If the axial ratio of an electromagnetic wave is zero dB. Then, the wave is:
circularly polarized
What the axial ratio is. As a polarised wave travels, the tip of its electric-field vector traces an ellipse in the transverse plane. The axial ratio (AR) is the ratio of the major axis of that ellipse to its minor axis, so by definition \(AR \ge 1\). In decibels:
\(AR_{dB} = 20\log_{10}(AR)\)
Step 1 — convert the given 0 dB back to a ratio.
\(0 = 20\log_{10}(AR) \Rightarrow \log_{10}(AR)=0 \Rightarrow AR = 1\)
Step 2 — interpret AR = 1. Major axis = minor axis, so the ellipse degenerates into a circle: the field vector keeps a constant magnitude and simply rotates at the wave frequency. That is circular polarisation, which physically requires two orthogonal components of equal amplitude and 90° phase difference.
Step 3 — check the other polarisation states against the same measure.
| Polarisation | Axial ratio | In dB |
|---|---|---|
| Circular | 1 | 0 dB |
| Elliptical | between 1 and ∞ | 0 to ∞ dB |
| Linear (horizontal or vertical) | ∞ (minor axis = 0) | ∞ dB |
So the given 0 dB rules out the elliptical case (which needs AR > 1) and both linear cases (horizontal and vertical polarisation have an infinite axial ratio, since the field simply oscillates along a line).
Why it matters in practice. Circular polarisation is used in satellite links and GPS because the received signal is insensitive to the relative orientation of transmitter and receiver antennas and is immune to Faraday rotation in the ionosphere. Real "circularly polarised" antennas are never perfect, so their specification quotes an axial ratio of a few dB (typically AR < 3 dB) over the operating band.
Hence, an axial ratio of 0 dB means the wave is circularly polarised.
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