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[Technical article, rfforum Editorial Team]
Most GNSS and many satellite links use circular polarization. The reason is not fashion: it keeps the link working when the relative orientation of the two antennas is unknown or changing, and it rejects multipath that flips the sense of rotation.
1. What circular polarization is
A circularly polarized wave has equal orthogonal E-field components in phase quadrature. Looking toward the incoming wave, right-hand circular (RHCP) rotates clockwise by IEEE convention (note that some optics texts reverse this). Left-hand circular (LHCP) rotates the other way. GNSS L1/L2/L5 and most GPS/Galileo/BeiDou signals are RHCP.
2. Why it matters on the link budget
A perfect RHCP antenna receiving a pure RHCP wave has 0 dB polarization mismatch. The same antenna receiving a pure LHCP wave has theoretically infinite rejection (in practice 15–25 dB for a good patch). A linearly polarized antenna receiving circular polarization always loses 3 dB, because it only couples to one of the two equal field components. That 3 dB is free if you use the matching sense of CP.
3. Axial ratio
Real antennas are never perfect. Axial ratio (AR) is the ratio of the major to minor axis of the polarization ellipse, usually in dB. AR = 0 dB is perfect CP; AR = ∞ dB is linear. A common datasheet claim is AR < 3 dB over the band of interest. Polarization mismatch loss between two antennas both with finite AR is larger than the ideal 0 dB even when the senses match; a rough rule is that two antennas each at 3 dB AR can cost about 0.5–1 dB extra compared with perfect CP.
4. How to generate CP on a patch
Two common approaches on a single-feed square or nearly-square patch: truncate two opposite corners, or add a diagonal slot, so that the two degenerate modes split slightly in frequency and sit in quadrature at the design frequency. Dual-feed patches with a 90° hybrid give better AR bandwidth but need the hybrid network. For AR bandwidth of more than a few percent, stacked patches or sequential rotation arrays are usual.
5. Multipath
Reflection from the ground or a metal surface often reverses the sense of circular polarization. An RHCP receive antenna therefore rejects a large part of the odd-bounce multipath. That is one reason GNSS receivers prefer RHCP even on handheld devices where orientation varies.
Discussion: for a single-feed GNSS patch, what AR bandwidth do you actually measure once the antenna is on your product plastic and ground plane, compared with the free-space simulation?
— rfforum 编辑部 / Editorial Team
Most GNSS and many satellite links use circular polarization. The reason is not fashion: it keeps the link working when the relative orientation of the two antennas is unknown or changing, and it rejects multipath that flips the sense of rotation.
1. What circular polarization is
A circularly polarized wave has equal orthogonal E-field components in phase quadrature. Looking toward the incoming wave, right-hand circular (RHCP) rotates clockwise by IEEE convention (note that some optics texts reverse this). Left-hand circular (LHCP) rotates the other way. GNSS L1/L2/L5 and most GPS/Galileo/BeiDou signals are RHCP.
2. Why it matters on the link budget
A perfect RHCP antenna receiving a pure RHCP wave has 0 dB polarization mismatch. The same antenna receiving a pure LHCP wave has theoretically infinite rejection (in practice 15–25 dB for a good patch). A linearly polarized antenna receiving circular polarization always loses 3 dB, because it only couples to one of the two equal field components. That 3 dB is free if you use the matching sense of CP.
3. Axial ratio
Real antennas are never perfect. Axial ratio (AR) is the ratio of the major to minor axis of the polarization ellipse, usually in dB. AR = 0 dB is perfect CP; AR = ∞ dB is linear. A common datasheet claim is AR < 3 dB over the band of interest. Polarization mismatch loss between two antennas both with finite AR is larger than the ideal 0 dB even when the senses match; a rough rule is that two antennas each at 3 dB AR can cost about 0.5–1 dB extra compared with perfect CP.
4. How to generate CP on a patch
Two common approaches on a single-feed square or nearly-square patch: truncate two opposite corners, or add a diagonal slot, so that the two degenerate modes split slightly in frequency and sit in quadrature at the design frequency. Dual-feed patches with a 90° hybrid give better AR bandwidth but need the hybrid network. For AR bandwidth of more than a few percent, stacked patches or sequential rotation arrays are usual.
5. Multipath
Reflection from the ground or a metal surface often reverses the sense of circular polarization. An RHCP receive antenna therefore rejects a large part of the odd-bounce multipath. That is one reason GNSS receivers prefer RHCP even on handheld devices where orientation varies.
Discussion: for a single-feed GNSS patch, what AR bandwidth do you actually measure once the antenna is on your product plastic and ground plane, compared with the free-space simulation?
— rfforum 编辑部 / Editorial Team
