Broadband Monopole Array with Reduced Half-Power Beamwidth Variation

This paper reports on the design and practical validation of monopole antenna arrays with reduced half-power beamwidth (HPBW) variation in a bandwidth (BW) greater than one octave of the RF spectrum. The HPBW variation is reduced by feeding the antenna elements with frequency-dependent feed networks. The feed networks are designed in such a way that the element power is gradually shifted from the outer elements at low frequencies to the inner elements at high frequencies. The operating principles of the proposed concept are explored through simulated examples of four-element arrays made of isotropic elements and then realistic monopole elements. Scalability of the concept to eight-element arrays is also explored using ideal simulations. For practical validation purposes, a four-element monopole antenna array and its corresponding feed network are designed and measured. The prototype resulted in a HPBW variation of 78% in a 2.6:1 BW, which is significantly reduced compared to the uniformly fed array, having a 216% variation.