Compact wide-band baluns for active electronically scanned array antennas

Baluns are an essential item in the feed structure of many antenna elements requiring a balanced input. They may be trivial structures such as simple lengths of co-axial line or a `sleeve', but when required to function over a broad bandwidth, at significant power, well matched and with minimal losses, within the element pitch of an electronically scanned array, the task of designing such a device can become complex. The development of a broad band balun for an Active Electronically Scanned Array (AESA) based on Highly Coupled Dipole (HCD) elements is described. The immersed element was designed to function over two octaves or a 4:1 impedance bandwidth, (covering either the 2.7-10.8GHz or 4.5-18GHz frequency range) at the 2:1 VSWR level, and hence a broad-band balun with at least this bandwidth was required.