Loop Antenna - Small Loops

Small Loops

Small loop antennas are much less than a wavelength in size, and are mainly (but not always) used as receiving antennas at lower frequencies.

The small loop antenna is also known as a magnetic loop since it behaves electrically as a coil (inductor) with a small but non-negligible radiation resistance due to its finite size. It can be analyzed as coupling directly to the magnetic field (opposite to the principle of a Hertzian dipole which couples directly to the electric field) in the near field, which itself is coupled to an electromagnetic wave in the far field through the application of Maxwell's equations. Because of this fact it is somewhat immune to noise affecting the electric field ("static") generated in the near field. Since at low frequencies, such as the AM broadcast band, the near field region is physically quite large, this provides a considerable benefit in relation to static generating devices (such as sparking at the commutator of an electric motor) in the vicinity. Contrary to myth, however, this immunity does not extend to noise sources outside of the near field: such noise is received as an electromagnetic (propagating) wave and would be received equally by any antenna sensitive to a radio transmitter at the location of that noise source.

Since the small loop antenna is essentially a coil, its electrical impedance is inductive, with an inductive reactance much greater than its radiation resistance. In order to couple to a receiver, that inductance is normally cancelled with a parallel capacitance. Since a good loop antenna will have a rather high Q factor, this capacitor is made variable and also functions as the front end tuning capacitor, determining the station to be received.

Surprisingly, the radiation pattern of a small loop is quite opposite that of a resonant loop. Since the loop is much smaller than a wavelength, currents along the conductor are essentially in phase. By symmetry it can be seen that the voltages induced along the various sides of the loop will cancel each other when a signal arrives along the loop axis. Therefore there is a null in that direction. Instead, the radiation pattern peaks in directions along the plane of the loop. Although a similar argument may seem to apply to signals received in that plane, that voltages generated by an impinging radio wave would cancel along the loop, this is not quite true due to the phase difference between the arrival of the wave at the near side and far side of the loop. Thus increasing that phase delay by increasing the size of the loop has a large impact in increasing the radiation resistance and the resulting antenna efficiency.

Another way of looking at this is to view the small loop antenna simply as an inductive coil coupling to the magnetic field in the direction normal to plane of the coil according to Ampère's law. Then consider a propagating radio wave normal to that plane. Since the magnetic (and electric) fields of an electromagnetic wave in free space are transverse (with no component in the direction of propagation), it can be seen that this magnetic field and that of a small loop antenna will be orthogonal, and thus uncoupled. For the same reason, an electromagnetic wave propagating in the plane of the loop, with its magnetic field normal to that plane, is coupled to the magnetic field of the coil. Since the transvserse magnetic and electric fields of a propagating electromagnetic wave are at right angles, the electric field of such a wave is in the plane of the loop, and thus the antenna's polarization (which is always specified as being that of the electric, not magnetic field) is said to be in that plane. Thus mounting the loop in a horizontal plane will produce an omnidirectional antenna which is horizontally polarized; mounting the loop vertically yields a weakly directional antenna with vertical polarization.

The radiation resistance R_R of a small loop is often much smaller than the loss resistance R_L due to the conductors comprising the loop, leading to a poor antenna efficiency. Consequently, most of the transmitted or received power will be dissipated in loss resistance. However in a receiving antenna, this inefficiency may not be of great concern since atmospheric noise and man-made noise dominate thermal (Johnson) noise at lower frequencies. (CCIR 258; CCIR 322.) For example, at 1 MHz, the man-made noise might be 55dB above the thermal noise floor. If a small loop antenna's loss is 50 dB (in effect, the attenna includes a 50 dB attenuator), the electrical inefficiency of that antenna will have little impact on the receiving system's signal-to-noise ratio. In contrast, at quieter VHF frequencies, an antenna with a 50 dB loss could degrade the received signal-to-noise ratio by up to 50 dB, resulting in terrible performance.