Further Analysis of Fading

Radio Record, Volume I, Issue 15, 28 October 1927, Page 12

Further Analysis of Fading

Ia this further analysis of the fading reports ‘""Megohm’’ gives some interesting material, and states that City listeners average more fades than country listeners, The question of land absorption is also death with,

PRELIMINARY REMARKS. Numerous explanations have been riven in the ‘Record’? of the manner it. which the modulated carrier wave fa sent out from a broadcast station. Such a modulated high-frequency wave, analysed into its elements and studied in detail, is revealed as an intricate fabric of elemental waves, so interwoven with each other that none of ¢hem can be disturbed without changing in some degree the complexion of the whole. For perfect results the whole band must arrive at the receiver gith an amplitude continuously proportional to that leaving the transmitter, or the expression of speech and music will not be correctly reproduced. All the component frequencies within the band must be unchanged in their relative amplitudes, lest the character of the sounds be altered. Even the relafive phase relations of the various frequencies must be preserved, for it can be shown fhat otherwise the interseckion ot the two side bands in the regeiving detector will result in the partial loss of some of the frequency components. Put into simple language, this means that the complex broadcast wave must, ‘in its passage to the receiver, meet with Ho influences that will change its shape or frequencies in the slightest degree, ‘if reception is to be as nearly perfect as is possible. If the ether, the existence of which is presuimed, was not contaminated by the presence in it of other matter of a denser type, the probfem of undistorted: transmission might be a much easier one. The ether, as supposed to exist, is a thing apart, et pervades everything, and is a medtum for the transmission of heat, light, electro-magnetic, and other’ vibrafions not only around this earth, but from sun to planet, and planet to planet. . The broadcast waves ‘in their passage from transmitter to receiver have to encounter many infiiences not of the ether ‘itself; and these obstacles may take the form of gaseous ions, a stratum of which forms the Heaviside layer, at a varying height from the earth, or it may be ‘that varying composition or form of the earth itself has a detrimental infiunence, and there are others between these two extremes. It has been found in research connected with the Transatlantic radio telephone, that the earth’s magnetic field, which is eyer varying, has a cousiderable effect upon radio transmission, » magnetic @isturbance tending to weaken night signals, and to slightly strengihen daylight reception. The actual effect of daylight upon very distant reception is too well own to need more than mention. EFFECTS OF MAGNETIC DISTURBANCE. © The writer remembers during the transmissions from VLDN at the Dunedin Exhibition, reception in Wellington was normally fairly good, but one evening a group of friends wére listening for particular item to be presented about the middle of the programme. As the

concert proceeded, a mysterious blur‘ring came over reception, at times quite blotting ont intelligibility. As the expected item approached, the distortion and fading became more intense, and apart from a snatch of the announcement very little of the solo came through, and the same trouble continued through succeeding items with more or less improvement. So mysterious and unusual was the trouble that the receiving set came under grave suspicion, but everything appeared to be in order. The newspapers next morning threw a light upon the mystery, announcing that on the evening in question a severe magnetic storm accompanied by a vivid display of the aurora australis, had raged in the south, being so severe as to put several land telegraph lines out of commission for the time. This happening is pointed out because it appears quite possible that such magnetic influences of lesser intensity may frequently be having some effect upon radio reception, although not making themselves manifest in other ways. But there are other still more subtle and evasive influences at work, chiefly during night transmission, and it is around these that a great amount of speculation and experiment is centred. The theory of reflection of waves from an ionised gaseous surface known as the Heaviside layer, situated at about fifty miles above the surtace of the | earth, appears to be borne out by many experimenters, some of whom have been able to actually measure the strength of the reflected waves, which at night is added to the volume from those that travel at all times near to or through the earth. Another theory adds that the surface of this reflecting layer is in a constant state of undulation, an idea that appears quite feasible, and one that would make it impossible for a transmission to be received in exactly the same way at atty two distant points, that is, both with regard to both the time of happening, duration, and- intensitv of fading or blurring, supposing it to be caused by a non-agreement in timing between the reflected wave and the earth wave. A great deal more mirht be said on the subject, but the chief matter now is to see what can be learnt from the reports that have been sent in. FADING EXPRESSED IN FIGURES. Tt is desired now to give a numerical expression of the amount of fadine experienced in different parts, such numbers to be the direct result of the recoatds on the charts. It is auite evident from the number of slioht fades on some of the charts, and.from the written supplementarv renorts accomnanvine verv manv of them, that ‘in some cases the slichtest detectahl> weakenine of sionals has heen recorded. This is shown bv the fact that. n few. renorts mentioned that it was AiGcult at times to distinonish hetween fadine and intentional soft nassaves in sones. Tt is thus seen that fadine that may he classed as vyerv. slioht and perhans in lone-distance recention anite neolicihle, denends very much —

