Aligning linear polarisation with only the wanted signal being observed and measured
If you are trying to align linear polarisation and only have the wanted signal being measured it is definitely not possible to align the polarisation with sufficent accuracy by adjusting for maximum signal strength, as the maximum is very wide.
One possibility is to misalign the polarisation by a considerable amount till a significantly lower level, but accurately recordable level, is observed. Record the this level accurately and also mark the angle on the feed throat. Then repeat the process on the other side of the maximum. Then carefully rotate the feed to a final position half way between the two marked positions. At this point, for a receive only antenna you are finished. If it is transmit antenna you can now contact the network operations center (NOC) and ask for permission to transmit. They will ask you to transmit a low level CW (unmodulated) carrier and they will observe what cross-pol interference you are causing. Hopefully this will be acceptably very low but they may ask for small adjustments for you to make. If they do ask you to make manual adjustments do it under phone contact with the NOC and be very patient and be prepared to wait for the NOC to make accurate measurements, which each may take several seconds to stabilise and record.
If you have a spectrum analyser and can observe multiple carriers over say 70 - 200 MHz you can try boldly rotating the feed and watching for cross polar carrier levels (typically visible inbetween your wanted polarisation carriers), which will rapidly drop to zero as you go through the sharp cross-polar null. This is not always the case if there is litle or no traffic on the opposute polarisation.
If you have a spectrum analyser and can observe the satellite beacons then prepare a note of the status of the various beacons and their frequencies. From this you can determine what polarisation you are using and also verify the satellite identily!. For example, a satellite might transmit one circular polarisation beacon on one freqnbecy and one linear polarisation beacon on another frequency.
Never transmit until you are certain of the satellite identity, your polarisation is approximately right and you are in phone contact with the NOC for permission so they can see your signal come up in the right place. If in any doubt or loss of phone link, turn off your transmit right away.
Feed systems vary. Many types comprise a linear polarisation OMT that combines two rectangular waveguides one TX, the other RX) into a single short circular or square cross-section waveguide. Between this and the feed horn is a polariser tube which contains a row of slots, pins or similar devices. This line is set at 45 deg to the linear polaristions of the OMT. Depennding on which way (+45 deg or -45 deg) the OMT to polariser junction is set make the feed operate clockwise transmit or anticlockwise transimt and the opposite on receive.
Which circular polarisation you are set to will be confusing. Not that one way will work perfectly and the other way no at all. So if you cant find the wanted signal at all then try the opposite polarisation. Confusion is common due to the polarisation changing each time the signal is reflected (once for frond fed antennas, twice for Cassegrain of Gregorian geomentries. Documentation is often not clear about the +45 or -45 deg and scales may be marked 0 - 360 or +/-180. Don't worry, just try one way is if no good try the other. Once it works document or photograph carefully so you can repeat at other sites.
My experiences of interference in satellite communiations include:
Interference to and from nearby satellites
Interference to and from nearby satellites in the geostationary orbit. To mitigate this antennas, both transmit and receive, need to have low sidelobe levels. International agreements and bilateral intersystem coordination agreements restrict off-axis eirp limits for transmit antennas.
Interference from terrestrial sources
Interference from terrestrial sources such as nearby microwave towers and radar are examples. Also much closer serious interferers can include neon signs, hand held cell phones and walkie-talkies, even clicks / spark noise bursts from air conditioning motors. Hiding a dish using site shielding is sometimes necessary.
Sun noise
Around the spring and autumn equinoxes, as the sun daily moves across the sky it will briefly go behind the satellite. During this time (up to several minutes) high noise levels occur. The sun appears a noise source at approx 8000 deg K.
Atmospheric effects
If you attempt satellite communications with satellites at low elevation angles the signals vary in level, called scintillation. It is exactly the same phenomena as the twinkling of a star near the horizon. It is caused by layers of different moisture and temperature content in the lower atmosphere or troposphere. The layers bend the radio signals rather like ripples on a water surface cause moving patterns of sunlight on the base of pool.
Rain and melting snow cause both attenuation of the signals and also increase in noise level due to the absolute temperature of the water.
Ionospheric effect
The polarisation of radio signals is rotated by the ionosphere. The problem is greater at lower frequencies like L band (~1.5 GHz) and is reason why circular polarisation is preferred at such frequencies.
It is about how to check that the rim of the antenna is accurately flat. Aim for an accuracy approaching 1/10th of the wavelength of the frequency in use.
You need the maximum depth of the dish as well as the two diameters. Put an accurately straight edge up/down the front, and measure the maximum depth as accurately as possible.
It it works let us know. The calculation method is a bit uncertain and it would be helpful to get feedback. wxw
