Dealing With Cross-Polarization (Xpol) or Mispointed Antenna

Cross-Polarization (Xpol) or Mispointed Antenna

Objective: Restore orthogonality between polarizations and clean up cross-pol interference caused by incorrect feed or antenna alignment.

Typical Symptoms

Satellite operator or NOC reports:

• High XPI or Xpol leakage from your terminal or beam

You see:

• Lower C/N with a ghost of the opposite polarization on the same frequency

Root cause is usually misaligned ground antennas or poor installation.

Immediate Actions (0-15 Min)

• Mute or back-off suspected offending uplinks if possible
• If you have a beacon or test carrier:
  • Lock on to it and freeze all non-essential changes until pointing and polarization are verified

Investigation (15-120 Min)

Step 1: Verify polarization settings

Confirm configured polarization (H/V, LHCP/RHCP) in:

• Modem
• Upconverter or RF chain
• Antenna feed assembly

Step 2: Xpol alignment procedure (linear pol)

Follow sat-operator guided peak and pol process:

• Peak antenna in azimuth and elevation
• Rotate the feed or pol angle to maximize desired pol and minimize orthogonal pol—typically targeting greater than 30-40 dB isolation on the satellite side

Step 3: Hardware checks

Inspect:

• Feed horn, polarizer and OMT for mechanical shift, corrosion, water ingress
• Mounting hardware for mechanical slop (wind-induced rotation)
• Any recent work at the site (tower climb, feed swap, repaint)

Mitigation and Recovery

Once correct polarization is confirmed:

• Re-enable uplinks at reduced power, then step up while the operator monitors XPI and ASI
• For sites that chronically drift, add mechanical locking (better clamps, anti-rotation brackets) or replace weak mounts

Prevention

Make Xpol verification a hard requirement for:

• New installs
• Moves or re-points
• After any tower work or feed service

Use documented procedures similar to those recommended by operators and equipment vendors for uplink and downlink polarization alignment.

Periodically run network-wide Xpol audits, prioritizing:

• Sub-1 m dishes
• Mobile or maritime terminals
• Harsh environments (high winds, extreme temperatures)