# Modified 16-Quasi Log Periodic Antenna Array for Microwave Imaging of Breast Cancer Detection
## 4 Analysis and Discussion  
LPKF protomat (S103) circuit board plotters were used to fabricate the proposed antenna prototype and a Tektronix USB Vector Network analyzer (TTR506A) was used to evaluate its performance. An experimental prototype of the antenna can be seen in Figure 4. It appears that the simulated and measured return loss for the antenna design, shown in Figure 5, are almost in good agreement. The observed measurement results indicate that the antenna operates in the frequency range of 2-5 GHz (\(-10\) dB criteria). An apparent slight deviation between experimental results and simulation data can be attributed mainly to the effect of the SMA connector, SMA soldering, cables, and fabrication tolerance. In simulation, the antenna is excited using a waveguide port, but in practice it is excited via the SMA connector. The characteristics thus obtained aid the PLPA in its role as a sensor in MIS.  
For better understanding of the designed antenna performance, the antenna current density distribution at 3.5 GHz and 5 GHz are shown in Figure 6. The current distribution at 3.5 GHz can be seen to be highly concentrated on the large dipole and along the transmission strip line, resulting in a directional radiation pattern towards the smaller dipole elements. A similar current distribution can be seen at higher frequencies of 5 GHz. Over the entire radiating patch, high density is concentrated around the larger and adjacent dipoles, resulting in a directed pattern.  
Figure 4: (**a**) LPKF (S103) protomat and (**b**) PLPA prototype.  
Figure 5: Simulated and measured return loss curves.  
The length of the large dipole element (L\({}_{\text{dipole}}\)), is given by the following equation:  
\[\text{L}_{\text{dipole}}=[\text{w}_{1}+\text{w}_{\text{f}}+\text{w}_{\text{6}}] \approx\frac{\text{c}}{2\text{f}_{\text{H}}\sqrt{\varepsilon_{\text{eff}}}} \tag{1}\]  
\[\varepsilon_{\text{eff}}=\frac{\varepsilon_{\text{r}}+1}{2}+\frac{\varepsilon_ {\text{r}}+1}{2}\frac{1}{\sqrt{1+12\text{h}/\text{l}_{7}}} \tag{2}\]  
where c = 3\(\times\)10\({}^{8}\) m/s, \(\varepsilon_{\text{eff}}\) is the effective permittivity, \(\varepsilon_{\text{r}}\) is the substrate dielectric constant, h is the height of substrate, l\({}_{7}\) is the width of the dipole element (all dipole widths are equal), and f\({}_{\text{H}}\) is the highest frequency in the operating band. According to Equation (1), the theoretical and practical lengths of the larger dipole element are 16.2 mm and 17 mm, respectively.  
Figure 7 depicts the 2D and 3D radiation patterns at 3.5 and 5 GHz. The 2D patterns at both frequencies are taken from XZ plane (\(\Phi\) = 0\({}^{\circ}\) plane). The proposed antenna is directional, with the main radiator patch pointing in the direction of the boresight as seen in the far field measurement. Over the operational band, the primary lobes of the patterns are fixed towards the end-fire direction. Hence, the radiation patterns are stable over the operating frequency band and thus enhance the sensor element in MIS for tumor detection. The CPW fed PLPA covers bandwidth of 3 GHz (\(-\)10 dB criteria) from 2-5 GHz with maximum peak gain of 4.6 dBi. The peak gain over the entire band is shown in Figure 8. At a lower frequency, the antenna has upward gain while it achieves gain stability at higher frequency.  
Figure 6: Surface current distribution on PLPA at (**a**) 3.5 GHz, and (**b**) 5 GHz.