Advertisement

The size of the antenna for a given application does not depend purely on the technology, but on the laws of physics where the antenna size with respect to the wavelength has the predominant influence on the radiation characteristics. With modern day communication devices becoming smaller and lighter, demand for low-profile antenna designs is greater than ever.¹

One way of realizing a low-profile antenna design is to use a high impedance ground plane in place of the conventional metallic ground plane.^2-7 Metallic plates are used as ground planes to redirect the back radiation and provide shielding to the antennas. By nature, the conventional ground planes that are perfect electric conductors (PEC) exhibit the property of phase reversal of the incident currents that result in destructive interference of both the original antenna currents and the image currents. To overcome this effect, antennas are to be placed at a quarter wavelength above the metallic ground plane, making the size of the antenna bulky at low frequencies. To reduce the size of the antenna, a ground plane that is a dual of the conventional PECs is needed; in other words, a perfect magnetic conductor (PMC) is required. But how can a PMC that is not available in nature be realized? The answer to this problem is provided in the form of high impedance surfaces (HIS), which can essentially be considered as artificial magnetic conductors.^8,9

HISs are popular for their widespread applications in reflect array antennas, low-profile antennas, electromagnetic absorbers and polarizers.^10-13 These surfaces exhibit unique properties like the in-phase reflection of incident waves and the suppression of surface waves. Different antenna parameters such as gain, impedance and size can be enhanced by incorporating the HISs into the antenna structures. The design of the HISs can be optimized to tailor their electromagnetic properties depending on the operational requirements. Computer-aided design tools have enabled the solution of complex problems by means of numerical optimization algorithms.^14,15 A large number of optimization methods are presently available for solving electromagnetic problems. Deciding the most appropriate method for a given problem, however, is a non-trivial task and depends on factors like the number and range of the varying parameters, the goal of the optimization, the model size and the resources available.

Two-dimensional (2D) arrays of periodic resonant elements (printed or complementary slot) interact with electromagnetic waves within certain frequency band(s) and can be characterized as frequency selective surfaces (FSS). Different FSS elements have been described in the literature, where each design has its own advantage over the other. The multiband dipole structures are polarization dependent, the symmetric designs are dual-polarized and the fractal structures are compact designs. The HIS is realized by printing the periodic array of FSSs over a metal-backed dielectric substrate. It is important to optimize the performance of HIS over the frequencies of interest. It is observed that the characteristics of the FSS and HIS follow each other. This correlation can be exploited to speed up the optimization process by carrying out the design in two steps: