Question about LNB skew

[Eddie King]

Good morning,

hopefully one of the resident experts here might be able to help me.

I am installing a new Gilbertini 150cm Satellite dish as I am on the
very edge of the footprint of the Astra 2x UK Beam and at present, with
my 110cm Triax dish lose reception of some transponders in the evenings.

The online Sat.-Calculator states that I should have an LNB skew of
approx. -7° (minus 7°).

When standing IN FRONT of the dish, should I rotate the LNB in a
clockwise or in an anticlockwise direction?

I have been advised that in marginal reception areas adjusting for the
correct skew can provide a significant improvement in signal strength.

Thank you for any assistance.

[Andy Burns]

Eddie King wrote:

> When standing IN FRONT of the dish, should I rotate the LNB in a
> clockwise or in an anticlockwise direction?

Looking at mine, anti-clockwise.

[Woody]

Eddie,
Are you still in Celle or have you relocated? If you are still there
according to Dishpointer you should be at:-
Elevation 27.5
True azimuth 157.6 or magn 154.1
Skew -5.9 which it shows as anticlockwise.

IME setting up sat when on holiday using the instrument that I carry, I
find that skew affects the signal quality more than the signal strength.

[Brian Gregory]

On 22/02/2021 12:07, Woody wrote:
> IME setting up sat when on holiday using the instrument that I carry, I
> find that skew affects the signal quality more than the signal strength.

Yes. If the skew is wrong you'll get signals from the opposite polarity
degrading your signal to noise ratio.

--
Brian Gregory (in England).

[S]

To the OP,

Its worth investing in a spectrum analyser that can give you a carrier
to noise figure or even a constellation map so you can adjust the skew
to give you the minimum modulation error rate. (MER).

Satlink is worth looking at without paying the prices of Promax.....

[Brian Gregory]

AFAICS it's clockwise if you're to the West of the satellite (by
longitude of your location) and anti-clockwise if you're to the East of
the satellite.

[Brian Gregory]

On 22/02/2021 17:22, Brian Gregory wrote:
> AFAICS it's clockwise if you're to the West of the satellite (by
> longitude of your location) and anti-clockwise if you're to the East of
> the satellite.
>

Cancel that.
I was WRONG.
It seems to be more complicated than I thought.

[Eddie King]

Hi Woody,
yes still in the area although moved to a small village between Celle
and Hannover.

It is signal quality which is more important to me. My online calculator
states -6.63° but I find it very confusing and am unsure which way to
turn the LNB. I guess it will be suck it and see although I did find a
post on the interweb which said "standing in front of dish a negative
skew equates to turning clockwise".

Thanks all for your replies. And any further comments.

[Eddie King]

I do have a Satlink analyser but not sure how to interpret the
constellation screen. Is it important to get all the dots in the 4
quadrants as close together as possible?

[R. Mark Clayton]

The satellites are in an arc, so as you look at them the ones to your east will be turned anti-clockwise and the ones to your west clockwise.

You need to turn the LNB the same way allowing for how many reflections you have (refractor 0;prime focus 1; Gregorian (like mine) 2), and then if you are facing the dish again because you are looking towards the dish not the satellites.

The objective is not to improve the signal of the polarity that you do want (which will change very little with misalignment), but to nullify the signal in the other polarity of the next transponder, which is half way between adjacent channels of the same polarity and will cause a lot of interference even for slight misalignment if this is not done.

[Eddie King]

Excellent, many thanks, learnt something new. Will have a play tomorrow.

[williamwright]

On 22/02/2021 10:04, Eddie King wrote:

Really you should ignore numbers and adjust it whilst checking signal
quality.


Bill

[Brian Gregory]

On 22/02/2021 18:34, Eddie King wrote:
> I do have a Satlink analyser but not sure how to interpret the
> constellation screen. Is it important to get all the dots in the 4
> quadrants as close together as possible?

Yes, four perfectly still dots would indicate an impossibly perfect QPSK
signal.

When there is a poor signal to noise ratio the dots typically look like
they are wiggling about or are spread thinly over an area rather than
just being more or less fixed points.

[Brian Gregory]

> The objective is not to improve the signal of the polarity that you do
> want (which will change very little with misalignment), but to nullify
> the signal in the other polarity of the next transponder, which is
> half way between adjacent channels of the same polarity and will cause
> a lot of interference even for slight misalignment if this is not done.

Actually with digital they don't even seem to bother with the halfway
between thing that they did with analog satellite TV.

For instance there are separate horizontal and vertical signals both on
11344MHz on Astra 2F.

[Andy Burns]

Brian Gregory wrote:

> Actually with digital they don't even seem to bother with the halfway
> between thing that they did with analog satellite TV.
>
> For instance there are separate horizontal and vertical signals both on
> 11344MHz on Astra 2F.

My tuner picks them up as 11343 V and 11344.5 H
but kingofsat lists them as 11344.5 V and 11344 H

But it's not unknown for my tuner to think it's 250 or 500kHz different.

[Brian Gaff (Sofa)]

I have really never got my head around skew, but it has to have something to
do with the angle of the sat from where you are I suppose and the last
thing you want is for the opposite polarisation to come in and interfere
with the signal.
It always amazes me how well this works considering the large lump of
earths atmosphere with its variability's in between you and the sat
depending on the angle.
Brian

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
bri...@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!
"williamwright" <wrights...@f2s.com> wrote in message
news:i9igka...@mid.individual.net...

