| Spiral antennas are of great interest for many broadband applications because of their inherent large bandwidth. This dissertation explores novel techniques for reducing size of conformal antennas with particular emphasis on spirals. Specifically, a new scheme based on distributed reactive loads is developed for concurrent control of the spiral impedance and wave velocity, without increasing the antenna volume. We also show that a properly terminated spiral element with material and absorber loading can achieve 0 dBi realized gain even when its size is only 0.15 lambda per side and lambda/20 in height at the lowest operational frequency (with ground plane backing), which is 3 times smaller than previous publications. The proposed size reduction techniques were adapted to develop a ground plane-backed six-element L-band spiral array delivering circular polarization. This L-band aperture is 1.9 times smaller than earlier versions and approached optimal size miniaturization. An extension of Chu's antenna limit theory is also presented subject to a given impedance mismatch factor. This extension provided, for the first time, a relation between antenna size and maximum achievable realized gain for a source with real impedance. Our examination of spiral miniaturization further indicated that the concept of wave slow-down is both an enabling and a limiting factor. |
