Twist-and-Radiate: A Reconfigurable E-Band Polarizer That Flips Antenna Polarization on Demand

At the crowded frontier of millimeter-wave communications, where 5G and future 6G networks are racing to exploit the vast bandwidths above 60 GHz, one of the most stubborn engineering constraints has been polarization itself. Once a horn antenna or any other waveguide-fed transmitter is built, its polarization is effectively frozen into the hardware. A team at the University of New Mexico, working with the Air Force Research Laboratory’s Space Vehicles Directorate, has now demonstrated a way around that limitation: a reconfigurable polarizer integrated into an E-band horn antenna that can be set to radiate linear polarization, right-handed circular polarization, or left-handed circular polarization, depending only on how two waveguide sections are oriented relative to each other during assembly. The work, published in Frontiers in Antennas and Propagation, was fabricated, measured, and verified across the full 60 to 90 GHz E-band.

The significance of the design lies in the frequency regime it targets. Polarization agility is a prized attribute in modern communication systems because it enables polarization diversity, frequency reuse, and robust links that can survive fading and interference. At lower frequencies, engineers routinely achieve this flexibility using PIN diodes, varactors, MEMS switches, or mechanical tuning elements embedded in the antenna or its feeding circuit. Those electronic switching approaches, however, cannot simply be transplanted to millimeter-wave bands, where the physical dimensions of the switching elements become comparable to the wavelength itself and losses mount rapidly. As communication systems migrate toward millimeter-wave technologies to capture higher data rates and lower latencies, the need for polarization diversity in these bands has become imperative, yet the antenna choice is often fixed in practical deployments, such as when a horn antenna or another waveguide-fed transmitter must be used.

Previous attempts at agile E-band polarizers have taken several forms, each with drawbacks. The oldest approach in the literature is the meandering line polarizer, which covers the full E-band but does not offer reconfigurability among all polarizations in a single device. Other designs have relied on periodic structures such as equally spaced dielectric gaps, circular cavities, and cross slots, but these likewise fail to cover all polarizations in one unit. Lens-based polarization conversion is an attractive alternative, but lenses can degrade the efficiency and directivity of the antenna, and they require an additional structure to be mounted on top of the antenna, which may not always be possible. Liquid crystal devices have recently been demonstrated for electronic polarization change at millimeter-wave frequencies, but liquid crystals may not always be practical and can be difficult to implement. The new design sidesteps all of these compromises with an elegantly mechanical solution.

The heart of the polarizer is a septum: a thin film of polytetrafluoroethylene, better known by its trade name Teflon, inserted into slots within a circular waveguide. Dielectric septum loading has long been known to induce circular polarization, and stepped septum polarizers built on this principle have been used to feed horns and even to construct a duplexer at 225 GHz. The New Mexico team expanded on this established approach by adding a second similar dielectric film in the feeding waveguide. Because the two dielectric films can be arranged in six different orientations, the device offers six distinct configurations. Depending on how the two waveguides containing the septums are aligned, the transmitted wave emerges as linearly polarized, right-handed circularly polarized, or left-handed circularly polarized. The working principle can be extended to any waveguide-fed antenna, and the authors note that elliptical polarization could even be included by creating new alignment positions of the septum within the waveguide structure.

The physics of the conversion is a study in controlled phase manipulation. When a wave enters the waveguide, the septum separates the electric field into two orthogonal components of about equal amplitude, one propagating through the septum and the other along the inner walls of the waveguide. The fields passing through the dielectric film experience a phase delay relative to the rest of the wave. At the end of the septum, these orthogonal degenerate TE11 modes recombine with a phase difference of 90 degrees, which is precisely the condition for circular polarization. Flipping the orientation of the septum to minus 45 degrees reverses the handedness, converting right-handed circular polarization into left-handed circular polarization. In the linear configurations, the phase shifts either cancel out between the two opposing septums, which are of the same length, or fail to add constructively with the required 90-degree difference. The length of the septum, which governs the phase difference between the orthogonal modes, was optimized to obtain the minimum axial ratio throughout the entire bandwidth.

