A new design reduces the radar cross section (RCS) of a microstrip antenna by using artificial magnetic conductors (AMCs) and a defected ground structure (DGS). RCS reduction is achieved by phase cancellation. Compared with the traditional method of only AMC loading, simultaneous AMC and DGS loading reduces the monostatic RCS at low frequencies, while gain is significantly improved. 10 dB RCS reduction is from 8.19 to 15.84 GHz (63.7 percent) and maximum gain enhancement is 3.7 dB at 10 GHz.
Antenna RCS is an important component of total RCS on a low observable platform. The requirement that its own radar wave is transmitted and received, however, makes antenna stealth difficult. A microstrip antenna is widely used in communication systems due to its small volume, light weight, low profile and ability to conform to a surface. In recent years, several methods have been proposed to reduce microstrip antenna RCS, one of which uses metamaterials.
Figure 1 AMC1 and AMC2 dimensions (mm): P = 5, h = 1.575, L1 = 4.9 and L2 = 3.6.
Because the reflection phase difference between electromagnetic band gap (EBG) structures and perfect electrical conductors is 180 degrees at resonance, their reflected waves cancel. RCS reduction is obtained by partially distributing the EBG substrate around the antenna patch, at the cost of antenna radiation performance.1 For wideband antenna RCS reduction, when the phase cancellation units are replaced by two different AMCs and arranged in a checkerboard configuration, 180 ± 37 degree phase difference can be achieved over a wider frequency range.2-4 The structure of a frequency-selective surface (FSS) can also be used to reduce antenna RCS, but it is usually limited to out-of-band reduction.5 To reduce both in-band and out-of-band antenna RCS simultaneously, the FSS structure is often combined with other methods, like loaded microstrip resonators6 and EBGs.7
This work reduces antenna RCS by using AMCs. Different from previous work, a DGS is incorporated into the antenna design. A DGS is commonly used to improve antenna performance by reducing mutual coupling,8, 9 broadening impedance bandwidth10 and suppressing cross-polarization.11-13 In this design, an AMC structure is combined with a DGS to reduce the antenna RCS at low frequencies and simultaneously improve gain.
AMC DESIGN
A conventional AMC structure is composed of periodically arranged metal patches on the upper surface, with an intermediate medium and metal ground. According to the equivalent LC resonant circuit, reducing the AMC center frequency with appropriate patch dimensions does not guarantee its bandwidth once the substrate thickness is determined. The center frequency is assumed to be the frequency corresponding to 0 degree reflective phase, and the bandwidth is the frequency range of the reflective phase from -90 to +90 degrees. Etching a DGS on the ground plane introduces additional inductance, which reduces the center frequency without changing the size of the upper patch, leaving the bandwidth unchanged. As shown in Figure 1, square patches of different sizes are selected as the AMC units, AMC1 and AMC2. The substrate material is Rogers RT5880 (εr = 2.2), and the ground plane is etched with a rectangular aperture.
Figure 2 AMC1 reflection phase vs. frequency, aperture width and polarization.
Figure 3 Reflection phase (a) and reflection phase difference (b) of the AMC units.
Figure 2 shows the reflection phase of AMC1 with different aperture widths and polarizations. The case w = 0 mm corresponds to a conventional AMC, with L3 set to 3 mm. As expected, the etched aperture on the ground plane has a large influence on the AMC phase characteristics. The center frequency decreases with increasing aperture width, especially when the polarization direction is parallel to the direction of w. Figure 3a shows the reflection phase of two AMCs with w = 2 and 0 mm for different polarizations. Because the AMC phase characteristics are related to polarization, the phase difference curves for different cases are shown in Figure 3b. The effective phase difference band of two different conventional AMCs spans 9.52 to 16.43 GHz, while the maximum AMC and DGS aperture band spans 8.61 to 15.47 GHz. The bandwidth is almost unchanged and the phase difference curve moves lower in frequency with the etched aperture on the ground plane.
ANTENNA DESIGN
The substrate material for the prototype antenna (see Figure 4) is the same as that of the AMC, and it is fed by a coaxial cable. The AMC is placed around the antenna patch in a checkerboard configuration, and the symmetric DGS is etched on the ground plane. DGS dimensions are optimized to achieve the maximum cross-polarization suppression in the H-plane without affecting the co-polarized gain and impedance bandwidth, defined as |S11| < -10 dB. As mentioned, the DGS structure reduces the AMC center frequency near the patch.
Figure 4 Prototype antenna dimensions (mm): S = 60, L = 8.6, cx = 1.7, W1 = 4, W2 = 9.2, W3 = 2 and t = 16.5.
Figure 5 Simulated |S11| (a) and realized gain (b) vs. frequency.
Simulated |S11| is plotted in Figure 5a. Antenna 1 is the antenna loaded with conventional AMCs, antenna 2 represents this work and an unmodified patch antenna is a reference. The resonant frequencies and impedance bandwidths of the three antennas are only slightly different. The simulated gain of antenna 2 is significantly higher than that of antenna 1 from 9 to 11 GHz, the maximum gain enhancement compared to the reference antenna is 3.7 dB at 10 GHz (see Figure 5b). The simulated antenna 1 patterns almost coincide with the reference antenna patterns, while the main beam of antenna 2 is narrower, indicating higher directivity (see Figure 6).
Figure 6 Simulated xoz (a) and yoz (b) plane radiation patterns at resonance.
Figure 7 Monostatic RCS vs. frequency.
