Low-frequency Antennas

Author: Edmund A. Laport

Low-frequency antennas are characterized generally as being electrically short. This means that they operate at frequencies that are low with respect to the fundamental frequency of the antenna system. This is because realizable structures are small in proportion to the radiated wavelength. One of the design objectives is to have an antenna of a given mechanical size appear electrically as long as possible within the economic limitations of good investment.

The means one may adopt to achieve this end depend very much upon the specifications for the communication system, of which the antenna is one element. If communication over long distances in the presence of high electrical noise is the objective, very high power may be necessary. The need for high power brings with it the need for special design techniques for the naturally large antenna currents and antenna potentials that result. The bandwidth required, depending on the type of emission used, may also be a determining factor in the design of an antenna. In another case, the range of transmission may be simply that which is possible within a specified capital cost.

Of course, the operating frequency itself is a dominant factor. The band embracing the so-called "low" and "very low" frequencies is from 300 kilocycles down to the lowest that have been used, something of the order of 12 kilocycles. For the purposes of the present book, we shall regard low frequencies as those below about 500 kilocycles, for the reason that the same basic techniques are usually employed for antennas within this range. Cognizance is taken of certain opportunities to apply the special techniques which have been developed for the medium-frequency broadcast band for frequencies below 500 kilocycles, where steel masts and towers are used as radiators instead of systems of wires. However, aerial wire systems constitute the majority of antennas for the low frequencies.

The design of antennas for frequencies below about 30 kilocycles is a very specialized field of engineering, and problems within this range arise very infrequently in general practice. Except for casual mention of certain details as they arise in connection with our general subject, we are omitting reference to this very low range.

Radiation engineering, in the sense of controlling the radiation pattern of the system in special ways for special purposes, is virtually absent from low-frequency-antenna engineering. To a limited degree, radiation control is applied to low-frequency navigational aids such as the four-course radio-range systems (see Sec. 1.13.3). In general, however, low-frequency-antenna engineering is principally a problem in circuits and how to obtain maximum efficiency from an electrically short antenna.

FIG. 1.1. Relative current distributions on electrically short vertical radiators.

The principles of the electrically short antenna are better understood from Fig. 1.1, in which the natural sinusoidal current distribution along a straight uniform-section quarter-wavelength vertical antenna is used for reference. A straight uniform vertical antenna with a height of 20 degrees would have the relative current distribution shown for the sine curve above the 20-degree level A. In the same way, one with a height of 30 degrees would have the relative distribution above the level B. An antenna with a vertical height of 10 degrees, but with top loading sufficient to give it a total electrical length of 30 degrees, would have the current distribution shown between A and B, and the top loading would be equivalent to an additional 20 degrees of vertical antenna so far as the shaping of the current distribution on the vertical portion is concerned.

A vertical 5-degree antenna, with a relative current distribution like that shown above the level C, is a comparatively ineffective radiator because of its very small exposure to space and consequent small resistance due to radiation.

In low-frequency-antenna design there is no optimum design. One can "squeeze" here and there to get a little more performance. The designer must decide where to stop. This decision may be the most important of all, since the performance-cost relations run into the law of diminishing returns. No one can specify in a general way how far one should go in this direction. The tendency is to go too far into diminishing returns as a part of the squeezing process.

It is helpful at this point to think of the results in decibels, since any improvement having appreciable cost should yield not less than 1 decibel of increase in radiation. If one can afford to be exhaustive in his preliminary engineering, which means that engineering cost is not a factor, detailed cost estimates can be obtained for a succession of design variations in the approach to an optimum investment.