High-gain SIW-fed slot antenna with realistic AZIM coating
In order to design a feeding antenna for the AZIM coating, a halfwave slot fed by a substrate-integrated waveguide (SIW) is adopted here due to its low profile compared to cavity backed or conventional waveguide-fed slots [50, 51]. The schematic view of the SIW-fed slot antenna is shown in Fig. 1.10a. It is composed of a 50 П microstrip and a shorted SIW with a longitudinal slot cut on its broad wall. A tapered microstrip is used for impedance matching between the 50 П feedline and the SIW. In order to achieve a magnetic dipole mode in the slot for efficient radiation, the length of the slot is chosen to be around Ag/2 at 5.8 GHz. The distance between the center of the slot to the shorted wall of the SIW in the x-direction is set to be about 3Ag/4 at 5.8 GHz, which allows the standing wave peak to be located at the center of the slot. The geometrical dimensions of the SIW-fed slot antenna were optimized to achieve 511 < -10 dB in the 5.6-6.0 GHz band (see Fig. 1.11a). The Я-plane (y-z plane) and Я-plane (x-z plane) gain patterns at the center frequency 5.6, 5.8, and 6.0 GHz and are presented in Fig. 1.12. It can be seen that the Я-plane has two peaks located at around 40-45° off broadside due to the diffraction of the finite-sized ground plane. The simulated broadside gain varies from 3 to 3.8 dBi.
Figure 1.10 (a) Schematic view of the SIW-fed slot antenna with the actual
AZIM coating. All the dimensions are in millimeters. (b) Photograph of the fabricated SIW-fed slot antenna covered by the AZIM coating. The inset shows the SIW-fed slot antenna alone.
Due to the fact that the AZIM coating has its near-zero permittivity tensor parameter only in the z-direction, the impedance of the slot is well maintained when it is covered by the AZIM (see Fig. 1.10b). As shown in Fig. 1.11b, the simulated 511 is below -10 dB from 5.6 to 6 GHz, which is very similar to that of the slot alone. This robust input impedance behavior ensures that the AZIM coating can be readily added onto or taken away from the slot to achieve different radiation properties without any additional modification to the slot antenna itself. The Я-plane (y-z plane) and Я-plane (x-z plane) gain patterns of the slot antenna with/without the AZIM at 5.6, 5.8, and 6.0 GHz and are presented in Fig. 1.12. Distinctly different from the two-peak patterns for the slot without the AZIM coating, a well-defined single beam at broadside is observed in the Я-plane with a beam squint less than 2° off broadside and an half-power beam width (HPBW) of about 35° ~ 40°. Notably, with the presence of the AZIM coating, the broadside gain is significantly increased to 10.2~10.6 dBi, which indicates an improvement of about 7 dB. It should also be noted that the front-to-back ratio is greatly reduced by about 10 dB throughout the entire frequency range. Both the broadside-gain increase and the front-to-back ratio drop are primarily attributed to the reduction of fields at the edges of the ground plane. This is facilitated by the presence of the AZIM coating, resulting in much weaker diffracted fields at the back side of the antenna.
Figure 1.11 Simulated and measured 5n of the SIW-fed slot antenna (a) without and (b) with the AZIM coating.
The SIW-fed slot antenna and the AZIM coating structure were fabricated and assembled, as shown in Fig. 1.10b. The 511 of the slot with and without the AZIM coating was characterized by a network analyzer. As shown in Figs. 1.11a,b, good agreement can be found between simulations and measurements. The measured 511 of the slot with and without the AZIM coating has a -10 dB band from
5.52 to 6.03 GHz and from 5.54 to 6.01 GHz, respectively. They are both slightly broader than the simulations predict due to the minor frequency shift of the resonance at 5.6 GHz and the lower quality factors of both resonances within the -10 dB bandwidth. The radiation patterns and the gain of the slot antenna with and without the AZIM coating were characterized in an anechoic chamber, as shown in Fig. 1.12. Overall, the measured gain patterns in both the Я-plane and Я-plane agree well with the simulated results, confirming the proposed antenna design. Specifically, patterns exhibiting a double-peak can be seen for the slot alone, while a single sharp beam pointing at broadside can be observed for the slot with the AZIM coating. The measured HPBW in the Я-plane is about 40-50°, which is slightly broader than the simulated beam width, especially in the high-frequency band. This is mainly attributed to fabrication and assembly imperfections, which result in a non-ideal symmetry in the actual MM structure. The gain at broadside for the slot with and without the AZIM coating is in the range of 9.8-10.4 dBi and 2.9-3.5 dBi, respectively, which indicates an improvement of about 6.9 dB.
Figure 1.12 Simulated and measured E-plane and H-plane gain patterns of the SIW-fed slot antenna with and without the AZIM coating at (a) 5.6 GHz, (b) 5.8 GHz, and (c) 6.0 GHz.