Small Loop Antenna

The small loop antenna is a closed loop as shown in Figure 1. These antennas have low radiation resistance and high reactance, so that their impedance is difficult to match to a transmitter. As a result, these antennas are most often used as receive antennas, where impedance mismatch loss can be tolerated. The radius is a, and is assumed to be much smaller than a wavelength (a<< ). The loop lies in the x-y plane. Figure 1. Small loop antenna. Since the loop is electrically small, the current within the loop can be approximated as being constant along the loop, so that I= . The fields from a small circular loop are given by: The variation of the pattern with direction is given by , so that the radiation pattern of a small loop antenna has the same power pattern as that of a short dipole. However, the fields of a small dipole have the E- and H- fields switched relative to that of a short dipole; the E-field is horizontally polarized in the x-y plane. The small loop is often referred to as the dual of the dipole antenna, because if a small dipole had magnetic current flowing (as opposed to electric current as in a regular dipole), the fields would resemble that of a small loop. While the short dipole has a capacitive impedance (imaginary part of impedance is negative), the impedance of a small loop is inductive (positive imaginary part). The radiation resistance (and ohmic loss resistance) can be increased by adding more turns to the loop. If there are N turns of a small loop antenna, each with a surface area S (we don't require the loop to be circular at this point), the radiation resistance for small loops can be approximated (in Ohms) by: For a small loop, the reactive component of the impedance can be determined by finding the inductance of the loop, which depends on its shape (then X=2*pi*f*L). For a circular loop with radius a and wire radius p, the reactive component of the impedance is given by: Small loops often have a low radiation resistance and a highly inductive component to their reactance. Hence, they are most often used as receive antennas. Exaples of their use include in pagers, and as field strength probes used in wireless measurements.