RADIO

Southland Times, Issue 20499, 30 May 1928, Page 11

RADIO

SHORT WAVES. WORK OF EXPERIMENTERS. Contrary to the generally accepted opinion, the short wave lengths which are attracting such interest in the radio field are not a recent discovery. The earliest experiments in radio, during the last quarter of the nineteenth century, were carried out in the band that we now know as “short waves.” The simple apparatus devised by the pioneer investigators of radio was of such proportions and design that only very short waves were radiated. These were measured and their behaviour determined in the confined space of the laboratory. As attempts were made to radiate higher power to cover greater distances the radiating surfaces (that is, aerial and ground systems) were increased in size and consequently the wave lengths increased. It was then felt that to cover greater distances it was necessary to have enlarged radiating systems, and until recently the development of short wave lengths was neglected. Although limited to low power and 200 metres, it took only a few years for the amateur to perfect apparatus to enable him to communicate up to the considerable distance of 2000 miles. While not consistent, these results were very encouraging. Except during the period of the world war, when amateur activities were suspended, continuous progress was made. With the development of the vacuum tube for transmitting and its practicability for radio telephony new interest was aroused, and the first practical broadcasting was accomplished. Following these developments commercial companies became interested in broadcasting and it was not long before broadcasting stations were numerous. To make room for these additional stations and to remove the possibility of interference from amateur transmitters, a radio conference some time ago at Washington extended the amateur wave band from 200 down to 150 metres.

After operating on the lower portion of this band for a short period, however, it was found that greater distances could be covered than were heretofore possible. These indications led many to believe that the wave lengths below 150 metres held further possibilities. A few amateurs then obtained special licence which permitted them to operate on wave lengths as low as 100 metres, and on this wave length, using a very small amount of power, they succeeded in carrying on communication with many foreign countries. This remarkable work encouraged many of the amateur experimenters to drop to still lower wave lengths. Tests and experiments as low as 10 metres indicated that extremely short waves offered a fertile field for research and was useful for long distance communication with low power. Realizing the importance of the low wave lengths many countries are now utilizing the shorter wave lengths and the number of short wave stations throughout the world has increased rapidly. The fascination of the short waves has taken hold of every tvpe of radio enthusiast. There is no telling to what extent the development of the short waves will finally lead. Short wave activities are so diversified as to be of interest to everyone. THE VALVE VACUUM. The more complete the vacuum in a radio valve, the better, and no other commercialized product requires such complete exhaustion. Such air pumps as are familiar in school laboratories are quite useless for making the “hard” vacuum needed, and even the mercury pumps found in ordinarily well-equipped laboratories can go only part of the way. The completeness of a vacuum is stated in terms of the air pressure remaining in the exhausted space. Normal atmospheric pressure averages 30 inches, or 860 millimetres, of mercury (that is, the height of the mercury column of a barometer; equal to approximately 14.7 pounds per square inch). This pressure is reduced as the exhaustion proceeds, so that when nine-tenths of the air is removed, the pressure is 86 millimetres of mercury, and so on. One would suppose that when the pressure reached one-tenth of a millimetre, meaning that only one part in 8600 of the air remained, a fairly good degree of exhaustion could be claimed; but this is a very rough job. In modern practice, the first pumping, made by means of electrically-driven rotary pumps, produces a vacuum of about onethousandth of a millimetre of mercury, which means that only about one-millionth part of the air remains behind. This is about the same proportion as one drop bears to twelve gallons, but the wiping-up of the last drop in a tank is child’s play compared with removing the last millionth from the inside of a vessel in which one cannot handle a sponge.

The process is continued by various means, one of the most satisfactory being the mercury vapour aspirator. This is an apparatus in which a stream of mercury vapour is driven at high speed past an opening which communicates with the vessel being exhausted. As it goes by it picks up small quantities of the air, and makes a remarkably clean job of the exhaustion. But on the surface of the glass bulb and the metal parts of the valve a certain amount of air is firmly held, and in time would leak off and reduce the vacuum. During the exhaustion, therefore, the valve is “cooked” at a high temperature. The glass is heated almost to the softening point, and by an electrical method the internal metal parts are raised to a bright red heat. This cooking drives the air out of the surfaces.

