Issues with the cross-polarization of a remote VSAT site are quite common and can be caused by a combination of reasons. The satellite operator typically requires isolation of 30dB or greater to pass this test so that there is no interference on the opposite pol or any network related issues with poor performance.
Cross Pol Issues
The antenna is mis-pointed – A mis-pointed antenna could cause Adjacent Satellite Interference (ASI), high Tx power levels, and poor performance. Take the time to fine-tune the antenna with the help of the NOC and satellite operator during the cross-pol test.
RF feed assembly is damaged – Inspect the entire feed assembly for damaged parts, any waveguide joints are secured with no gaps, no corrosion inside the feed horn and the Mylar cover is intact with no holes.
An obstruction in the line of sight to the satellite – obstacle in the line of sight (LoS) to the satellite will affect the signals levels and the isolation. You may have to relocate the antenna if trees, buildings, masts, etc. are in the way. You should allow 10° of clearance to any obstruction. Check that there are no RF cables in front of the feed or antenna.
RF Feed not set correctly – Calculate the pol angle, also called skew, and set accurately as you can. When testing the cross-pol make tiny movements (1° or less) of the feed as the isolation value will vary significantly if you move the feed too much. Try and rotate the feed by 180° and try again. Also, check you have the correct feed assembly for the antenna model!
Inclement weather to the remote site – The cross-pol should be completed in clear sky conditions as clouds and rain could affect the isolation result.
The antenna is damaged or not constructed correctly – Look for signs of damage or distortion of the reflector.
- Cracks in the surface
- Cracks in the rim
- Mis-alignment of the seems
- Stressed seem fasteners
- Seems are flush and aligned
- Any inserts are in the correct location
- Any spacer washers are present and correct
- The antenna passes the ‘Crossed Line’ test
Crossed Line Measurement – The antenna face should be flat and true. An easy method of confirming the reflector is flat is by using two lengths of the fishing line crossed over the face of the reflector. The lines should perfectly be touching in the middle, with no gap. If they do not slightly touch, then the solution is to “birdbath” the antenna, loosen all the reflector hardware, and start again.
