Re: Receiver shows higher signal at edges of alignment

It is possible to measure an antenna pattern and see two equal peaks, but I've never seen it on an antenna as small as 80 or 100cm at Ku band.
The explanation, which is probably not relevent in this case, is as follows:
There is a single big main beam shaped like a bell or parabola, but surrounding this is the first sidelobe ring, typically 10 to 15 dB lower. Many lower level sidelobes rings exist further out.
If you make an antenna pattern cut across the main beam you see a picture like that on the left, below:

The above (left) is the normal situation and it is quite easy to work out where to move the antenna to peak up.
There is a special case however to be aware of, particularly with large antennas: You see two similar height 'main' beams, and this pattern is similar in both azimuth and elevation. See right hand picture. Why ?
The reason is that the nearest you are getting to the main beam is when you are in the central low trough (first null) between the main beam and the first sidelobe ring. When you 'peak up' you are in fact 45 deg away from the main beam and sitting on the top of the first sidelobe ring, in one of four possible 45 deg quadrant positions.
As you move, you simply cut through the first sidelobe ring in two places and miss the main beam altogether. The same applies when you do the azimuth or elevation cut. Think of potentially moving in a full square around the main beam, missing it all the way!
The cure is to put the antenna into the bottom of the null between the two approximately equal first sidelobe peaks and then sweeping the antenna at right angles. You will then get dramatic improvement as you find the main beam itself.

The above phenomena is very visible on large teleport dishes where the receive system is likely to work suprisingly well, even if the dish gain is 15 dB low, when peaked on the first sidelobe.

In the case of a small antenna having two such obvious peaks is a mystery to me.

I would try:

a. Check out the above (improbable) theory !

b. Check the front rim of the dish is flat using the crossed threads test. Pull the two threads tight and check they just touch in the middle.

c. Optimise the polarisation. Turn the feed one way and the other, until severely degraded and measured signal quality levels are noted. Record the angle. Repeat going the other way, and then half the angle between the two equally degraded points. The ASTRA satellite polarisation may differ from calculation (e.g. 7 deg error?).

Note:

Severe distortion of the dish shape could result in a double hump main beam.
Severe distortion of the dish shape or out of focus feed distance or feed position could result in high sidelobes and adjacent satellite interference, where the first sidelobe hits an adjacent satellite.
Poor polarisation adjustment could result in x-pol interference being severe on axis. Signal power level wou

Ignore the crossed strings test. Your dish is obviously designed such that the front rim is NOT flat.

The LNB arm may well have sagged down due to part broken bracket just below the dish. If this can be tightened up, and the LNB raised slightly, note that the beam will go down and the elevation will need adjusting.

Polarisation adjustment is the rotation of the LNB in its clamp. With a motor driven polar mount the LNB is normally set, and fixed, exactly upright when the motor is centralised (aimed at a due south satellite). Astra may be off by 7 deg so if you spend most time watching Astra it will help to optimise the LNB polarisation for Astra.

A polar mount requires considerable skill to set up and may take several days work to re-align, once it gets misaligned. There are three adjustments: two elevation adjustments, behind the dish itself and the motor axis angle. The clamp to the pole needs to be adjusted exactly north-south.

Take great care working on the antenna in view of risk of falling from height and risk of dropping tools on people below. If in any doubt get a local satellite TV supplier/installer to help.

Best regards, Eric.

Ok, the second graph you posted ("Starting just below main beam") is almost exactly what I'm seeing. I'll try to get the feed arm up a little higher and see what happens. I won't be on site there for a couple of days, but will post later on how it goes.

On Polar Mount Setup

I see how challenging polar mounts can be, even for an engineer (albeit in a different field). The whole thing took a weekend, for my first time. For the benefit of others, or if you want to point out if I did something wrong, here's what I did (at the risk of repeating information from elsewhere):

1. Read about polar mounts here: https://www.satsig.net/polmount.htm. Read about geostationary orbits and communications satellites on Wikipedia (https://en.wikipedia.org/wiki/Geostationary_orbit), for background. Found my longitude and latitude here: https://www.satsig.net/maps/lat-long-finder.htm.

