A CPW-fed planar log-periodic dipole antenna with suppressed cross polarization

Abstract

In theory, log-periodic antenna (LPA) is a linearly polarized antenna with frequency independent properties on the input impedance and gain patterns [1]-[2]. Owing to these characteristics, LPA has gained research interests and been employed for many wideband applications, like EMC and antenna chamber test. The planar PCB log-periodic dipole antenna (PLPDA) has gained increasing attention recently. The planar single end-fire PLPDA have been proposed in different ways [3]-[4]. Since these antennas radiate toward the end-fire (vertex) direction, their phase centre may vary as frequency changes [5]-[7]. This problem might cause a problem of pulse distortions in time domain of UWB communication systems [8]. A vertex-to-vertex LPA fed with out-of-phase power divider can alleviate this problem because the antenna mainly radiates toward broadside and its phase center remains fixed over the wide frequency range. Several vertex-to-vertex LPAs have been reported and fabricated, but they have unfavorable cross-polarization level [9]. In this paper, we propose a new design of CPW-fed PLPDA which has the fixed phase center and low cross polarization level. In addition, the design is extended with a conductor-backed coplanar waveguide transmission line and an offset vertical balun [10] to achieve a fully planar structure incorporating the antenna feed.