4438 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 9, SEPTEMBER 2013 Differential Fabry–Perot Resonator Antennas Kai Lu and Kwok Wa Leung, Fellow, IEEE Abstract—The differential Fabry–Perot resonator (FPR) an- tenna is investigated for the first time. It has advantages of suppressing the cross polarization and enabling integration with differential circuits. Its basic structure consists of two vertical parallel metal plates, which are connected by a horizontal ground plane. The differential FPR antenna is fed by a pair of differential -probes protruding from the ground plane. It is found that the basic differential FPR antenna has considerable sidelobes in the -plane. To suppress the sidelobes, a modified differential FPR antenna is proposed which has a pair of ridges at each side-opening of the parallel plates. In addition, a second modified differential FPR antenna is also proposed to reduce undesirable backward radiation. The second modified antenna has a pair of ridges placed at the top of each metal plate. It is found that all of the basic and modified antennas have low cross polarizations. The two modified differential FPR antennas were fabricated and measured, and the measurements agree reasonably well with HFSS simulations. The first and second modified antennas have calibrated measured gains of 14.2 dBi and 15.4 dBi, respectively, with measured 10-dB impedance bandwidths of . Index Terms—Backward radiation reduction, cutoff structure, differential antennas, Fabry–Perot resonator antennas, sidelobe suppression. I. INTRODUCTION O WING to the advantages of its high directivity, high efficiency, low profile and ease of fabrication, the Fabry–Perot resonator (FPR) antenna has received tremendous attention in the last decade [1]–[5]. A conventional FPR an- tenna has a metal ground plate and a parallel partially reflective plate, along with a primary radiator. Its resonance frequency is determined by the separation between the parallel plates [5]. To improve the performance of the FPR antenna, some design techniques have been introduced to the ground plane and partially reflective surface. For example, an artificial magnetic conductor or high impedance surface has been used in place of the metal ground plane to reduce the antenna height [6]–[8]. To broaden the bandwidth, the partially reflective surface can have non-uniform patterns [9], [10], multi-layer structures [11], or even non-planar shapes [12]. Usually, the partially reflective surface of a FPR antenna has a very high reflectivity (smaller than but close to 1), resulting in a highly directional radiation pattern with a narrow beam [2], [4]. For mobile Manuscript received July 25, 2012; revised March 30, 2013; accepted June 03, 2013. Date of publication June 07, 2013; date of current version August 30, 2013. This work was fully supported by a GRF research grant from the Research Grants Council of Hong Kong SAR, China (Project No.: CityU 116609). The authors are with the State Key Laboratory of Millimeter Waves and De- partment of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong. (e-mail: kailu2@cityu.edu.hk; eekleung@cityu.edu.hk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2013.2267196 communications, however, a broad-beam [13] or fan-beam [14] antenna can provide larger coverage than a narrow-beam [15] antenna and should therefore be more attractive. Over the past years, some FPR antennas with moderate gains have been proposed [6], [16]–[18], such as fan-beam FPR antennas with rectangular radiating apertures [17], [18]. The beam solid angles of the antennas in [16] and [17] are 650 (25 26 ) and 576 (12 48 ), respectively, where the superscript is the square degree of a solid angle [19]. However, it is diffi- cult to obtain a larger solid angle from the conventional FPR antenna because of its inherent narrow-beam characteristic. To broaden the beamwidth, a new class of FPR antennas has been recently proposed [20], which can provide a wider beam solid angle of 896 (16 56 ). The new FPR antenna has two vertical parallel metal plates, which are connected by a horizontal ground plane. The strongest radiation direction of this FPR antenna is normal to the ground plane, instead of the parallel plates as found in a conventional FPR antenna. Since the new FPR antenna simply employs solid conducting plates instead of partially reflective plates, its design and fabrication are very much easier than for the conventional FPR antenna. The differential signaling technique is widely used in modern radio-frequency/microwave circuit designs because it can pro- vide high signal-to-noise ratios. Also, it can improve radiation patterns when applied to antennas, such as reduce the cross-po- larized field [21] and eliminate the tilting angle [22]. Further- more, it enables direct integration of antennas with differential circuits. As a result, a number of differential antennas have been investigated over the last decade [21]–[27]. In this paper, the differential FPR antenna is investigated for the first time. The basic differential FPR antenna is based on the recently proposed design [20]. This generic structure, however, undesirably produces significant sidelobes in the -plane. To suppress the side lobes, a modified differential FPR antenna is introduced which has a pair of ridges fabricated at each side- opening of the parallel plates. The modified antenna can reduce the sidelobe level at by about 22 dB. For a unidirectional antenna, it is desirable to reduce its backward radiation to avoid wasting energy. An approach in achieving this is to make the main beam more directional. In general, this can be done by enlarging the radiating aperture, which can be realized in [20] by increasing the spacing between the parallel plates. However, changing the spacing will also affect the operating frequency of the FPR antenna. In this paper, a second modified differential FPR antenna for suppressing backward radiation is also introduced. It has a pair of ridges located at the top of the parallel plates. The spacing between the top ridges is wider than that between the parallel plates to provide a larger radiating aperture. Since it is needless to change the spacing between the parallel plates, there is no virtual effect on the resonance frequency of the FPR antenna. 0018-926X © 2013 IEEE