Design Of A Planar Equiangular Spiral Antenna And The Balun

In this paper, we have studied the design, optimization and application of the ultra-wideband planar equiangular spiral antenna and its matching balun. Thesis work is divided into three parts: 1, we introduced the main performance of the antenna, and outlined the main calculation of electromagnetic field: finite element method, then designed and optimized a planar equiangular spiral antenna which worked in the 0.4 ~ 2GHz band. 2, According to the balanced feed characteristics and impedance characteristics of the antenna, we have designed a balanced - unbalanced converter which was known as Balun. 3, we discussed the applications of the planar equiangular spiral antenna.First, this paper outlined the main performance of the antenna, including: pattern, directivity, gain, polarization, VSWR, the effective length and effective area and bandwidth etc., then described the main methods of calculating the electromagnetic field of the antenna: finite element method, also briefly introduced the simulation tools: Ansoft HFSS, and then designed a planar equiangular spiral antenna which worked at 0.4 ~ 2GHz, using FR4 printed circuit board (which relative dielectric constantεis 4.4) as the basic material. The antenna's diameter D was 600mm, thickness was only 1.6mm. Antenna's circular polarization axial ratio, return loss and other important parameters were calculated by Ansoft HFSS V.10, the results showed that: in the 0.4-2GHz band, the antenna's gain was 3-7dB, circular polarization axial ratio was less than 2dB, impedance characteristics was about 140Ω. We also discussed the influence of the dielectric thickness, relative dielectric constant to the antenna's impedance and gain. This paper also described the application of such antennas. To make the antenna input impedance match with the coaxial cable (which impedance is 50Ω), according to the antenna impedance characteristics, we designed a balun for the antenna's balanced feed, and impedance transformation. The structure of the balun was trapezoidal double-sided microstrip line, and the simulation results showed that the balun of this structure had excellent ultra-wideband characteristics and the impedance conversion function, Simulation results also showed that the antenna return loss was significantly reduced, from -8dB down to -10dB,and the lowest point reached -48dB,the same time, the gain and the axial ratio of circular polarization of the antenna had little effect after connecting to the balun.