ON ALL WAVELENGTHS

Evening Star, Issue 20422, 1 March 1930, Page 8

ON ALL WAVELENGTHS

By

A POWER AMPLIFIER. Since the advent of the moving coil loud speaker, and as the characteristics of the power tubes have been improved, the push-pull principle has been used to give as perfect amplification as possible. There are many advantages with a push-pull amphnez. The tubes add their outputs; direct current magnetisation of the iron core of the output transformer is cut out; only 200 volts are required on the plates of the tubes; and motor-boating is considerably reduced. When constructing the amplifier, use a metal mounting plate for the components. The preceding audio stage feeds into the push-pu l input transformer. The terminal P is connected to the plate of the preccdnig audio tube and the terminal “B” to the ninety volts. A3O ohm potentiometer is used to obtain the electrical centre of the filament circuit as a lead is taken from this centre point to a resistance the other side of which is earthed. The plate current flowing through ,this resistance causes a voltage drop across its extrcmeties. The centre point of the input transformer’s secondary is earthed similar to one side of the resistance previously mentioned. Thus a negative voltage is applied to the grids of the tubes. The higher the value of this bias resistor the greater the _ potential difference across its terminals, and thus the higher the negative bias applied to each grid.

The value of a bias resister for 245 type tubes in push-pull should be 787 ohms, and should be by-passed by a 1-microfarad condenser. The resistor should be capable of handling about seventy milliamperes, otherwise excessive heating will cause the burning out of the resistor. The filaments are excited by a filament supply transformer delivering 2.5 volts at three amperes. . As the amplifier has been designed to operate a dynamic speaker, a plate power output choke is used to feed plate.,current to the valves. The output of the valves is fed_ through two 4-microfarad condensers into the output transformer of the moving coil speaker. This transformer is included in all makes of moving coil speakers. The wiring of the amplifier should be carried but with a good class of single flex. The amplifier finished, a lew preliminary tests are always advisable in case everything is not operating correctly, raise readings may result if the constructor has not carried out his test correctly.

There are three mam measurements necessary: (1) Plate vpltage, (2) grid voltage, (3) plate current. With a plate voltage of 200 volts, and each tube drawing' approximately thirty milliamperes, the resultant bias should bo about forty-eight volts. The output of the amplifier will bo in the region of two watts. The first diagram this week slmws where the voltmeters and the milliameters should be inserted to insure correct readings. The second diagram details the circuit of the push-pull amplifier. DEEP INTEREST IN RELIGION. “ Seven years’ experience of public reaction to broadcast religion enables one to draw some fairly definite conclusions f and it is good to speak from the facts of experience > as well as from the assurance of conviction ” (writes Sir John Reith, Director-General of the 8.8. C., in an English "i-eviow). “The popularity of the services on the Sabbath, the readings from the Bible, the daily morning service, and the Epilogue would be little short of astonishing had it not been anticipated beforehand. Even so there is something remarkable in it. “The facts are obvious; so should also bo certain conclusions. Put briefly, there is still a place of priory for religion, a response to it and a need for it; the country is religious at heart or capable of being so; and this holds in spite of seeming indifference on the part of a large proportion of the people, irrespective of diminishing attendance at church and the growth of materialism and irresponsibility.” Sir John expresses surprise that “ so little, if anything, seems to have been done by the church to take advantage of the immense interest in religion which broadcasting has awnkaned oi reawakened among many millions of listeners. , “ A criticism I have made and make is that innumerable people, stirred to take an interest in or give some thought to religion—either once again or for the first time—by the sort of services atid addresses they listen to by wireless, and in pursuance of that interest gb to church, and are disappointed and bored by what they get there. ' “There‘are faults in pulpit and pew, but it is usually of the former only that we hear. There does sometimes seem to be a lack of personality and sincerity in the conduct of the services.” !

NOTES AND COMMENTS

CORRESPONDENCE. “Magnet” (Dunedin) asks: (1) Would an English radio set, which is ran off a 250 volt a.c. supply in England, be suitable for Dunedin’s 200230 volt supply? (2) What constitutes a prohibited circuit in New Zealand? (3) What is the frequency of Dunedin's a.c. current?—Yes, an English set would work on the a.c. supply in Dunedin, hut there would be a big risk as the filaments of the tubes may be fed with too high a voltage unless a satisfactory adjustment were fitted. In a prohibited circuit an oscillating tube is connected straight on to the aerial. The frequency of Dunedin’s a.c. current is 50 cycles. s.o.s. A RADIO DRAMA. Broadcasting was, of course, originally developed to provide entertainment and instructions; and nobly has it fulfilled its purpose. Witli the advent of an organised system of broadcasting other duties have been aded, and the 8.8. C. stations now regularly broadcast official announcements at the request of the police, urgent messages to the relatives of sick folk and so forth.

