Experimental Validation of the Theory
Much of the very early work in coupled oscillators for phased-array applications involved both theory and experiment. Probably the earliest was the work of Karl Stephan in which he studied a linear array of coupled oscillators for beam-steering of a linear array of radiating elements . In his concept, the phasing was controlled by injection locking the end oscillators to signals whose relative phase was controlled with a phase shifter. While still earlier work was published, Stephan points out that it did not involve mutual injection locking, the defining feature of the concepts treated in this book.
6.1 Linear-Array Experiments
Stephan’s pioneering experiment in 1986 involved three very high frequency (VHF) transistor Colpitts oscillators coupled together by a network of lumped elements. The end oscillators were also injected with external signals derived from a master oscillator signal that was split into two signals, one of which was phase-shifted relative to the other by a variable length transmission line (coaxial line stretcher). Stephan measured the oscillator phases with varying amounts of phase shift of the injection signals and verified that the behavior conformed qualitatively to the theory. However, he noted several issues that have persisted throughout the ensuing development of this technology. He noted that manufacturing variation among the oscillators resulting in variation in their free-running frequencies ...