Two element active antenna array mode stability

It is known that when a passive antenna array is arranged to enable quasi-optical power-combining, a single phase distribution or mode will exist between the individual elements. However, when active antenna arrays are considered the individual active antenna oscillators can exhibit a number of array modes. In this paper, closed form analytical equations for the stability condition for the two modes available within a two element collinear array as the element separation is increased are presented. This information enables the correct prediction of the output characteristics of the array which is important since the far-field radiation pattern depends on the stable mode (i.e. boresight null or peak).