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In this article, a radiant metal block structure is proposed for designing reduced-size cavity-backed filtering antenna. The method of characteristic mode analysis is used to investigate the characteristic current modes of the cubic metal block, one of which can maintain a high inherent radiation capacity over a wideband. In addition, the radiation quality factor of the utilized mode is described in detail. Based on this characteristic current mode, metal block antennas with different bandwidths can be designed to realize broadside radiation. The proposed metal block antenna element can maintain stable radiation characteristics in the passband and the realized gain is greater than 9 dBi. To design the filtering antennas, the proposed metal block is cascaded with traditional resonant cavities. A third-order metal block filtering antenna array is fabricated for verification. The measurement results are basically consistent with the simulated ones. The maximum measurement gain is 12.85 dBi and the gain in the passband is flat. [ABSTRACT FROM AUTHOR]

In this article, a method for designing a tunable coaxial filtering antenna is presented. The proposed filtering antenna has a simple structure, consisting of a standard waveguide and some metal cylinders. Instead of utilizing traditional coupling irises, an all-cylinder coupling structure is employed for flexible tuning. Thus, the center frequency and bandwidth of the coaxial filtering antenna can be conveniently tuned by adjusting the lengths of the cylinders. Theoretical analysis and design process of the filtering antenna are given in detail. A third-order tunable filtering antenna with Chebyshev response is designed to verify this method. The tunable range of the center frequency is from 9.25 to 10.75 GHz, and the tunable range of the fractional bandwidth (FBW) is from 2% to 8%. The measured gain of the antenna is flat and greater than 6.5 dBi along the tuning range. In addition, the antenna maintains good impedance and radiation characteristics throughout the tuning range. [ABSTRACT FROM AUTHOR]

A high selectivity and high efficiency filtering antenna is presented in this communication. A double-layer filtering patch antenna is first proposed, which can generate a radiation null due to the mixed electric and magnetic coupling between the two patches. By changing the length of the slot line on the patch, the position of the radiation null can be flexibly set at the lower band edge or upper band edge. Based on this mechanism, a three-layer patch antenna with two radiation nulls is designed. Two radiation nulls are introduced on both sides of the passband, consequently, a quasi-elliptic bandpass response can be obtained. Moreover, the proposed antenna features high efficiency since no extra filtering circuit is utilized. A prototype antenna operating at 1 GHz is designed, fabricated, and measured for validity verification. The measured results agree well with the simulated one, showing that the prototype antenna has a fractional impedance bandwidth of 16% (0.91–1.07 GHz), a flat passband gain response with peak value of 8.83 dBi, a high efficiency about −0.41 dB in the passband. The band-edge selectivity is highly improved by the radiation nulls near the passband edge. [ABSTRACT FROM AUTHOR]

In this communication, a high-selectivity and high-gain filtering slot antenna operating in $X$ -band is designed based on a triple-mode cavity resonator. Three modes of the cavity resonator, namely, TE101, TE011, and TM110, are excited by properly selecting the position and size of the coupling aperture. By adjusting the frequency and external coupling of the three modes, transmission zero can be introduced on both sides of the passband, achieving high selectivity of antenna gain. Because the cavity has a high unload quality factor, the radiation efficiency in the passband is high and flat. In order to increase the gain of the filtering waveguide antenna, a grid-slotted patch is added on the top of the slot. To demonstrate the concept, an $X$ -band waveguide filtering antenna is designed and fabricated. Measured results show the filtering antenna can obtain a gain of greater than 8.5 dBi in the frequency range of 9.8–10.2 GHz. [ABSTRACT FROM AUTHOR]

A method of designing a printed filtering dipole antenna using codesign of bandpass filter (BPF) and antenna is proposed in this article. The printed dipole antennas consist of two identical arms, which are placed on the opposite sides of the substrate. One arm of the dipole is used to construct the half-wavelength resonator of a typical BPF, and thus filtering and radiation performance can be obtained simultaneously. A third-order filtering dipole antenna is designed and analyzed in detail to demonstrate the codesign method. The measured S11 of the filtering dipole antenna is less than −11 dB in the range of 0.94–1.094 GHz and the peak gain is 7.8 dBi. To extend this design method, a third-order ±45° dual-polarized filtering dipole antenna is designed and measured. The measured S11 and S22 of the ±45° dual-polarized filtering dipole antenna are both less than −11 dB in the range of 0.94–1.08 GHz. Besides, the isolation between them is higher than 20 dB in the passband. The measured radiation patterns show that the dual-polarized filtering dipole antenna would be a potential candidate for base stations. [ABSTRACT FROM AUTHOR]

A novel broadband three-input four-output ($3 \times 4$) Butler matrix is proposed in this communication. The proposed Butler matrix achieves equal power division and −90°, 0°, and +90° phase differences between output signals in the 2–4 GHz frequency range. By using the $3 \times 4$ Butler matrix as a four-element antenna array feed network, a broadside beam and two symmetrical beams are realized over a wideband frequency range. The proposed Butler matrix has been verified by measurements of S-parameters and radiation patterns using the broadband four-element antenna array. The measurement results are in good agreement with the simulation results. In addition, the proposed $3 \times 4$ Butler matrix can be extended for use with antenna arrays with a higher number of beams. [ABSTRACT FROM AUTHOR]

In this communication, an approach for designing multi-band microstrip slot antennas is proposed. Multiple resonators are loaded on a conventional microstrip slot antenna to introduce multiple resonances. The number of the operational bands of the antenna can be easily controlled by varying the number of the loading resonators. The locations of each resonance can be adjusted by varying the positions and lengths of the loading resonators together with the geometry of the slot if necessary. For validation, prototypes with dual-, tri-, and quad-band operations are designed, fabricated, and measured. Since no modified slot is used in this design method, all the operating bands show good bidirectional radiation patterns. The measured and simulated results agree well. The proposed antenna features low profile, small radiating slot, and easy fabrication, showing promising application prospect for modern communication system. [ABSTRACT FROM AUTHOR]