This paper investigates the number of required data points to accurately represent the vector far-field information over the radiation sphere and its frequency variation for array antennas. The method needs to be explicit in terms of the excitation of the elements. We compare representations of far-field data on the sphere with two different approaches to spherical wave expansion representations and apply a data-driven model-order reduction to find a small-size representation. As is well known, a spherical wave expansion strongly reduces the number of required data points. Here, we show that accounting for the phase center of the elements further strongly reduces the number of required coefficients in the tested cases. The Loewner framework datadriven model-order reduction shrinks the required far-field data by an additional factor of 8–14 per mode coefficients, depending on the acceptable level of error. This reduction works well for the two investigated cases.

The near-field contribution in the high-frequency method Shooting and Bouncing Rays (SBR) is investigated for installed antenna performance. Three SBR solvers, the in-house solver SIENT and two commercial solvers, are compared to a commercial full-wave solver. SIENT and one commercial solver have an option to disable near-field effects, which allows the strength of these effects to be investigated. The last solver also includes near-field effects. The results indicate an increase in accuracy when including near-field effects. The most notable difference in the far-field phase and the induced surface current are obtained for the in-house solver. Improvements for the surface current density is seen close to the antenna. On the tested platform, the difference in gain is not as notable but it is overestimated for both the in-house solver and the commercial solvers when excluding near-field terms.

A new wide-scan active direct-integrated phased array antenna (AIPAA), designed for Tx/Rx mm-Wave applications, is introduced in this paper. This novel AIPAA offers seamless switching performance between transmitting (Tx) and receiving (Rx) modes without the need for lossy intermediate RF components, such as RF switches, circulators, and duplexers. The unitcell of the AIPAA consists of three miniaturized tapered slot elements operating in the K-band frequency range. In the Tx mode, a GaN high electron mobility transistor serves as the power amplifier (PA), while in the Rx mode, a GaAs MMIC low noise amplifier (LNA) is employed. The unticell's center antenna element is reshaped to match closely to the optimal load impedance of the PA ( $\mathrm{Z}_{opt}=6+\mathrm{j} 36 \Omega$ ) while the other elements are tuned to maintain a $50 \Omega$ input impedance suitable for the LNA. This direct integration approach enhances system efficiency and reduces the cost and size by eliminating the need for any intermediate impedance matching networks and RF switching components. The proposed Tx/Rx AIPAA achieves a matching at both the PA (optimum load) and LNA ( $50 \Omega$ ) ports with a 6% and 16% fractional bandwidth over ±50° scan coverage, respectively. The switching capability is incorporated by utilizing the ON/OFF modes of the PA and LNA DC-biasing. The proposed AIPAA's cell size is $9.2 \times 6.5 \times 1.8\ \text{mm}^{3}(0.67 \times 0.5 \times 0.13 \lambda^{3})$ .

In Internet of Things (IoT) applications the antennas are often electrically small at their radiation frequency. This makes it hard to design antennas that meet desired bandwidth requirements. It is therefore interesting to consider the trade-off between antenna size and antenna position within the terminal with respect to its available bandwidth, as characterized by the Q-factor bound. To determine these quantities are associated with solving a non-trivial optimization problem. Recent development of model order reduction techniques can include parametric dependencies. Here we apply the data-driven Loewner framework to the Q-factors as a function of size and position parameters, to investigate how well these methods work for the Q-factor bounds in IoT applications. We show that the Loewner framework can deliver a reduced-order model of the bound. Properties of the reduced model can be used to indicate how well the reduced order interpolates the parametric dependence.

A convex optimization program is presented for wideband arrays. Constraints are imposed on the frequency variation of excitation coefficients to ensure that the optimal solution can be realized in a wideband active electronically scanned array (AESA). AESA implementation with true time delays (TTDs) and phase shifters are handled separately. We also discuss the general case of combining TTDs and phase shifters. Contrary to single-frequency optimization, the wideband optimization method presented here ensures that the computed excitation is optimal over a specified bandwidth. It is shown that there is a trade-off between instantaneous bandwidth and side-lobe level. The proposed method works for both narrow and wideband arrays, as illustrated with examples. In addition to regular arrays, the method is also applicable to monopulse arrays. The optimization program is implemented in terms of embedded element patterns to account and compensate for mutual coupling, radome and platform effects.

