Low-Profile Slot Transmitarray Antenna 175
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174 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 1, JANUARY 2015 Low-Profile Slot Transmitarray Antenna Bahman Rahmati and Hamid R. Hassani Abstract—A novel ultrathin slot-based transmitarray antenna is the aperture, at least 360 of phase range is required. The mag- presented. The related unit cell consists of two thin dielectric layers nitude of transmission is an additional design consideration for placed on top of each other, with annular ring slots placed on the transmitarray elements. In addition to a large phase range, large outer layers and a PBG element placed on the common interface. The combination of the two annular ring slots and the PBG element bandwidth, low reflection, low insertion loss, and small phys- acts as a bandpass filter for the transmitted wave. The design of the ical profile are also desirable. elements can be carried out through a simple circuit-based anal- Up-to-date transmitarray antennas have been designed using ysis approach. The proposed two-layer structure, the unit cell, has multiple dielectric layers with printed patch antennas on each a thickness of and provides the required 360 of phase layer. The designs are through two approaches: the layered scat- shift. Furthermore, over the bandwidth, the unit cell can have up to 7 of phase error for incident angles ofupto50 . The unit cell terer approach (LS) and the guided-wave approach (GW). and the full transmitarray prototype are fabricated, and the results In the LS approach, the required phase shift for each array el- were compared with simulation and discussed. The full transmi- ement is achieved by varying the element dimensions. Multiple tarray has 1 dB gain bandwidth of 5.7%andefficiency of 38% dielectric layers with printed patch antennas on each layer are at the center frequency. The structure is simulated via HFSS and coupled through slots in the ground plane of the patches [1]–[3], CST software package. air gap layers [4], [5], or printed cross-elements [6] controlling Index Terms—Antenna array, lens antenna, microstrip array, re- the level of coupling between the two patches, resulting in a unit flectarray, transmitarray. cell acting as a spatial phase shifter. In the case of [1] and [2], unit cells that have and thicknesses, respectively, were proposed. The transmitarray of [3] provides 7% bandwidth I. INTRODUCTION with a thickness of . The transmitarrays of [4] and [5] that use coupled patch rings on a 4-dielectric layered structure N wireless technology, in the area of antenna, the need (approximately thick) provides 1 dB gain bandwidth of I for low-profile, lightweight, low-cost, broadband, and 7.5%. The unit cell described in [6] uses four dielectric layers, high-gain antennas has increased recently. Among the possible with each layer having a thickness of . A full review of options, the transmitarray antenna is a promising potential the various phase-shifting surfaces developed in recent years is technology to meet these requirements. reported in [7]. Comprising a planar array of printed elements, the transmi- In the guided-wave approach, the required phase shift for tarray avoids the fabrication complexity inherent in parabolic each element is achieved through a physical phase shifter, reflectors, and being usually less than one wavelength thick, it whereas the element dimensions are kept constant throughout has size and weight advantages over shaped dielectric lenses. the array. Two dielectric substrates are separated from each Free-space feeding improves the radiation efficiency by elimi- other by a middle layer. Two patch antennas are on the outer nating the losses that occur with corporate feed networks of a layer of the substrates, and either a simple printed transmission large planar array of printed elements. In a reflectarray with the line phase shifter [8], [9] or a circuit-based phase shifter [10], feed being in front of the array aperture, the efficiency is reduced [11] connecting the two patches physically, is on the middle because of feed blockage losses, whereas in a transmitarray, be- layer. In [10], a unit cell with an overall thickness of is cause the feed is placed on the opposite side of the array, the given, whereas in [11], a thick transmitarray design providing blockage does not exist. 