THE WIRELESS WORLD

Otago Daily Times, Issue 21649, 20 May 1932, Page 11

THE WIRELESS WORLD

NEWS AND NOTES By Magna Vox. Items of local interest are invited by “Magna Vox” 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.. 0. 353 metres. 2PC, Sydney.—6Gs K.C., 422 metres. 3AR, Melbourne.—62o K. 0., 464 metres 3LO. Melbourne.—B3o K.C.. 371 metres SCL, Adelaide.—73o K.C., 395 metres. 4QG Brisbane.—76o K.C., 385 metres. IYA, Auckland.—B7o K.C.. 333 metres ■ Every day except Monday, 2VA Wellington.—72o K. 0., 420 metres Every day except Wednesday. 3YA, Christchurch. —880 K.C., 306 metres Every day except Tuesday. 4 Va. Dunedin.—6so K.C.. 463 metres Every day except Thursday. 4ZM, Dunedin —IOBO K.C., 277.8 metres Tuesdays and Saturdays. 4ZO, Dunedin.—loßo K.C.. 277.8 metres Monday and Friday. <ZB. Dunedin.—loßo K.C., 277.8 metres Wednesday. Thursday, and Sunday morning

ANSWERS TO CORRESPONDENTS. : R. -F: O’C.j Dunedin. — (1) Correct voltages must be used on both the plate and screen of a screen-grid valve in order to get the best possible results. (2) The greater part of leakage due to a wet lead-in can be overcome by the down lead from the aerial dipping below the lead-in point.

“ X Y Z,” Kurow—(l) 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 are concerned, it is sufficiently high. Possibly, the detector valve has aged, but more than likely your trouble is 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 bv volt meter, in which case there will be a preliminary kick, and a return to zero of the needle. If a reading is maintained, the condenser is faulty. “Low-frequency,” Dunedin. —You should bear in mind that transformers of the type you mention are inclined to have their own special peculiarities. One or these is that with the high magnetic permeability of the metal case the transformer generally gives its best performance when there is only a very small d.c. current in the primary. If you use it under conditions where primary current is too large, this will probably reduce the effective inductance so that the performance of the instrument will not be so good. Of course, if the transformer is specially made to carry a current of several milliamps, everything will be OK, but many of them are not designed to carry more than one or two milliamps in the primary. Such a transformer would not give its best if it were used, for instance, with a detector valve passing, say, four or five milliamps through the transformer. A USEFUL RULE. A good rule to remember, and one'that is applicable to most wireless receivers, is that each thousand ohms of resistance absorbs one volt at one milliampere. |or example, a 1000-ohm resister, passing Sve milliamperes, will absorb five volts. Take another case that is likely to arise m practice; that of the anode circuit of a power-grid detector, estimated to pass eight milliamps, and where it is desired to’fit a 20,000-ohm decoupling resistance. Applying the simplified formula, we see that the voltage loss will amount to 20 x 8, equalling 160 volts. When dealing with bias resistances, it is, perhaps, more convenient to remember that 0.1 volt is developed across each 100 ohms for each milliamp flowing in the circuit. TESTING VALVES. The plate current reading is a most important gauge of efficiency, as it is a monitor of the major valve characteristics which determine the efficiency of the valve, Plate current readings of valves of similar design should be approximately uniform . and come within the limits specified by the manufacturer. It is unwise’ to select valves with an abnormal reading, since this is an erroneous, though prevalent, _ practice. The filament emission reading is a check on mutual conductance. Mutual conductance, above all checks, determines the merit of the valve in so far as its ability to provide the required amplification or gain. Filament emission itself indicates the extent of useful life the valve may possess. Some attempts have at various times been made to check valves in an oscillating circuit, but, unfortunately, no correlation between’ the ability of the valve to perform in a fixed oscillating circuit and its ability to function in a radio receiver has been established. The fallacy lies in the fact that an oscillating circuit used in a test board may be properly designed for one particular type of valve in one particular circuit, but still be valueless in testing valves of other types in other circuits. Even the oscillating circuit in a superheterodyne will vary too greatly from a test board circuit to determine whether performance in one will indicate performance in the other. REACTION CONTROL. ’ A good deal is said about tone and quality of reproduction as being dependent on the components used in the set, the layout of the circuit, and various i other matters connected" with the receiver. These, of course, are very impor--1 tant, and, if there is any inherent defect in the set, it is obvious that no amount of operating skill will entirely overcome it. But at the same time I think bad quality is much more due to bad operation of the receiver than to defects of the instrument itself. The most flagrant example of this, of course, is the misuse or excessive use ■ of reaction. It very often happens that a sot will, in proper hands, give excellent quality of reproduction, but in the hands of an unskilled or careless user give just the opposite. I do not think it is necessary to say much aboul the use of reaction, because in these days .everyone knows that reaction is to be regarded as a sort of medicine, only to be used sparingly, and not to he relied upon as a regular thing. When listening for distant or weak stations yon will generally have to use some reaction, and you are very apt to get in the habit of relying upon the reaction and using far too much of it when receiving local or fairly strong stations. Remember

