Broadband Linear Antenna Arrays with Frequency-Invariant Half-Power Beamwidth

This paper reports on the design of broadband four-element antenna arrays with frequency-invariant half-power beamwidth (HPBW) within a bandwidth (BW) of 3:1. They are based on frequency-selective feednetworks that gradually shift the power from the outer elements to the inner ones as frequency increases. The operating principles of the proposed concept are presented through a four-element array of isotropic radiators. For practical validation, a 3:1 elliptical monopole antenna array fed by a feednetwork that comprises two second order low-pass and two fifth-order high-pass filters was developed. It exhibited HPBW variation of +/- 26.5% in the frequency range 2-6 GHz.