What are the loop antenna applications?

Loop antenna overview

A loop antenna is a structure in which a metal wire is wound into a certain shape, such as round, square, triangular, etc., with both ends of the conductor as the output end. A multi-turn (such as spiral or overlapped winding) is called a multi-turn loop antenna.

For antennas formed by bending wires into a loop, the terminal load impedance of the loop antenna can be zero or equal to the characteristic impedance of the loop, and the current distribution on it is similar to that of parallel transmission lines. When the circumference of the terminal shorted ring is not greater than 0.2 times the operating wavelength, it is called a small loop antenna, and the current on the ring is approximately distributed in equal phase in phase. When the radius of the short ring is large, the current on the ring is a standing wave distribution. When the impedance of the terminated load is equal to the characteristic impedance of the loop, the current on the loop is a traveling wave distribution.

According to the duality principle of electromagnetic radiation, the radiation field of the small ring antenna and the small electric dipole antenna placed perpendicular to the torus are similar except for the exchange of the amount of electricity and magnetism, that is, the pattern is a circle on the plane of the torus, the pattern on the plane where the ring axis is located is a figure 8, and the radiation along the ring axis is zero.

Loop antenna principle

When the diameter of the ring increases, the current is distributed according to the standing wave, the most commonly used is a ring with a working wavelength circumference, and the shape of the ring is rectangular, diamond and triangular in addition to the circle. The square ring fed at the midpoint of one side radiates most strongly in the direction perpendicular to the torus, the polarization of the field in this direction is parallel to the ring edge of the feed point, radiates zero in the direction of the axis of the ring edge of the ring with feed point, and the input impedance is about 100 ohms at resonance, and the gain is about 3.09 dB. Other shapes of loop antennas are similar. This kind of ring is commonly used to form ultra-short-wave antenna arrays, such as forming double-ring or four-loop TV antennas and loop-shaped Yagi antennas.

Loop antenna application

The ring can be hollow or core; Single or multi-turn. Theoretical and experimental results show that the radiation field is proportional to the area, number of turns and current on the ring, and inversely proportional to the square and distance of the working wavelength; It has little to do with the shape of the ring. Small ring antennas have low radiation efficiency and are commonly used as receiving antennas, widely used in direction finding, radio compasses, and medium and shortwave broadcast receivers.

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