Design of a Archimedean Spiral Antenna with Low-Profile

An Archimedean spiral antenna with low input impedance is proposed in this paper.The input impedance could be decreased by modeling an Archimedean spiral antenna with exponentially spiral lines of a constant width.The balun used for feeding can work well with a small length in a large bandwidth.The antenna proposed has a input impedance of 75Ω,and performs well in the frequency range of 2~10GHz.The feed is a exponentially tapered microstrip balun whose length is 22.5mm(only 15% of the largest operating wavelength).The antenna is backed by a 22.5mm deep traditional cavity.In the frequency from 2GHz to 10GHz,VSWR smaller than 2.5,axial ratio at the main direction less than 4dB are measured.