Radio Waves and Natural Fading

Radio Record, Volume I, Issue 11, 30 September 1927, Unnumbered Page

Radio Waves and Natural Fading

Simple Explanation of the Heaviside Layer and its Effects = Beginners in Radio, and others, will find here a simple and understandable explanation of the transmission of radio waves, and will be particularly interested in the phenomena of natural fading. This subject is now attracting a great deal of attention. The explanation here given represents the fundamentals of matural fading, in relation to which, in some.aspects, other factors may have a bearing,

(By

R. J.

Orbell

BE. Assoc. AIE.E.)

Radio receiving sets may Le divided into two distiuct categories, those of the crystal type, designed primarily for reception of programmes from a local Station, and those employing valves with, as a consequence, a more ex- . tended range. Owners of crystal! sets experience steady an uninterrupted reception from the nearby station; whereas the more ambitious possessors of tnulti-valve receivers, anxious to find out what their sets can do, try to bring in the more distant broadcasts. While doing this, the valve user frequently meets with a variety of conditions which are unknown on the crystal set, princinal among these being difficulty in eliminating the local stations, fading and atmospherics or static. It is the purpose of this article, therefore, to give, with a minimum of technicality, the thory of what causes some of these effects, together with some idea of the fundamental principles of wave transmission as applied to broadcasting. In order to understand what happens when radio waves travel between a transmitter and a receiver, let us consider an analogy. Imagine, firstly, a large pond of water with.a perfectly calm surface. Now imagine two wooden sticks weighted each at one end, so that these will float vertically in the pond with about half of the sticks abova water as in the accompanying sketch at "a" and "b" (See top figure of block). Suppose that one of these sticks, which are to represent receiving aerials, is weighted in such a manner that, if given a momentary push on its top end, it will oscillate up and down in the still water exactly once in half a second. Let the other stick be so proportioned that it will move in this manner at the rate of once per second. This stick will naturally be longer and heavier than the first one. Let us call the number of times per second at which these "nerials" oscillate their ‘natural frequencies." These will be 2 for the first one, and 1 for the second one. Now for our transmitting aerial. This we can conveniently represent as a tapered stick or plunger,-mount-ed. vertically in the water some distance away, as shown at "c" in the diagrara, capable of being agitated vertically by any mechanical contrivance at any desired speed. Having arranged the above analogies of transmitting and receiving aerials in our imaginary pond, we can now set things in motion. Firstly imagine the "transmitter" to oscillate up and down exactly twice per second, so that every second two waves-will be ' generated on the surface of the water. These will travel outwards in all directions in the form of a seriés of concentric rings gradually diminishing in intensity. At this stage we should note two important things, firstly, that no matter how small these waves become they always travel at the same velocity; and secondly, that a3 a consequence of this and the fact that the waves are being produced at a constant rate, there is a definite. distance between any two consecutive rings, This definite distance is

knOwn as the wave-length, and will, of course, depend on the number of wayes produced per second (or frequency) and on the velocity at which they travel. We could thus establish, experimentally, the fundamental ‘equation for wave propagation .in any medium, viz.V equals NL, Where V is the velocity of the waves, N is the number of waves per second, or frequency. Where L is the wave length (or distance between successive waves). Let us now see what happens when the two waves per second on our pond pass the two vertical floating sticks. Since one of them has been seen to bob up and down naturally twice per second, this one will be energised by the passing waves and will respond

freely at that frequency, while the other one will not be greatly affected unless one wave per second passes it, in which case the first one would not respond, . The above anology indicates in principle what happens when a broadcasting station transmits energy to a receiver, and it also illustrates why a receiver will respond only when it is tuned to the frequency of the transmitter. The chief differences are . that the oscillations are electric currents flowing up and down the aerials to the order of a million a second, and that the velocity of the invisible radio waves in ether is 186,000 miles per second instead of the slow velocity of the waves on the pond. Nevertheless there is still a fixed distance between them or wave-length which in the case of a frequency of a million

amounts to 800 metres (about 328 yards). This can be verified by substituting the correct valves in the above equation. This series of waves of constant amplitude continuously radiated from a broadcasting station is known as @ carrier wave. We shall now investigate the manner in which actual speech is trans. mitted. As already stated, the carrier frequency is, in actual practice, in the neighbourhood of a million per second, the exact figure depending on the wave length decided upon. This is at much too high a rate of oscillation to produce an audible effect in a receiver (about 15,000 vibrations per second being the highest audible note). Suppose, however, that at the station end this carrier is modulated or partially interrupted in accordance with words spoken into a microphone, which are, of course,

of a pitch sufficiently low to be audible. It is not difficult to understand that these lower pitched sounds of the voice, superimposed on the carrier frequently can become andible in the receiver, due to fluctuations ' which they make in the otherwise inaudible carrier. currents . passing through the receiver. It is not within the scope of this article to discuss fully the actual operation of the receiyer, which is not so simple as that touched on here. It is necessary, however, to obtain a clear idea of the ‘above principles by which speech is transmitted, in order to more fully understand the explanations which are to follow, THE FALLACY OF BROAD TUNING, When a receiver is situated within

