A single-arm rectangular spiral antenna is analyzed using the finite-difference time-domain method. The spiral is printed on a finite-size dielectric substrate backed by a finite-size conducting plane. Both the substrate and conducting planes are square with a side length of less than 0.6 0 ( 0 : wavelength in free space). The radiation pattern is dependent on the outermost arm peripheral length . The spiral whose peripheral length is within 2 3 ( : the guided wavelength of the current)

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... a tilted beam of circular polarization. When the peripheral length is decreased to 2 , the spiral radiates an axial beam. The axial beam has a wide half-power beam width of approximately 102 (for 0 369 0 ) with a gain of approximately 6.7 dB. The axial beam shows a 15% frequency bandwidth for a 3-dB axial ratio criterion. Over this bandwidth, the voltage standing-wave ratio (VSWR) is less than two, as desired. The experimental results for the radiation pattern, gain, axial ratio, and VSWR are also presented. Index Terms-Beam formation, finite-difference time-domain (FDTD) analysis, printed spiral antenna.