Bellini–Tosi direction finder

A Bellini–Tosi direction finder (B–T or BTDF) is a type of radio direction finder (RDF), which determines the direction to, or bearing of, a radio transmitter. Earlier RDF systems used very large rotating loop antennas, which the B–T system replaced with two fixed antennae and a small rotating loop, known as a radiogoniometer.

Source: Wikipedia — Bellini–Tosi direction finder (CC BY-SA 4.0)