THE WIRELESS WORLD

Otago Daily Times, Issue 21537, 8 January 1932, Page 5

THE WIRELESS WORLD

NEWS AND NOTES By Maona Vox. Items of local interest are invited by “ Magna A 7 ox ” for publication in this column. It is necessary that such matter should reach this office by Tuesday of each week for insertion on the following Friday. 2BL, Sydney.—Bss K.C., 353 metres. 2FC, Sydney.—66s K.C., 422 metres. 3AB. Melbourne.-—O2O K.C., 484 metres 3LO, Melbourne. —830 K. 0., 371 metres SCL, Adelaide.—73o K.C., 395 metres. 4QG. Brisbane.—76o K.C., 385 metros. XYA, Auckland.—B7o K.G., 333 metres Every day except Monday. 2YA Wellington.—72o K.C., 420 metre* Every day except Wednesday. 3YA, Christchurch.—9Bo K.C.. 306 metres Every day except Tuesday. 4YA, Dunedin.—6so K.C., 463 metres Every day except Thursday. 4ZM. Dunedin 1080 K.C., 277.8 metres Tuesdays and Saturdays. 4ZO, Dunedin.—loßo K.C., 277.8 metres Monday and Friday. 4ZB, Dunedin.—loßo K.C., 277.8 metres Wednesday. Thursday, and Sunday morning. ANSAVERS TO CORRESPONDENTS. “Dial,” Port Chalmers.— (l) It is virtually impossible to calculate the setting of the dial to pick up any given station, but by observing the readings and wavelengths of known stations, it is possible to prepare a curve from which readings for unknown stations may be estimated. (2) A fixed condenser of .Imfd in scries with a variable resistance of 10,000 ohms across the primary of the transformer will servo the purpose,. “Farad,” Dunedin. — (1) If the filament temperature were too low you would experience the trouble, but the fact that the performance is satisfactory would show that as far as the a.f. amplifiers were concerned, it is sufficiently high. Possibly the detector valve has “ aged,” but your trouble is more likely due to a faulty grid leak, or perhaps a high resistance in the plate circuit. (2) Connect the condenser, a pair of headphones, and a battery in series. On making the circuit there will be a loud and distinct click in the phones which should not be audible on breaking the circuit. If its intensity is unaltered on breaking, there is a fault. The phones may be replaced by a voltmeter, in which case there will be a preliminary kick anu a return to zero of the needle. If a reacting is maintained, the condenser is faulty. AVAR ON PIRATES. The recent steps taken by the Post Office in and about London to catch pirates has caused quite a jump over the ordinary number of applications for licenses. It is considered that the revenue increased by more than £IOO,OOO at a cost of some £650. The number oi prosecutions arising out of the campaign totalled nearly 120, and evidently this would have been multiplied by a large figure but for the rush of applicants who feared that they would be detected as pirates. Systematic searching for unlicensed stations is to be extended to other parts of the country.

POWERFUL CZECHOSLOVAKIAN STATION. One of the smallest countries in Europe now operates the most powerful broadcasting station, though full output is not at present used. Czechoslovakia is the country, and the new station will operate with 200 kilowatts. The next big station is at Warsaw, which previously hold first place with 158 kilowatts. Self-supporting lattice towers are employed, and are almost 500 ft high. The entire wireless transmitter is virtually screened and the screen network grounded. Balanced pushpull circuits are used throughout, thus avoiding amplifier distortion and reaction. The oscillator and modulator form practically a small 250 watt station in themselves, and drive the high frequency amplifiers and associated • circuits. The amplifiers use a plate voltage of 20,000 volts derived from a 16,000 volt 50-cycle high tension supply. The wavelength used is 488 metres, so that when on full power the signals should be picked up at gre.at distances. The station was officially opened on November 21 on half power.

CURING THRESHOLD HOWL. The phenomenon known as threshold howl generally makes itself known as a grunt or hoot which occurs just as the receiver is going into oscillation. This unpleasant noise, which partakes of the nature of low-frequency oscillation, is not only annoying, but has also a very definite limiting effect on the range of any receiver which relics largely upon reaction for its distance-getting properties. In a bad case it becomes impossible to bring reaction up to a point where the maximum sensitiveness of the set is developed. The behaviour described is in most cases due to high-frequency circuits which, flowing in the low-frequency circuits, are picked up on the aerial or in the tuning circuits ■of the set —a form, really, of stray reaction. It is particularly likely to make its appearance in a portable set where the frame aerial and loudspeaker are often in the closest juxtaposition, but it is not unknown in receivers of less compact design. If the loudspeaker leads are very long, for example, there is greater feedback to the aerial circuits, and therefore a greater tendency to threshold howl. The recognition of its cause at once indicates the cure) it is only necessary to keep all high-frequency currents out of the lowfrequency part of the set by inserting a proper filter into the anode_ circuit of the detector, reinforcing this, if it is found necessary in a bad case, by a resistance (of 109,000 ohms or go) in series with the grid of the first low-frequency vaivo. The filter usually consists of a high-frequency choke connected directly to the anode of 'lie detector; from each terminal of this choke a condenser, of capacity ,0003.t0 .0005 mfd, runs directly to the filament of the valve. In the case of a set connected to its speaker by lends many feet long, it is often advisable to take still a further precaution by connecting another highirequency choke between the negative loudspeaker terminal and the lead, with a condenser of about .002 mfd between this point and filament.

