A superdirective 3-element array for adaptive beamforming

A small array composed of three monopole elements with very small element spacing on the order of lambda/6 to lambda/20 is considered for application in adaptive beamforming. The properties of this 3-port array are governed by strong mutual coupling. It is shown that for signal-to-noise maximization, it is not sufficient to adjust the weights to compensate for the effects of mutual coupling. The necessity; for an RF-decoupling network (RF-DN) and its simple realization are shown. The array with closely spaced elements together with the RF-DN represent a superdirective antenna with a directivity of more than 10 dBi. It is shown that the required fractional frequency bandwidth and the available unloaded Q of the antenna and RF-DN structure determine the lower limit for the element spacing. (C) 2003 Wiley Periodicals.