Mohamed K. Emara

Global Navigation Satellite Systems (GNSS) are increasingly vulnerable to intentional jamming, making efficient interference mitigation essential for robust navigation. Analog null steering provides a low-complexity anti-jamming solution by steering radiation pattern nulls toward interference without requiring high-speed ADCs at each antenna element. Conventional Power Inversion methods rely on pre-computed Look-Up Tables (LUTs) storing nulling weights for all jammer directions, but face a critical trade-off: coarse-resolution LUTs enable rapid searches with shallow nulls and incomplete coverage, while fine-resolution LUTs achieve deeper nulls at prohibitive computational cost and latency. This paper introduces an adaptive LUT optimization framework based on Iterative Greedy Optimization (IGO) that dynamically adjusts the search space according to measured jammer power. Unlike population-based methods such as genetic algorithms and simulated annealing, IGO exploits the submodular structure of the null coverage problem to achieve deterministic convergence with guaranteed spatial coverage in a single forward pass. IGO incrementally selects only the most effective entries from a fine-resolution baseline, minimizing null overlap while guaranteeing complete spatial coverage at required suppression depths. Simulations with a four-element Uniform Circular Array demonstrate predictable LUT scaling with target null depth and search-time reductions up to 20× compared with exhaustive search. Experimental validation using a Calian Controlled Reception Pattern Antenna (CRPA) confirms that IGO-optimized LUTs meet or exceed target suppression levels while reducing search space by up to 96.5%. Hardware-in-the-loop navigation testing with a Syntony GNSS simulator under severe jamming (J/S = 65 dB) demonstrates that IGO maintains full positioning availability while preventing complete outages experienced by unprotected receivers. Navigation analysis reveals IGO achieves twice faster average recovery following jammer onset compared to exhaustive search (3.56 s vs. 7.11 s), while delivering equivalent carrier-to-noise ratio and satellite tracking. These results establish the proposed approach as a highly effective and near-optimal solution for resource-constrained GNSS receivers.

A novel reconfigurable metasurface reflector with a simple architecture is proposed based on either PIN or varactor diodes for magnitude and phase control, respectively. Each metasurface reflector is based on a single-dielectric two-metal-layer printed-circuit board (PCB), with a tunable resonator on the top metal layer and biasing on the bottom metal layer. The metasurface reflectors are designed for reconfigurability on a column-by-column basis (i.e., single-plane beamforming). Biasing is integrated in the design of the unit cell and RF-DC isolation is implemented at the ends of each reflector column. The bias lines are designed to have minimal effect on the resonator characteristics, while appropriate architectures are devised for each configuration involving PIN or varactor diodes. To demonstrate various functionalities of the metasurfaces, the PIN-based metasurface reflector is used to demonstrate magnitude control, side-lobe level control, and binary amplitude grating to generate three symmetrical beams. The varactor-based metasurface reflector is used to demonstrate single beam-steering and binary phase grating to generate two symmetrical beams.

A metasurface reflector unit cell is proposed for achieving near-complete-range of complex reflectance (i.e., magnitude and phase) for versatile beamforming capabilities. The unit cell is based on tunable coplanar coupled resonators: a split ring resonator (SRR) with a lumped capacitor and a lumped resistor, and a dipole ring resonator (DRR) with another lumped capacitor. The DRR inserted inside the SRR creates a coupled resonance configuration which results in an enhanced complex reflectance range at the desired frequency. To provide a physical insight and explain the operation principle of the structure, the response of the unit cell is modeled as a coupled Lorentz oscillator via the effective surface susceptibilities, where a unique plasma frequency, damping coefficient, and resonant frequency can be attributed to each resonator. The proposed unit cell is demonstrated in an array configuration for linear-polarized beamforming, where full-wave simulations are used to demonstrate beam-steering, gain control, side-lobe level control and dual- and triple-beam generation, as illustrative examples. Finally experimental demonstration is performed to validate the full-wave results and obtain in-depth electrical characterization of the reflectors.

A novel metasurface reflector unit cell based on two coupled resonators is proposed and demonstrated for real-time reconfigurable beamforming in the X-band (8-12 GHz). The unit cell is composed of a split-ring resonator (SRR) with tunable capacitance and a dipole-ring resonator (DRR) with tunable resistance, whose collective variations allow for control over the complex reflectance at a desired frequency. To gain physical insight into its working mechanism, the proposed unit cell is modeled as a coupled Lorentz oscillator using the surface susceptibility description of the unit cell. Thereafter, a metasurface reflector based on the proposed unit cell is demonstrated in full-wave simulations to achieve various beamforming examples, such as beam-steering, side-lobe level control, beam-steering with amplitude control, and multi-beam patterns, from a single normally-incident plane wave excitation. Three metasurface reflectors are fabricated to experimentally demonstrate the proposed concept; the first is based on an SRR with a varactor diode, the second is based on a DRR with a PIN diode, and the third is based on the proposed coupled SRR-DRR configuration with both varactor and PIN diodes, for simultaneous magnitude and phase control. The metasurface reflector based on the coupled resonator unit cell is experimentally demonstrated to achieve versatile beam transformations including beam-steering with amplitude control and multi-beam patterns.

