RADIO AND ITS RECEIVERS

Sun (Auckland), Volume II, Issue 511, 14 November 1928, Page 14

RADIO AND ITS RECEIVERS

Conducted fer THE SUN by

R. F. HAYCOCK

Improved Electric Sets

MORE RELIABLE DESIGNS

Less Likely to Burn Out

ELECTRIC radio set operation using alternating current is destined to become more firmly entrenched in the favour of listeners than ever before, according to radio engineers, despite an annoying proclivity on the part of various last 'year’s AC set models to allow tubes to burn out prematurely.

A year’s observation of the sets in actual operation through out the country has afforded designers an opportunity of gathering information which was largely lacking for last year’s sets. When some of the electric sets were introduced during the past season, according to the engineers of a leading radio valve makers, very little information was available to set designers concerning electric line voltage conditions throughout the country. As a result, many set manufacturers •were responsible for their products being severely overloaded,” resulting in an abnormally high mortality of some of the new alternating current type of tubes. Overload Causes Trouble “Conditions of overload cannot be detected by listening to the loud speaker,” said Mr. R. M. Wise, chief engineer of the Cunningham Company, recently. “Overload on a radio valve has the effect of shortening its life. •Dealers were not fully informed as to the proper methods of meeting unusual voltage conditions until trouble developed, either in the form of unsatisfactory volume or short valve life. As dealers obtained additional information, and began to make it a practice to obtain voltage 'londitions before sets were installed, these initial difficulties have been overcome. The case is similar to that experienced with the electric lamp on abnormally high voltages. The lamp burns brighter, but their lives are correspondingly decreased. It is even more severe with the radio valve. “One condition which has been difficult to improve is that of extreme voltage fluctuation in certain localities. If

the lighting line voltage .hanges over a range of 20 to 30 volts during the day, the A.C. set may work unsatisfactorily when the voltage is lowest, while the valves may be overloaded when the voltage is highest. In some cases such conditions have been remedied by the power companies, which have been as a rule willing to co-operate with radio dealers. When such conditions cannot be remedied by power companies, voltage regulating devices now coming on the market may be used to secure proper A.C. set operation.” Radio dealers are advised by valve manufacturers to ascertain that the new A.C. sets will work without overload, notwithstanding the promise of greater tolerance in design, by undertaking tests with one or more samples of the receivers in localities where they will be eventually installed in homes. The first test is to determine accurately just how high the line voltage can go without valve overload by measuring power line and tube filament voltages simultaneously with the proper A.C. voltmeters. The second test is to have the service man read the voltage of the house-lighting circuit before the receiver is installed, and where voltages are found normally too great to instal a resister in the power leads to the set. Valve manufacturers point out that a “variation of 10 volts in the line will not affect the performance of the A.C. set, provided the valves are not overloaded.” If the line ariation throughout the day, where the set is to be installed, is found be to 10 volts, the safe plan is to reduce the line voltage by 10 volts, by furnishing a fixed power line resister with all installations. This will prevent the voltage from ever exceeding the approximate safe value.

VALUE OF SHIELDING METAL BOXES HELP HINTS FOR HOME SET BUILDERS The importance of shielding a radio receiving set is stressed by the Copper and Brass Research Association, America, which has compiled some valuable suggestions to aid radio set builders. It is pointed out that when an electric current runs through a wire magnetic lines of force are set up about the wire, creating a magnetic field. If the wire is wound into a coil the magnetic field is increased, and if iron is inserted into the coil, the field is further increased. This field is not confined to the centre of the coil or the iron core, but spreads out around the coil in the form of magnetic flux in accordance with the shape of the coil. In coils of the solenoid type the field is most intense along the axis of the coil, but flux lines spread outside the coil from its ends. The toroidal or doughnut type coil, which is a solenoid bent in the form of a ring, is designed to confine these stray flux lines, as the coil has no free ends.

