Numerical and experimental results
A slab of MM was constructed as a 12x12 array of unit cells to be placed over a horizontal crossed-dipole antenna, which is itself placed a quarter-wavelength above a metallic ground plane. The AZIM slab tends to spread out the radiating energy to fill the entire aperture, so in order to mitigate this effect, a perfect electric conductor (PEC) band was placed around the edge of the lens to contain the fields and prevent leakage from the sides of the lens. The feed element, ground plane, and lens together form a cavity antenna. The vertical spacing between the ground plane, feed antenna, and MM lens was selected to optimize the return loss and achievable gain.
The prototype antenna was modeled and measured using a single linear dipole in two separate orthogonal orientations, with no difference in response when the lens was rotated 90°. The combined antenna showed a 6~8 dB measured gain improvement across a 17% bandwidth over the dipole and ground plane alone, and also demonstrated low cross-polarized fields that are approximately 25 dB below the peak gain throughout the band. Figures 1.14a,b show the gain and 511 spectra for the AZIM lens with both PEC and AMC ground planes, showing the performance trade-offs for the reduced profile of the AMC design.
The MM lens antenna system has a relatively thick profile due to the required spacing between the ground plane and feed, and the feed and the lens. The thickness of the entire system can be reduced by replacing the PEC ground plane beneath the feed antenna by an AMC ground plane, as shown in Fig. 1.13a. Since the feed antenna may be placed arbitrarily close to the AMC ground plane, the ground- feed distance may be reduced to almost zero. This change does not affect the feed-lens thickness requirement however, and the AMC has a finite thickness, which limits the minimum height of the lens. Replacing the PEC with an AMC ground plane enabled about a 20% overall thickness reduction in the prototype lens. The AMC ground plane may be designed according to a number of possible design variations [32, 57, 58], but the simplest is the mushroom-type surface , whose template is illustrated in Fig. 1.13c.
The trade-off for thickness reduction is a decreased bandwidth. In the original lens, the dipole and its associated spacing requirements, the electric MM, and the magnetic MM were each independently tuned to operate at the same frequency. Of these effects, the electric permittivity was the limiting constraint on operational bandwidth. The addition of another strongly dispersive and narrowband element to the structure limits the possible bandwidth of an optimized design compared to the original optimized antenna (10% versus 17%), and also decreases the peak gain of the antenna from 14 to 12 dBi. Although the achievable profile reduction is limited, such a design decision may still be worthwhile for selected applications.
Figure 1.14 (a) Peak gain versus frequency for the original dipole, MM lens
with PEC ground plane, and MM lens with AMC ground plane. (b) Return loss for the original dipole, MM lens with PEC ground plane, and MM lens with AMC ground plane. The MM lens with PEC ground plane shows better performance and bandwidth in both impedance and radiation properties but has a larger height profile.
Simulations of both antennas, with and without the AMC ground plane, were performed using HFSS, while measurements of the same structures confirmed the performance predictions. Each of the two designs demonstrated better than 10% impedance and pattern bandwidth, enabled through careful tuning of the geometric and electric parameters of the feed, electric MM, magnetic MM, and the AMC. While each additional dispersive and frequency-limited component increases the design constraints and reduces the possible operational bandwidth, these designs demonstrate that it is possible to create complex, coupled systems of resonant MMs while maintaining a useful operating bandwidth.