## 3 Simulation and Experimental Results
Also shown in the figures is the RCS response against frequency of the reference antenna, which has the same lateral dimensions as of the proposed antenna. In addition, the calculated RCS frequency response of a perfect conductor of similar size is also presented. The calculation is based on the relation given as: \(\sigma_{\text{C}}=4\pi\text{a}^{2}/\lambda^{2}\), where \(\sigma_{\text{C}}\) is RCS of the perfect conductor, a is the area of the conductor, and \(\lambda\) represents wavelength of interest. For both polarizations shown in Figure 8a,b, wideband RCS reduction was achieved, including in-band frequencies. RCS reduction bandwidth (BW) extends over 4-16 GHz (120%), and an almost identical frequency response was achieved for the VP and HP wave incidences. This is owed to the symmetric unit cell design. For VP, average RCS reduction over the bandwidth is about 8.5 dB, and for HP, it is about 8.8 dB. Maximum achieved RCS reduction values are 25 dB (for VP) and 24 dB (for HP), respectively, appearing at 14.3 and 14.4 GHz value. The results shown in Figure 8a,b correspond to the co-polarized RCS frequency performance. The cross-coupled RCS monostatic performance of reference as well as proposed antenna, considering VP and HP wave incidences, is displayed in Figure 8c. As can be seen, the cross-coupled radar echoes are significantly low. The cross-coupled RCS performance of the proposed antenna obeys the cross-coupling reflection results presented for the unit cell in Figure 2a. Hence, the function of the absorber for reflectivity suppression is validated. Figure 7: Radiation beam plots (H-plane) at three different frequencies (f). The results presented in Figure 8 are for the case of 50 \(\Omega\) antenna termination. This is because for most of the practical cases, the antenna would be matched terminated to its transmitter (Tx) or receiver (Rx). However, simulations for the cases of open/short termi  
Figure 8: Frequency response of monostatic radar cross section (RCS) for (**a**) vertically polarized (VP) and (**b**) horizontally polarized (HP) wave incidences. (**c**) Cross-coupled monostatic RCS response for proposed and reference antenna. nations (worst case scenarios) were also performed for the proposed antenna, and it was observed that there was no significant change among different cases (short/open/matched). This might be due to the reason that the short or open primarily affects the antenna mode scattering, which is a function of antenna gain given as [41]:  
\[\sigma_{\text{ANT}}=G^{2}\;\Gamma^{2}\;\lambda^{2}/4\pi, \tag{3}\]  
where \(\sigma_{\text{ANT}}\) represents antenna mode scattering, G is antenna gain, \(\Gamma\) is reflection coefficient, and \(\lambda\) is the wavelength at the frequency of interest. The out-of-band frequencies will be unaffected by open/short terminations as these frequencies are being significantly absorbed at the absorber surface (and hence leave an insignificant energy that would reach the antenna surface). For the in-band frequencies, the in-band absorption is impaired (as evident from Figure 2a,b), but the in-band antenna mode scattering (frequencies reflected from the mismatched load and getting re-radiated by the antenna) would still be insignificant as the gain towards broadside is considerably low. The measurement of frequency response of monostatic RCS was performed using two sets of in-lab developed parabolic reflector antennas that had lambda/2 dipole as their excitation source. Each set contained two such antennas (one for transmit (Tx) and one for receive (Rx) operation). Within the desired frequency range, six different frequencies were tested. In comparison to horn antennas, the size of these antennas is smaller; hence, in a real scenario, they can better replicate the ideal monostatic simulation setup--a merit that is worth noting. Figure 9a represents the simulated vs. measured monostatic RCS suppression for VP incident wave, whereas Figure 9b displays the same for the HP incident wave. The measurement setup is displayed in Figure 9c.