Wideband balun bandpass filter explored for a balanced dipole antenna with high selectivity

A novel microstrip wideband balun bandpass filter (BPF) is presented for development of a balanced dipole antenna with wide operating band and high frequency selectivity. By utilising the standing-wave property of current distribution along a half-wavelength (λ/2) open-ended microstrip line, out-of-phase signals are ideally obtained at two output ports of a balun. Meanwhile, proper coupling topologies between the open-circuited λ/2 microstrip transmission line and the quadruple-mode resonators are selected to achieve a balun BPF with both characteristics of balanced performance and wideband filtering properties. A balun BPF operating at 1.65 GHz with 32% fractional bandwidth is at first designed. The derived results not only exhibit good performance with 0.49 dB amplitude imbalance and 7.2° phase imbalance, but also achieve high selectivity relying on two transmission zeros. Next, this balun BPF is integrated with a balanced dipole antenna with a flat shape towards wideband and high-selective radiation. Both simulated and measured results achieve higher than 10 dB return loss and 2.4 dBi radiation gain over a band of 1.4–1.9 GHz.