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A novel full-band (40% relative bandwidth) waveguide polar modulator (WPM) design based on a new architecture with two turnstile junctions and a nearly frequency-independent differential 180° phase shift between orthogonal arms is presented. The use of suitably designed turnstile junctions together with antiphase waveguide-coaxial-waveguide transitions in reduced-height waveguide technology enables the improvement of bandwidth over previous WPMs with a very compact and simple solution. This design enables continuous rotation about its central axis at 40 revolutions/s, which makes the modulator suitable for receivers with significant $1/f$ noise such as cryogenic radiometers. The polar modulator presented in this paper is designed to be cooled down to cryogenic temperatures in order to reduce its contribution to the receiver noise. This design is suitable for radio astronomy applications due to its cost-effective, compact design, and high quality performance as well as being readily scalable. A polar modulator in the Ka-band (26.5–40 GHz) has been designed to show a reflection better than −20 dB, an insertion loss around 0.3 dB, and a cross-coupling near −35 dB measured at room temperature ( $T = 298$ K). This insertion loss is reduced by 30%, while the phase difference and cross-coupling maintain similar values when the polar modulator is cooled to approximately $T = 80$ K. [ABSTRACT FROM AUTHOR]