Modulated Arm Width (MAW) Spiral: Theory, Modeling, Design and Measurements

The design and performance is discussed of equiangular four-arm modulated arm width (MAW) spiral antennas. Effects of expansion factor, modulation ratio, termination and feedpoint region on cross-polarization, pattern symmetry, gain, and impedance for both forward and reverse mode 1 operation are studied. The bandstop region is characterized using the transmission line theory and full-wave modeling. Three composite ideal beamforming measurements are developed and are shown to be in excellent agreement with theoretical predictions. Obtained results indicate that the expansion factor should be less than 1.5 and the modulation ratio needs to be higher than 4 to achieve high purity radiation patterns in both modes of operation. Beamformer design, effects of the dielectric loading, and non-ideal cavity-backing are not discussed.