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While the first equiangular spiral antennas were slots cut from a thin conductive sheet, the widespread availability of cheap photoetching fabrication has made it more common for the spiral to be printed on a dielectric substrate. This paper examines the effects of the substrate on the spiral's performance. The finite-difference time-domain (FDTD) technique is used to model the spiral over a range of configurations. The results are used to construct a design graph that shows that the substrate significantly affects the impedance of the antenna. The results also show that the substrate can negatively impact the bore-sight gain and radiation patterns. Measurements from two spirals are used to verify the accuracy of the numerical model.