An ultrawideband unidirectional modified foursquare antenna

This study introduces an ultrawideband unidirectional modified foursquare antenna. The antenna consists of two radiating loops and two truncated parasitic patches. A microstrip‐to‐stripline transition is used as a balun to feed the two radiating loops. A square cavity works as a reflector to realise a unidirectional radiation. This antenna has a total size of 0.56λ L × 0.56λ L × 0.15λ L (λ L : free space wavelength at lowest operating frequency). The measured results show that a broad operating bandwidth of 103.8% (2.67–8.44 GHz) for return loss being higher than 10 dB is achieved. Meanwhile, measured stable unidirectional radiation patterns with a gain (6.9–11.3 dBi) in +z direction, a front‐to‐back ratio better than 14.5 dB and low cross‐polarization level (<−17 dB) are also obtained over the whole band. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:522–529, 2015.