## 5 MTM Antenna Performance Analysis and Microwave Imaging
The measured radiation pattern in the major planes for resonant frequencies is depicted in Figure 9. In the Satimo measurement lab (UKM StarLab), the antenna is measured according to Phi axis rolling and Theta stepping. The measurements are logged in tables of Theta and Phi spherical coordinates. The spherical coordinates relate to the Cartesian axes as follows: XZ Cut is Theta = 0 to 360 & Phi = 0, YZ Cut is Theta = 0 to 360 & Phi = 90 and XY Cut is Theta = 90 & Phi = 0 to 360. The xz-plane (\(\phi\) = 0)  
Figure 8: (**a**) Simulated reflection coefficient of the proposed MTM antenna with and without MTM array; (**b**) Simulated gain of the proposed MTM antenna with and without MTM array; (**c**) Simulated efficiency of the proposed MTM antenna with and without MTM array; (**d**) fabricated prototype with measurement setup at the satimo lab (UKM-star lab). and yz-plane (\(\phi\) = 90) are considered as the E-plane and H-plane. The near field performance shows that the proposed MTM antenna is directive. The direction of the main lobes is fixed in the endfire direction. Interestingly, the radiations are improved in both the E-plane and the H-plane. The radiation pattern of the co-planar is almost bi-directional, whereas the antenna in co-planar is maximum along the broadside 0\({}^{\circ}\). The directivity is improved at about 5 dB. At a higher frequency, the directivity is shifted due to the introduction of a small back lobe. At a higher frequency, especially at 8.71 GHz, few nulls exist in the radiation pattern due to the excitation of the higher order current mode [19; 20; 21]. The above discussion proves that the use of the MTM structure reduces the energy leakage of the proposed antenna. Figure 9: Measured radiation pattern of cross-polarization and co-polarization of the proposed antenna at (**a**) 4.01 GHz (**b**) 7.63 GHz (**c**) 8.71 GHz. Surface current distributions for resonant frequencies of the proposed MTM antenna are shown in Figure 10. At a lower frequency, the main current conducting area is around the feed line. At higher frequencies, the current is disseminated around a triangular stripe of patch, with the MTM copper stripe over the substrate and the ground. We observed that by using the MTM structure, the feedline current does not spread out and it is well suppressed by the MTM unit cells. In this case, the feedline current is mostly used to energize the radiating patch. This edge-to-edge coupling between patch and ground helps to create extra radiation over the bandwidth. The MTM structure blocks the surface waves along the vertical cut and assists with improving the mutual coupling between elements. By considering the physical size of the antenna, the minimum quality factor can be obtained from [22; 23]\(\text{Q}_{\text{lb}}=\eta\text{Q}\), where \(\text{Q}=\frac{1}{\text{k}^{3}\,\text{a}^{3}}+\frac{1}{\text{ka}}\), the wave number \(\text{k}=2\pi/\lambda\) and a is the sphere radius, the smallest sphere that encloses the antenna. The upper band of bandwidth efficiency product \(\left(\text{Bnr}\right)_{\text{ub}}=\frac{1}{\sqrt{2}}\left[\frac{1}{\text{ ka}}+\frac{1}{\text{k}^{3}\text{a}^{3}}\right]^{-1}\). Figure 11 shows the \(\text{Q}_{\text{m}}\) and \(\left(\text{Bnr}\right)_{\text{ub}}\) for the antenna with 70% average efficiency and for different values of ka. Figure 10: Surface current distribution at **(a)** 4.01 **(b)** 7.63 and **(c)** 8.71 GHz **(d)** Scale. With the value of using the resonant bandwidth of the proposed antenna ka = 1.42. From the curve, it can be noted that for ka = 1.42, the minimum limit of Qm is 0.68 and limits of (Brr)ub is 0.98. The quality factor of the presented antenna is estimated as:  
\[\text{Q}_{\text{a}}=\frac{2\sqrt{\beta}}{\text{B}}\text{ Where }\sqrt{\beta}=\frac{\text{s}-1}{2\sqrt{\text{s}}}\leq 1 \tag{10}\]  
For the maximum allowable voltage standing wave ratio s = 2, the proposed antenna achieved Qa = 0.64 and Brr = 0.93.