This paper introduces a novel filtering approach that employs integrated periodic structures with a conventional Vivaldi antenna to achieve a fully integrated bandpass filtering antenna. The approach results in a wide out-of-band suppression, high passband selectivity, adjustable operational bandwidth, and low insertion loss. The proposed filtering approach maintains the original size of the conventional Vivaldi antenna (base antenna) without requiring additional modifications. To validate the approach, we present two filtering Vivaldi antennas: filtering antenna I (center frequency: 18GHz, fractional bandwidth: 21%, insertion loss: 0.32dB) and filtering antenna II (center frequency: 6.5GHz, fractional bandwidth: 12%, insertion loss: 0.6dB). Their wide out-of-band gain suppression (typically $\geq 15$ dB) covers the conventional Vivaldi antenna’s frequency range (4-24GHz). A prototype of the filtering antenna I is manufactured. Its measurement results validate the proposed approach and show good agreement with the simulated reflection coefficient, realized gain, and radiation patterns. The features of the proposed filtering antenna approach, make it suitable for various applications requiring efficient frequency filtering.

In this paper, we propose a new wide-scan active direct integrated phased array antenna (AIPAA) for mm-Wave applications. The AIPAA’s unit-cell comprises three K-band miniaturized taper slot elements, a GaN high electron mobility transistor (HEMT) as a power amplifier (PA), a stabilizer, an input matching network, and biasing components. The tapered slot antenna element is reshaped so that its input impedance closely matches the optimal load impedance of the HEMT (Zopt=6+j38 Ω@22 GHz), which enhances the system efficiency. The peak AIPAA’s Power-Added Efficiency (PAE) is ≥ 56% with ≤ 9% variation over scan coverage (±50⁰) at 1.5dB power backoff from P1dB. The relative frequency bandwidth with PAE above 25% is between 9%-13% over the scan range. The proposed AIPAA demonstrates less than 0.9dB and 1dB scanloss over the scan coverage in terms of antenna array gain and PAs’ power gain (Gp), respectively. The peak PA-integrated array gain and EIRP @P1dB of 24dBi and 51dBm are achieved, respectively. The proposed AIPAA is 18 × 58 × 17 mm3 with a cell size of 9.2 × 6.5 × 1.8 mm3 (0:67 × 0:5 × 0:13 λ3). The measurements are in good agreement with electromagnetic and circuit co-simulation results.

This paper presents a novel method for analyzing and reducing the total coupled power in the Active Highly Integrated Phased Array Antennas (AIPAA), incorporating both the nonlinear impacts of the power amplifier (PA) transfer functions and the normalized scattering matrix. The proposed method introduces a new degree of freedom to reduce the coupling level utilizing two key factors: (i) the balance between the PAs' transfer function gain and the coupled power difference to the PAs' gate and drain/antenna ports, and (ii) the out-of -phase sum based on PAs' transfer function phase response. The proposed method is theoretically demonstrated by accounting for all tones and also is subsequently simplified for the main tone. The method is practically validated by applying the method to a 1 x 5 AIPAA structure to reduce the total coupled power at its drain/antenna port. The results demonstrate a coupling reduction of -5 dB as an average with a maximum reduction of -20 dB, over a scan coverage of & PLUSMN;30 degrees and 10% fractional bandwidth at 22 GHz for the center cell.

Circular polarization is an essential feature for small antennas designed to connect with satellite navigation systems. Here, a Q-factor optimization problem constrained with a circular polarization radiation requirement is formulated and solved. The constrained problem is shown to be solvable as fast as calculating the Q-factor bound without the constraint. Two antenna models are designed to assess the bounds. One aims for bandwidth, and the other for circular polarisation. The well-known relation between the Q-factor and the available bandwidth is used to evaluate the designs. The circularly polarised antenna is close to the bound and has a wide field of view. The Q-factor bound provides insight into desirable antenna positions on the small device.

Mutual coupling between antennas is a key parameter in multi-antenna systems. The present paper describes a method to map contributions to the magnitude of the mutual coupling spatially on surfaces between the ports. The method utilizes the reaction theorem for electromagnetic fields to obtain the contribution of the coupling from different regions. The derived result is valid for and applied to the strong-coupling regime, where the mutual impedance is of the same order as the self-impedance.

Communication at mmWave frequencies is currently being developed to achieve higher data-transfer capacity combined with a larger coverage in an energy-efficient way. Highly integrated TX phased array antennas have recently been proposed to improve the performance of such systems. The high-density integration results in an ultra-compact array system in which the knowledge of an accurate port-to-port mutual coupling and the associated power-transfer under operation is essential. The impact of the mutual coupling determines the quality of the overall system performance. This includes areas such as operation linearity, system efficiency, total output power, and the stability of the system. The investigated case has a non-standard impedance at each port of the antenna where an amplifier is directly integrated into the antenna elements in the array (direct-integration). Thus as a difference to passive antennas [1], a full multi-port electromagnetic-circuit co-simulation is required to characterize the system. Note that all ports need to be included, e.g. all the ports of the power amplifier, biasing, antenna for each element in the array. This increases the complexity of the system simulations due to the following facts: 1. the unit-cell consists of multi-ports with often unequal complex termination impedance, 2. active components are non-linear and their behavior depends on both the input power and the considered frequency. 3. although the transferred power to each port through mutual coupling is essential, the particular ports, like the input port of the amplifier, are more sensitive. 4. The transferred power depends on the scan angle of the array. This effect is similar to that of a passive antenna where the active-reflection coefficient depends on the scan-angle.

