Abstract: This paper introduces a miniaturized planar spiral antenna with a very wide bandwidth. In the frequency range from 0.95 GHz to 15.20 GHz, the measured return loss is less than -10 dB. Good circular polarization performance (axial ratio<4 dB) is obtained between 1.4 GHz and 10.2 GHz. Compared with the conventional spiral antenna, the proposed antenna significantly reduces the size and efficiently broadens the 3 dB beam width to about 130° by adding a conical back cavity beneath the antenna. Moreover, a wideband balun is designed using an exponentially tapered microstrip parallel-line wire. Simulation and experimental results show that the proposed antenna is of good circular polarization and wideband characteristics. 

Key words: back cavity, miniaturization, wideband balun, planar spiral antenna