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[Technical article, rfforum Editorial Team]
A GNSS antenna that looks fine as a bare element can fail the axial-ratio (AR) gate once the plastic radome, paint, or bracket is on. RHCP gain may still look acceptable while AR has already walked into the "multipath magnet" region.
1. What the radome does
Dielectric loading detunes the two orthogonal modes differently. A slight imbalance between the 0° and 90° (or 45°/135°) paths turns a designed RHCP element into elliptical polarisation. Paint thickness, rib geometry, and metal fasteners near the patch edges are common culprits.
2. S11 is not AR
A clean return loss after retuning the feed does not restore AR. You can recentre |S11| and still have 6–10 dB AR at boresight. Treat AR (or cross-pol discrimination) as a separate sign-off metric with the production radome installed.
3. Bench AR check that catches it early
- Measure with the real radome and mounting, not a foam stand-in
- Use a known CP source or a spinning LP probe method; record AR vs elevation if the product claims roof/vehicle use
- Compare bare vs radomed on the same fixture so the delta is obvious
- Watch for AR that is fine at zenith and collapses at 30–60° elevation (common after asymmetric plastic)
4. Habit
Freeze a radome revision in the AR report the same way you freeze a PCB revision in the S-parameter report.
Discussion: at what AR (dB) do you fail a timing or survey GNSS antenna with radome on, and do you measure AR only at zenith or across elevation?
— rfforum 编辑部 / Editorial Team
A GNSS antenna that looks fine as a bare element can fail the axial-ratio (AR) gate once the plastic radome, paint, or bracket is on. RHCP gain may still look acceptable while AR has already walked into the "multipath magnet" region.
1. What the radome does
Dielectric loading detunes the two orthogonal modes differently. A slight imbalance between the 0° and 90° (or 45°/135°) paths turns a designed RHCP element into elliptical polarisation. Paint thickness, rib geometry, and metal fasteners near the patch edges are common culprits.
2. S11 is not AR
A clean return loss after retuning the feed does not restore AR. You can recentre |S11| and still have 6–10 dB AR at boresight. Treat AR (or cross-pol discrimination) as a separate sign-off metric with the production radome installed.
3. Bench AR check that catches it early
- Measure with the real radome and mounting, not a foam stand-in
- Use a known CP source or a spinning LP probe method; record AR vs elevation if the product claims roof/vehicle use
- Compare bare vs radomed on the same fixture so the delta is obvious
- Watch for AR that is fine at zenith and collapses at 30–60° elevation (common after asymmetric plastic)
4. Habit
Freeze a radome revision in the AR report the same way you freeze a PCB revision in the S-parameter report.
Discussion: at what AR (dB) do you fail a timing or survey GNSS antenna with radome on, and do you measure AR only at zenith or across elevation?
— rfforum 编辑部 / Editorial Team
