# A VHF Circularly Polarized Turnstile Antenna with Balun for Meteor Radar
## 2 Antenna Topology and Simulation Analysis  
To meet the circularly polarization demand of the meteor radar, a turnstile dipole antenna is used to provide orthogonal electric fields. The antenna can be considered as two integrated sub-units. Additionally, equal amplitude feeding and 90\({}^{\circ}\) phase shift can be achieved by the Wilkinson power divider and phase shifter inserted in the feeding network. The circularly polarized antenna was simulated by using the ANSYS High-Frequency Structure Simulator (HFSS) software. Figure 1 shows the simulation model. Figure 1: The simulation model of the designed turnstile antenna. The antenna consists of two orthogonal dipoles and two reflectors. The overall structural arrangement is more or less the same as two turnstile Yagi-Uda antennas without directors. A support rod is used to raise the antenna to a proper height, and a feeding network is fixed between the two dipoles. In the simulator, balanced feeding and 90\({}^{\circ}\) phase shifting are set. Therefore, the balun and Wilkinson power divider do not need to be considered at the stage of computer simulation. Figure 1 shows that the radiator of the designed antenna has a T-match connection [12]. Dipole1 or Dipole2 is fed by a pair with a length of \(c\) and a radius of \(r\). The parameter \(s\) and \(d\) can be conveniently changed to match the transmission line. To further understand the effects of the T-match, parametric studies on three vital parameters: \(b\), \(s\) and \(d\) of the T-match depicted in Figure 1 are simulated. Figure 2 presents the simulation results. The simulation shows that tuning these 3 parameters will significantly affect the impedance matching of the 2 excited resonant modes, whereby the optimum impedance bandwidth is achieved when \(b\), \(s\) and \(d\) are 1330, 182 and 235 mm, respectively. Then, according to the system port characteristic impedance \(Z_{0}=50\)\(\Omega\), the dipole needs impedance matching to \(Z_{0}\). Thus, the impedance of the ports of the antenna is set as 50 \(\Omega\) in the simulator. After a series of calculations and corrections, the parameters of the turnstile dipole antenna are shown in Table 1. The simulated Smith Chart and Voltage Standing Wave Ratio (VSWR) of the antenna are depicted in Figures 3 and 4. Before tuning, the impedance of the antenna is about \(Z_{L}=48.945+155.68\) at 39 MHz (black points in Figure 3), hence capacitors are needed to be tuned. With the proper capacitors, the VSWR of the antenna decreases to below 1.2 within the operating frequency band. Figure 2: The simulation results of the designed turnstile antenna with varying parameters (**a**) \(s\), (**b**) \(b\) and (**c**) \(d\). The simulated radiation patterns are shown in Figure 5. The maximum circularly polarized gain of 5.9 dBic is obtained at 39 MHz, which is quite desirable for a meteor radar antenna [6]. In Figure 5b, the simulated circular polarization (Co-pol.) and cross polarizations (X-pol.) in different vertical planes are plotted. Within the operation frequency band of 38.5-39.5 MHz, the circularly polarized radiation patterns maintain a stable shape, and the co-pol. exhibit high polarization purity. It can be seen from Figure 5b,c that the \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline a & 1725 \\ b & 1330 \\ c & 1010 \\ d & 235 \\ m & 3900 \\ n & 25 \\ s & 182 \\ l & 3660 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optimized dimensions of the antenna; all dimensions in mm. Figure 4: The simulated VSWR of the designed turnstile antenna. Figure 3: The simulated Smith Chart of the designed turnstile antenna. 3 dB beam width can reach about 96\({}^{\circ}\). It is also found that the gain above 0 dB extends from \(-68^{\circ}\) to 68\({}^{\circ}\) (about 136\({}^{\circ}\)) along the radiation direction of the antenna. These far-field characteristics, such as wide beam width and polarization gain, are well-suited for detection systems.