In this paper, a novel omnidirectional array antenna based on Substrate Integrated Waveguide (SIW) technology is proposed. The proposed antenna is realized by removing some vias on one side of the narrow wall of a conventional SIW which results a slot antenna. The gain of an antenna with 8 slots is 9.5 dBi with a gain variation of 2 dB over 360°. Besides, the antenna polarization is horizontal with the cross-polarization level of less than −40 dB in comparison with the co-polarization level. The bandwidth of 80 MHz at 6.66 GHz is achieved with the antenna's dimension of 300×22.86×1.575 mm 3 . The measurement results of the antenna have a good agreement with the full-wave simulation results. The proposed antenna is well suited for 6 GHz wireless access applications due to the low cost, low complexity, simple integration, and high gain.

Mutual coupling, or equivalently, the isolation between antennas, is a key parameter in antenna system design. In this work, the previously defined impedance density is generalized, and it is demonstrated how it can be used to obtain spatial information about the mutual coupling. The generalized impedance density is a real-valued scalar and it can be visualized as a three-dimensional density in space. It is shown that there is a strong connection between regions with a positive (negative) generalized impedance density and a decrease (increase) of the coupling when an absorber is placed in that region. This predictive ability is a useful feature, which is tested for three numerical cases. The results are robust to the shape of the platform, and it can be compared across frequencies. By placing absorbers based on the generalized impedance density, it is possible to reduce the required amount of absorbers needed to obtain a certain reduction in mutual coupling. The visualization results and predictions of absorber positions are compared with a Poynting vector based method. Placing absorbers based on the generalized impedance density had a larger impact on the mutual coupling, compared to the predictions with the Poynting vector based method in the investigated cases.

Full-wave simulation of antennas together with fabrication and measurement of their electromagnetic properties are vital steps in the design of antennas. The complexity and cost of measurements push an increasing part of the design into the simulation domain. This is, in particular, true for highly-integrated arrays and antennas installed on large platforms, where direct measurements of all desired quantities are difficult or impossible. With the rapid development of high-quality electromagnetic simulation, it is nowadays accepted that well-conducted simulations and well-designed antennas closely agree for electromagnetic problems of reasonable electromagnetic size. Robustness and stability of electromagnetic quantities against small perturbations are vital in preserving simulation results into realization and measurements. This type of examination has a long history, with early results on shape-tolerances for the far-field behavior associated with radome antennas.

In this paper, we propose a unit-cell element suitable designed for wide-angle scanning active phased array antennas. The design method utilizes the sub-array factor theory inside the unit-cell. The generalization shows that the unit-cell architecture can be applied to different elements to improve their embedded radiation pattern. The presented unit-cell is comprised of three similar radiating elements. It consists of one excited element and two unloaded/open passive parasitic ones. The 1 dB beamwidth of the embedded radiation pattern of the designed unit-cell with a miniaturized vivaldi element is around 130 o in the E-plane. It results in a gain reduction of -1.2 dB over ±60 o scan angles for a linear array. The total array factor of the proposed architecture is similar to the dense arrays with half-wavelength interelement distances and smaller (high-density array), with the same physical size. Another advantage is that the active impedance variation per scan angles has improved in comparison with half-wavelength arrays unit-cell. The number of excited ports is 1/3 of the equivalent high dense array with the same interelement distances. The whole aperture-size of the designed unit-cell is about half-wavelength in which there are three radiating elements with only one excited element.

This article considers a small embedded planar antenna in a square-shaped terminal of 25 cm(2) at a frequency band near 900 MHz, intended for long-range communication. This article aims to shows how the Q-factor bounds can be used to predict the performance of such an antenna. Both to determine the optimal bandwidth and the variations in the total efficiency but also to help to inspire the antenna design shape. The choice of shape and position impacts both bandwidth and efficiency. The latter is illustrated by a center-edge positioned folded inverted F-antenna with higher efficiency, as compared to, a more bandwidth optimal meander antenna at the corner. Fabrication and measurements show that the corner positioned antenna is close to bandwidth optimal, and also that it and the associated optimal current have a similar radiation pattern.