A waveguide polarizer changes the polarization of a microwave signal, commonly converting linear polarization into circular polarization or converting circular back to linear. It introduces a relative phase shift of approximately 90° between two perpendicular electric-field components. Circular polarization requires those components to have equal amplitudes as well as the correct phase relationship. The device changes polarization, not the signal’s carrier frequency. [1]
In a dual-circular satellite feed, the polarizer works with a port-separating structure so one antenna can handle right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP). ESA’s Q/V-band engineering model uses a polarizer and an orthomode transducer (OMT), with measured axial ratio below 0.4 dB and isolation above 31 dB between its opposite-polarization ports. Those are results for that feed, not universal polarizer specifications. [2]
Table of Contents
How the Polarization Changes
A linearly polarized wave has an electric field that oscillates along a fixed direction. For circular polarization, the field rotates with time at a fixed observation point. Inside the polarizer, two perpendicular field components experience different phase shifts. With equal amplitudes and a relative phase of +90° or −90°, their combined field traces a circle.
For a conventional differential-phase polarizer, a linear input oriented at 45° to its principal axes excites both components equally. Metal geometry or dielectric loading then creates the required phase difference. Incorrect input orientation, unequal transmission amplitudes, or phase error produces elliptical polarization instead. The relevant axes depend on the design: diagonal-iris structures can avoid the separate 45° waveguide twist used in other feed arrangements. [1]
The common guide must support both orthogonal modes. A standard rectangular waveguide carrying only its dominant mode cannot carry that two-mode circularly polarized field. This is why the conversion section often uses a square or circular cross-section, with suitable transitions to the radio-side ports. [1]
Polarizer vs. OMT
A polarizer sets the relationship between the field components. An orthomode transducer separates or combines two orthogonal linear modes through independent ports. Used together, they convert incoming RHCP and LHCP waves into separate receiver channels, or combine transmitter channels into the corresponding circular polarizations. ESA’s Q/V feed is a documented example of this arrangement. [2]
A septum polarizer integrates polarization conversion and port separation. It typically has a common dual-mode aperture and two separate single-mode ports. The labels RHCP and LHCP describe the polarization each port produces or receives at the common aperture; they do not mean circular polarization travels down each rectangular branch.
Main Polarizer Designs
Septum Polarizers
A septum polarizer uses a shaped metal partition to control modal amplitudes and phase while forming separate channels. Construction details matter: a 2023 design integrated the common waveguide and inline transformers to avoid a split-block air gap associated with mode leakage and resonances. [3]
Iris Polarizers
Iris polarizers use metal diaphragms inside the guide to load the two modes differently. Their dimensions and spacing control differential phase and matching. A published Ka-band design uses 12 diagonal irises in a square guide; its optimized internal cross-section is 13.08 × 13.08 mm and its total length is 60 mm. These are dimensions of that simulated 17.7–22.2 GHz design, not a standard waveguide size or a prescription for other bands. [1]
Dielectric Polarizers
A dielectric plate inside a circular waveguide changes the propagation of the two orthogonal components differently. In a 2004 design, the authors used fixing and compensation slots to control the plate’s electromagnetic effects, and reported that adjusting its length could correct phase errors caused by fabrication or dielectric-constant inaccuracies. Material properties and mounting geometry are therefore part of the RF design. [4]
What Published Performance Looks Like
These examples have different test boundaries. A simulated component, a measured component, and a complete feed are not interchangeable benchmarks. None of these results is a performance guarantee for a Dolph Microwave product.
| Published design | Operating band | Reported performance | Evidence |
|---|---|---|---|
| Diagonal-iris polarizer, 2021 | 17.7–22.2 GHz | Axial ratio <0.45 dB; XPD >32 dB; VSWR <1.04; differential phase 90° ±2.9° | Numerical simulation and optimization. [1] |
| ESA Q/V feed, project status January 2017 | 37.5–43.5 GHz and 47.2–51.4 GHz | Axial ratio <0.4 dB; XPD >32 dB; opposite-polarization port isolation >31 dB | Measured engineering-model feed. [2] |
| Broadband septum polarizer, 2023 | 198–260 GHz | Insertion loss approximately 0.28 dB; return loss >15 dB. Port isolation >23 dB applies to 206–260 GHz. | Measured component. [3] |
| Dielectric-septum polarizer, 2004 | 60–62.25 GHz | Axial ratio <0.5 dB; return loss >13 dB for both orthogonal polarizations | Measured component. [4] |
Axial Ratio, XPD, and Loss
Axial ratio (AR) describes the shape of the polarization ellipse: major-axis field amplitude divided by minor-axis field amplitude. Perfect circular polarization has a ratio of 1:1, or 0 dB. A 3 dB axial ratio corresponds to approximately 1.41:1. It does not mean the component loses 3 dB of signal power.
