Afrl-osr-va-tr-2014-0101 Protecting Superconducting Hts-antennas by Meta-material Cloaks

This project addressed the analysis and design of a cryogenic microwave anisotropic wave guiding structure that isolates an antenna from external incident fields from specific directions. The focus of this research was to design and optimize the radome's constituent material parameters for maximizing the isolation between an interior receiver antenna and an exterior transmitter without significantly disturbing the transmitter antenna far field characteristics. The design, characterization, and optimization of high-temperature superconducting metamaterials constitutive parameters are developed in this work at X-band frequencies. A calibrated characterization method for testing arrays of split-ring resonators at cryogenic temperature inside a TE10 waveguide was developed and used to back-out anisotropic equivalent material parameters. The artificial material elements (YBCO split-ring resonators on MgO substrate) are optimized to improve the narrowband performance of the metamaterial radome with respect to maximizing isolation and minimizing shadowing, defined as a reduction of the transmitted power external to the radome. The optimized radome is fabricated and characterized in a parallel plate waveguide in a cryogenic environment to demonstrate the degree of isolation and shadowing resulting from its presence. At 11.12 GHz, measurements show that the HTS metamaterial radome achieved an isolation of 10.5 dB and the external power at 100 mm behind the radome is reduced by 1.9 dB. This work demonstrates the feasibility of fabricating a structure that provides good isolation between two antennas and low disturbance of the transmitter's fields. Introduction This work presents a microwave cryogenic wave guiding structure that provides isolation between an interior receiver and an exterior transmitter without significantly distorting the transmitter antenna far field pattern. A sketch illustrating the problem and approach is shown in Frig. 1 . An arbitrarily-polarized transmitting antenna polarized is able to communicate with an intended receiver some distance away from the isolation structure, minimally affected by its presence. In the figure, the antenna at the center is shown as cross-polarized with respect to the transmitting antenna, which means there is already isolation between the two. However, some power could be coupled into the center antenna. In our measurements and simulations, these two antennas are co-polarized to simulate the worst case. We will show that isolation can be achieved even in the co-polarized scenario. Small antennas can be made in superconducting technology for improved gain and low noise amplifiers can be cooled for improving the noise figure. This means that the wave guiding radome can also be implemented in superconducting technology, resulting in low loss. It will be shown that a radome that meets the operating requirements has electromagnetic properties that are not found in natural materials. Thus, it will be constructed of metamaterial elements, such as split-ring resonators. The structure is multi-layered and cylindrical in shape, with each layer constructed from arrays of high-temperature superconducting split-ring resonator metamaterials, artificial materials that have properties that are not found in regular materials. Figure 1: A superconducting receiver antenna is located inside the isolation metamaterial radome in the path of a pair of antennas and is receiving radiation in the zenith direction undisturbed by the other two antennas. The communication link between the pair of dipole antennas (left and right) is unaffected by the radome. To understand the functionality of the structure, imagine a plane wave propagating in free space with no object in its path. See Figure 2a, where the gray region shows the placement location of the structure. The straight arrows show the direction of the Poynting vector field lines. When the isolation radome is placed in the path of propagation (gray region in Figure 2b), the structure isolates its interior from the outside fields, while outside of the radome the fields are not altered. Such structures have been demonstrated in simulations [1-3] and similar “metamaterial cloaking'' structures have been constructed and measured using normal conducting resonant elements [4,5]. The structure in [4] has properties such that the permeability is radially varying and is measured inside a parallel plate waveguide. The structure in [5] has a radially varying permittivity and is measured in free space (anechoic chamber). In both cases, the amount of isolation in the interior of the cloak and field distortion exterior to the cloak are not quantified. Thus, one can only visually judge the performance of the cloaking ability of the structure. In this project, a probe measures the