# How is the Axial Ratio of a Polarizer Measured?

SAGE Millimeter often receives questions on measuring the axial ratio for polarizers. In this blog post, SAGE Millimeter (SAGE MM) reveals the step-by-step process of axial ratio measurement for its SAS series polarizers.

A Vector Network Analyzer (VNA), Orthomode Transducer (OMT), and Mode Transition will be used to test the axial ratio.

Figure 1 shows the port designations of an OMT. The OMT is a device which can conduct the linear polarized waveforms from its common port, the antenna port, to its designated ports or separate circular or elliptical polarized waveforms into two orthogonal linearly polarized waveforms at its designated ports. The common port of the OMT offered by SAGE MM is equipped with a square waveguide which can support linear, circular, and elliptical waveforms. The vertical and horizontal ports of the OMT are standard rectangular waveguides. The SAT series OMT covers full waveguide operations.

Due to the circular waveguide nature of the polarizer, a square to circular and a circular to rectangular waveguide mode transition is required. The transitions are shown in Figure 2 and Figure 3, respectively.

System Calibration:

1. Connect the Square to Circular (S/C) Waveguide Transition and the Circular to Rectangular (C/R) Model Transition together to form an assembly.
2. Connect the assembly by connecting the square waveguide port to the OMT’s antenna port (square port). Now the test fixture is formed. The block diagram is shown in Figure 4.

3. Terminate the V-port of the OMT with a waveguide matching load. Insert the test fixture into the VNA test ports by connecting H-port of the OMT between the VNA test ports for H-plane path calibration. Make sure the H-path of the fixture is aligned. Calibrate the system.
4. Rotate the text fixture by 90 degrees. Terminate the H-port of the OMT with a waveguide matching load. Insert the test fixture into the VNA test ports by connecting V-port of the OMT between the VNA test ports for V-plane path calibration. Make sure the V-path of the fixture is aligned. Calibrate the system.

Polarizer Axial Ratio Testing:

1. Add the DUT (polarizer) into the test fixture. The new assembly is shown in the following block diagram and is ready for testing.

2. Insert the above assembly into the VNA test ports. The linearly polarized signal transmitted by VNA will become circularly polarized after it passes through the polarizer. The circularly polarized signal is then divided into V and H polarized waveforms by the OMT.
3. Terminate the H-port of the OMT for vertical component testing then terminate the V-port of the OMT for horizontal component testing. If the polarizer is capable of converting the linearly polarized waveform provided by the VNA perfectly, this test shall yield two equal magnitude components at V and H ports, simultaneously. In this case, the axial ratio is 1.

Polarizer Axial Ratio Testing Example:

1. The following is the test results of SAS-603-14115-F1, SAGE Millimeter’s V-band linear to circular polarizer. The vertical and horizontal port amplitudes of the polarizer are shown in Figure 6. The resultant Axial Ratio can be calculated by converting the insertion loss to numbers, then dividing. This is show below in Figure 7.