This paper presents a low switching-loss Dicke radiometer for W-band passive imaging systems. The equivalent switching loss introduced by the passive single-pole-double-throw (SPDT) switch in the conventional radiometer is significantly reduced by the proposed single-pole-double-throw distributed amplifier (SPDT-DA), which leads to radical improvement on the receiver's noise performance. The Dicke radiometer consisting of a SPDT-DA, a four-stage low noise amplifier (LNA) and a power detector is fully integrated in a 0.13-mu m SiGe BiCMOS chip. With the 0.93-dB equivalent switching loss at 91 GHz of the SPDT-DA, the total noise figure (NF) of 8.4 dB at 91 GHz is achieved by the SPDT-DA followed by the LNA. With a power consumption of 28.5 mW, the radiometer obtains an overall RF gain of 42 dB and a noise equivalent temperature difference (NETD) of 0.21 K with 30-ms integration time. The two-dimensional imaging experiment with object distance of 0.7 m is successfully carried out with the radiometer chip.