Investigation of anomalous beamwidth variation in the sinuous spiral antenna

The sinuous antenna [DuHamel, 1987] was developed to fill a long time need for a flush-mountable antenna that would respond simultaneously to opposite-sense polarizations over a very wide band. Although geometrically related to logarithmic spirals, the sinuous spiral antenna is a log-periodic structure that is self-scaling only for a discrete set of scale factors. Hence, frequency-independent performance is not guaranteed even in the absence of truncation effects. Indeed, variations with frequency of the half-power beamwidths have been observed experimentally for some sinuous spirals [Ingerson et al. 1991]. These variations have no analog in a spiral antenna of the same parameters. Numerical analysis of the current distributions on sinuous spirals has been undertaken to provide insight into the cause of the beamwidth variation.<>