METAL SPRAYED VALVES. One of the latest developments in valve manufacture is the application of u metal coat or screen to the exterior of the glass bulb. Valves treated in this manner are referred to as metallised or metal-sprayed, as the coating consists of some such metal as zinc sprayed on to the exterior of the glass ami connected internally to the cathode in the case of a.c. operated valves, or to one of the filament pins in the ease of battery operated valves. The result of this treatment is to provide an earthed screen or shield entirely surrounding the valve, and for a valve so treated many advantages are claimed. Primarily the object is to achieve perfect shielding and thus enable greater amplification to be obtained than would otherwise be the case. Obviously the screen so formed does away with the necessity for using the separate cylindrical screen which is a part of many modern sets. Among the advantages claimed for the metallised valve is the elimination of couplings between the anode and any nearby component as a result of which stability combined with greater amplification may be more easily obtained. Further, the external coating extends the internal screening between tne grid and anode to the exterior of the

valve, so that the working inter-electrode capacity (between grid and plate) is reduced appreciably. This also allows an increase in amplification. While this means for enabling greater amplification to be obtained is, in itself, very desirable, it must not be forgotten that no matter how good a valve may be, its effective amplification is governed by many other factors, such as the design of the coil, the construction of the valve base and valve socket. Under working conditions the impedance or a.c, resistance and the amplification of any valve, particularly if it is of screen grid type, are usually very different from those given in the table of characteristics or obtained under ideal conditions, and there js only one value of screen-grid voltage which will give maximum amplification, and this varies with the nature of the coil with which the valve is associated. If the coil is a poor one, the valve has to supply more power to overcome the losses therein. Measurements have shown that a thoroughly up-to-date type of seraoned'grid valve of the indirectly heated type, with a rated amplification factor of 1500, could only give a stage gain of from 830 to 1000 when the best available type of coil and circuit were used, and the following valve had its cap removed so that the losses associated with the cap and the valve holder could be eliminated. With more ordinary types of coils and the usual components, such as valve holders and bases, the stage gain dropped to between 200 and 300, while with a plug-in type of coil of no particular merit the wonderfully high theoretical gain became a mere 75. When one realises that the best can only be got out of a valve when every other component js most carefully designed, it will be seen that, although metal spraying is a most desirable feature, it is likely to be wasted unless the circuit is specially designed for maximum efficiency, a condition which is but rarely encountered. Further features of the metal sprayed valve are its ability to increase selectivity by preventing signals leaking through the inter-electrode capacity, which is usually small ip most modern valves, and the possibility of reducing hum in the case of a.c. operated valves. The valves are of advantage in detector stages, and in the intermediate frequency amplifiers, but it should be remembered that screening boxes must not touch the metal coating, otherwise the grid bias resistor, if in the lead to the cathode, will be short-circuited, while in d.c. valves the A battery may be shortcircuited.

PICTURES BY RADIO. Mr David Sarnoff, president of the Radio Corporation of America —a man who rose from newsboy to head of the I greatest radio firm in the world—-has been expressing his views on television Cstatcs an exchange). They are intensely interesting, and, coming from such a man as Mr Sarnoff, must be given close and respectful attention. Mr Sarnoff thinks that television is still in the laboratory stage, although rapid progress is being made. “Television,” he says, “has been brought definitely nearer commercial dovelopment by the research and technical progress of the Radio Corporation of America during this period,” which, naturally, as an American, Mr Sarnoff believes; but in fairness to Mr Baird it must be remembered that he also has done very much valuable pioneer experimental work. Mr Sarnoff says that one year ago television was a subject of engineering conveVsation and a topic. for technical dispute, It now has progressed beyond that point. To-day, transmission of sight by radio is a matter of accomplishment, not of speculation. He emphasises, however, the fact that the present sporadic activities in this direction cannot be classed as a practical service. They are purely experimental, but as such deserve encouragement and merit public interest. In fact, he likens the present status of television to the condition of radio in the immediate pre-broadcasting era, when amateurs were beginning to hear faint sounds through the air. Mr Sarnoff anticipates that by the end of next year television should be comparable to the earphone stage of radio, and at this point the public may well bo invited to share in its further unfolding, “ By that time,” he says, “television should attain the same degree of development as did radio sound broadcasting in the early period of the crystal set. _ This does not mean that the actual physical structure of the first television receiver will be similar in any way to the crystal receiver; the similarity will lie in the class and condition of the

service; the visions which first come through the air to the public will be of the same embryonic quality as the first faint sounds which sent mother hurrying to the earphone of the boy’s crystal set in the attic. When television reaches this stage rapid strides may bo expected, comparable perhaps with tho growth and development of broadcasting of sound. The progress to follow should make possible the projection of moving images on a screen on the wall, Reception of sight by radio then will be comparable to the loud-speaker stage of sound reception.” We understand from Mr Sarnoff that the Radio Corporation of America contemplates building several experimental television stations by the end of this year. One will be in Now York city, and another station will be located on the Pacific Coast. Through the _ operation of these, experimental stations Mr Sarnoff expects to obtain exact information and practical field experience which are required before definite plans can be developed for-a television service of nation-wide scope. Regarding the benefits that television will confer, Mr Sarnoff says: “The great works of painting and sculpture in the art galleries of Europe and America lie buried there, in so far as the vast majority of the earth’s population is concerned. Television, advanced to the stage when colour as well as shadow may be faithfully transmitted, would bring these treasures vividly to the home. Conceive the exhibition of such works of art in the home, accompanied by comments and explanations by the proper authorities. Just as sound broadcasting has brought a new sense of musical appreciation to millions of people, so may television open a new era of art appreciation. When television has fulfilled its ultimate destiny, man's sense of physical limitation will be swept away, and his boundaries of sight and hearing will be the limits of the earth itself, With this may come a new horizon, a new philosophy, a new sense of freedom, and greatest of all, perhaps, a finer and broader understanding between all the peoples of the world."