Wideband Pattern Nulling With Multiarmed Spiral Antennas

The wideband modes of multiarmed spiral antennas are studied, with the goal of obtaining insight into their potential for stable pattern nulling over extremely wide bandwidths. Using the mode theory, it is found that for stable and extremely wideband operation, a maximum of N-2 nulls may be inserted into an N-armed spiral's pattern. Single and multiple null insertions are demonstrated numerically and experimentally, using three-, four-, and eight-armed spirals. Spiral mode theory is also utilized to determine limitations imposed by the unequal mode impedances of the multiarmed spiral. Techniques for compensation of frequency-dependent null rotation are also discussed, where it is shown that a simple delay-line compensation technique is satisfactory for stabilizing the null locations over modest bandwidths. It is also shown that the number of nulls that can be practically inserted in the pattern is limited not only by the number of arms, but also by the size of the antenna.