Cross-polarization discrimination (XPD) compares the desired polarization with the unwanted orthogonal polarization. For a single fully polarized field resolved into opposite circular senses, let A = 10AR(dB)/20. For A > 1, XPD(dB) = 20 log10((A + 1)/(A − 1)); perfect circular polarization is the infinite-XPD limit. Calculated values are about 30.8 dB for 0.5 dB AR and 15.3 dB for 3 dB AR. This conversion describes polarization purity; it does not predict electrical isolation between physical ports.
Insertion loss measures signal-power loss through the specified path. Return loss measures matching: a larger positive return-loss value means less reflection. A polarizer can have low insertion loss and still have poor axial ratio, so a transmission-loss measurement alone cannot establish that polarization conversion is correct.
A Worked Alignment Example
Consider an ideal differential-phase polarizer intended for a linear launch at 45° to its principal axes. Suppose the launch is instead at 50°. The component amplitudes are proportional to cos 50° and sin 50°, so their larger-to-smaller ratio is tan 50° ≈ 1.192. If the differential phase remains exactly 90°, the resulting axial ratio is 20 log10(1.192) ≈ 1.52 dB.
This is a calculated installation scenario, not a measured customer case. It assumes ideal orthogonal modes, equal transmission magnitudes through the polarizer, and no reflections. Its practical lesson is specific: if matching looks acceptable but circularity is poor, check launch orientation before blaming insertion loss or changing the component.
Phase error has a separate effect. With equal output amplitudes and phase difference 90° + δ, the ideal axial ratio is:
AR(dB) = 10 log10((1 + |sin δ|)/(1 − |sin δ|)), for |δ| < 90°.
A ±3.5° phase error gives approximately 0.53 dB AR. A 0.5 dB AR target allows only about ±3.30° when phase error is the sole imperfection. Amplitude imbalance consumes part of that allowance; the two limits cannot both be treated as independent full error budgets.
How to Check an Installed Feed
A useful acceptance test records the configuration, reference planes, unused-port terminations, frequency sweep, and measurement uncertainty. IEEE Std 149-2021 provides the broader framework for antenna measurements and test facilities. [5]
- Sweep the operating band. Record amplitude balance, differential phase, matching, and polarization performance. Do not assume the best circularity occurs at center frequency: the published diagonal-iris simulation has AR minima at 18.36 and 21.61 GHz within its 17.7–22.2 GHz band. [1]
- Verify handedness separately. A rotating linear probe can establish ellipse axes under suitable range conditions, but amplitude-only rotation cannot distinguish RHCP from LHCP. Use a known circular-polarization reference or a calibrated phase-sensitive measurement for that check.
- Record the viewing convention. State propagation direction, observation direction, excited port, and reference plane on the test drawing. Rotating an entire circularly polarized feed about its axis does not by itself reverse handedness.
- Test the assembled hardware. Include the feedhorn and relevant transitions. Compare component and assembly results before attributing a mismatch to the polarizer alone.
ESA’s measured Q/V feed illustrates why the assembly boundary matters: the polarization network was combined with a project-specific horn and triplexers before the engineering-model feed was assessed. For an antenna feed network, specify whether the acceptance limits cover the polarizer, the complete feed, or the antenna radiation pattern. [2]
What to Specify Before Choosing One
Give the supplier the exact transmit and receive bands, required polarization senses, port arrangement, mating flanges, and mechanical envelope. Set separate limits for axial ratio, port isolation, insertion loss, and return loss across those bands. Where off-axis performance matters, specify the angular coverage as well as boresight performance.
Power handling also needs its own specification: peak and average power, pulse duration or duty cycle, operating pressure, and temperature range. The RF performance results in the table do not establish a high-power rating. A dielectric design also needs material-loss and thermal data for the intended conditions.
The polarizer’s job is to provide the required polarization at a defined interface. A useful selection therefore starts with the complete feed requirement—not a universal “good” axial ratio or a performance number copied from a different frequency band.
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References
- Stepan Piltyay. “Square Waveguide Polarizer with Diagonally Located Irises for Ka-Band Antenna Systems.” Advanced Electromagnetics, 10(3), 31–38, October 26, 2021. Publisher article and full paper.
- European Space Agency. “Q/V-Feed — Q/V-Feed System Development.” Project status dated January 24, 2017; accessed October 24, 2026. Engineering-model results and frequency plan.
- B. Zhang, H. Zhu, Y. Chen, and Y. Zhang. “Broadband Resonance-Free Septum Circular Polarizer Operating Above 200 GHz.” IEEE Transactions on Antennas and Propagation, 71(5), 4461–4469, 2023. doi:10.1109/TAP.2023.3243847.
- “A Circular Polarizer Designed With a Dielectric Septum Loading.” IEEE Transactions on Microwave Theory and Techniques, 52(7), 1719–1723, July 2004. doi:10.1109/TMTT.2004.830487.
- IEEE Std 149-2021. IEEE Recommended Practice for Antenna Measurements, published February 18, 2022. IEEE standard scope and publication details.
