Comparison of Compact Monopole Antenna Arrays With Eigenmode Excitation and Multiport Conjugate Matching

The compactness of antenna arrays is highly restricted due to the mutual coupling between the radiation elements, which degrades the performance of impedance matching, radiation efficiency and antenna diversity. For this reason, networks based on eigenmode theory or conjugate matching methods have been introduced in a number of previous works to overcome these effects. With this objective, a matching and decoupling network is required between the input ports and the antenna array, whose function is to improve power matching and ports decoupling. In this paper, three designs with one-layer microstrip line structure of matching and decoupling networks based on a balun technique and neutralization lines are discussed. Two two-element monopole antenna arrays with
$0.1 \lambda $
separation and a four-element monopole array with
$0.07 \lambda $
separation operating around 2.5 GHz are fabricated and measured. The theoretical analysis is based on the scattering matrix. A comparison of the two approaches is illustrated in the paper. Good matching and decoupling behavior of the antenna arrays is found. Overall, it is found that arrays with only
$0.1 \lambda $
element separation or even less can provide reasonable antenna performance.