Undirectional tuned loop antennas using combined loop and dipole modes

This paper presents a simple computer simulation model that assumes that both the loop and dipole radiation modes can exist simultaneously. The model computes the antenna pattern and directivity for any chosen combination of the two modes. The patterns obtained by varying the parameters defining how the modes are combined (in relative amplitudes, phase and position) and then matched for best fit with the measurements for various loop sizes and shapes. The results in this paper show that both dipole and loop modes do exist simultaneously in most loop configurations. Also it has been found that there are some loop configurations where the modes combine to give antenna patterns that are significantly and usefully unidirectional. Also in practice these unidirectional patterns are retained over a wide band of frequencies for a given loop. This indicates that the loop mode radiation resistance varies similarly with frequency as the dipole mode and not as the fourth power of the frequency according to existing theoretical formulas. Otherwise the directional properties would be expected to exist only over the narrow band of frequencies for which the amplitudes of the two modes are approximately equal.