This communication examines the relationship of metal width and arm spacing of a planar printed spiral antenna on its constant input impedance. Observations on the current and electric field distribution are made first to establish the physical mechanisms responsible for the impedance behavior. An analytical model is proposed to analyze the excited mode on the spiral antenna, which can be used to analytically obtain the constant input impedance in the spiral's radiation region. The expression in this work can be applied to evaluate the input impedance for a variety of non-self-complementary spiral structures. \ 1963-2012 IEEE.