Design of multioctave spiral-mode microstrip antennas

The design of spiral-mode microstrip antennas with a bandwidth of 6:1 is demonstrated. The antenna has a small dissipative loss due to a resistive loading outside its active region. As a specific measure of bandwidth at microwave frequencies, the spiral-mode microstrip antenna gain is higher than that of the conventional loaded-cavity spiral over a 5:1 and if the spacing d between the microstrip and the ground plane is 0.25 in. or larger over the 2-12 GHz range. Cursory efficiency calculations show consistent improvement over a loaded-cavity spiral. A study of ground plane spacing showed that the antenna gain remains above the conventional spiral gain as d is reduced until it is as small as 0.1 in