The Cause and Elimination of Fading

Radio Record, Volume I, Issue 24, 30 December 1927, Unnumbered Page

The Cause and Elimination of Fading

The fading question is one of intense interest to all radio fans and the investigation by "Megohm" into the fading experienced from 2YA has enabied listeners to appreciate some of the causes responsible. Anything that would tend to decrease the "fades" would be an untold boon to those who live at some distance from our broadcasting station. This article, which appeared in this month’s issue of the "Radio News," details the results of a series of experiments conducted by foremost radio authorities in England. Listeners who have closely followed the subject will see that Dr. Appleton covers many of the points brought forward by "Megohm" in our own investigation. Since this was carried out certain radio enthusiasts who consider they know all that is to be learnt on this question, have criticised "Megohm’s" finding, and blamed the transmitting station. If they will carefully read this article they will find that Dr. Appleton has discarded the theory that the station is responsible.

amy. HT, discovery that radio waves, unlike their first cousins, light waves, are able to travel in curved paths around the surface of the earth and thus be received in the Antipodes, was regarded with astonishment. In the first place, it was definitely contrary to the theory then existing and, in order to explain it, the so-called Ken-nelly-Heaviside layer was postulated. It is reasonable to suppose that sunlight, falling upon the earth’s atmosphere, may ionize it, and produce a layer of electrons at a high level. It has been shown by Larmor and others, that a radio-wave, entering this region, will be either absorbed, retarded, or bent back toward the earth, depending nartly upon the amount of ionization, partly upon

the wave-length of the radiation itself, It is thus possible to explain many facts about radio reception. No theory of the propagation of radio waves over the earth’s surface will be complete, however, unless it also explains the large and rapid variations of signal intensity and direction of propagation of the waves as observed at night, and, to some extent.during daylight, particularly in winter. It Las been recognised for some time that the phenomenon of "fading" may possibly be due -to

interference between the direct wave along the earth and the wave reflected from the Heaviside layer; but experimenters have found great Ccifliculty both in proving this and in accurately describing the exact mechanism of the phenomenon. Dr. E. V. Appleton, the well-known professor of Physics at Cambridge, England, has performed a series of briltiant exp2riments which deal with the crux of the question. He and his collaborators first set out to discover definite exper’mental proof of the existence of the Heaviside layer. MEASUREMENTS OF RECEPTION. The fact that temporary variations of signal intensity are experienced at night for short-wave transmissions is well-known to many broadcast listeners. In a normal case of signal fluctustion,

such as may be experienced with a receiver 150 to 200 miles from the sending station, the variation of signal intensity is so marked as to be noticeable in the ’phones. In order to measure accurately, however, the variation in intensity, Dr. Appleton discarded audible estimates as unreliable and obtained an accurate gauge of intensity by means of a galvanometer. The circuit used is shown in Fig. 1. It employs tuned radio-frequency amplification which is coupled on the ov'put side to a detector circuit with a crystal rectifier and a galvanometer in series. In the preliminary experiments, galvanometer readings were taken every six seconds and plotted to obtain a curve. In the later investigations, however, greater accuracy was obtained by

recording the beam of light reflected from the swinging galvanometer mirror, on a moving photographic film. Fig. 4 shows the general character of the fading and its increase after sunset. In order to explain th: %bserved phenomenon, it is necessary to assume that rays of appreciable intensity are reflected at night from the Heaviside layer. These interfere with the ground wave at the receiving station in such a way as to produce fading. -(Continued, on page 2.)

FADING (Continued from page 1.) WAVE INTERFERENCE, ! In grder to picture the process, let the reader refer to Fig. 3, where the two paths are represented. Let us suppose, to make the calculation easy, that the wavelength is 500 metres and the distance between the two stations

100 kilometres (100,000 metres) ; there will be, then, 200 waves in the direct. path, If the reflected wave traverses exactly twice the distance, the two waves .will be "in phase"; i.e., vibrating similarly at the receiver, as pic-. tured in Fig, 2A, Since the crests of the waves coincide, the two waves will assist each other and the reception will be good. Suppose, now, that the radio-station’s wavelength is changed from 500 to 498.75 metres. It is easily calculated that we will have 200.5 instead of 200 direct waves, and 401 instead of 400 indirect waves. The extra half-wave changes conditions, as shown in Fig. 2B. The waves will arrive; "out of phase," vibrating in opposite directions, and tend to annul each other. If the wavelength be further decreased, we have another maximum of intensity, and so on. Professor Appleton arranged with a broadcast station that its wavelength should .be systematically"and regularly varied through a small range, say, ten metres, in the period of half a minute. The coils used on. the receiver were specially wound with high-resistance wire, so that broad, flat tuning was obtained. The results showed definitely the maxima and minima predicted. by theory and further enabled the eminent investigator to place the height of the. Heaviside layer at about 50 miles. He obtained also some results which indicated the. presence of some triplereflected rays. He also found that the