2. Made sure the mounting pole is as vertical as possible using a level on multiple locations around it. It was.

3. Adjusted the elevation on the motor mount based on my latitude (mine had latitude markers on one side).

4. Adjusted the offset angle behind the dish. My original SG2100 motor had a 40deg crank, whereas DM3800 I now have has a 35deg crank, so the setting on the dish for me is 35deg - "polar mount dish offset tilt", where the offset tilt is available here: https://www.satsig.net/ssazelm.htm.

5. Found true south using a mobile phone compass (the iPhone one allows switching between magnetic and true south), making sure to calibrate it first and being careful about interference. Verified true south by using Google maps and a distant southern landmark. I used https://www.satsig.net/maps/lat-long-finder.htm. Rotated the entire motor/dish assembly towards true south as best as possible.

6. Set the receiver to USALS mode and input my longitude and latitude, so it can calculate where to point to other satellites for me. Told the receiver to go to Astra 3B (it already knew it's at 23.5degE).

7. Made small physical adjustments to the motor mount's azimuth and dish's declination angle until I found Astra 3B. This I knew by doing a channel scan. This took a while!

8. Tried to maximize the signal reported on my receiver by making iterative, very tiny adjustments to the azimuth and declination angles. For me, the center of the signal was surprisingly a minimum, which is why I posted in the first place.

9. Verified the motor could track to other satellites, like Astra 1L at 19.2degE. It could.

On Polarization

I can rotate the dish around with the motor and safely reach the LNB, so I'll try checking the polarization. I think I see what you mean, that the circular part of the LNB itself looks like it can rotate. and in fact, I think polarization should start properly set before adjusting the elevation and azimuth, right? Then it might need to be optimized later to maximize signal quality.

Thanks again..

In a polar mount the LNB polarisation is physically set to zero relative to the LNB clamp ring mark and a line up/down the middle of the dish.
When the motor is centralised and you are looking at the due south satellite then the polarisation angle will actually be zero.
When the motor turns either way the polarisation tilts automatically to the side, as the whole dish and LNB arm is tilted sideways as a whole.
The above works perfectly assuming all satellites have their vertical polarisations aligned north-south parallel with the earth axis or at 90 deg to the equator.
A few satellites do not follow this normality. Eutelsat and Astra may be off by 3 or 7 deg. Not sure.

Best regards, Eric

Before any arm bending of the LNB sort, I'll make a table of clicks vs signal and quality to see if it says anything.

I was actually initially having trouble picking up 11797. I thought I was pointed properly because I was receiving ČT2 (which according to https://www.skylink.cz/kontaktni-formulare#Jak+naladit+programy is supposed to be on the same satellite at 12525) but channels on the 11797 transponder, like the Golf Channel, weren't working. I still had to make a small elevation adjustment, then optimize the azimuth adjustment for 11797 to work, which it now does. Unfortunately I can't be more scientific than that because I was doing it by hand and not documenting my steps at the time.

It doesn't make sense to me that orientation adjustments would have to be made for different transponders on the same satellite, I thought. I then thought of Astra 3A, which is now at 23.7deg and in an inclined orbit, according to https://en.wikipedia.org/wiki/Astra_3A. Was Astra 3A still transmitting and confusing me somehow? I reckoned that no, they can't have both transmitting on the same frequencies at the same time. They probably wouldn't even split services between the two. And in fact, the line on Wikipedia that says "As of October 2012, Astra 3A is in an inclined orbit at 23.7degE with all services carried by the adjacent Astra 3B craft." suggests to me that Astra 3A is just a floater at this point.

So the mystery of how I could find an orientation to receive 12525 but not 11797 is not yet solved. But I know now to keep an eye out for adjacent satellite signals in case they also confused me, and think this will get sorted out...

Is it possible you are using the same boom and LNB for your new larger dish as you were for the old small dish? If so, would this be your problem? The "reflective" pattern from the larger dish could be different than the pattern from the small dish. Ed.