Magnetic loop antennas (also known as small transmitting/receiving loops) are small compared to other antennas for the same wavelength. A magnetic loop is one in which the current amplitude is constant round the circumference, and it is therefore small enough to avoid a significant standing wave in the current distribution. This implies a circumference of 1/10 wavelength or less. Small transmitting loops are typically smaller than 1/4 wavelength in circumference at the intended frequency of operation. Since full-size antennas for shortwave communication can be very large -- sometimes several hundred meters in size -- the loop's ability to operate with smaller size albeit reduced radiation efficiency gives it some advantages, especially for mobile use and military use. Conceptually, the magnetic loop can be thought of as a high Q tank circuit having very large diameter to length ratio to facilitate the magnetic "leakage" that makes it effective as an antenna.
Analysis of the design by antenna professionals, confirmed by controlled experiments, has shown that high radiation efficiency is not obtained, and that the main advantage of the antenna is its compact size. These antennas are usually operated close to ground and therefore produce high-angle radiation at the lower frequencies, which is well suited to NVIS (near-vertical incidence skywave) propagation.
Usually a capacitor is used to "enlarge" the antenna by tuning it to resonance in a parallel L-C circuit. The disadvantage of this method is the low bandwidth of the antenna (high Q) which limits operation to a narrow frequency range. A high Q can also be advantageous, however, since tuned magnetic loops work within a narrow frequency range, when used for reception they reject noise produced in the receiver from intermodulation products. This reduces the noise level as compared with wider-bandwidth antennas. However, because all transmitted signals require a finite bandwidth, the high Q of magnetic loop antennas means that they cannot be used for higher-bandwidth applications.
A further advantage of magnetic loops used for receiving is that they respond to the magnetic-field component of the arriving signal; locally generated rf noise (within 1/6 wavelength) has a generally weak magnetic component so the noise tends to be rejected. This effect is more marked at lower frequencies.
As a result of the narrow operating bandwidth of the antenna, if the frequency of operation is changed, the antenna must be retuned by changing the value of the antenna's tuning capacitor. Bandwidth is the usable frequency range of an antenna. When the antenna is operated outside of its bandwidth, energy from the transmitter is reflected back from the antenna, down through the feedline, and back to the transmitter, and so the antenna obviously fails to radiate correctly.
In addition to narrow bandwidth, magnetic loops have very low radiation resistance, often one or two orders of magnitude less than a full size antenna such as a dipole, and only a fraction of an ohm. Efficiency thus depends on low-loss construction, typically use of thick conductors, and low loss air, mica, or vacuum dielectric capacitors to raise the Q to as high a value as possible consistent with the required bandwidth.
In addition to the high currents resulting from the low radiation resistance, high voltages appear across the tuning capacitor when the loop is used for transmission; a kilowatt transmitting loop can have currents of the order of 100 Amperes and voltages at the capacitor of several tens of kV.
Magnetic loops are often fed with 50 ohm coaxial cable connected across a smaller coupling loop that is 1/5 to 1/8 the size of the antenna. This feed loop provides an impedance match to the loop's low feed resistance over the widest frequency range when it is located on the side of the antenna opposite the tuning capacitor. A less common feed system breaks the tuning capacitor into a two series capacitors with the feed across one of them.
The magnetic loop antenna is an old design which is in limited use because of its low radiation efficiency and narrow bandwidth. However many military, commercial, and amateur radio operators still use them today because of the advantages conferred by small size, high-angle radiation and easy transportability. The magnetic loop was widely used in the Vietnam war due to its high portability.
"Performance of a small loop antenna in the 3-10 MHz band", Boswell, Tyler and White, IEEE Antennas and Propagation Magazine, Vol. 47, No. 2, April 2005.