upon the human factor. It was quite good for these fadings to be recorded, however, as they are helpful in conjunction with other reports. Now when we come to consider. the fading where volume decreases below half the normal, there is room for very little doubt or uncertainty, so it will be seen that on the whole the intense fading, more decided, but not so frequent in occurrence, will form a safe basis on which to build the decision now sought. — The total number of intense fades on each chart for September 26 have been counted in groups and the average for that group calculated, the result being the average number of intense fades per report. The list shows as follows, the average distance from 2YA being shown in parentheses :- AVERAGE INTENSE FADES PER REPORT. North Island. , North Auckland (850 m.).......00.. 44 Auckland City (800 m.).......0++ . 10 Country, including South Auckland, Waikato, King Country, Rotorua, Thames, Coromandel, Te Awha (200 to 250 m.)...... 7.4 Taranaki (excluding New Plymouth) (125 mm.) ......6. eeecee veer OD New Plymouth (150 m.) ............ 12 East Coast (Bay of Plenty, etc.) (250 31.) ceeeeseeceece eeese arenes 3.2 Hawke’s Bay (excluding Napier) 4.3 Napier -cesssesereees dveceeeeees a ssee cease . 13 South Island. Marlborough (50 mm.) ...ceceeeeeeesees Nil Havelock, Rai Valley, Pelorus Sound only (50 m.) cesccsseceee 7.5 Nelson district (100 mx) .....ccseceees 11 Christchurch City (190 m.) ......... 2.7 Canterbury (excluding Christchurch) (250 m1.) crccccesecceees .. 6 Westland (200 m.) cescsccecsesoeeee 10 Dunedin (875 mi.) Seeeeenee ateavercaree 5 Otago (850 32.) ccccccsecreeee ecccosececse. 6.5 Timaru (270 1m.) c.soresese eesscsecseeeee TC Southland (450 m.) c.ceccccesecceere 4.7 From the above it will be noticed that, on the‘ Whole, recéivers situated in the country get less fading than ‘those in thickly populated areas, But notable exceptions are Christchurch and Diinedin, which both receive the wave from 2YA direct over water, the favanrahle nath — ,

sesvewselC OOOO lS eee , es Reports from Marlborqugh state that no fading is experienced there either night or day, but three separate reports from distinct parts of Marlbor-ough-Havelock, Rai Valley, an¢ Pelorus Sound show a considerable amount of fading, averaging 7.5 intense, but only three fades are shown on the Havelock report, so most are on the other two As these places are all situated within sixty miles of 2YA, or not further away than is Shannon, they appear to be worth special attention On looking at the map we find that the Pelorus Sound receiver 1s atl air-line distance of 47 miles, and following this line from Wellington, it strikes land to the north of ‘Tory Channel entrance, encounters the summit of Arapawa, 15983 feet, and further on, peaks of 978, 1531, and 1795 feet, all within about a mile of the line. In the case of Rai Vallev, which shows the most fades, there are peaks of 1762, 2095, 3198, 2288, and 2367 feet close on the airline. ‘this receiver is 68 miles from 2VA. A good detailed report was sent in, and has been comnared with a similar one from Russell, North Auckland. There is agreement hetween the two in recard to the ravioritv of fades and increase of voluma., but the southern one also shows several intense fades that were not noticed even as faint ones at Russell, and some that were faint at Russell are reported from Rai Vallev as "faded rieht out." Tt ig auite clear that most of the fades were noted at hoth places simnitaneously. hut with different intensities. Anart from the neaks mentioned. the country traversed by the wave to reach thie receiver is anountsinons and irreeuiar in the extreme A+ Shannon, an eanat Vetanee from OVA, in a northward Mirection. no fading is exnerienced tort a canal! amoannt of distortion is 1¢satted Navlicht recention is nat mentianed in the shove renott, hit nt Patarie Sannd is anite ned. Morn. taing anrear tn have some influences "non nioeht fransmiceion, and if thee have anv tnon davtime recention i mrst he of a different trind, or lessened intensity, MANAWATU CONDITIONS. Reception must be satisfactory at Palmerston North, for there are no 7 complaints from that quarter, but, strange to say, from Feilding, only a ‘|few miles further, there are several complaints of distortion and mushiness, accompanied by intense fading. | This appears to point to local conditions of some kind, either geographical or otherwise. As very few reports came from here, it is quite possible that most Feilding listeners are getting good re- | ception, which is quite presumabJe, as | Marton, a few miles further away, unanimously reports all well. NELSON DISTRICT, In the track of the wave from 2YA to Nelson City, there are peaks of : 1762, 2095, 3183, 2288, 2367, 3420, 3976 feet, all within four miles of the airline, so that so far as mountains are concerned, Nelson and the district to the west of it are well screened, This fact appears to be well borne out by the repotts, which shows a larger number of intense fades than are experienced in other country districts. The average distance of this district from 2YA is +100 miles, Nelson being 60, Woodville,

