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Low Profile UHF/VHF Metamaterial Backed Circularly Polarized Progress In Electromagnetics Research C, Vol. 60, 11–20, 2015 Low Profile UHF/VHF Metamaterial Backed Circularly Polarized Antenna Structure Taulant Rexhepi1, *, Igor Bendoym2, Ada-Simona Popescu1, Andrii Golovin1, Johnny Daniel3, Kate J. Duncan3, and David Crouse2 Abstract—In this work, a low-profile metamaterial backed planar antenna structure designed to work in the UHF/VHF range is presented. The antenna has right-hand circular polarization. It is ideal for satellite-based communications and radar systems. An artificial magnetic conductor was designed using a metamaterial composed of a split ring resonators to reduce the size of the planar antenna and ground plane system. The proposed artificial magnetic conductor has more confined surface waves at the reflecting plane than previous designs and is suitable for circular polarization. Through numerical simulations, performance characteristics including return-loss, and realized gain of the antenna systems are calculated and analyzed in the VHF range. The proposed antenna system is narrowband and is linearly scalable in the range of 100 MHz–1 GHz. 1. INTRODUCTION Over the recent decade, a major area of interest for microwave researchers has been the replacement of metallic electric-conducting ground planes of antennas with artificial magnetic material surfaces made from metamaterials [1–7]. With such structures, it was demonstrated that the distance between the antenna and the metal ground plane can be significantly reduced while maintaining the optimized gain [2]. The radiation emitted by a planar antenna has two components: a forward propagating component emitted in the direction of the receiver or target, and a backwards propagating component that, unless captured, would be wasted. Antennas typically use metal ground planes to capture this second component of the signal. However, the ground plane must be placed at a particular distance away from the antenna to ensure that the backwards propagating component after being reflected constructively interferes with the forward propagating component, thereby increasing the gain of the system by 3 dB [8]. With the ∼ 180◦ phase change that an electromagnetic beam undergoes upon reflection from a metal, the minimum antenna-ground plane separation distance is λ/4 when the ground plane is a metal [8]. This requirement of a λ/4 separation between the emitting element and standard metallic ground plane becomes problematic when one needs to design compact lightweight antennas. It is also highly desired in applications such as GPS and SATCOM communications that the antenna is circularly polarized, hence special consideration has to be made for the ground plane to ensure that the circular polarization of the antenna is maintained upon reflection off the ground plane. With metamaterial ground planes that mimic Artificial Magnetic Conductors (AMCs), the phase change of the beam upon reflection can be made ∼ 0◦ over a certain frequency bandwidth. This allows the antenna-ground plane separation to be close to zero for that frequency band. The first generation Received 9 September 2015, Accepted 16 November 2015, Scheduled 23 November 2015 * Corresponding author: Taulant Rexhepi ([email protected]). 1 Electrical Engineering Department, City College, The City University of New York, 160 Convent ave, New York, NY 100310, USA. 2 Phoebus Optoelectronics LLC, New York, USA. 3 US Army, Weapons and Materials Research Directorate, Aberdeen Proving Grounds, MD 21005, USA. 12 Rexhepi et al. of AMC arrays described in [1–4], consisted of an array of square patches connected through via holes to the ground plane to prevent the excitation of surface waves. This configuration came to be known as the mushroom structure. These structures work well for circularly polarized antennas in the higher RF spectrum, however when applied to antenna systems in the UHF/VHF band, the structures become problematic to fabricate. The AMC presented in this work is easier to fabricate since it does not require any via-holes and shows better performance, in terms of surface wave excitation. 