[S]

for QPSK...

this is the best graphical view I can find explaining how to interpret a
constellation map......


https://www.ntt-review.jp/archive_html/201804/images/ra2_fig06.jpg


for 8PSK, they have 8 points rathetr than 4 (more common for DVB-S2
transponders)


https://www.promaxelectronics.com/assets/images/news/407-constellation8PSK.jpg

[PeterC]

That's what I do - saves trying to understand the more technical points.
I also use 'rock tuning' - adjust to max. quality, mark that point, carry on
until the quality drops, another mark, go back to the same level of drop,
mark that, then finess it between those points. Do the same for the other
adjustments.
Used to do that when calibrating pyrometric instruments to MoD standards, as
the analogue resistance load had it's own hysteresis. Digital version was
easier but I still checked the drop off to each side.
--
Peter.
The gods will stay away
whilst religions hold sway

[Brian Gregory]

Yes I don't know if they are exactly the same frequency but they
definitely overlap more than just the edges because they're around 20-30
MHz wide.

[Brian Gregory]

On 23/02/2021 07:48, Brian Gaff (Sofa) wrote:
> I have really never got my head around skew, but it has to have something to
> do with the angle of the sat from where you are I suppose and the last
> thing you want is for the opposite polarisation to come in and interfere
> with the signal.
> It always amazes me how well this works considering the large lump of
> earths atmosphere with its variability's in between you and the sat
> depending on the angle.

Yes. You don't need an amazingly good S/N ratio for digital satellite TV
but I think it must be reliably getting at least 10dB ratio between
wanted and unwanted polarization because you need some extra noise
margin for the actual ordinary noise.

[Brian Gaff (Sofa)]

It is really amazing how well it works. I don't have it here, but people who
do only seem to suffer if there is a big thunderstorm in the way. I'd guess
perhaps a bigger dish might help there.

Brian

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
bri...@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!

"Brian Gregory" <void-invalid...@email.invalid> wrote in message
news:i9ksl7...@mid.individual.net...

[Brian Gregory]

Yes. Good diagrams.

Just note that the numbers shown in the first one seem to be for some
other application of QPSK and as far as I can see are not at all what
you might expect with satellite TV.

[NY]

"Brian Gregory" <void-invalid...@email.invalid> wrote in message

news:i9nikf...@mid.individual.net...
>> https://www.promaxelectronics.com/assets/images/news/407-constellation8PSK.jpg

Spot graph (d) with an axis labelled "Wavelength (THz)". Er no - either it's
wavelength in (fractions of) a metre or else frequency in (multiples of) a
hertz. And come to think of it, frequencies of around 200 THz are *very*
high. I thought satellite frequencies were around 10 GHz.

[Brian Gregory]

On 24/02/2021 10:42, Brian Gaff (Sofa) wrote:
> It is really amazing how well it works. I don't have it here, but people who
> do only seem to suffer if there is a big thunderstorm in the way. I'd guess
> perhaps a bigger dish might help there.
> Brian

What amazes me the most is the speed with which an experienced installer
can aim the dish correctly and fix it firmly in place so that it'll
probably stay pointing in the right direction for quite a few years. The
margin for error must be about a degree since the satellites are only
spaced apart in the "Clarke belt" by around 3 to 4 degrees.

The brackets that hold satellite dishes in place don't seem to have
anything special about them that would help you manage to tighten all
the bolts without messing up the aim of the dish.

[Brian Gregory]

It's probably something to do with some kind of high speed signalling
via fibre optics.

Yes most satellite TV including Sky UK is in the 10.7 - 12.75GHz range
of a normal "universal" LNB.

[williamwright]

On 24/02/2021 18:18, Brian Gregory wrote:

> The brackets that hold satellite dishes in place don't seem to have
> anything special about them that would help you manage to tighten all
> the bolts without messing up the aim of the dish.
>

No, they can be very annoying.

Bill

[S]

Yes that is correct, but constellation maps can be used on QPSK or 8PSK
signals irrespective of signal frequency.

Basically a QPSK signal is demodulated to 4 phase angles which in a
perfect world would be at 45, 135, 225 and 315 degrees and also with
equal amplitude (distance) from the centre.

No demodulation is exact so you never get the ideal perfect 4 "dots" but
rather a set of 4 "clouds" but with robust error correction, you can
regenerate a usuable QPSK signal provided the S/N, C/N and BER are
within accetpable limits (they are dependent on tx power, FEC & Symbol rate)

If those "clouds" increase too much in size, then some of the
demodulated bits then get too close to being mistaken for another phase
angle so some bits at 45 get msitaken for being at 135 or 315 degrees....

[Indy Jess John]

On 24/02/2021 18:18, Brian Gregory wrote:

> The brackets that hold satellite dishes in place don't seem to have
> anything special about them that would help you manage to tighten all
> the bolts without messing up the aim of the dish.

That might be true for the Sky dish. I bought a Fortec Star dish and
installed it myself. I could position it and tighten the bolts so that
they were stiff but I could do fine adjustments by hitting the dish with
the heel of my hand until I had the perfect position, then I fully
tightened the bolts. It didn't take long and it has never moved, even in
gale force winds.

Jim

[PeterC]

Same here - Fortec 65cm dish, put bigger bolts into the wall (supplied ones
were small for a Sky dish) about 2010 and it's been stable since.
One major factor in aligning it was that I could see the dish on the
alignment site - got a pretty good line from that!

[Brian Gregory]

On 23/02/2021 09:22, PeterC wrote:
> I also use 'rock tuning' - adjust to max. quality, mark that point, carry on
> until the quality drops, another mark, go back to the same level of drop,
> mark that, then finess it between those points. Do the same for the other
> adjustments.

It used to be that one of the satellites was at 28.5°E and the other two
were at 28.2°E.

I find conflicting information on the web about what the situation is
now that the last Eutelsat at that position has died and been replaced
by an Astra satellite.