angle of the downcoming tays not con-,; stant, The most ¢bviaus explanation of fad‘tog would be to ascribe it to the fre-quency-variation of the transmitter, but a little thought will show that it would. be a poor station ‘indeed which would ‘he unstable by a full metre. An alternative theory, and one which has niet with wide acceptance, is that rapid

vatiations in the height of the Heaviside layer are the cause of fading. It will be shown later, however, that this view is untenable. RECEPTION AT SUNRISE. The effect of daylight was extremely well shown. The observations were taken respectively at 3.51, 4.47, and 5.12 a.m., and illustrated the type. of fadirg at the transition period, from dawn to sunlight. The wavelength of the transmitter (375 metres) was gradually increased by ten, During this stage the intensity of the signal varied periodically, and the changés were exactly duplicated as the wavelength returned to its origina) value, Sunrise occurred at 4.04 a.m., and with it came an increase in atmopheric ionisation, a lowering of the Heaviside layer to a height of 30 miles or so, and a consequent "attenuation" (thinning out) of the reflected ray. The fading was less at this time. It has been urged by some that the reflected ray, instead of being deviated vertically, a» in Heaviside’s theory, has suffered horizontal defection instead. Appleton has shown that if the first theory be true, fading should be stronger on a vertical line than on a loop antenna; while, if the second be correct, the reverse should be the case. In an investigation of the question, he finds evidence decidedly in favour of

Heaviside. The galvanometer deflec- , tions are in opposite idrections, This tesult is one of particular interest to radio fans; especially since it indicates & possible solution of the fading problem. A COMBINATION ANTENNA. It is not too well known that a com‘bination of leop and. aerial provides a simple means of elminating signals

from one direction, The sensitivity of reception of the ordinary antenna is shown at the left, that of the loop in the centre, and their combination at | the right. in a curve known as the. cardioid. Employing a special device, » Dr. Appleton found it possible to elm-_ inate the ground ray entirely and: study the indirect ray by itself, The results appear quite conclusive. They show, primarily, that the intensity of the reflected ray is by no means constant; and that its variation is definitely allied with fading. This is direct evidence against the view that fading may be attributed either to the variation of the station’s' wavelength or to fluctmations in the height of the Heaviside layer. We must, instead, postulate varying absorption of the indirect wave as the canse of the intensity change. During the course of the investigation it was determined that the re'flectivity of the ionised layer is smalifrom 0.2 to 6 per cent.-hence it is easy to see how a small change here will produce a marked effect in the receiver. Dr. Appleton has shown that about 100,000 electrons per cubic centimetre must exist in the Heaviside layer to account for the observations. During the winte: and during the night, | When the ionising power of sunlight is at a minimum, the reflecting region becomes tigher and more attenuated, and, while longer distance reception is possible, fading is more marked than in daylight hours or in summer. Dr. Appleton has conducted some experiments with a view to eliminating . fading. While he has attained con‘siderable success, he feels that the system will not completely do away with this trouble. Briefly, he reasons thus: ‘Since fading is shown to be due mainly to fluctuation in the intensity of the atmospheric ray, it shouli be pos‘sible, by eliminating this, to receive a fairly steady ground signal, even in the night-time. "WOBBLING" OF THE REFLECTED WAVE, He uses the same type of antenna system employed to suppress the ground ray, and finds, on the whole, signal reception much steadier, He ascribes his failure in obtaining complete success to the variation of the angle of incidence of the indirect wave, as mentioned before; i.e., while he may he able to stop a ray received at a coustant atgle, it is impossible to do this when the inclination of the ray is rapidly and irregularly changing. Since the suppressed-atmospheric-ray system described. above will not receive rays striking the groun] in a given direction, it is also incapable of radiating waves in the same direction, If this type of antenna be used at the transmitting station, te eliminate the indirect ray at its origin, it may be possible to reduce or completely get rid of fading; but Dr. Appleton, in the absence of experimental evidence, is somewhat sceptical regarding the probable success of such a scheme, owing to the lack of symmetry of the

radiation, | Nevertheless, there is no) question but that it shonld be tried. Dr, Appleton’s researches have thrown considerable light on the causes, | nature, and elimination of fading, His} results are verified by experiments con-.| ducted over relatively short distances, mot exceeding a few hundred miles; and it is uncertain whether or not the! fading of a very distant station is to

be attributed to the same causes. We shali look forward expectantly to seeing more of the brilliant results which are being obtained by this eminent investigator.