Planar spiral antennas are excellent prime-focus reflector feeds for certain applications due in part to the frequency stability of their beamwidth, phase center, and axial ratio. Phase center is a particularly important parameter, but one that is often neglected in the available literature on spiral antennas. In the 1960s and 1970s various studies of spiral antenna phase center were reported, but these studies were often narrow in scope. For instance, the numerical models often neglected any possible contribution from the arm termination or cavity backing, and measurements relied on limited information from principal-plane cuts. Today, commercial numerical antenna analysis tools allow the consideration of additional effects in the model, as well as the output of dense pattern data. This pattern data can be used to gain an improved understanding of phase center. This work studies the effects that field-of-view, mode of excitation, number of arms, arm termination, and reflective cavity backing have on the phase center and its associated error of the planar spiral antenna. An algorithm that calculates a unique three-dimensional phase center position is discussed and used to account for possible modeformer amplitude and phase imperfections.
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