A compact dual-band antenna element is presented that operates in the bands 0.617-0.894GHz (36.7% fractional bandwidth), and 1.69-2.4 GHz (34.7% fractional bandwidth). The dual-band antenna element consists of a low-band bowl-shaped antenna with a ring for the low-band and a high-band dipole antenna, placed in the center of the bowl. The ring is used to reduce the size of the dual-band antenna element without compromising the bandwidth. The presence of the ring can introduce spurious resonance modes in the structure that impairs the performance of the antenna. These resonances are suppressed by placing stubs on the ring. The proposed dual-band antenna element consists of robust metallic components, with some dielectric supports. As a result, the antenna provides a high radiation efficiency and power handling, making it suitable as a base station antenna. Furthermore, the presented antenna is also demonstrated to be suitable for array configurations, which is critical for the targeted application. A prototype is manufactured and tested to corroborate the simulation results, and a good agreement is found. Importantly, the measured radiation efficiency is higher than 96% and 93% in the targeted low and high bands.

This paper presents a geodesic H-plane horn antenna with switchable circular polarization. The polarization diversity is realized by stacking two identical geodesic horn antennas and joining them at the aperture using a parallel plate waveguide septum polarizer. Depending on which of the two horns is fed, the antenna can radiate either left-hand or right-hand circular polarization. The proposed antenna operates in the K, -band, with a center frequency of 29 GHz and a 3 dB axial ratio bandwidth of 15 %.

This paper discusses forward and backward modes in one-dimensional (1D) periodic bounded structures. Importantly, we derive a phase velocity for general 1D scalar waves and use this to highlight some challenges in characterizing a mode as forward or backward. The discussion is illustrated by analyzing a 1D-periodic doubly corrugated parallel plate waveguide. The results of this work are important for an accurate understanding of the propagation characteristics in bounded periodic structures.

The spurious transmission dips that occur in glide-symmetric holey gap waveguides (GSHGWs) are systematically characterized in this work, and the obtained information is used to suppress them in the intended operating band of the gap waveguide. The analysis relies on the dispersion characteristics of the waveguide segment with electromagnetic bandgap (EBG) holes. These characteristics are explored through the multimodal transfer matrix approach, particularly focusing on identifying relevant edge and waveguide modes. We find four types of unwanted dips in the transmission coefficient within the intended operation frequency band of the gap waveguide under study. The first three types are all associated with the edge mode mostly concentrated in the small air-gap region between the waveguide and the EBG holes, whereas the fourth type is caused by a narrow stopband in the waveguide mode. Based on a thorough understanding of all dips, we propose three viable solutions: placing EBG holes away from the waveguide channel, intersecting EBG holes with the waveguide channel, and intersecting additional small holes with the waveguide channel and the EBG holes. After comparison, the last solution with two small holes per EBG hole along the waveguide channel was demonstrated to be the most advantageous in terms of transmission properties, compactness, and flexibility. This solution was also experimentally validated using a WR-19 GSHGW operating from 35 to 63 GHz.

Radio telescopes rely on calibration to provide high-quality images. Here, we present a polarizing metasurface that can be used to aid the calibration of all-sky radio telescopes. The metasurface is intended for the REACH telescope, which will be used to study the early epochs of the Universe. The polarizing metasurface successfully transforms the polarization of a radiation pattern of a test antenna within the frequency range 90 to 165 MHz (apart from a narrow band around 155 MHz). The dielectric losses in the metasurface are estimated to be below 2%. Future work to improve the performance of the polarizer is outlined.

We present a low-loss shared-aperture dual circularly polarized 4x4 lens array antenna for full-duplex geostationary satellite communications in the K/K-a-band. The proposed array has a periodicity of approximately 2 lambda in the K-a-band and 1.5 lambda in the K-band, with the resulting grating lobes suppressed by highly directive array element patterns. Two separate sequential rotation feed networks (for the K and K-a-bands) are implemented using a combination of H-plane ridged waveguides and E-plane rectangular waveguides. The feed works for the two bands are stacked. The performance of the proposed lens array is validated using full-wave simulations in CST. In the receiving band of 17.7 - 20.2 GHz, the array achieves a right-hand circular polarization with an axial ratio lower than 2 dB, a realized gain of 23 dBic, a total antenna efficiency of more than 85%, and a return loss higher than 10 dB. In the transmitting band of 27.5 - 30 GHz, the array achieves a left-hand circular polarization with an axial ratio lower than 1.5 dB, a realized gain of 27 dBic, a total antenna efficiency of more than 80%, and a return loss higher than 10 dB.

This communication presents a Luneburg-like geodesic lens antenna with a defined footprint operating from 60 to 70 GHz. Traditionally, geodesic lens antennas are rotationally symmetric and consequently possess a large footprint. By defining the outline of the lens footprint, the overall size of the lens can be reduced while introducing new degrees of freedom to optimize its performance. An in-house ray-tracing algorithm was used to aid the design process of the lens. In the case presented, the geometric parameters were selected for the overall port performance. A final prototype was manufactured using laser powder-bed fusion (LPBF), with measurements agreeing closely with the simulated results. The proposed antenna finds applications in future wireless communication networks.

Glide-symmetric holey electromagnetic bandgap (EBG) structures have found wide applications in high-frequency gap waveguide components because of their demonstrated wide stopband and easy manufacturing. However, potential dips in the transmission through the gap waveguide at certain frequencies limit the effective bandwidth. Here, we perform a Bloch analysis of the unit cell, a rectangular waveguide segment implemented with the glide-symmetric holey EBG, using a multimodal transfer matrix method. We find two main spurious dips in the transmission coefficient in the recommended operating frequency band of the investigated WR-15 standard rectangular waveguide. The first transmission dip is found to correspond to the coupling of the waveguide mode and the edge mode formed in the air gap between the waveguide and the EBG holes. The second transmission dip is caused by a small open stopband in the waveguide mode.

In this paper, a novel dual-slant polarized three-dimensional (3D) periodic Luneburg lens with a diameter of 390 mm ( $4.6\lambda{0}$ at 3.55 GHz) is presented. Copper-plated cubes with truncated corners are placed in a body-centered cubic (BCC) lattice and held together with layers of Rohacell foam. The sizes of the cubes are varied to realize the gradient refractive index (GRIN) profile of a generalized Luneburg lens at a low cost, with a low weight and loss. The designed Luneburg lens operates from 3.3 to 3.8 GHz with a total weight of 1 kg. The quasi-isotropic response of the proposed periodic structure allows a wide angle coverage. Measured results show that the lens antenna can achieve a peak gain of 22 dBi with a scanning loss of less than 0.4 dB in a wide angular range in both the azimuth and the elevation plane. Importantly, the proposed lens antenna design achieves a cross-polarization level below peak gain of less than -19 dB at all angles for the two slant polarizations of the feed, while comparable designs report up to -11 dB at given angular directions. This high-gain multi-beam antenna with a commercially viable design is suitable for wireless communications at low microwave frequencies.