that even quite a small amount of reaction, or perhaps I should say of un- \ necessary reaction, will have the effect of ‘ cutting , down the higher notes and also ! probably of sharpening up the tuning and making the set still more difficult to control. It is almost an axiom nowadays that if you , have to rely upon a large J amount of reaction to get the desired \ signal strength, especially con nearby i stations, there must be something wrong, with the set or with the operation of it. If it is the operation that is wrong it is most probably the tuning, but at the same time you should take a careful look over the coils to make sure that there is no undue loss there, and also be certain that losses are not occurring due to high , resistances in the aerial circuit. As regards the tuning, this also is one of the i most fruitful sources of distortion, many j listeners making a bad habit of using tlie j tuning for,.the purposes of volume control, j Strictly speaking, a’set should be accur- i ately tuned always, and any’ variations, i either in quality or quantity, which you f want to make should; be carried out in- : t dependency of the tuning. In other words, the tuning control should, be used for tuning and nothing else. If the set ■ is particularly selective—as most sets are: in these days—the distortion brought about by improper tuning of the main circuits will be all the greater.; OCEAN NEWSPAPERS. ' The ocean traveller need not abandon what now amounts to an hereditary craving for news. . Wireless has made it possible lor him to have a daily newspaper which differs in little else but size from its prototypes on land., It contains news and literary pages, and, in addition, a bright collection of advertisement's, which in themselves create in the mind of the reader the feeling that he is still in touch with" civilisation' and. can slip into Bond Street or. Fifth Avenue at any • moment when the mood takes him, writes L. C. Thomson in, the Wireless • World. , While the ocean newspapers are strictly non-political, care is taken to give a comprehensive survey of world happenings in u manner calculated to interest all types of voyager, from the commercial magnate eager to read the market reports, to the society debutante whose thoughts may tend more in the direction of the latest wearing apparel. What is never forgotten is that most of the journal’s read- j ers find themselves in an unfamiliar environment and’ are not ' prepared to concentrate for long periods on weighty ■ leading articles or dissertations which woiild be quite acceptable to the landlubber in his carpet slippers at home. The supply of news never ceases. Daily throughout the year Rugby Radio transmits three news bulletins—in the early hours, at mid-day, and during the evening. The cordial co-operation of the Post Office is, of course, essential in the conduct of such a service, and it says much for the efficiency of the staff concerned that the news supply _ is maintained year by year without a hitch. 1 Lucky though he may be, the ocean traveller has only one newsiuiper, and if the news services were to weak down he could not buy a competing journal. In addition to the regular news bulletins, Rugby Radio transmits a Satur day night message giving the League football results.. This message is in geeat demand among, the crew’s on cargo boats all over the world. Lest it should be thought that the ship’s newspaper ■ is a ( new- creation, lit is worth noting that' No. I, Vpl.. 1, of the Transatlantic Times, the forerunner. of the existing publications, appeared bn November 15, 1809! The : issue was made in the :; form of "a leaflet on>. board the’ s.s/ St; Paul, on which Senatore Marconi had 1 fitted his apparatus. Communication .was established with the Needles station, 36 miles away, and reports were printed, in a somewhat telegraphic style, of the progress of the South African War. The modern ocean newspaper is an excellent reproduction in miniature of the “ real thing,” but. already th’ere are indications that it may, sboner or later, give, place to an exact replica of journals on land. This should, ultimately, be achieved- by means of , facsimile telegraphy, which could repi-oduee on. board ship a complete page, printed on, land, in a shorter time than would be required to transport the actual newspaper from one end of Fleet Street to the other. Whether anyone wants this luxury is another question. MICROPHONES. By virtue of its position at the head of the