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High-frequency Antenna Development
Author: Edmund A. Laport
In the earliest days of radio communication, the frequencies below about 200 kilocycles were employed for high-power transmission. For marine communication, frequencies as high as about 1,000 kilocycles were used. At the time broadcasting was becoming established on the medium frequencies, about the middle of the 1920's, high-frequency transmission for long-distance services made its appearance.
The use of high frequencies began at about the time of a sunspot maximum and was ushered in by some phenomenal results with low power. This provided a strong incentive to make extensive use of the high frequencies. However, by the time a large-scale conversion had been made, the low part of the sunspot cycle had been reached and there was some disappointment in the results achieved during this period in the late 1920's and early 1930's. The reason was lack of knowledge of the ionosphere. The relation of radio transmission to sunspot activity was established about 1926, but even in this field of study it was not until 1948 that the discordance between total sunspot numbers and ionosphere conditions was explained.
The importance of ionospheric study was recognized at a very early date in high-frequency history, and many experimenters in the field of pure science and in communication engineering devoted their efforts to this intricate problem. Measurements were made of layer heights and critical frequencies, and the effects of skip distances were observed. Multipath transmission began to be understood. There is a great deal of literature on this subject which emphasizes the magnitude of the puzzle that had to be solved. This field of research is necessarily endless and must be continued into the future without interruption. Great impetus was given this work during World War II, when the strategic value of ionospheric information for operational purposes led to the establishment of ionospheric sounding stations in many parts of the world. The accumulation and correlation of these data provided the kind of information that permitted the most effective use of frequencies on a day-to-day engineering basis. It is now possible to predict some months in advance the conditions of the ionosphere throughout the world with considerable exactness. After the war, this service was made available to everyone under the Central Radio Propagation Laboratory of the National Bureau of Standards. Almost all the major countries of the world cooperate in providing data for this service.
The vagaries of the ionosphere posed some serious problems for the antenna engineer, which could not be alleviated by his backlog of theoretical antenna information. It took several years to decide, from long experience, whether to use vertical or horizontal polarization. The earliest high-frequency beam antennas used vertical polarization, but subsequent evolution has caused the almost universal use of horizontal polarization. There may be a reversion to vertical polarization in the future for certain applications.
Long studies were made of the best vertical beam angles and whether to use broad or sharp vertical beams. In point-to-point services it was at first believed that very sharp horizontal beams could be used advantageously with consequent gain, but rather wide deviations were observed in the azimuths of arrival of signals, which limit the beam sharpness that can be used at the receiving point.
Finally, quantitative study of the multipath problem was possible for the first time with the availability of modern ionospheric information, and this led to the important discovery of a method for reducing multipath distortion by using complementary antennas at both ends of a circuit, the right frequency at the right time to obtain selective angular penetration, and the need to change the antenna characteristics as ionosphere conditions change by seasons and by years. Engineering practice has not yet made extensive use of these latest principles.
It is virtually impossible to measure the complete performance of a beam antenna system. In the past, it was customary to obtain statistical performance by measuring the fields at some distant point. This required the interpretation of performance of a transmitting antenna, for instance, to be made through a veil of uncertainty introduced by the propagation medium and by the receiving antenna. The difficulty can be comprehended if we consider what is involved in the attempt to measure a few decibels difference in gain through a medium varying at random, even over short intervals of time, from 20 to 80 decibels. With better propagation data now available, the desired radiation patterns can be selected and the required array designed. If there are any direct measurements wanted, a scale model is made and measured using the ultrahigh-frequency techniques and instruments.
The development of antennas proceeded along two main lines - those using standing waves and those using traveling waves. The various dipole and harmonic-wire arrays are of the standing-wave type, while the Beverage, fishbone, and rhombic antennas exemplify the traveling-wave type. Evolution of antenna design has included both these types for vertical and horizontal polarizations.
Among antennas of the standing-wave type there were developed broadside and end-fire arrays. The former is the type where the beam is normal to the plane of the radiators, and the latter is the type where the beam is in the plane of the radiators. Evolution seems to have eliminated end-fire arrays from high-frequency practice. The nearest approach to the end-fire condition is the use of long-wire radiators with both standing and traveling waves. Such systems have their main radiation lobe at a small angle to the wire when their length is several wavelengths.
Many of the early beam antenna designers went to great extremes to achieve very high directivities and gains. The passage of time brought the economic factors into prominence, and there followed an era of simplification in design, with gradual compromise of technical perfection in the interests of lower capital costs. This was fully justified in one respect, because designs had actually been pushed far into the region of diminishing returns.
The growth of high-frequency broadcasting induced certain new studies of the transmission problem because there were basic differences from fixed point-to-point working. One of the principal differences was a complete lack of control over the design and location of the myriad individual receiving stations. Listening hours were fixed not by technical conditions but by personal habit. The coverage of a substantial geographical region also differed from delivery of the best signal at some fixed point. Most receivers were naturally located in places of high noise level. The only engineering possible from a system standpoint had to be done at the transmitting end. Broadcasting antenna design made considerable use of radiotelegraph experience but went on from there to satisfy its own requirements as well as it could.
In broadcasting there is still a need to increase power; and when very high power comes into use, there will be some new antenna-design problems. Furthermore, the economics of broadcasting are completely different from those in the fixed services, and the amount of capital one is justified in investing in antennas is not wholly determined by technical considerations.
The 1947 Atlantic City Conference of the International Telecommunications Union brought about two points that may have a profound influence on future antenna design. The first was the limitation in the number of frequencies assigned to fixed services (actually a considerable reduction from previous assignments), which means that more and more traffic must be handled with fewer channels. This will require the utmost utilization of the propagation medium by multiplexing in various ways, which in turn requires a better solution of the multipath problem that limits working speeds.
The second point is the adoption of a plan for world-wide sharing of frequencies on an engineered basis as a means of more intensive utilization of the available frequencies. This brings a need to clean up some of the antenna patterns by elimination of large parasitic side lobes and backward radiations that now cause interference without having any beneficial value. The presently known way to achieve this cleanup of radiation patterns is to adopt some of the principles that have been applied to ultrahigh-frequency antennas, such as neutral-screen reflectors to suppress backward radiation and graded current distributions to reduce side lobes. This will require more extensive and costly systems of the dipole type if these particular principles are followed. Any further influence on antenna design will include the newer facts of high-frequency propagation as they appear, which in turn may result eventually in low-power one-hop relaying on routes that have land masses at the desired relaying points. It is already known that in such a way very high telegraphic speeds can be maintained over long distances with great economy of power, but at the expense of using more frequencies. Relaying via the tropical latitudes will benefit from greater ionosphere stability due to large auroral-zone clearance and much higher usable frequencies, with lower attenuation. Refined antenna engineering will play an important role in the eventual achievement of better utilization of the high-frequency spectrum.
A review of high-frequency antenna engineering in the light of present-day developments in very-high- and ultrahigh-frequency antenna engineering makes it evident that the former is in a very elementary state. The difference in the wavelengths naturally excludes certain very-high-frequency techniques from being applied to the high frequencies. Nevertheless there are many techniques that can be applied in the future as the pressure of circumstances justifies greater investments in new antennas for the fixed services.
Another conclusion from such a review pertains to the significance of antenna gain. Gain was for many years a primary objective in antenna designing, on the supposition that this augmented the effective transmitted power and gave the maximum received signal energy, both with consequently improved signal-to-noise ratios. This concept is valid only with an absolutely stable propagation medium. It is now recognized that the matching of the transmitting and receiving antenna patterns to the conditions of the very unstable propagation medium is the dominant engineering objective, and antenna gain is actually an incidental consideration.
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