A Compact Broadband Horizontally Polarized Omnidirectional Antenna Using Planar Folded Dipole Elements

A compact horizontally polarized (HP) omnidirectional antenna with broadband characteristic is presented. The antenna is composed of four planar folded dipole antennas (PFDAs) as radiation elements of a square array and a broadband microstrip feeding structure composed of baluns and matching networks. The folded dipole antennas are chosen in order to achieve wide bandwidth operation within a compact size. The folded dipole elements are modified to compensate for the mutual coupling effects and also to improve the bandwidth and radiation performance of the antenna. The antenna is fabricated and its input impedance and radiation pattern are measured. The measured 10 dB return loss bandwidth is about 53.2% (1.19–2 GHz) and the gain variation in all directions of azimuth plane is less than 2 dB over the frequency band 1.2–1.9 GHz, and is increased to 2.8 dB at 2 GHz. The gain is almost constant over the entire band with a peak gain about 1.2 dB. The co-pol and cross-pol isolation is more than 20 dB for all azimuth directions and frequencies over the entire band.