When the exhaustion is complete the glass tube by which the valve is connected to the pumping apparatus is sealed off by melting it in a flame. An indication of the “fine points” of the business, and of the importance of the surface-bound gases is that special care has to be taken that the sealing of the tube does not liberate gas and damage the vacuum. Even this elaborate exhaustion process does not complete the work. It is quite impossible, by any imaginable form of pump, to take all the air out of a vessel; and the little that remains is regarded by the best people in the business as a bit too much. Hence the looking-glass effect. A scrap of magnesium is fixed to the plate of the valve, and, being ingeniously heated up, it flashes off into vapour, which is deposited as a film on the glass. This metal has a very powerful affinity for gases, and as the cloud of magnesium molecules flies through the vacuum it picks up some of the now thinly distributed molecules, carries them with it, and cements them on the glass. HERE AND THERE. The amateur transmitters of New Zealand some time ago formed an Association of Radio Transmitters in order to safeguard their interests. The association has its headquarters in Auckland and meetings are held at frequent intervals to discuss matters and questions having relation to radio. The association publishes a monthly journal and is doing useful work in fostering amateur radio in the Dominion.

The British Broadcasting Corporation for a number of nights asked for the opinion of listeners respecting the continuance of religious broadcasts. So great was the demand for the continuance of such broadcasts that the corporation decided to have religious talks for 15 minutes every morning in addition to the usual Sunday broadcasts. The week-day transmissions consist of sacred music, mostly choral, and the reading of a prayer. There is no address. It is estimated that more than 7,000,000 people listen to the Sunday broadcasts from the chain of stations of the British Broadcasting Corporation.

TELEVISION PROBLEMS. IMPRACTICABLE SYSTEMS. After recent spectacular and, to some extent, successful tests of their apparatus, television workers in Great Britain appear to have received a distinct reverse. There is a growing feeling that in spite of the initial success achieved in television experiments | the methods being adopted by inventors working on the problem are too crude, and will never be capable of producing a successful commercial system. It is believed that important new discoveries which will make it possible to adopt an entirely new method will be necessary, before real success in the transmission of moving images will be possible. The weakness of the present methods of television lies in the apparatus for the process known as “searching.” This consists of breaking-up the image to be transmitted into a large number of small sections, which are transmitted one after the other from the transmitter to the receiver and reassembled at the receiver in the correct positions to form an image. The apparatus to analyze and later build up the image again must work extraordinarily rapidly. A single picture is transmitted as tens of thousands of successive impulses, which must *follow’ each other so quickly that at the receiver they give the illusion of being simultaneous. Further, so that a good moving image can be obtained, between 12 and 16 complete pictures must be sent every second. This means that some hundreds of thousands of impulses, each representing a small section of the image being transmitted, must be sent from transmitter to receiver every second.

Great difficulty has been experienced in designing mechanical apparatus, which usually consists of revolving shutters carrying lenses to work at the required speed. Various methods of carrying out this process electrically have been suggested, but so far none has been found practicable. Nevertheless, the use of searching apparatus operating purely electrically seems to promise a definite advance, and it is likely that a practicable electrical method will be discovered before long.