long line of: equipment necessary I for the transmission and -subsequent reception of .broadcast speech aijU music, . the microphone is a very important unit! It must not fail to pick up any part of the transmission, for, if it does, the subsequent reception can never be perfect, and as far as possible it must be prevented from introducing errors, for,' although they may be more or less corrected in subsequent stages, -the ultimate reproduction may be seriously affected. It is, in a measure, the electrical equivalent of the human'ear, but it lacks the desirable quality of: distinguishing be-; tween: wanted and unwanted sounds, and deals as faithfully as 1 it cam with every sound within its reach.. For studio work this lack is not as important as it is for broadcasts from theatres or recordings in talking-picture studios. In order to improve the results from such places as these, much work has been done recently in the development of microphones having directional qualities, and as the result of one line of research the reflector microphone has been produced/. This, in a few words, consists of a large parabolic mirror which picks up the sound waves reaching it from a certain direction and focuses them upon a point where the microphone itself is placed. It avoids, to a large extent, extraneous noises and produces a good output from the required quarter, and at the same time it allows the device to be placed in a position where it cannot obstruct the view. Another form of microphone which has directional properties is the “ ribbon ” type, iu which the operating element is a flat metallic ribbon vibrating in a magnetic field. It responds more to sounds which strike it upon the flat side, as those which come in sideways, meet only a relatively thin edge. As far as the design of the units is concerned theproblem has been to produce something which will respond faithfully to sounds of all frequencies. This is all the more difficult. because whatever principle is used' .the unit must have mass, which means that it requires a certain amount of sound energy to operate it and that there arc likely to be certain frequencies at which it will vibrate more freely than others. It is a long step from the microphone of to-day to the successful pick-up device developed in 1876 by Hughes. This arrangement was remarkably simple and effective, and consisted of a' carbon rod supported between two carbon cups. A small current was sent, through the combination. and sound waves striking the loose rod varied the contact and hence the current. From this early beginning the carbon-button and carbon-granule-type of device was developed. It is practically the standard equipment in t telephone transmitters, and, until recently, in its more refined forma, in the broadcasting stations. In its commonest form it consists of a box of loosely packed carbon with a diaphragm pressing against one side. When the sound waves strike the, diaphragm the carbon is subject to corresponding pressure, with -.the result that its conductivity i« varied, and so the

sound waves create variations in a minute current flowing through the carbon. Today the condenser type of-microphone is found in the best .broadcasting and ’talking picture studios. It is, iu principle, practically as old as the carbon tvpe, .but for many years it remained undeveloped. In it the diaphragm forms one plate of a small.condenser, and when this plate is moved by sound waves striking it the capacity changes. Its great drawback is that its output is relatively weak, which means that the first amplifying valve must be very close to it, and the only, practical way of overcoming this difficulty is to place the amplifier in .the same bos as the microphone, which, .of course, means the supply of filament,’plate; and other voltages with-their attendant .difficulties; The trouble, is'due’to the fact that, because

it relies upon a change of a very small capacity, the capacity of the connecting; leads must be kept down.- The disabilities' of these two types have; directed attention to the development of the electro-dynamic microphone, which, in its general principles, resembles the moving coil loud speaker operated iri the reverse direction'. A small coil is attached to the diaphragm and caused) to move in a magnetic field to generate voice currents of a strength and form similar to the movement, of the diaphragm. This type of pick-up device promises to provide the host solution ,of the general difficulties.