# A Wide-Angle Scanning Leaky-Wave Antenna Based on a Composite Right/Left-Handed Transmission Line
## 3 LWA Design and Experiment Results
### Simulated Results of the LWA with Finite Unit Cells  
An LWA with 20 cascaded unit cells has a length of about 4\(\uplambda\)(\(\uplambda\) is the operation wavelength in free space). The scanning performance of the main beam can be predicted using the study described in Section 2.1. However, there are some deviations between the characteristics of the actual LWA and the infinite periodic TL from Section 2.1, in which the mutual coupling effects are ignored among the unit cells. The \(S_{11}\) and \(S_{21}\) of the designed LWA were calculated when the \(C_{L,var}\) changed from 0.35 pF to1.9 pF. Figure 6 shows the simulated \(|S_{11}|\) and \(|S_{21}|\) in dB with \(C_{L,var}=0.45\) pF, 0.6 pF, and 1.0 pF, respectively. Figure 6 and the results with other loaded capacitances show that the designed 20-unit cell LWA can be matched well at 3.0 GHz; the fractal bandwidth is 13% with \(|S_{11}|\) less than -7.5 dB. The slight shift in the frequency comes from the mutual coupling among the unit cells. Besides this, high radiation efficiency was also obtained; the lowest radiation efficiency was about 50% when the \(C_{L,var}\) changed. The efficiency can be improved by increasing the number of unit cells in the LWA. According to the authors' calculation, the LWA could reach a high radiation efficiency of over 80% when the length of the LWA is extended to more than 80 unit cells. The scanning patterns of the LWA at 3.0 GHz are shown in Figure 7. Figure 7 shows that the scanning angles \(\theta_{0}\) of\(-66^{\circ}\), \(-33^{\circ}\), \(-20^{\circ}\), \(0^{\circ}\), \(8^{\circ}\),\(29^{\circ}\), \(42^{\circ}\), and \(62^{\circ}\)wereachievedwhen the \(C_{L,var}\) was equal to 0.38 pF, 0.45 pF, 0.5 pF, 0.6 pF, 0.7 pF, 1 pF, 1.3 pF, and 1.9 pF, respectively. As the capacitance increased, the main beams were steered from the back to the fore gradually; a scanning range of \(128^{\circ}\) (from \(-66^{\circ}\) to \(62^{\circ}\)) was obtained when the capacitance of the varactors changed from 0.38 pF to 1.9 pF. The cross-polarization components are shown here due to their lower level as compared with the co-polarization radiation. In addition, the side lobe level (SLL) was also low over the entire beam-scanning range because of the gradual distribution of the current amplitude in the LWA aperture. It should be noted that broadside radiation is achieved when the \(C_{L,var}=0.6\) pF in Figure 7. Generally speaking, an unbalanced condition is easy to generate because of the sensibility to broadside radiation. Consequently, it is a common phenomenon in the periodical LWA that the realized gain decreases significantly at the broadside [5,6]. However, for the proposed LWA, the balance condition is satisfied at \(C_{L,var}=0.6\) pF. Therefore, the stop band and decline of the realized gain do not happen for broadside radiation. In Figure 7, the peak gain of the scanning beams is 9.9 dBi; the minimum gain of 5.0 dBi appears at scanning angle \(\theta_{0}=62^{\circ}\). Gain fluctuation over the entire scanning range is about 4.9 dBi. The main reason for the lower gain at larger radiation angles off the broadside direction is the decrease in the effective aperture area to the direction of low elevation (equivalent to low leakage efficiency). As mentioned in the discussion of radiation efficiency, the gain can also be improved by increasing the number of unit cells in the LWA. Figure 8a shows the scanning angles versus the \(C_{L,var}\). As mentioned above, the proposed LWA achieves a beam-steering range of 128\({}^{\circ}\) with a small gain fluctuation. This excellent beam-scanning ability in a wide angle range comes from the particular LWA structure and the wide capacitance variation range of the loaded varactors. Figure 8b shows the half-power beam width (HPBW) of the scanning beam in the scanning plane (E-plane) with different loaded capacitance values. When the LWA radiates to a larger angle off the broadside direction, the HPBW becomes wider.