# A Split Ring Resonator-Based Metamaterial for Microwave Impedance Matching with Biological Tissue
## 4 Experimental  
The antenna and metamaterial prototypes have been fabricated by employing the dielectric substrates Rogers Duroid 5880 with \(\varepsilon_{r}=2.2\) and \(\tan\delta=0.0009\) and Rogers RO4350B with \(\varepsilon_{r}=3.48\) and \(\tan\delta=0.004\), respectively, and by using a standard PCB process. Top-views of the SIW cavity-backed antenna and the metamaterial lens are shown in Figure 9a, the four holes are fabricated to fixing with suitable screws the metamaterial lens to the SIW antenna and the nuts are screwed in order to the obtain the distance \(d=6.9\)\(\mathrm{mm}\), as shown in Figure 9b. The designed thickness of the metamaterial lens, \(t=3.04\)\(\mathrm{mm}\), is achieved by overlaying two layers of dielectric substrate each of thickness 1.52 \(\mathrm{mm}\). The scattering parameter \(S_{11}\) of the device has been measured with the Agilent Technologies N5224A Network Analyzer. In Figure 10, the scattering parameter modulus \(|S_{11}|\) of the SIW antenna versus the frequency with metamaterial lens is illustrated, the measurement is performed with the device that irradiates in air (dashed line) and in the biological tissue of a hand (continuous line) placed at a distance close to \(D=10\)\(\mathrm{mm}\) from the SRR superstrate. The comparison highlights a good impedance matching of the microwave applicator loaded with the biological tissue in the frequency range 10.4-11.5 \(\mathrm{GHz}\). A scattering parameter modulus \(|S_{11}|\) minimum of \(-42\) dB at the 10.6 \(\mathrm{GHz}\) frequency has been measured. Moreover, the measured results are in good agreement with simulated one of Figure 7b, although we can observe a shift of the resonance frequency of few hundred of \(\mathrm{MHz}\), in fact a scattering parameter modulus \(|S_{11}|\) minimum of \(-36.9\) dB at the 10.9 \(\mathrm{GHz}\) frequency has been simulated. The second resonance frequency at \(f=12.7\)\(\mathrm{GHz}\) of the simulated results is less pronounced respect to the experimental results around at \(f=12.3\)\(\mathrm{GHz}\). These differences may be due to manufacturing tolerances and the possible air film between the two layers of the dielectric that constitute the metamaterial lens. Figure 11 shows the measured scattering parameter modulus \(|S_{11}|\) of the SIW cavity-backed patch antenna, with (continuous line) and without (dashed line) the metamaterial superstrate that irradiates in the biological tissue. We can observe that the electromagnetic field of the antenna without the metamaterial lens is almost completely reflected from human tissue. We underline that the simulations of Figure 8 perfectly agree with the experimental results of Figures 10 and 11. In particular, in Figure 11, the measured scattering parameter modulus \(|S_{11}|\) in the case of the antenna with SRR metamaterial and tissue (full curve) exhibits a strong dip close to the frequency \(f=10.7\)\(\mathrm{GHz}\). At the frequency \(f=11\)\(\mathrm{GHz}\), the impedance matching is good enough since the measured scattering parameter modulus is close to \(|S_{11}|=-14\) dB. In fact, Figure 8a shows a homogeneous propagation/distribution of the simulated electromagnetic field. Moreover, Figure 11 shows that the measured scattering  
Figure 9: **(a)** The metamaterial lens (at left) and SIW cavity-backed patch antenna (at right) prototypes. **(b)** SIW antenna and metamaterial superstrate assembled and connected to the Agilent Technologies N5224A Network Analyzer. parameter modulus in the case of antenna and tissue without SRR metamaterial (dashed curve) is close to \(|S_{11}|=-4\) dB. This implies an impedance mismatch, the presence of the stationary wave and a not homogeneous distribution of the simulated electromagnetic field with nodal regions. It is in perfect agreement with the simulation in Figure 8b. In other words, the SRR metamaterial provides an impedance matching and a consequent suppression of the reflected wave.