A device and method for detecting motion using an adjustable detection shell. The motion detector includes an antenna; a reception circuit configured to receive a reflected radio frequency (RF) signal via the antenna; a time gate circuit electrically connected to the reception circuit and configured to generate a control signal for the reception circuit based on a timing setpoint signal; and an electronic processor electrically connected to the reception circuit and the time gate circuit. The electronic processor is configured to receive a signal from the reception circuit indicative of motion occurring within a detection shell that is adjustable via the timing setpoint signal. The signal is based on the reflected RF signal. The electronic processor is further configured to generate a notification when the signal received from the reception circuit is indicative of motion occurring within the detection shell.