# A VHF Circularly Polarized Turnstile Antenna with Balun for Meteor Radar
###### Abstract  
This paper presents a circularly polarized turnstile antenna for a meteor radar system. The antenna employs a pair of turnstile radiators, a 1:2 power divider and a 90\({}^{\circ}\) phase shifter. After a series of simulations and optimizations, the maximum circularly polarized gain at 39 MHz is 5.9 dBic with 96\({}^{\circ}\) 3 dB beam width. The simulated axial ratio below 3 dB is from \(-\)39\({}^{\circ}\) to 39\({}^{\circ}\) within the band of interest. Then, the proposed antenna is fabricated and applied to a meteor radar system. Two tests are carried out for radiation characteristics demonstration. In the first test, the VSWR of the proposed antenna is below 1.2 at 38.5-39.5 MHz and keeps good agreement with simulation results. However, the meteor azimuth distribution is illogical when the output results are compared with the Australian meteor detection radar (ATRAD) meteor detection. To address the problem, baluns are investigated and introduced for balanced feeding. The 1:1 unbalance-balance transformer has been used to improve the radiation characteristics of the designed antenna. The final test results show that the meteor radar system with the improved antenna achieves better performances in meteor counts and meteor azimuth distribution.  
antenna balun; circular polarization; end-fire; turnstile antenna; VHF meteor radar; Wilkinson power divider 2023350203502 necessary for the dipole antenna feeding network. The term balun is a portmanteau of balance and unbalance, indicating that a balun can transition between a balanced transmission line and an unbalanced transmission line.  
For any asymmetric antenna feeding with coaxial cable, a balun is essential and significant to ensure the radiation characteristics of the antenna. Balun can not only be used as an unbalance-to-balance converter, but can also be used as an impedance transformer. There are many kinds of baluns, such as coaxial [14; 15], microstrip [16] and LC baluns, currently applied in the antennas industry. Microstrip baluns are low profile and lightweight, easy to fabricate, conformable to mounting structures, and compatible with integrated circuit technology. Many efforts have been made to achieve ultra-wideband (UWB) microstrip baluns for antennas or low-noise amplifiers (LNA) [17; 18; 19; 20; 21; 22]. However, for very high frequency (VHF) band antennas, microstrip baluns will cause more insertion loss, and elaborative design of microstrip baluns are needed to avoid the influence of distribution capacitance and inductance.  
In this work, a circularly polarized turnstile antenna with baluns for meteor radar is presented. The antenna operates at 38.5-39.5 MHz and can realize unidirectional radiation by using reflection structures. In addition, a coaxial cable form Wilkinson power divider is designed as a balun. A fabricated prototype is validated in Section 3. To address the problems found in the meteor radar, a voltage balun is applied to each feeding port of the turnstile antenna, which improves the ability to search for meteor numbers and uniformity of the detection area of the meteor radar.