But in addition to the material broadcast officially from the studios is a wealth of unofficial messages continually on the air. Many listeners have derived considerable pleasure in picking up messages sent out from Croydon airport to aeroplanes making the passage between England and the Continent. Moreover, those who are expert at reading the Morse code occasionally amuse themselves by listening to ships at sea. The following true incident occurred only a fow weeks ago, and indicates that even the much-maligned Morse messages which sometimes cause interference with the , ordinary broadcast, especially in coastal districts, may have their interesting and even dramatic side.

Like many other naval officers who have undertaken a business career ashore, Mr Cyril E. Baron has maintained his interest in naval affairs, and, having specialised in wireless operation, has fitted up a room at his home in Surrey as a complete replica of a ship’s wireless cabin. Thither he frequently repairs, and keeps himself in practice by tuning in to the wireless messages sent out by shipping in the Channel.

On December 8 last Mr Baron discovered that liis. aerial was in need of repair. As the woathei was extremely boisterous and the work involved climbing a tree more than 50ft high. Mr Baron was sore y tempted to leave it until the storm had abated, but such was his interest in radio that he felt compelled to carry out the repair. This was eventually completed, and in ordei to assure himself that all was now in order Mr Baron went into his study. switched on his Master Three Star receiver, and listened. No broadcast programme was being radiated at the

time, and Mr Baron therefore tuned in to the ships in the Channel. Traffic on that particular date was extremely heavy, and the ether congested with shipping signals of every description. After a few minutes, however, one call appeared to stand out clearly above the buzz of jumbled messages. It was the dread 5.0.5., and was followed by a statement that the Russian steamship Jakov Sverdlov was in difficulties. “Please, everybody, come and hejp,” was the laconic but poignant appeal. The message was repeated several times, but Mr Baron could bear no reassuring answer. His knowledge of conditions at sea gave the clue to the situation—the vital S.O.S. was jammed by ordinary traffic, witli the result that the North Foreland coast station bad not been able to pick it up! Realising the seriousness of the Jakov Sverdlov’s position, Mr Baron immediately put through a trunk telephone call to the North Foreland station, and repeated the S.O.S. message. With a curt “Thank you ” the North Foreland operator cleared his telephone line, and ho must have acted promptly, because by the time Mr Baron had again resumed the head phones all the ships in the Channel, acting on instructions from the North Foreland, had closed down their wireless, and communication had boon established with the Jakov Sverdlov. Mr Baron was informed later that tho ship, which was 1.812 tons burthen, was eventually towed to safety, undoubtedly as the result of tho fortunate picking up of the signal of distress by a private listener. There are two epilogues to this drama of' the seas—one still more dramatic, the other mildly humorous. Within a few moments of tho incident Air Baron’s aerial was again blown down by the wind. A few days later Mr Baron received a letter of thanks for his prompt action from the Post-master-General. which concluded with the whimsical assurance that tho charge for tho telephone call to North Foreland would not ho debited to Mr Baron’s telephone account. By a curious coincidence this letter was signed by the official, who sixteen years previously had signe 1 Mr Baron’s certificate as a ■ t-e ,;ss wireless operator.

AVIATION AND RADIO. In Australia, aviation is advancing by leaps and bounds; yet, with it all, there is a phase in aviation that does not appear to receive the attention it demands. That is the use of radio. Systems have been developed that will facilitate the navigation of the_ pla'ne. By having an ordinary receiving set on the flying machine, it is possible to fly direct to the required station, irrespective of side drift, etc. Radio beacons are a boon to commercial aviation Then comes the possibility of communicating from the plane to the ground, and again between two planes. Aircraft radio has tremendous possibilities, and it is the object of this article to illustrate a few of the problems in its development. ; Conditions met with on aircraft demand that most of the radio equipment carried be of a highly specialised design, both electrically and mechanically, in order to meet the severe requirements imposed. Space and weight restrictions arc of paramount importance.

