I. Introduction
In the design of microwave systems using solid-state components the need often arises for more microwave power than a single active device can supply [1]–[3]. If the power from two or more devices is combined steps must be taken to insure that the outputs of the individual devices are in phase [4]. The topic of this paper is the creation of a circular radial cavity, that is, a radial waveguide with short circuit walls used to feed an active antenna array circuit which in turn controls the output phase of each device by the method of injection locking. In a radial cavity waveguide antenna, a coaxial feed is used to launch an outward-traveling axially symmetric wave inside the radial cavity which is then coupled to matched slots for radiation to free-space, [5]. This leads to a low loss feed network which does not suffer from the design complexity or inherent losses associated with standard transmission line based corporate feed networks. In addition for the application to be discussed here the radial cavity coaxial feed offers a convenient means for inserting an injection locking signal. By slot coupling individual active antenna oscillators to the radial cavity an entire system consisting of many oscillators can be phase locked by a single injection-locking input signal distributed by the radial cavity. Thus in this paper we develop a quasioptical spatial power combining device which relies on a circular array of weakly nonresonant slot coupled injection locked active field-effect transistor (FET) microstrip patch radiating elements fed from a radial cavity. To our knowledge, this method of injection locking an active transmitting array using single a coaxial fed source has not been deployed before.