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An 18 to 110 GHz two-arm Archimedean spiral antenna directly fed with a Dyson balun is designed, fabricated, and measured. The feed line includes a monolithically integrated rectangular coaxial line impedance transformer designed to reduce the nominal antenna impedance from 180 to 80 V and to match that impedance to the nominal system's impedance of 50 V. Full-wave modelling and impedance measurements are used to assess antenna performance. A VSWR , 2.5:1 is obtained throughout the measured band.Introduction: Spiral antennas were conceived in 1954 by Turner [1]. Their high and low frequencies of operation are determined by the finite size of the feed region and the outer diameter, respectively. The feeding method becomes an additional limiting factor to consider when the frequencies of operation extend up to millimetre-waves. Some specific considerations such as loss, dispersion, embedded impedance matching and mechanical assembly of the feed line to the radiator are required. Monolithic feeds decrease the complexity of the antenna assembly and have been recently investigated [2]. Multi-octave wideband millimetreand sub-millimetre-wave spirals (.40 GHz) have been demonstrated with integrated bolometers, diodes, or photoconductive switches [3][4][5]. As receive only elements, they are configured to extract a DC output from an incident RF signal. However, these types of feeds lack the ability to allow meaningful data to be extracted from the carrier signal.In this Letter, a two-arm Archimedean spiral antenna with a monolithically integrated rectangular coaxial (recta-coax) Dyson balun and impedance transformer is introduced. The antenna is manufactured using the PolyStrata TM process [6], thus facilitating the monolithic integration of the spiral aperture, Dyson balun, and impedance transformer. The fabricated device has measured VSWR , 2.5:1 over the measured frequency band (up to 67 GHz) with excellent agreement between simulated and measured results. The design is carried out with the finite element method (FEM) and method of moments (MoM) codes HFSS [7] and FEKO [8], respectively.

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