It will be seen, then, that if coils are inserted in a radio receiver and are not spaced sufficiently far apart, the free fields will interlink so that the radio frequency currents in one coil will set up interfering currents in another coil, producing what is known as feed-back or radio frequency interstage coupling. This manifests itself in the form of whistles. Shielding, to be effective, must be complete. The smallest crack or opening will spoil the whole receiver. It is difficult to shield a receiver already designed and built without shielding, and the most satisfactory and practical plan is to tear down tlie circuit and rebuild the set. For example, in many five-valve receivers the radio frequency valves are not placed directly behind their coils and condensers. Instead they are placed beside tlie condenser inductance unit and are staggered with the audio frequency valves. For this reason it is difficult to make a box to surround the condenser-in-ductance and valve of each radio-fre-quency valve. WHY COPPER IS USED The ideal theoretical shielded receiver Is one in which the inductances are so widely spaced that their fields cannot interlink. Because oil size limitations and feed-back due to wiring etc., this condition is impracticable, if not impossible. and it is therefore necessary to sacrifice efficiency slightly for practicability by using a shielding material that will dissipate these stray fields in the form of small eddy current losses. As the efficiency of this dissipation is proportional to tlie conductivity of the shielding material, copper is found to be the most practicable material for the purpose .-- n , n laying out the design of the shielded receiver the size of the box tor the radio-frequency unit must be considered first. This is determined by the equipment it is to hold—the condenser, inductances and tubes being the essential factors. The most important of these is the inductance, which must have sufficient clearance to the walls of the box. It is best to make these boxes just as large as possible. The sub-panel should sit one-sixteenth of an inch behind the main panel. Allow room enough between the boxes for rheostats and volume controls that are usually mounted on the main panel between the boxes. It is not necessary that the rheostats be shielded, providing that the wires go directly into the shielded box below the sub-panel or into one of the interstage boxes directly adjacent to the controls As a rule, the set will wire best if the jacks are mounted on the panel below the sub-panel so that they project into tlie sub-panel box. HOW TO MAKE THE LAYOUT This layout will determine the length of the main panel and sub-panel. Now lay out the audio tubes and transformers to determine the depth of the subpanel. and make a front view of the set and boxes to determine the necessary main-panel height. If it is found that the Danel will be toe long rearrange the jacks to be located directlv under the rheostat and volume control

and, if necessary, decrease the width of the boxes a little to gain space. Make a schematic diagram and from it work out a full scale working wire diagram, rearranging the parts to make the leads as short and direct as possible. The shielding should be connected to the ground binding post of the set. Plan on insulating the parts, etc., where necessary, with bushings or blocks of hard rubber. The most practical shielding material will be found to be sixteen-ounce soft copper sheet, which may be procured from any tinsmith, plumber or hardware dealer, as it is a standard size which is invariably carried in stock. It is best to use sheet that has not been coiled, to keep the surface as flat and true as possible. In soldering, be sure to have the copper clean and bright. Use a very hot iron, as large surfaces of copper dissipate heat rapidly. Use as little flux as possible and take pains to wipe off all excess flux with alcohol after the work is complete. Practically all circuits can be altered slightly so that the rotors of all the condensers may be grounded on the boxes. This is accomplished in the case of the detector circuit by returning the grid of the tube through the grid condenser and inductance to A—instead of A-f-, with the grid leak connected to A+ instead of across the grid condenser. All wiring passing through the copper shields should be covered with a good grade of spaghetti or other insulating tubing, and all burrs should be removed from the shields where the wires pass through. RADIO ON AIRSHIPS Radio experiments on airships duringl the past year have met with considerable success, according to the Air Corps of the TJ.S.A. War Department. Previous experience with signal corps sets demonstrated the feasibility of excellent transmission, hut no results were obtained for reception, the principal trouble being interference caused by motor ignition. During the past year, however, satisfactory results were obtained in reception aboard airships with a special shielded seven-tube set. The installation in its present form is crude, but it has proved very satisfactory. “Some of the troubles which were encountered in the first experiments were traced out, and it was discovered that by the use of a ten-foot counterpoise and 100 to 125 feet of aerial trailing from opposite sides of the ship and insulated from the ship by means of rubber mats practically all of the interference from the motor ignition was eliminated,” the report said. “The airship had been fairly bonded before the experiments were made, and a navy type reel was used with a special bracket to fasten the reels to the side of the ship. However, interference was picked up through the wood of the ship and the reel and brought into the set. “The interference was eliminated by insulating the bracket from the wooden side of the car by a rubber mat. The set and batteries were jilaced in one container and suspended from the sides of the ship by shock absorbers. Further vibration of the vacuum tubes was prevented by the use of caps with pads surrounding each group of tubes. Practically all experiments were conducted on the TC type of ship. No attempt was made to transmit from the local station until the principal difficulties of reception were eradicated. “With the use of head telephones commercial broadcast was received from Cincinnati, Chicago, Des Moines and other stations of equal distance during after-noon flights, and the most distant stations came in powerfully. “Communication was held for three or four hours on one flight and the crew in the control cabin was able to hear every word broadcast from the station. The reception at the station was also satisfactory. Further experiments are contemplated, it was said in which refinement of installation will he effected to reduce the weight involved.”

