictures.the dish is a reflector and is like a lense.
the area reflects the microwave energy to the lnb.
the antenna is in the lnb and is the size of a nail head.
so the greater the area the more the gain.
thats why an 1 meter offset has a better gain that a 1 meter prime.
even though they are 1 meter the offset has slightly more surface area.
there is also a dead spot in the center of a prime dish but none in the prime focus..
the lnb can be twisted in the holder to offset the satellite's tilt in space.
ralph

 

[archive10]

Hi Ralph,

thanks for you post, informative and very valid, but my point is that the Telesystems PE57 has a strange mounting of the LNB.

When assembled correctly (and there is very little room for variation here!), the LNB is not pointing to the center of the dish in the vertical diretion.
It is pointing above halfway between the center and top rim of the dish.
This loses out on parts of the signal (although only about 1.7 d, and is a bit strange.
This is even clearly visible on the website of the mfg.

_http://www.telesystem-world.com/web/it_en/electronic/oem/58/fibre-line/197/satellite-antenna.html

My current guess would be that it's a reuse of an arm design from a non-oval dish, which would mean the LNB was pointing at the correct point if the dish was a bit taller. (But if it still works for customer, and is cheapest option for provider to give to customer, then it woudl make sense).

 

[archive10]

My current guess would be that it's a reuse of an arm design from a non-oval dish, which would mean the LNB was pointing at the correct point if the dish was a bit taller. (But if it still works for customer, and is cheapest option for provider to give to customer, then it woudl make sense).

AHA! Should've taken the next logical step. There is indeed a "normal" offset dish in the Telesystem range where the arm is used AND points the LNB to the center of the dish:

_http://www.telesystem-world.com/web/it_en/electronic/oem/58/fibre-line/200/satellite-antenna.html

So there, it's simply a matter of "it'll do"...

_http://www.telesystem-world.com/web/it_en/electronic/oem/58/fibre-line/200/satellite-antenna.html