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This paper discusses the development of a compact light-weight planar spiral antenna for use in amplitude comparison and interferometer direction finding (DF) applications covering 1 to 18 GHz. The antenna uses a slow-wave (zigzag) spiral radiator which results in a cavity diameter which is only 68% of the wavelength at 1 GHz. The two-arm spiral is fed by a Marchand balun and achieves a VSWR of less than 1.5:1 over almost the entire 1 to 18 GHz frequency range. The spiral antennas are supplied in phase and amplitude tracking sets for wideband DF applications. Amplitude and phase tracking performance for a fully integrated linear interferometer DF panel which includes the radome and feed cable effects are presented. Phase DF performance for the panel with pulse receivers as measured in an anechoic chamber is discussed.