English: An AT-4 dipole antenna of an ARN-1 radar altimeter system from an advertisement in a 1947 radio magazine. It was mounted on the bottom surface of an aircraft and transmitted radio waves which reflected from the ground. The altitude of the aircraft was calculated from the time it took the radio waves to return.

This is a half-wave dipole; composed of two symmetrical elements each 1/4 wavelength long, insulated from each other with the white insulator. It is mounted 1/4 wavelength below the aircraft's body, so the aircraft's aluminum skin functioned as a reflecting surface to reinforce the outgoing wavefront. It is made of painted brass tubing, and was mounted parallel to the airstream to reduce air drag. The text gave no information about the operating frequency, but if the antenna is roughly 20 cm, long as it appears to be, the wavelength is around 40 cm so the operating frequency was in the UHF range, around 700 MHz. A coaxial cable feedline attached to the connector, bottom.