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This work presents a single-antenna self-injection-locked (SIL) radar to reduce the hardware complexity of continuous-wave (CW) Doppler systems. The theory provides a basis for determining the signal-to-noise spectral density ratio (SNDR) with the effects of clutter. Experimental results agree closely with the theoretical predictions, showing that the clutter does not affect the optimal SNR performance in an SIL radar. A single-antenna SIL radar array is designed to detect vital signs with random body movement cancellation. To this end, a subject is placed between two single-antenna SIL radars to measure the rates of respiration and heartbeat using Doppler shift, and the effects of random movement of the subject are cancelled by wireless mutual injection locking (MIL) of the two radars. In an experiment, a prototype of such a two-radar array with a spacing of 2 m was implemented at 2.4 GHz, providing accurate and reliable cardiopulmonary monitoring of a subject who jogged on a treadmill with random body motion of many centimeters.