— Hunterville, Wanganui, and Patea are about the sate distance, and only from those places: where transmission travels overland, that is, Feilding, Pahiatua, and Woodville, have there been reports of fading. THE CASE OF NEW PLYMOUTH. Authorities . have placed the maximum distance for perfect and uninterrupted broadcast reception at from thirty to fifty miles from the transmitting station. This may be quite an accurate estimate based on careful experiment, taking the power of the station into account. It is found in practice, however, that passably good reception can be obtained at much greater distances than those mentioned, and upon a commercial basis -a much larger distance must be covered by practically all broadcast stations at the present time, particularly in New Zealand, where a comparatively small revenue is calle1 upon to provide fofir separate transmitting centres, When distance from the transmitter is accentuated by seographical conditions. that weaken the signals received, it certainly becomes somewhat of a hardship, although natural conditions of many kinds must have to be suffered in many countries. New Plymouth has been a source of insistent complaints regarding fading of transmission from 2YA, and an at‘tempt is now to be made to show approximately the conditions of reception there. The small plan herewith shows that New Plymouth is about 150 miles from QYA, whilst Mount Egmont, frequently capped by an enveloping bank of

heavy clouds, lies almost directly in the air-line from 2YA.. It has been shown already in the case | of Nelson district that a great amount. of fading is experienced there, pre--sumably on account of the mountain ranges over which the 2YA wave must travel. And Nelson district averages fifty miles nearer to 2YA than is New Plymouth. Looking through the reports for the two districts they seem in a general way to show a similar amount of slight fading. On reference to the plan it will be seen that practically no fading is experienced antil. the influence of Mount Egmont is added to overland distance. At Inglewood this effect begins to show, six fades being reported, Lepnerton four, Waitara 21, and an isolated’ set at Bell Block 19. In New Plymouth town the average number is 12 intense fades per listener, whilst a sinele renort from Moturoa, rather more in the radio shadow of Eemont, shows 15 intense fades. A complaint from Hamilton which has been referred to ‘‘Megohin’’ sugests that the fadine trovhle is caused bv the station ifself.. Perusal of a very interesting revort of proceedings of the Institute of Pedio Fneineers of America shows results of exneriments carried out in connection with fadine Station variation daes not enter into the anestion at all) for it is slown that definite but irreenlar areas at a given distance from the transmitter are

troubled with fading and distortion much more than other areas at the same distance. That fading can be caused anart from any possible variation at the station is made anite evident bv tire test now being investisated, there. being freauent cases where one receiver registers a fade whilst another gets no variation worth noting. Putting the blame unon the station is anite a natural move for thase who have not made a study of the auesHon, and whilst it would nerhens he ening too far to exnect iné-ttihilitv fram ativ statien, a great amount of fading or interference with the transmitted wave oc wrs during its passage throush snace.