a radius of a mile or so from a broadcasting station difficulty is frequently met with in tuning out that station, and when an attempt is made to receive a distant broadcast, both stations come through together. This gives to the novice the erroneous impression that the local station is broadly tuned. He quite naturally believes that the modulation is being radiated many metres to either side of the wave length assigned to the station. The spark or damped (i.e. not continuous) wave stations, commonly in use on ships for morse com~ munication, are capable of being somewhat broadly tuned, especially if a tight coupling to the aerial is employed. Broadcasting stations, how- . ever, utilise a continuous series of modulated waves as has been shown above; hence the frequency in the aerial must keep in step with that. in the transmitter itself where the oscillations are generated. «°° . Owing to an interaction of -the speech and carrier frequencies (which are, as has been explained, of a greatly different value) there is a narrow margin on each side of the unmodulated earrier frequency, over which the modulation ranges. These two limits, the upper and the lower, are called sidebands, and consist of the sum and difference respectively of the carrier and speech frequencies. In actual practice these limits amount to approximately one meter as a maximum. It is apparent that speech (or music) cannot be radiated over a greater range than that determined by the limits of these narrow sidebands. In order to show why nearby unselective receivers pick up the trans‘mission ever a wider scale of wave lengths, let us refer once again to our analogy of water waves on the pond. Imagine that the two weighted sticks previously referred to are both situ. ated a short distance only from the source of the waves. At this point, the impact of the passing of the waves will be so great that both sticks will be forced to rise and fall in sympathy with them, independent of the manner in’ which the sticks would naturally oscillate in still water. We thus have'an illustration of two receivers, each tuned differently, picking up energy from a transmitter radiating one frequency only. If a xeceiver is designed so that the coils in it are screened by metal shields to prevent waves from the transmitter from affecting them directly, and if a good wave-trap is use in the aerial wire, little difficulty will be had in tuning to a distant broadcast, not many metres different in tuning from the local station. A wave-trap alone will very greatly improve even a simple unscreened set, A simple and effective trap may be made at a cost of less than ten shillings as follows:-To the ends of a two inch diameter coil of about 385 turns of any guage wire, join the two sets of plates of an’ ordinary variable condenser. Then, having removed the aerial from its terminal on thea set, join the latter to one end of the coil and the aérial to the other end. To use it, first tune. in the loral station in the ordinary way, (Continued on pags 5 under "Yading,."

RADIO IVAVES AND NATURAL FADING ---

(Continued from Cover.) using no reaction; then slowly turn the condenser of the trap till a point is found where the volume of the local is reduced to a minimum. This point will be quite sharp and should be earefully found. Then, leaving. the wave trap thus set, proceed to tune in distant stations. This will now become comparatively easy, and little or no interference should result. FADING AND ITS CAUSE, When radio waves travel over long distances, they, unfortunately, do not always reach the receiver with uniform intensity, but rise to full volume after a period of weakness and vice versa, sometimes accompanied by a peculiar mushiness at times of greatest fading. . These effects are due primarily to the fact that the waves, from the transmitter reach the distant receiver after having travelled to the upper atmosphere, where disturbed electrical conditions exist, instead of following the surface of the earth. (See bottom figure of block. This represents diagramatically conditions exist‘ing at night time and shows waves travelling from a broadcasting station B, to two receivers, R, situated from 50 to 150 miles away, and R2 (up to 2000 miles distant), It will be seen that no surface waves reach the distant receiver R2. These are so greatly attenuated by the resistance due to obstacles on the -earth’s surface that they are lost. ~The waves which do reach R2 have travelled to great altitudes and have been bent down again, partly by reflection from a more or less gradually defined layer of ionised and semiconducting gases, known as_ the Heavyside Layer, and partly by refraction. The upper atmosphere, being very rarified, is at ‘times subject to extremely rapid changes in ionisation, caused primarily by-the sun {although the sun is not shining at the time). The result is that R2 experiences vary reception, as the reflec(Continued next. Column.)

tion becomes less perfect at certain times than at others. Different conditions exist for the moderately close receiver R1, as waves reach it by two paths, one following the surface of the earth, and the other by reflection as before, but this time at a sharper angle. Owing to this sharper angle, there is greater absorption by che Heavyside Layer, which it should be remembered is not sharply defined, also more energy passes through it than before. Hence less perfect reflection results, also refraction is now non-existent. Here also a change in the distribution of the vertical and horizontal components of the waves, known as polarisation, occurs. The surface waves reaching R are largely attenuated as explained above. These unfavourable conditions combine to produce, at night time, results which are actually poorer at R1, than those to be had by the more distant receiver R2, the waves to which follow a path with fewer obstacles. Still another effect occurs at Rl, which seldom exists at R2. We have seen that waves reaching R1 do so by two routes, one of which is longer than the other. It is not difficult to understand that waves which leave the transmitting aerial at‘the same time and following their two paths will not arrive at the receiving aerial together. This introduces further complications in the receiver which have the effect of producing an apparently slight distortion at Ri, noticeably only at night. *° These distances from a station where poor results are thus had vary according to the wave length used, as well as to the season of the year, and is also modified according to whether the transmission is over land or sea. They are usually referred to as skip-distances. Receiving conditions in the day time are entirely different. The Heavyside Layer is then broken up and dispersed by the direct effect of the Sun’s rays and little or no reflection occurs, with the result that the distant receiver R2 receives weakly or not at all. Ri on the other hand usually experiences clearer reception by day, with no fading or distortion, as this station now receives surface waves only, these having greater energy than before, and it is unaffected by reflection from above. very close receiver of course receives direct waves either by day or by night. As a@ consequence no atmospheric fading can occur at any time from a local station.