1. IntroductionΒΆ

In the case of loop antennas, the dipole consists of a usually circular or rectangular current loop which acts like a coil and functions in response to the magnetic component of the electromagnetic field [49].

A small loop (circular or square) is equivalent to an infinitesimal magnetic dipole whose axis is perpendicular to the plane of the loop. That is, the fields radiated by an electrically small circular or square loop are of the same mathematical form as those radiated by an infinitesimal magnetic dipole [8].

Loop antennas can be seperated into two categories: small loops and large loops, as seen in Fig. 1.1.


Fig. 1.1 : Loop Antennas

  • Electrically small loops: overall length (circumference) less than about one-tenth of a wavelength.

    • Small radiation resistance that are usually smaller than their loss resistances. They are very poor radiators and usually in the receiving mode [8].
    • The radiation resistance of the loop can be increased by increasing (electrically) its perimeter and/or the number of turns. Another way is to insert a ferrite core [8].
    • In view of their small dimensions the current in the entire loop can be regarded as locally constant. Consequently, they generate the same sort of field as a Hertzian dipole, i.e. a very short electrical antenna on which the current distribution is a function of position to an equally small extent [49]. The field pattern of electrically small antennas of any shape is similar to that of an infinitesimal dipole [8].
    • Used as probes for field measurements, directional antennas for radiowave navigation [8].
  • Electrically large loops: circumference is about a free-space wavelength.

    • Used primarily in directional arrays.
    • To achieve such directional pattern characteristics, the circumference (perimeter) of the loop should be about one free-space wavelength.

They have become less significant as transmitting antennas for frequencies below 30 MHz. An exception to this is the tuned transmitting loop, which can be equipped with a remotely controlled capacitor to make a resonant circuit (a receiving loop can be provided with additional selectivity and sensitivity in the same way). However, such loops are extremely narrowband systems and therefore have to be retuned whenever the frequency is changed [49].