This paper presents a gap waveguide-based compact monopulse array antenna, which is formed with four unconnected layers, for millimeter-wave tracking applications at $W$ -band (85–105 GHz). Recently developed gap waveguide technology removes the need for galvanic contact among metallic layers of waveguide structures, and thereby, makes the proposed antenna suitable for easy and low-cost manufacturing. In this context, a low-loss planar Magic-Tee is designed to be used in a monopulse comparator network consisting of two vertically stacked layers. The gap waveguide planar monopulse comparator network is integrated with a high-efficiency $16\times 16$ corporate-fed slot array antenna. The measured results of the comparator network show the amplitude and phase imbalance values to be less than 0.5 dB and 2°, respectively, over the frequency band of interest. The fabricated monopulse array antenna shows relative impedance bandwidths of 21% with input reflection coefficients better than −10 dB for the sum and difference ports. The null in the difference radiation pattern is measured to be 38 dB below the peak of the sum radiation pattern at 94 GHz. The measured gain is about 30 dBi for the same frequency. The low-loss performance of the comparator network and the feed network of the proposed array, together with the simple and easy manufacturing and mechanical assembly, makes it an excellent candidate for $W$ -band compact direction-finding systems.

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$W$ -Band Low-Profile Monopulse Slot Array Antenna Based on Gap Waveguide Corporate-Feed Network

substrate-integrated gap waveguide (SIGW)-fed metasurface antenna (metantenna) array with nonuniform excitation is proposed for wideband operation and sidelobe-level (SLL) reduction at millimeter-wave Ka -band. To achieve wideband and low profile, a $2 \times 2$ square patch-based metasurface is introduced based on characteristic mode analysis (CMA). The ridged microstrip-fed slot is employed to excite the metasurface, while the mushroom-like patches are positioned around feeding lines to operate as the SIGW for suppressing the undesired modes. Due to the compactness of the proposed SIGW, the amplitude of each element is controllable using unbalanced T-junctions. A nonuniformly excited $4 \times 4$ antenna array is designed and fabricated for validation. The measured results show an impedance bandwidth ( $\vert \text{S}_{11}\vert dB) of full Ka -band with a 3 dB gain bandwidth of 28–40 GHz (35.3%) and SLLs less than −12.1 and −15.6 dB in the E- and H-planes, respectively.

Wideband Sidelobe-Level Reduced $Ka$ -Band Metasurface Antenna Array Fed by Substrate-Integrated Gap Waveguide Using Characteristic Mode Analysis

The synthesis of frequency-scanned monopulse radiation patterns using an array of two leaky-wave antennas (LWAs) is demonstrated The LWA array must be designed with appropriate shifted frequency-dispersion responses to fulfill the desired monopulse-scanning specification over the required bandwidth and number of channels This is demonstrated using two manufactured LWAs in half-width microstrip technology, designed to scan in the band from 239 to 265 GHz covering a field of view (FoV) from −80° to +80° and using three frequency channels Also, this paper addresses the procedure to estimate the direction of arrival (DoA) of an incoming beacon RF signal composed by prescribed tones distributed in the scanning band This type of multicarrier beacon signals can be produced in wireless systems which allow for frequency-hopping operation It is demonstrated that the proposed monopulse DoA-estimation technique combines good angular resolution and wide FoV using a simple passive antenna structure

Frequency-Scanned Monopulse Pattern Synthesis Using Leaky-Wave Antennas for Enhanced Power-Based Direction-of-Arrival Estimation

This article presents the design and fabrication of low-loss, -weight, and -cost, modified Butler matrix in groove gap waveguide technology at W-band, implemented by additive manufacturing. The design, simulations, and measurements of each of the elements that conform to the modified Butler matrix are presented. The proposed modified Butler matrix is asymmetric and oriented to an easy assembly with a radial antenna in order to obtain simultaneous sum and difference patterns for a narrowband radar application. The measured individual components of the modified Butler matrix fit very well with the simulations. As for the full modified Butler matrix, measurements provide a 0.7% bandwidth below −20 dB for sum and difference ports centered at 94 GHz. The amplitude and phase imbalance are below 1.5 dB and 20°, respectively. This narrow bandwidth is due to the difference in length between input and output ports due to the asymmetry of the design. The total losses of the modified Butler matrix are below 1 dB.

