A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line (or possibly a balun). 

There are two distinct antenna designs: the small loop (or magnetic loop) with a size much smaller than a wavelength, and the much larger resonant loop antenna with a circumference close to the intended wavelength of operation. 

These antennas are ideal for the magnetic field measurements as required e.g. by CISPR 11 (reference CISPR 16-1-4 and CISPR 16-2-3) and FCC 18. The built-in preamplifier matches the extremely low impedance of the loop to the measuring receiver.