5.8% of the bandwidth is described. To form a pencil beam in transmitarray design, the phase A summary of comparison between previously reported shifts from the source to the transmitarray elements and, subse- transmitarray designs is presented in Table I. In the above-men- quently, to a planar surface must be equal for the wave beams on tioned works related to the LS and GW approaches, the required a planar surface before the aperture. Transmitarray collimates phase shift that can be achieved from a unit cell typically re- an incident wave by manipulating the phase shifts at different quires at least three dielectric substrates, making the overall positions on the planar surface. To produce arbitrary phases on antenna structure thick and expensive. To minimize the overall transmitarray antenna thickness, Manuscript received April 15, 2014; revised August 05, 2014; accepted Oc- while keeping the bandwidth almost similar to the previously tober 07, 2014. Date of publication November 07, 2014; date of current version reported wideband structures, this paper uses the LS approach December 31, 2014. The authors are with the Electrical & Electronic Engineering Department, and presents a new design based on slot elements that provides Shahed University, Persian Gulf Highway, Tehran, Iran (e-mail: brah- the required phase range with less number of dielectric layers. [email protected]; [email protected]). The unit cell comprises two dielectric layers with annular ring Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. slots on the outer layers and a uniplanar compact photonic Digital Object Identifier 10.1109/TAP.2014.2368576 bandgap (UC-PBG) element on the inner layer that acts as 0018-926X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. RAHMATI AND HASSANI: LOW-PROFILE SLOT TRANSMITARRAY ANTENNA 175 TABLE I COMPARISON BETWEEN REPORTED TRANSMITARRAY ANTENNAS UC = UNIT CELL,LS=LAYERED SCATTERER,GW=GUIDED WAV E Fig. 2. Simulated transmission coefficients (magnitude and phase) and ADS equivalent circuit response of an annular ring slot resonant element, Fig. 1a, for various slot gaps, ( and ). Fig. 3. Simulated transmission coefficients (magnitude and phase) of an an- nular ring slot resonant element for various slot side lengths, Fig. 1. Unit cell configuration and its equivalent circuit: (a) annular ring slot ( and ). resonant element on a dielectric substrate, (b) UC-PBG coupling/resonant ele- ment, and (c) the two-layered structure. A. Annular Ring Slot Element the coupler and as an additional resonator. In the following It is known that a slot antenna element can provide a low-pro- sections, the design of a thin two-layer slot transmitarray unit file, lightweight, low-cost, and broad bandwidth. As such, in the cell providing 360 of phase shift along with the transmitarray design of a transmitarray, one can use slot element rather than is provided. The design is shown to be easily extendable to the usual printed patch element to provide wave transmission larger number of layers providing a multiple of 360 of phase with the least possible loss and with the maximum amount of shift. phase shift. For this purpose, a very simple annular ring slot can be selected that would allow wave transmission and provide the II. UNIT CELL DESIGN required phase shift. In this section, the design of the proposed slot transmitarray For the simple single-layer annular ring slot chosen in this unit cell is provided, the structure of which, is shown in Fig. 1. work, Fig. 1a, the transmission magnitude and phase are plotted Thin unit cell with minimum number of layers to achieve the in Figs. 2 and 3 for various slot gaps and slot side lengths. In desired phase shift is the main goal of this unit cell design. Ac- doing so, when one parameter is changed, the other parameters cordingly, the overall design is based on a pair of annular slot of the slot are kept constant. ring elements, each is replaced on the outer surface of two at- For the above single-layer annular ring slot, to have knowl- tached dielectric substrates, and a printed PBG element is on edge of the overall phase response and the passband behavior, the inner surface of the substrates. The PBG acting as a res- a simple model based on a parallel-type LC circuit [Fig. 1(a)] onant/coupler element reduces the requirement of extra layer. that can provide similar results to that of the full-wave results Parametric studies of dimensions of the annular slot and a study can be introduced. Fig. 2 also shows the transmission magnitude of different coupling element types to provide a compact unit and phase of the single annular ring slot based on the equivalent cell with the proper range of phase shift angles are provided. LC model as obtained through Advanced Design System (ADS) ThedielectricsubstratesusedinthispaperareRO4003witha package. As can be seen, the CST simulation and the model re- permittivity of 3.55 and a thickness of 0.8 mm. sults are very close to each other for both narrow and wide slots. 176 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 1, JANUARY 2015 TABLE II EQUIVALENT LC MODEL PARAMETER VALUES FOR THE UNIT CELL OF FIG.1(A) FOR VARIOUS GAP SIZES G In obtaining such close results, the quality factor (QC and QL) of the lumped elements should also be taken into account.