2. GEOMETRY OF THE ANTENNA Planar antennas have attracted the attention of many researchers and industry engineers in the recent years due to the ease of the fabrication. There is an increasing demand for both military and industry applications for low profile, compact size, conformal and circularly polarized antennas. Another attractive feature of planar antennas is the fact that they can be fabricated with standard printed circuit board (PCB) manufacturing methods that allow for a wide variety of geometric shapes [5–8]. Together with a ground plane, these planar antennas can provide very good directivity [9]. For applications in the higher RF spectrum, namely the S-X bands, they are also very compact in size, however for the lower frequencies they are bulkier, which depending on the application can be problematic. The main focus of this work is the design and performance of the AMC surface and not the primary radiator. In spite of this, care was taken to optimize the geometry of the antenna such that it has a well-defined gain in the frequency range of interest. The antenna described here is a four clover shaped planar antenna, (Figure 1(a)). The clover or teardrop shape has been widely used, particularly for broadband technologies in wireless communications [7, 8]. Concerning the physical dimensions of the structures in this work (i.e., antenna and metamaterial ground plane), all results are normalized relative to the lowest frequency f0. Depending on the desired operating frequency, all structural dimensions of the primary radiator are given in terms of the operating wavelength, λC and can be scaled identically using the same scale factor that scales the operating frequency, fC. Good performance of the antenna is maintained for scaling factors that yield fC values in the range of 100 MHz–1 GHz. The bandwidth of the antenna system is primarily dictated by the bandwidth of the AMC structure which also scales well with frequency. The substrate for the antenna is λ/1000 thick with a dielectric constant value of 2.17 and loss tangent of 0.0008. Being that the antenna is a cross-dipole, its gain in free space (i.e., no ground plane) is L A B T W G S (a) (b) Figure 1. (a) Geometry of the four clover shaped planar antenna: dimensions A = λ/5andB = λ/7. The feed of the antenna is the small crisscrossing structure in the middle, all four clovers are same dimensions and rotated 90◦. (b) The geometry of the SRR unit cell, the dimensions are based on the wavelength, λ of the resonant frequency, fR as follows: L = λ/6.25, S = G = λ/2000, T = λ/48, and W = λ/110. The dielectric thickness is on the order of λ/315. The separation between adjacent rings is λ/2000 on all four sides. Progress In Electromagnetics Research C, Vol. 60, 2015 13 around 2.5 dB. The metal reflector adds another 3 dB in the direction of desired gain increase. Further optimizations techniques, such as employing the clover design and rounding sharp edges add another 2 dB making the total gain at boresight roughly 7 dB. 3. ARTIFICIAL MAGNETIC CONDUCTOR SURFACE AMCs can be of various configurations. The most widely used is the mushroom structure which consists of square patches interconnected to a metallic ground plane. Another way of implementing such a system is by using two layers; the top layer consisting of a capacitive array which would not be connected to the ground plane. In this form the top layer consists of an array of periodic elements with lateral dimensions on the order of λ/10 [10, 11]. The characteristics of AMCs are dictated by the geometries, orientations and configurations of the metal and dielectric structures of which they are composed; such characteristics and their dependencies have been the focus of many research efforts in the microwave field in recent years [1–4, 10, 11]. In this work, the Split Ring Resonator (SRR) was chosen as the unit cell of the AMC. While different in important ways than previously reported AMCs based on split- ring resonators (SRRs) [10–15], the resulting AMC described in this work (Figure 1(b)) still uses this important structure as its key component. AMCs based on SRRs have been used as an innovative method for reducing the overall thickness of planar antennas [11–14]. In [16], Ayed et al. demonstrated that this method can be used to reduce the profile of a simple linearly polarized dipole antenna. In their work, SRRs were also chosen over the conventional square patches. It is important to note that in this configuration the magnetic moment of the SRR is not being excited, the geometry serves to provide mainly capacitance. Other similar works have also used SRRs for antenna systems [12, 14, 16], however the applications were limited to linearly polarized antennas. Mushroom structures have been shown to work well with circularly polarized antennas for GPS applications in the L band [17, 18]. To the best of our knowledge, no previous research has been reported describing SRR AMCs operating in the VHF band for circularly polarized antennas. Our work described in his paper verifies that such structures work well in VHF and UHF antenna systems. The unit cell of the AMC given in Figure 1(b) was based on the work done by [19]. The dimensions of the unit cell are presented in terms of the wavelength, λ corresponding to the resonant frequency, fR of the AMC. Good performance of the AMC is maintained for scaling factors that yield fR values in the range of 100 MHz–1 GHz. The fractional bandwidth of the AMC is maintained through this given range. In this design the substrate that was used was the Rogers TTM13i dielectric (εR =12.2).
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