Ground-wave Propagation of Low Frequencies

Author: Edmund A. Laport

Ground-wave transmissions in general require vertical polarization of the wave field; that is, the electric vector lies in the vertical plane through the direction of propagation.

Low-frequency antennas therefore are designed for vertical polarization, the useful radiations being derived from currents in the vertical portions of the antennas. Radiations from horizontal portions of antennas at low frequency are lost by cancellation of their image radiations. Currents in the conductors of a flat-top should be balanced with respect to the center where the down lead is taken off. For this reason, T antennas are preferable to inverted-L types¹ except for the case where multiple tuning is used.

The fact that, at low and very low frequencies, practical vertical heights are usually a very small portion of a wavelength is the cause for two important basic facts in low-frequency-antenna design: (1) the vertical-radiation pattern is always that due to a very short vertical radiator and follows the equation where a. is the angle above the horizon, and (2) the radiation resistance is always very low, often very much lower than any other resistances in the system. For that reason, the radiation efficiency, defined as the ratio of power radiated to total power input to the antenna system, is generally low.

The attenuation of a ground wave passing over a given path increases with frequency, as one may observe from the data of Table 1.1. This table lists the attenuation for a ground wave propagated over a smooth, spherical earth for sea water, soil of good conductivity, and soil of poor conductivity. Most soils encountered in practice will come between these "good" soil and "poor" soil limits. The specifications for these soils are given in the table.

Relatively low attenuation constitutes one of the advantages to the use of a low frequency from the propagation standpoint. But there are two opposing factors which penalize the low frequencies from a communication standpoint. One is the reduction in radiation efficiency which attends the use of lower and lower frequencies, due to the limited electrical size of practicable antennas. This factor is equivalent to loss of as much as 10 to 12 decibels at the source for the lowest radio frequencies or for other frequencies where the radiation efficiency may be of the order of 6 percent. The other factor is that relatively high noise levels generally prevail at these frequencies.

While these noise levels vary considerably with time and geography, they seriously limit the range of transmission by requiring relatively high field strengths for practical working signal-to-noise ratios. The only way of overcoming this range limitation is by employing high-power transmitters. Furthermore, it is not feasible to obtain appreciable power gains by using directive antennas as is so easily done at much higher frequencies. Some advantage of directivity can be realized at the receiving location by employing loop or, preferably, wave antennas.

1)	Many old textbooks tell us that an inverted-L antenna is directive in its horizontal pattern. The amount of such directivity is too small to consider.