Tham khảo tài liệu 'advances in satellite communications part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Helical Antennas in Satellite Radio Channel 19 b Fig. 11. a Standard conical and tapered BHAs and b their radiation patterns. F B dB Gain dB AR HPBW Standard BHA 111 Conical BHA 113 Tapered BHA 16 87 Table 1. Simulation results of radiation characteristics of standard conical and tapered BHA. F B dB Gain dB AR HPBW Tapered BHA nt n 3 90 Tapered BHA nt nu 3 120 Tapered BHA n 0 nt 6 85 Tapered BHA n 1 nt 82 Tapered BHA n 2 nt 87 Table 2. Simulation results of reduced size tapered BHA. 20 Advances in Satellite Communications a b Fig. 12. Geometry and radiation patterns of reduced size BHA a and b respectively. b a Fig. 13. Typical radiation patterns of bifilar scanning helical antenna a conical at GHz and b normal radiation pattern at GHz. Contrary to monofilar helical antenna the bifilar helical antenna yields scanning radiation mode when relative phase velocity p v c . This is confirmed with the comparison of the simulated results with the experimental and calculated results Nakano et al. 1991 Zimmerman 2000 of the lobe direction for the different values of phase velocity Fig. 14. Helical Antennas in Satellite Radio Channel 21 Frequency GHz Fig. 14. The comparison of the simulated calculated and experimental results for the lobe direction vs. frequency. The quadrifilar helical antenna The quadrifilar helical antenna QHA also known as the Kilgus coil is mostly used for telemetry tracking and command TT C satellite systems due to its simplicity small size wide circularly polarized beam and insensitivity to nearby metal objects. The QHA consists of four helical wires equally spaced circumferentially and fed from the top or the bottom. The open ended QHA generally uses the length of each wire of 2 4 or 32 4 with typical input impedance in the range 10 to 20 ohms while the short-circuited QHA uses 2 2 or 2 length of each wire which .
