Loop Antenna

(frame antenna), a directive antenna in the shape of one or more flat rings of wire forming a circular, square, or rectangular frame. It was proposed in 1913 by K. Braun.

In most cases, the perimeter of the loop is very small compared with the operating wavelength; thus, the input impedance of the antenna has the characteristics of an inductance. By connecting a loop antenna with a variable capacitor, it is possible to obtain an oscillatory circuit that is tuned to the operating wavelength. When the dimensions of the loop are small, the amplitude and phase of the current oscillations in the loop are practically constant over the entire perimeter. In a transmitting loop antenna, the direction of the current in the opposite elements of the loop is reversed, and the radiated electromagnetic waves are therefore shifted by exactly 180° in phase. As a result, the radiation in the direction perpendicular to the plane of the loop is completely canceled. In other directions the cancellation is incomplete, and the maximum radiation intensity corresponds to the directions lying in the plane of the loop.

The strength of an electric field E of an electromagnetic wave at some point located a great distance d from a transmitting loop antenna is calculated from the formula

where l is the current in the loop, n is the number of turns, S is the area of the loop, λ is the operating wavelength, and ɸ is the angle between the plane of the loop and the direction to the point under consideration. The electromotive force ∊ induced in a receiving loop antenna is calculated from the formula

where E is the component of the electric field strength of the received wave parallel to the plane of the loop and ɸ is the angle between the plane of the loop and the direction of arrival of the wave. In the plane perpendicular to the plane of the loop, the polar diagram of the antenna has the shape of a figure eight. The directivity factor is equal to 1.5. Loop antennas sometimes have dimensions comparable with λ. In this case, the polar diagram has multiple lobes and the direction of maximum radiation (or reception) is changed.

Loop antennas are primarily used as the receiving antennas for radio direction finders, including radio compasses, and for radio broadcast receivers operating in the low-, medium-, and high-frequency ranges.