A novel metallo-dielectric Huygens’ unit cell is proposed that can be designed to operate either as a quarter-wave plate (QWP) or as a half-wave plate (HWP). The unit cell is based on a single-layer perforated dielectric, with straight (non-metallized) and 45° (metallized) elliptical holes. The novelty of the unit cell is its suitability for millimeter-wave implementation in printed-circuit board (PCB) fabrication due to the absence of metallic patterns of small line widths and gaps. As an example of their applicability, the wave plates are demonstrated in the near-field of 60-GHz band diffusion-bonded slot array antennas. The unit cell operating as a QWP is demonstrated in full-wave simulations and experimentally for linear-to-circular polarization conversion, whereby the combination of a linearly-polarized (LP) antenna and the QWP produces a circularly-polarized (CP) wave. Measurement results of the antenna and the QWP show ≤ -10-dB reflection from 59 to 63 GHz (6.6%), ≤ 3-dB axial ratio from 59.5 to 61.5 GHz (3.3%), and an overall efficiency ≥ 77% from 59.5 to 61.5 GHz. The unit cell operating as a HWP is demonstrated in full-wave simulations for polarization rotation of the LP antenna, and for changing the sense of rotation of a right-hand CP antenna into left-hand circular polarization.

This paper investigates material and scattering loss from a general two-port leaky-wave antenna (LWA) in transmit and receive modes using scattering parameters. It is demonstrated that the total material and scattering losses depend on the LWA orientation and whether the LWA is used as a transmitter or a receiver, leading to a different radio frequency (RF) environment in each scenario. First, full-wave simulations are used to demonstrate this behavior for a patch-to-horn configuration. Then the behavior is experimentally demonstrated for two-port LWAs in different orientations with respect to a horn antenna.

Two different bidirectional periodic leaky-wave an-tennas (LWAs) are presented for the millimeter-wave (mm-wave) band with suppressed stopbands and orthogonal radiation di-rections: a bidirectional side-fire antenna with left/right in-plane radiation and a bidirectional slot-pair antenna with top/bottom out-of-plane radiation. The operations of these antennas are con-firmed using full-wave simulations, with a broadside frequency of approximately 28 GHz. These antennas can be individually engineered to form various radiation patterns in each of their radiation planes, and can further be combined to design a tetra-directional periodic LWA.

An analog, low-profile and shielded spectrum analyzer is proposed for operation at mm-wave frequencies around the 60 GHz band based on a novel side-fire leaky-wave antenna (LWA) configuration. The proposed side-fire periodic LWA is systematically developed from a conventional three-port waveguide T-junction which is modified to a LWA unit cell with an internal matching mechanism to suppress the stopband and enable broadside radiation based on unit cell symmetry considerations. The resulting periodic side-fire antenna radiates in the plane of the antenna, whereby the leakage power can either be allowed to radiate in free-space or kept confined inside a parallel-plate waveguide (PPW) structure. The proposed side-fire structure can therefore be completely shielded, making it useful as a broadband spectrum analyzer that is compatible with substrate-integrated waveguide (SIW) technology. Furthermore, a convex side-fire antenna is demonstrated to focus the radiated beams in the near-field of the structure to make the entire system compact. The integrated spectrum analyzer is experimentally demonstrated between 59 and 66 GHz providing 1 GHz frequency resolution. Furthermore, a simple mathematical model consisting of an array of line sources is used to efficiently model the beam-scanning characteristics of the curved side-fire LWA in the near-field of the structure.

A novel application of leaky-wave antennas (LWAs) as multiplexers/demultiplexers is proposed and experimentally demonstrated in the millimeter-waveband at 60 GHz. The first application is demultiplexing of an oblique-incident free-space wideband plane wave into 2N channels using N LWAs with different beam-scanning laws. The second application is N-channel multiplexing and demultiplexing using N pairs of identical LWAs in each other's far-field, where each LWA pair is designed such that the broadside frequency is the center frequency of its respective channel. The Friis transmission equation is first used to analytically demonstrate the two applications. The LWAs are then implemented using reflection-canceling slot pairs using substrate integrated waveguide technology. The two applications are demonstrated in full-wave simulations and experimentally by conducting horn-to-LWA and LWA-to-LWA transmission measurements. The proposed LWAs provide a simple, compact, and high-efficiency multiplexing/demultiplexing solution with high-integration capability with other circuitry. Compared with conventional multiplexers, LWAs do not require matching networks and can be directly scaled to higher frequencies using the same architecture.

An all-dielectric Huygens’ transmit-array is proposed and experimentally demonstrated in the 120-GHz-band. The proposed transmit-array is fabricated using a high-precision laser-drilling process providing high fabrication precision and accuracy. A non-uniform Huygens’ surface is next placed on top of a planar high gain slot-array antenna for far-field beamforming. Full-wave simulations are first used to demonstrate the beamforming capability by generating difference-pattern beams as an example. A uniform transmit-array was then fabricated and characterized to investigate the effect of the transmitarray on the antenna performance, along with showing very low fabrication tolerances and minimal dimensional variation due to the laser drilling process. Next, a phase-gradient transmit-array for beamtilting is experimentally demonstrated as a practical example of beamforming. The proposed all-dielectric structure is the first Huygens’ structure to be demonstrated at these frequency bands, and represents an attractive alternative to conventional metallic Huygens’ metasurfaces based on standard printed circuit board (PCB) solutions.