AR Measurement in Waveguide Polarizer | Axial Ratio Explained

In the context of waveguide polarizers, AR stands for Axial Ratio. Let us evaluate its role in circular polarization and how it is used to assess polarizer performance across frequencies.

What is Axial Ratio (AR)?

The axial ratio is a measure of purity of circular polarization. It is defined as ratio of major axis to minor axis of the polarization ellipse of electromagnetic (EM) wave.

• An ideal circularly polarized wave has axial ratio of 1 (or 0 dB).
• An elliptically polarized wave has axial ratio greater than 1.
• A linearly polarized wave has an infinite AR.

AR measurement in waveguide polarizer

A vector network analyzer (VNA) with circular or dual linear feed antennas is often used to extract the AR from measured S-parameters. Following points summarize why AR measurement is performed in different types of waveguide polarizers such as septum, iris or dielectric slab polarizers.

• To evaluate how well device converts linear to circular polarizarion or vice versa.
• To quantify deviation from ideal circular polarization
• To determine performance across frequency range

Key points

• AR measurement is plotted over frequency to show bandwidth over which polarizer performs well.
• For most applications, axial ratio below 3 dB is considered acceptable for circular polarization.

Summary

AR measurement in a waveguide polarizer is used to evaluate how well the polarizer generates or handles circular polarization, with an ideal AR being 0 dB. This measurement is crucial in high frequency RF systems like satellite communication, radar and microwave remote sensing, where polarization purity affects system performance.