Coupling considerations in the design of active dipole-transistor arrays

Since the inception of spatial power combining, active antenna arrays have always been a candidate technology. Generally, they consist of an array of elements comprising a number of radiators integrated with solid state devices. They are particularly attractive because of their planar design, compatibility with monolithic fabrication techniques and simple construction. However, exploitation of these structures have generally been hampered by the difficulty in designing and analysing them. In particular, the problems of frequency and phase synchronization between the elements have surfaced repeatedly. The oscillators are required to lock to a single frequency with a uniform phase progression across the array to produce useful, coherent and directional radiation. Some form of injection locking is needed to enforce this condition. A synchronising mechanism, showing promise for millimetre wave implementation, is the scheme of internal injection locking utilizing the natural electromagnetic coupling between radiators. This paper discusses the problem of designing an active array of simple dipole-transistor elements using mutual coupling to lock the oscillators to a specified frequency and force in-phase operation. The design equation, in conjunction with a method of moments analysis, is used to determine the microstrip dipole dimensions and reactive feedback needed for oscillation at the particular frequency and power output level.