Low-frequency Antennas

Author: Edmund A. Laport

Practical radio communication began with the use of the low frequencies and for several years the trend was toward lower and lower frequencies. It was believed at that time, early in the twentieth century, that the range of a station was a matter of a certain number of wavelengths, so that the longer the wavelength the greater the range. This continued until the middle 1920's, when the possibilities of high-frequency communication became evident. For a time, high-frequency transmission captured the imagination of the radio-communication world, with the consequence that for many years low frequencies were believed to be discarded. For this reason low-frequency techniques almost became a lost art, even though the applications for low frequencies have been constantly growing throughout the world. If it were not for the relatively congested condition of the low-frequency spectrum, there would be more extensive uses for this band from 15 to 500 kilocycles.

An important factor in the ever-increasing importance of the lower radio frequencies is that of their comparative propagation stability. The variations that occur are small with respect to those usually encountered on high-frequency circuits. There are circumstances where this characteristic outweighs all the disadvantages of the lower frequencies, such as when reliability is the dominant objective. Reliability in this case includes not only propagation stability but also relative immunity to jamming. Another characteristic value is the deep penetration of ground currents (which are really waves propagated in the ground or in water), which makes it possible to communicate at considerable depths under the sea or under ground.

It is well known that the strength of atmospheric static increases as the frequency becomes lower. This kind of interference is a controlling factor in the effectiveness of low-frequency communication. While the strength of the transmitted wave remains relatively steady, the rise and fall of the noise levels during a typical day may cover a range of 20 decibels. Thunderstorms in the general area of the receiving station may increase this range to 80 or 100 decibels on occasion. The variations for frequencies down to 100 kilocycles for the different noise zones for different hours of the day can be deduced from the curves reproduced as Appendix VII. These curves explain why the range of a low-frequency station is determined by ambient noise rather than by actual fading of the radio wave. These curves also indicate the very large differences in noise conditions between the various noise zones from grade 1 to grade 5. It is a fortunate fact that the world's auroral zones are regions of lowest atmospheric noise for low-frequency propagation, because these are also the regions of greatest instability for high-frequency propagation. This permits low frequencies to be easily substituted for high frequencies, and communication can take place with very moderate powers and with relatively inefficient antennas. This is one reason why the lower frequencies have exceptional importance in the high latitudes.

For many years the backbone of the North American airways navigation system was the four-course radio-range system using frequencies between 200 and 400 kilocycles. This basic system is being replaced by newer systems using the very high frequencies, but the older low-frequency system will remain for many years. In Europe this band has long been reserved for broadcasting. Marine communication makes extensive use of the low radio frequencies.

In presenting a chapter on low-frequency-antenna design it must not be thought by the general reader that this is a historical subject only. Low-frequency-antenna engineering is an active modern subject, and one where the sources of information are few and the problems difficult. The author has been impressed by this lack of specific information on low-frequency antennas even after so many years of engineering effort devoted to them, as well as enormous sums of money. The reason evidently is that the compromises that have to be made in design are so extreme that the designer has no particular pride in the result and says as little as possible about it in his technical reports to the profession. Almost all papers in technical journals about low-frequency antennas are strictly descriptive and lack the detailed discussions of how the designs were conceived and developed that one misses so much when searching for engineering guidance. This chapter will provide some useful information and explain something about the nature of the compromises that one must make in practice. Large low-frequency antenna systems involve large capital outlays, and one may pay dearly for ignorance of the practical importance of the several controlling design factors and the compromises the designer finally accepts.