3-D-Printed Modified Butler Matrix Based on Gap Waveguide at W-Band for Monopulse Radar

This paper addresses the constrained multi-objective optimization problem of sparse conformal arrays designing. The objective of array synthesis is to find an optimal element arrangement on a conformal surface and its associated excitation strategy, which generate the main radiation beam along a pre-selected spatial direction with maximum gain and, simultaneously, suppress sidelobe levels elsewhere. A hybrid algorithm particle swarm optimization (PSO)-second-order cone programming (SOCP), comprising of PSO and SOCP, each for a dedicated purpose, is proposed to fulfill this task in this paper. More specifically, the PSO algorithm is introduced to optimize sparse conformal array element positions, whereas the SOCP is applied to seek optimal excitation coefficients for each array layout obtained. After extensive simulation with the examples of sparse circular array and sparse conical arrays, we can find that our proposed method can synthesize better radiation patterns with regard to peak sidelobe levels, compared with those obtained through other traditional algorithms.

/pdf/low-sidelobe-pattern-synthesis-for-sparse-conformal-arrays-3d3j8mjofb.pdf

Low-Sidelobe Pattern Synthesis for Sparse Conformal Arrays Based on PSO-SOCP Optimization

An iterative method has been proposed for optimising element excitations in linear phased arrays for synthesising a cosecant-squared pattern (CSP). Compared with the other methods, this weighted alternating reverse projection (WARP) method allows having fewer ripples in the shaped region and lower sidelobe level (SLL). Some detailed examples are presented and results show that the WARP method has better performance in terms of SLL and ripple.

Cosecant-squared pattern synthesis using a weighted alternating reverse projection method

This article generalises a pattern synthesis technique to conformal arrays. An iterative method is used for optimisation of element excitations in an array of rectangular patch elements that are conformed to a hemispheric surface. The embedded-element co-polarisation and cross-polarisation patterns are evaluated using a full-wave method. These patterns are used in a newly introduced iterative synthesis algorithm to obtain the minimum sidelobe level (SLL) and cross-polarisation level (XPL). The mutual coupling effects on the synthesis results are investigated in this procedure. The synthesised patterns show adequate XPL and SLL comprised to other methods. The weighted alternating reverse projection (WARP) method is modified and it is shown that the WARP method can synthesise lower XPL and SLL than those of other methods. Compared with other synthesis method, the WARP improves SLL by 2 dB and XPL by about 1.5 dB in 19 element array.

Conformal array pattern synthesis using the weighted alternating reverse projection method considering mutual coupling and embedded-element pattern effects

Surface distortions change the radiation characteristics of reflector antennas. Using functional calculus, radiation integral is analyzed, and the first variational derivative (VD) of the radiation field with respect to surface profile is extracted. The VD can show the radiation pattern sensitivity to surface distortion. All formulations are based on the physical optics approximation. The final formula for VD depends only on the original undistorted antenna parameters, so it can be used for all types of distortions. It has been shown that the formulations are more general than those in previous works. Different polarization states of feed antenna are studied, and VD amplitudes are plotted to depict the regions that have greater effects on the main-lobe and the first few side-lobes levels. Some periodic, aperiodic, and local distortion profiles are considered, and the exact and approximated distorted patterns are compared to check the accuracy of the presented procedure.

Distorted Reflector Antennas: Analysis of Radiation Pattern and Polarization Performance

A bandwidth-enhanced $8\times 8$ cavity-backed slot array antenna for fixed beam applications is presented. The antenna consists of four $2\times 2$ subarrays fed by ridge gap waveguide and a modified bow-tie coupling slot in the cavity layer. The proposed bow-tie coupling slot provides additional $\text{10}\%$ impedance bandwidth as compared to the traditional rectangular coupling slot. The fabricated prototype demonstrates an $|S_{11}|\leq -10\,$ dB fractional impedance bandwidth and a $3\,$ dB gain-drop bandwidth of approximately $\text{28}\%$ and $\text{25}\%$ , respectively, with peak gain of $26.4\,$ dBi, aperture efficiency $>\text{60}\%$ and cross-polar discrimination $>40\,$ dB. Measurements show good agreement with simulations. Moreover, we show the impact of the dimensions and shape of the cavity coupling slot on the bandwidth enhancement. The bandwidth enhancement is explained by the double-ridge slot behavior of the bow-tie coupling slot, which shows a more wideband behavior than the traditional rectangular coupling slot behaving like a cross section of a waveguide.

/pdf/a-bandwidth-enhanced-cavity-backed-slot-array-antenna-for-1umudqg9nz.pdf

Abolfazl Haddadi

Papers

$W$ -Band Low-Profile Monopulse Slot Array Antenna Based on Gap Waveguide Corporate-Feed Network

Cosecant-squared pattern synthesis using a weighted alternating reverse projection method

Conformal array pattern synthesis using the weighted alternating reverse projection method considering mutual coupling and embedded-element pattern effects

Distorted Reflector Antennas: Analysis of Radiation Pattern and Polarization Performance

A Bandwidth-Enhanced Cavity-Backed Slot Array Antenna for mmWave Fixed-Beam Applications