SPEAKER PROBLEMS With so many types of loud speakers, varying much in design and price, on the market the selection of one of these instruments often presents great difficulties. Choice is considerably aided if one has a general understanding of the peculiarities of the various types and of the results which can be expected from each. Although loud speakers take many forms, they can be grouped into three general types possessing distinct characteristics. These are the horn type of speaker, and the rigid-edged cone type of speaker, both of which have been on the market for some years, and the new dynamic cone or floating cone speaker, which has been on the New Zealand market for only about three months. The dynamic cone speaker is regarded at present as the last word in loud-speaker design. There is no doubt that if properly used it is capable of giving reproduction so faultless that the limiting factor in giving perfect reception becomes the quality of the broadcast transmission itself. The dynamic cone speaker'is capable of giving uniform response at all musical frequencies. This means that no note or notes will be amplified more readily than others, while none will be unduly suppressed. The highest audible notes on the musical scale and the lowest bass sounds will be reproduced with equal facility, and if necessary the speaker is capable of reproducing band music with all the volume and all the fidelity of a full brass band. It is capable of giving an output of undistorted sound far in excess of that which can be obtained from any other type of speaker. Unfortunately, however, the dynamic cone speaker, although now fairly cheap as an element, cannot yet be regarded as the speaker for every wireless set. In the first place, it requires far more power to operate it than any other form of speaker, and must be operated in conjunction with a very powerful amplifier. A receiver driven by high-tension batteries is of little value in conjunction with a dynamic cone. Before the speaker is capable of justifying itself it must be operated from a main-driven set of more than ordinary power, and in order that the full fidelity of reproduction of which the instrument is capable may be achieved the associated amplifier must be faultlessly designed. These limitations will, for the time being at any rate, make the dynamic cone of little value unless operated from elaborate and expensive receivers. Of the other two general types of speakers more is known to most listeners. Prom the point of view of faithfulness of reproduction each has a clearly defined fault. The chief fault of the ordinary cone speaker is that, while it renders low notes with. great fidelity, it generally fails on the higher notes, either distorting them rather badly or so reducing their volume in comparison with lower notes that the perspective of the music is marred. The horn speaker possesses the opposite fault. It will reproduce the higher and middle notes with great clarity, but in its ordinary form it is incapable of faithfully rendering the lower ones. The reason for this failure is that the action of a horn speaker depends on the horn itself. The wave length of the higher sounds is very short a couple of inches or less —but the lowest of the musical sounds with which a loud speaker must deal has a wavelength of eight or ten feet. Hence it will be seen that the average large horn speaker, with a horn about 4ft. long, is capable of repeating all the higher notes well, but because the horn is not long enough to accommodate at least one wave of the lower notes it cannot reproduce them effectively. Recently this disability has been overcome to a great extent by considerably lengthening the horn of the horn type speakers, and to conserve space coiling it into a sort of spiral like the bodv of certain forms of trumpets. Speakers of this type which are now on the New Zealand market are capable of rendering accurately all types of music impressed on them. The horn type speaker has one important advantage over most forms of rigid-edged cone speakers. On a given receiver it is capable of giving louder signals than the rigid-edged cone. Hence, if one is using a relatively small set to operate

the speaker a better output of sound can be expected from a horn speaker than a cone speaker. It is interesting to note that the combination of the characteristics of an ordinary type of horn speaker, which will fail on the lower notes, and the rigid-edged cone speaker, which will fail on the higher notes, me |-s the requirements for practically faultless reproduction. Hence if the two speakers are used simultaneously on a given receiver, by connecting their terminals either in series or in parallel, and placing two speakers fairly close together. one will reproduce one-half of the musical scale and the other will reproduce the other. In this way almost faultless reproduction can be obtained. A great many listeners possess both types of speakers, and are therefore in a position to get the best music which their receiver can give by using the two speakers simultaneously. MUSICAL AND DRAMATIC COMMITTEE A meeting of IYA musical and dramatic committee was held last week, when the following were present: Mr. Karl Atkinson (Auckland Gramophone Society), in chair, Mr. C. B. Plummer (Auckland Choral Society), Mr. L. E. Lambert (Bohemian Orchestra), Mr. C. R. Straubel (University Students’ Association), Dr. Kenneth Phillips (Society of Musicians), Mr. J. P. Montague (Auckland Comedy Players), Mr. G. T. Lee (Aeolian Orchestra), Mrs. Carr Tibbits (League of Penwomen), Miss Delaney (Ainsley Operatic Society), Mr. S. J. Hayden (station director of IYA). Correspondence was received from the Radio Broadcasting Company intimating that they were pleased to adopt the committee’s recommendations regarding the radio play competition and title competition. A report was received from the company’s musical director dealing with the question of the proposed IYA choir, and the matter was referred to a subcommittee. A report was received from the League of Penwomen in which the league intimated that it was willing to supply a monthly agenda of forthcoming lectures to the station for broadcast of such lectures as might be suitable. The matter is to be referred to the league's committee for confirmation, and further discussion on the question of giving studio performances. The question of Christmas programmes was also discussed, and it was decided that a carol concert should be given, preferably on Christmas Eve. There is one radio to every three farms in the State of lowa, U.S.A., according to a recent report of the low-a Department of Agriculture compiled from figures gathered by township assessors. The number of radios on lowa farms increased from 10,566 in 1926 to a total of 76,032 in 1927.