Magnetic loop

Magnetic loop antennas (also known as Small Transmitting Loops) have a small antenna size compared to other antennas for the same wavelength. The antenna is typically smaller than 1/4 wavelength of the intended frequency of operation. Antennas for shortwave communication are normally very large sometimes covering hundreds of feet or meters in length. The advantage of the magnetic loop is that with its small size it maintains very high efficiency levels.

The technical mechanism is to use a capacitor to "enlarge" the antenna and bring it to resonance. The disadvantage of this method is the low bandwidth of the antenna, also known as "high Q". However, a "high Q" antenna also has advantages as well. Since Magnetic Loop antennas only function within a narrow range of frequency when tuned, they reject harmonic noise from other radio sources. This rejection of interfering noise from other harmonically related frequencies keeps the noise level down compared to other antennas like the common 1/4 wave vertical antenna.

As a result of the narrow operating bandwidth of the antenna, if the frequency of operation is changed, the antenna needs to be retuned by changing the capacitive value of the antenna. Bandwidth is the usable frequency range of an antenna in relation to the area of desired operation. When the antenna is operated outside of its bandwidth, the energy from the radio is reflected back from the antenna, down through the feedline back to the radio. The term bandwidth relates to the concept of Standing Wave Ratio or SWR. When the reflected power exceeds a 2.5:1 power reflection ratio (too much energy being reflected from the antenna back into the feedline) the antenna will not maintain its performance characteristics. This type of condition relates specifically to the antenna's ability to transmit radio energy from the transmitter to the antenna.

The magnetic loop antenna is an old antenna, however, many military, commercial, and ham radio operators still use them today. The Magnetic Loop was widely used in the Vietnam War due to its high portability.