14 DEAD ” AREAS

MAN-MADE STATIC. Certain localities are frequently said to be “dead” areas for radio. The statement is intended to convey the information that the reception of IYA, 2YA or any other station is out of the question, or at all events, is not worth the effort of tuning-in. The informant usually indicates there is something of the particular place which is of a physical or geological nature peculiar to certain localities which are described owing to those peculiarities as wireless “dead areas.” Upon investigation it will*'almost invariably be found that the simple explanation of the absence of satisfactory reception in terms of its earning the reputation of' a dead area is not justified. It is not dead; and the failure of some owners of sets to tune in satisfactorily is due to removable causes or to conditions which are remediable. The conditions at the typical district can be found in many localities; but the stigmatizing of the district as hopeless for reception does wireless a lot of harm. One of the greatest hindrances to satisfactory progress in broadcasting is the publicity that is given to ill-chosen or rash and partly correct expressions of a semi-t.echnical nature. It only needs someone who is better known than his neighbours in radio to speak of the revolutionary change in radio-receiv-ing equipments that is coming shortly, or the statement that such and such a place is quite unlike all other places in the behaviour of radio waves in their neighbourhood, when he is immediately believed. Not only that, but the story goes round and grows, until it is probably unrecognizable by the original perpetrator when next he hears it. One needs to be careful of one’s comments to such circumstances.

To return to the question of dead areas—it may be asked what has given rise to their reputation. There are some alleged dead areas where no indigenous trouble or local difficulty exists whatever, and the failure of one or a few owners of sets to get good reception is due to the sets themselves or the owners of the sets. Those cases are of course few, although they may be given undue publicity.

There are many instances, however, of localities where interference with broadcast reception undoubtedly exists; interference, annoying and puzzling, which causes listeners to give up the service or alternatively to carry on under conditions that are far from satisfactory. The causes of such interference do not justify the application of the reputation of a dead area, although the interference may be so severe as to have a deadening effect on efforts of reception.

The most prevalent form of man-made interference as distinct from natural interference, such as atmospherics, is that caused by electric machinery or supply systems. It is possible, of course, to obtain perfect reception near electric systems, but if they are not designed or maintained with any consideration of their influence as generators of radio interference, the listeners within the zone of influence have a bad time. Some country residents know too well how suddenly and completely their enjoyment of broadcasting ceases for the night immediately a picture show opens up; while others know that every night is spoiled for reception when a medical man with his electrotherapy apparatus starts his high frequency oscillators.

There are towns where the supply system is the cause of continual “static” and listeners’ efforts at obtaining satisfaction from their sets are practically nil. This form of interference is one that should be tackled by the authorities responsible for broadcasting. It seems wrong to collect a fee for listening when the enjoyment of the listening is ruined or very considerably lessened by causes quite capable of prompt removal.

TESTING BATTERIES. THE BEST METHODS. Failures in radio-receivers are being traced daily to the discharge of the B-battery. The A-battery is always under observation, because, unless it is fully charged, the valves do not light to customary brilliancy. It is, therefore, a simple matter to judge when it needs recharging, by noting the condition of the valve filaments, although this method is not recommended as a scientific test. There is no corresponding crude test which can be applied to the B-battery, and its condition, therefore, often escapes attention. The only satisfactory method of testing a B-battery is by the use of a voltmeter. Suitable voltmeters are now so cheap that one should form part of the equipment with every valve receiver. It is worth remembering, however, that some of the cheaper voltmeters are badly designed in that they impose a rather heavy drain on the battery undergoing test. They should, therefore, be used as little as possible, and left on the battery only long enough to obtain an accurate reading. It will usually be found that the voltage of a B-battery will fall off steadily until it has dropped to about two-thirds of its original value. After that point is reached the battery becomes noisy, and is likely to be unreliable, as the voltage may fall without warning to two or three volts. Thus, it is generally wise to discard a B-battery when it has been used until its voltage is only two-thirds of the initial voltage. In ordinary circumstances the life of the battery to this point should be between three months and a year or more, depending upon the conditions of use.

If a sudden battery failure occurs before the battery is about three months old, it is likely to be due to the failure of one section. Most batteries are now divided off by terminals into of different voltages, and these sections can be tested one after the other by the voltmeter. When the defective section is found, it can be bridged over by a piece of wire so that it is removed from the circuit, and the battery will then probably last for many weeks longer.