Satbeams, Channel Masters and real life obs.

So this is going to sound stupidly obvious but bear with me!

A Channel Master 1.2m, no matter what angle it is skewed over too on the arc the satellite will always be directly in line with the central LNB support arm, is that correct?

So here's why I'm asking, recently installed said dish at a friends house who has reception down to 75E. We've never actually checked where on the horizon it is other than looking on satbeams and that suggested it was clipping the rooftop of a nearby house. This evening we placed a laser along the central LNB support arm at a 0.7° elevation to confirm sat beams, this was to decide if it was worth moving the entire installation for more free space under 75E.

The test showed the actual direction was a full 5° further south than satbeams indicates, so much so that the signal is being partially obscured by a tree in his garden. I realise I may of been slightly out lining up the laser with the arm but there's no way would it be this far.

So either my reckoning that the central LNB arm is inline with the satellite is incorrect or satbeams can't be relied on, which one is it?

For what it's worth, what works out to be 1.5° east of 75E we are seeing a regular bump in the noise on 11670V, not enough to register any SNR but too regular to be random noise, unless of course it's a set of traffic lights 5 miles away.

Attached is a satbeams projection showing 75E in green, the red trace is what we observed assuming the dish arm theory is correct.

aceb said:

A Channel Master 1.2m, no matter what angle it is skewed over too on the arc the satellite will always be directly in line with the central LNB support arm, is that correct?

Click to expand...

Yes.
But what you may have to observe in this case is that the "beam width" of the CM120 is 120+ cm wide.
This means that the beam that the CM receives is as wide as the dish.
So ignoring trees and other obstacles, this beam will be clipped by the horizon at some point.
But when nearing the horizon, the height of the dish mount makes a difference (ignoring atmospheric absorption for a minute).
So the ground clerance really depends on where the dish is mounted - ground level (where I think satbeams and dishpointer is based), or mounted on the roof (which is different and much better).

HOWEVER, I have previously found on-line sighting sites (dishpointer) to be out several degrees when aligning dishes.
I cannot rule out that something similar happens here.

aceb said:

A Channel Master 1.2m, no matter what angle it is skewed over too on the arc the satellite will always be directly in line with the central LNB support arm, is that correct?

Click to expand...

Would you mean by this that you use a diseqc motor, with a bend in the rotating axis?
That would complicate things, I guess. However at this hour of night I cannot reason straight, I'm afraid.
A picture of the dish setup would be helpful, to visualize.

I've seen posts about dishpointer map-directions being wrong at some places, too, as @st1 mentioned.
Don't know the cause of that ~~ might be map projection matters?

greetz,
A33

aceb said:

So this is going to sound stupidly obvious but bear with me!

A Channel Master 1.2m, no matter what angle it is skewed over too on the arc the satellite will always be directly in line with the central LNB support arm, is that correct?

So here's why I'm asking, recently installed said dish at a friends house who has reception down to 75E.

So either my reckoning that the central LNB arm is inline with the satellite is incorrect or satbeams can't be relied on, which one is it?

For what it's worth, what works out to be 1.5° east of 75E we are seeing a regular bump in the noise on 11670V, not enough to register any SNR but too regular to be random noise, unless of course it's a set of traffic lights 5 miles away.

Click to expand...

Somewhere on the forum is a PDF of the CM1.2 line drawing which should confirm if the offset angle is the same as the angle that the support arm, though I cannot find it on the screen here.

I believe the lowest angle that can be observed with an installation at gutter level to horizon is 76 degrees, taking into account the small dips/rises in land, though this is from London, there will be a variation in Sussex owing to a different latitude and look angle to the arc.

If you are seeing just a 'bump' in the noise with a 1.2m, then realistically you should be persuading your friend to upgrade to a 2.4m as a minimum to achieve a lock on whatever traffic light is out to the East, add a quality elevation motor to the mounting system to fine tune, and invest in a rotating system for the feed.

