Richards, William F.; Long, Stuart A.; University of Houston-University Park (International Foundation for Telemetering, 1986-10)
      A dual mode microstrip antenna element has been investigated which has two independently excitable modes resonant at the same frequency. This element has been shown to be capable of producing a broadside maximum, a broadside null, or an end-fire type pattern by suitable choice of its reactive loads and suitable excitation of its degenerate modes. Appropriately located loads can be used to resonate modes normally resonant at quite different frequencies, at a single, common frequency. The results indicate that the nodal lines of the loaded element are accurately predicted by the generalized theory of loaded microstrip antennas, and that two modes can be excited independently of each other by feeding each mode along the nodal line of the other. To verify the theoretical predictions an actual dual mode microstrip element was fabricated and tested. The results of this experiment correlate well with the theoretical model with respect to the overall characteristics of the radiator.

      Richards, William F.; University of Houston, University Park (International Foundation for Telemetering, 1986-10)
      A study has been undertaken to determine the feasability of dynamically controlling the input impedance of a microstrip antenna element by changing its reactive loading. The major applications of such an element would be for use in a scanned phased array. By changing the loading of individual elements appropriately, one could alter the active array impedance of the elements to compensate to some degree for the onset of scan blindness. While the ultimate feasability of such applications cannot yet be firmly established, a single element can be controlled using PIN diodes to effectively alter its input impedance. The generalized theory of loaded microstrip antennas has been used to predict the impedance of a variety of microstrip, antenna configurations with multiple loads. This work has shown the possibilities of changing the input impedance of the radiator over a wide range of values without affecting its resonant frequency or radiation pattern by moving a set of short-circuited loads from one position to another. Actual printed-circuit antennas were fabricated based on this design and good correlation was found between theory and experiment.