A type of electronic instrument used to test and adjust radar equipment. It operates on the principle of a cavity resonator. A small amount of electromagnetic energy from the transmitting antenna is fed into a small cavity (or box), the volume of which can be adjusted to resonate electrically, or “ring,” to signals of this frequency, like a tuning fork responding to a musical note of proper pitch. This resonance is detected by the radar receiver. The amount of resonance appearing at the receiver output (the oscilloscope) is a function of the power transmitted, the tuning of the cavity, and the tuning and amplification of the receiver. Accordingly, the echo box provides a test of the overall efficiency of the radar system, eliminating atmospheric variables.