What is "loop antennas"

Usage examples of "loop antennas".

A designated operator would hide himself somewhere in or around the city and the contestants, using direction-finding loop antennas on their auto receivers, would attempt to trace out the hidden transmitter's location first.

The ham operators designated as the hunters, using RDFs or radio direction finders--usually loop antennas atop their cars--would start reading signal strengths on their S-meters, thereby eventually tracking the hidden transmitter's direction from their receiver.