Active antennas with periodic structures

Integration of a solid-state transistor oscillator with a radiative element has many advantages: it eliminates the feed network, replaces the expensive waveguide transition, reduces the size of the antenna system as well as providing active impedance matching. Active devices also offer promise for optical control of beam steering antenna arrays. The objective of this thesis is to determine the physical characteristics of an active antenna and understand its operational principles.;In our investigation of the active antenna, a noninvasive in-situ optical sampling measurement technique is applied. Several active antennas and antenna arrays were fabricated on GaAs substrates. HEMTs and MESFETs in the chip form were used as the active elements for these devices. The real time potential distributions on the active antenna structure were obtained. A Green's function for the charge on a thin dielectric substrate was developed for interpreting the experimental results. Numerous simulations of the antenna operation were performed and compared to the measurements. The operation mode of the antenna is identified within some degree of certainty.;An optical sampling technique is developed to determine the guided-wave circuit parameters which are inaccessible to other microwave measurements. Sampling an active radiating structure provides us with the opportunity to analyze the active antenna. A single active antenna with the same geometry as a single array element was built and experimentally characterized. It was found that the single active antenna has the same form of potential distribution, same stability condition, and the difference in oscillation frequency are within the locking range (which is about 0.4% of the oscillation frequency in our case). The same bias dependency is also found in a single antenna and an array. Sampling data on various sizes of the active array indicated that there is no edge effect for the finite array. The evaluations of the optical sampling combined with microwave measurements indicate that the oscillation condition of the antenna is determined by the geometry within the spacing of the grid elements, which are periodic. Coherent operation of the array is achieved by mutual coupling between the neighboring active elements. In short, the active antenna in the periodic structure is found to behave as a coupled oscillator array. Even though there is a lack of a ground in the back side of the antenna, an open cavity formed by placing a mirror in front of the array functions as an external cavity. As long as the mirror is further than half a wavelength away, the presence of the mirror would not affect the oscillation condition of the antenna. Thereby, the optimization procedure of the active array can start with optimization of a single active antenna. The performance of the array can be analyzed through coupled oscillator theory.;An experimental technique for determining the Van der Pol parameters of a single active antenna is derived. A simple analytical formula is presented for obtaining a damping constant of the active antenna. Experimental results agree with the analytical solution. With the aid of the Van der Pol coupled oscillator theory, the fundamental limit of the size and output power of the active antenna array can be defined. The approach should be useful for future design of active antenna arrays.