Your big problem is ground noise. The CM 1.2 has a beam width of 1.5 degrees so the ground will be in the main beam. Ground noise will be to add ~5dB or so to your noise floor i.e. it will be like using a 3dB NF LNB . Going to a larger dish won't help unless you can get the beam width down a long way (and it must have low side lobes) as you will bring up the noise as well as signal. A 2.4m CM might help a bit but even that has a -3dB beam width of .7 deg. At low elevations you will get large variations with time of day and weather. There may also be variations in skew so, as CH says, you need to be able to tweak it.

The 2nd plot here shows the noise increase when a dish is pointed at the ground K5SO home page

1.2CM manual attached.

Another thing - there may be changes in the perceived location of the satellite due to the long path through the Earth's atmosphere and its variability.

Channel Hopper said:

Somewhere on the forum is a PDF of the CM1.2 line drawing which should confirm if the offset angle is the same as the angle that the support arm, though I cannot find it on the screen here.

Click to expand...

Pointed my 1.2 CM to 4.8º E , elevation for my location 30.7º the angle of the lower feedarm is ± 24º

Trust1 said:

Pointed my 1.2 CM to 4.8º E , elevation for my location 30.7º the angle of the lower feedarm is ± 24º

Click to expand...

That would tie in with the geometry of the older Type 121 antenna, where the feed is not ponting exactly at the satellite but is slightly lower at 23 degrees.

Channel Hopper said:

Somewhere on the forum is a PDF of the CM1.2 line drawing which should confirm if the offset angle is the same as the angle that the support arm, though I cannot find it on the screen here.

Click to expand...

On normal CM 1.2m lower arm can not have same angle as an offset angle. It is couple of degrees lower. IMHO.

RimaNTSS said:

On normal CM 1.2m lower arm can not have same angle as an offset angle. It is couple of degrees lower. IMHO.

Click to expand...

We know

Oops, look like I've confused things by poor wording. I don't mean the elevation angle on the CM, I'm referring to the azimuth angle, so no matter what angle the dish is leaning over on the polar mount, 75E in this instance the satellite will be in the same azimuth direction as the lower LNB support arm. I wouldn't think to question it except the satbeams data is so far out and what we observed puts other satellites up to 66E behind a tree but he gets reception from 74/72/70 and 68.

I've attached another photo taken from the dish looking down the length of wood we used to show both azimuth and elevation, the green circle is where satbeams puts 75E. The branches directly in front of the view of 75E are no longer there ;)

Thanks for the interesting replies about low angle reception, as much as he would love to put up a 2.4M (it's about 10ft AGL into the back garden on a sloping plot) it would need major structural work to stop the dish falling through or ripping the entire roof off when it gets windy, and it really does get windy up there.

The plan now is to move the entire installation 1.5m to the NE at some point to maximise the window between roof and tree, plus a little more pruning!

A path to a satellite at 75 dgrees East is not an angle at 75 degrees from your longitude, it will be approximately a couple of degrees less.

Give me a town/latitude and I can work it out.

aceb said:

Attached is a satbeams projection showing 75E in green, the red trace is what we observed assuming the dish arm theory is correct.

View attachment 111427

Click to expand...

The green line would be about 102* Azimuth, which combined to the elevation of 0.7* would be right for the sussex area.
Your red line would be around 106* azimuth.

What is the magnetic azimuth (compass bearing) you measure? (Dishpointer sais magnetic deviation <1 degree in sussex area).

The arm theory btw would only be correct, when distance from left side of dish to end of the arm is equal to distance from right side of dish to end of arm. Have you measured that?

However my guess would still be that the cause would be map projecting issues: the map not being right.

Greetz,
A33

Using the sun method, to aim and to check Line Of Sight, would also be helpful.
(I use this calculator for that: Satellite Look Angle Calculator )

However, I guess the sun is now BELOW the clarke belt for sussex, so checking LOS to satellites near the horizon is a no go at the moment. You're a bit late in the season, alas.....

greetz,
A33