The complete antenna system consists of four parts: the horn antenna itself, two waveguides each containing a septum in the middle that together form the polarizer, and an adapter. The septum is created by a thin PTFE film inserted into two slots, with two additional slots of matching thickness created perpendicular to the septum to maintain symmetry. For ease of fabrication, the waveguides were designed with a partition in between so that the septum can be locked in place during assembly. This clever mechanical arrangement allows both waveguides to be independently removed and rotated in steps of 45 degrees before reassembly. An adapter, a standard WR-12 circular waveguide with UG-387 flanges on both sides, makes the rotation of both waveguides possible independently of the feed. In the default configuration, both septums make an angle of 45 degrees to the horizontal plane of the waveguide, feeding the horn with a right-handed circularly polarized wave; at minus 45 degrees, the wave becomes left-handed circularly polarized; and when the septums are parallel to the waveguide or perpendicular to each other, the linearly polarized wave continues to propagate unchanged.

The horn itself was created by designing an exponential taper between the diameter of the polarizer and the aperture diameter, and the whole assembly was fabricated on a CNC machine for experimental verification. The polarizer is tapered at both ends to smooth the transition, and all geometric parameters, from the septum thickness to the taper length, were optimized to work from 60 to 90 GHz. The team then subjected the fabricated antenna to a battery of measurements, beginning with impedance matching. The measured data showed good matching of the antenna throughout the entire E-band, with a small aberration for the right-handed circular configuration around 76 GHz that the authors attribute to a small loss caused by errors during fabrication or assembly of the setup. Critically, the impedance matching was maintained across all tested configurations, meaning that rotating the waveguides to change polarization does not disturb the antenna’s electrical behavior.

Radiation patterns were measured in a spherical near-field chamber, and the results confirmed the polarization performance with impressive precision. The line-of-sight axial ratio, the standard figure of merit for circular polarization purity, came in at 0.8 dB at 75 GHz and 0.4 dB at 90 GHz, with the abstract reporting a line-of-sight axial ratio around 1 dB overall. The 3 dB axial ratio beamwidth, the angular range over which the polarization remains acceptably circular, measured 56 degrees at the 75 GHz center frequency and increased to 69 degrees at 90 GHz. For the right-handed circular configurations, the patterns showed a single dominant main lobe with roughly 10 dB of isolation from the cross-polarized measurements. The linear configuration achieved even better isolation, reaching 20 dB at 75 GHz and 10 dB at 90 GHz. All configurations generated a dominant main lobe with a half-power beamwidth around 48 degrees at 75 GHz, steadily declining to around 36 degrees at 90 GHz, with small sidelobes existing around 25 to 30 dB below the main lobe.

The measurements were not without anomalies, and the authors are candid about them. The cross-polarization patterns consistently differed from simulations: the measured cross-polarization backlobe showed a significant decrease compared with predictions, and the main lobe of the cross-polarized field was narrower than simulated at 75 GHz but wider at 90 GHz. The team suggests that small bends in the septum during fabrication and assembly might be causing these differences, and that more precise fabrication and assembly of the polarizer may improve the cross-polarization pattern. A more suitable choice of dielectric material for a sturdier septum may also improve the accuracy of assembly. Indeed, the authors identify the delicacy of the septum and the antenna’s sensitivity to imprecise manufacturing as the principal drawback of the proposed design.

Even with those caveats, the demonstration opens a genuinely practical path to polarization diversity at millimeter-wave frequencies. Because the design makes no use of PIN diodes or other semiconductor switching elements, it is not constrained to lower frequencies the way electronic approaches are; its only fundamental limit is the smallest dimension at which the septum can be created, which means the concept can be scaled to radiate the desired polarization at any frequency. The polarizer can easily be included in any waveguide-fed design, from satellite communications terminals to point-to-point millimeter-wave links, and the ability to select among linear, right-handed circular, and left-handed circular polarization simply by choosing an assembly orientation, rather than by adding active components, offers a robust and low-loss solution for the E-band systems that will carry the next generation of wireless traffic.

Subject of Research: A reconfigurable dielectric septum waveguide polarizer integrated with an E-band horn antenna for switchable linear and circular polarization

Article Title: E-band reconfigurable polarizer for waveguide fed antennas

Article References: Cooper, J., Vijayamohanan, J., Ardelean, E., Lane, S. A., & Christodoulou, C. (2025). E-band reconfigurable polarizer for waveguide fed antennas. Frontiers in Antennas and Propagation, 3, Article 1615932. https://doi.org/10.3389/fanpr.2025.1615932

Image Credits: AI Generated

DOI: 10.3389/fanpr.2025.1615932

Keywords: E-band, millimeter wave, polarization reconfigurable antenna, waveguide polarizer, dielectric septum, horn antenna, axial ratio, circular polarization, PTFE, antenna propagation, 60-90 GHz, waveguide-fed antennas