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A ${V}$ -band heterodyne radiometer in SiGe BiCMOS employing a capacitor-reused absorptive inter-stage matching network is firstly proposed, which prevents from employing a circulator, a resistive attenuator or two quadrature couplers in the conventional high-sensitivity cascaded receiver. This radiometer consists of a five-stage low noise amplifier (LNA) with the proposed reflectionless output matching network (RLMN), and a double-balanced Gilbert mixer stage with integrated Marchand baluns. In the RLMN, the capacitor for in-band matching is reused as an impedance inverter in the lossy absorptive network to eliminate reflections. Utilizing the reflectionless LNA, the radiometer eliminates the out-of-band reflected signals round the interfaces of LNA and mixer. Fabricated in a $0.13~\mu \text{m}$ SiGe BiCMOS technology, the reflectionless LNA achieves a gain of 31.9 dB@56 GHz, an absolutely stable operation in-band and 11 times larger stability factor out-of-band than conventional LNAs. Moreover, the implemented heterodyne radiometer achieves a noise equivalent temperature difference (NETD) of 0.3 K which is even comparable with wideband direct-detection radiometers. [ABSTRACT FROM AUTHOR]