received power of the measurement environment with and without the wave guiding radome. This gives us the information to quantify the isolation and shadowing characteristics of the structure. Shadowing is defined as a reduction of external transmitted power in the presence of the radome with respect to the no-radome case. Using a cylindrical shape has the advantage that it can be opened on the ends to allows an antenna to be placed in its interior to receive and transmit radiation in the perpendicular direction. For our work, only the guiding structure has to operate at the same frequency as the transmitter. The receiver can operate at any frequency. Figure 2: (a) and (b) show the direction of the Poynting vector field lines. In (a) no object is placed in its path. In (b) a cylindrical electromagnetic cloak is placed in the path of the power flow. The interior is completely isolated from the exterior. In addition, the lines are undisturbed outside of the structure. For small antennas, the loss resistance, RL, in the conductor and substrate can dominate the radiation impedance, since the real resistance Rr is small. The radiation efficiency given by will therefore be small. To demonstrate the impact loss has on the radiation efficiency, consider an electrically small single copper loop antenna, with diameter d=10 mm and wire radius b=0.5mm. Generally, an antenna is considered electrically small if its largest dimension is less than or equal to onetenth of the operating wavelength (≤ λ/10). For example, the largest dimensions for a dipole, a loop, and a microstrip patch antenna are the length, diameter, and diagonal, respectively [6]. The radiation resistance Rr and loss resistance RL for a loop antenna at 1GHz are given by [7]: The term RL represents the loss resistance due to the conductor and dielectric loss, which is almost four times greater than Rr. The antenna radiation efficiency for this antenna is 22.3\%. However, if the normal conducting part is replaced with a superconductor, e.g. YBCO with RS=500μΩ, [8] the loss resistance can be brought down due to the much lower surface resistance of the superconductor relative to the normal conductor. Chalupka et al. [9] experimentally demonstrated a large increase in radiation efficiency by comparing a miniature copper patch antenna (6mm by 6mm on a 1mm thick substrate) to a high-temperature superconducting (HTS) YBCO-version at 2.45GHz. At 77K the measured efficiency was 3% for the copper patch and 45% for the YBCO patch. The increase in radiation efficiency from copperto HTS-antennas has also been reported in other literature (see e.g. [10] and [11]). A disadvantage of superconducting antennas is the need to cool them. In 1986, Bednorz and Müller discovered that LBCO has a TC in the 30K range [12]. In 1987, Wu et al. [13] discovered the HTS compound YBCO that has a TC of ≈ 92K, which was the first superconductor to have a TC greater than the boiling temperature of liquid nitrogen. Thus antennas made of high-TC superconductors can easily be cooled to below TC with LN2 (with a boiling temperature of ≈77K) or by a small cryocooler, e.g. from SunPower, Inc. [14], which was used in this project. Another problem superconducting antennas face is nonlinearity when a strong microwave field creates a surface current density that is comparable to the superconductor JC (≈ 2MA/cm 2 at 77K). Using the HTS antenna as a transmitter, Chalupka [9] measured a drop in |S21|/|S21 |, from unity at -20dBm input power into the feed to ≈ 0.4 at 0dBm input power. Such an antenna would be rendered unusable as a receiver in the presence of a strong transmitter. Figure 3: {Electric field plot of (a) an empty parallel plate waveguide and (b) a parallel plate waveguide with a copper cylinder placed some distance away. The E-field is greatly distorted by the presence of the copper cylinder. One method for isolating the receiver from a transmitter is by shielding them with a copper cylinder. The isolation in this case will be nearly perfect. However, this will greatly alter the transmitter's field characteristics. Figure 3a shows a snapshot in time of the electric field inside an empty parallel plate waveguide at 9.5GHz. The field plots are generated in the Ansys HFSS full-wave simulation where the source is a waveport excitation (center bottom). The four sides of the waveguide are assigned radiation boundaries to reduce reflection. To illustrate the field distortion in the presence of the copper cylinder, a 1mm thick cylindrical copper shell is placed in the radiation path, Figure 3b. The electric field amplitude is greatly reduced behind the copper shell, in addition to reflection on the transmitter side. An improved approach is to utilize a structure constructed of artificial materials to help guide the EM waves around the receiver. This approach was used in this project and was extensively investigated.