The advent of the short waves made communication over long distances much easier, for with it large power supplies and heavy equipment are unnecessary. A short-wave transmitter oml receiver, with their necessary powei can now bo made to lit into a comparatively small space. Good results are usually attained by using a single trailing wire aerial against the metal structure and bonding of the plane as a counterpoise. Stranded phosphor bronze. or copperclad steel wire is generally employed, making a total weight of from two to five pounds at the end. To reduce the possibility o! accidental loss, and to make possible ready renewals, tubular aerial weights, together with a large diameter fairlead, are used. After use the aerial can then bo returned to tho plane. Severe restrictions in space and weight are imposed on all aircraft radio equipment, and these can only be met ,by skilful design. All equipment should! be readily accessible for inspection and maintenance, and must bo constructed to' withstand the continued vibration and landing shocks without breakage. Equipment is generally supported on cushions of sponge rubber, or on spring suspensions, or is hung in place ny a suspension of rubber cord known as “ bunge cord.” Too springy a suspension cord, however, is unsuitable, as it may allow the equipment to bounce around, and perhaps suffer severe shocks by touching adjacent objects during a bumpy landing. A precaution, winch is necessary for efficient radio operation, and which is desirable for general safety reasons, is the proper bonding of aircraft. By this is meant a thorough electrical interconnection of all metallic parts of the plane, with permissible exception only of such isolated parts us are well insulated from the main structure. There are three reasons for bonding. First, it reduces the possibility of fire by preventing sparking between adjacent metallic parts. In the absence of bonding, such sparks may occur as a result of charges of atmospheric electricity, or from voltages induced by transmission. Obviously, special care must be taken in bonding all fuel tanks, and feed pipes, with the adjacent metal work; or if used in hydrogen-filled dirigibles, bonding ol all metallic parts near the gas cells, or envelope, must be given the greatest attention.

A second reason for bonding is the resulting increase of effective counterpoise area for transmission, with a resultant reduction in radio frequency resistance.

Lastly, the absence of bonding may cause many electrical disturbances m radio from intermittent slapping or rubbing together of separate conductors in the of the aerial-counterpoise system. To avoid noises, particular attention must be paid to the control wires. It is the modern practice to run such cables through sheaves and guides, which cither are well-grounded or well-in-sulated, and to cover one or both cables with insulating sleeves at points, where they _ cross, or may slap together in flight. There are many things to contend with in radio reception during a uight, and these are only recently being overcome to a satisfactory degree by specialised design and installation precautions. One of tho worst obstacles is the noise produced by the motors usu ally accompanied by noise from tho pro peller and whistling of exposed wires. These noises are naturally more pronounced in open planes, and can bo partly reduced by well-fitting radio helmets with pads or rubber cups surrounding tho ear pieces; nevertheless, a very strong signal is required to b ■ heard during flights. It has been found, by using an audibility meter, that or dinarily signals giving between 1.5- and 2 volts r.ni.s. across tho headphones are required for satisfactory reception in flight Radio disturbances caused by motor spark pings and leads often cause a considerable amount of interference. Theso can only bo avoided by carefu. and systematic shielding and bonding of tho entire ignition system.

There are serious microphouic noise-, set up by tho vibration of tube ele ments or condenser plates, and thes. parts often require special suspension and design. Special tubes with rigid elements have been developed for use in aircraft. Audio frequency tuning b\ peaked transformers or trap circuits is often of use in reducing disturbances, but this, obviously, is no use in radiotelephony, and is only suitable for telegraphy. Series condensers in the aeriai or ground circuits should be avoided or shunted by a choke or resistance leak, in order to prevent sparking from ac cumulated charges.

Wo now come to the transmitters. These must be of very compact, yet durable and accessible, construction, and the wiring should have the same attention as that given to a receiver. Intricate adjustments should bo avoided in order that the set may be operated by untrained personnel. High voltage circuits should be well protected against any accidental short circuits under vibration, and safety precautions should bo taken to minimise any danger to tho operator should it be necessary for him to work on his transmitter in the confined space during flight. In all enclosed planes it is especially desirable to guard_ against the dangers of fire by encasing all sparking switch contacts in flameproof boxes, to prevent igniting'any stray gasolene fumes that may be present. An added precaution against fire is that the insulating throughout must be of the highest quality and properly employed, with particular attention to avoid chafing under vibration. In radio telephony some disturbances in transmission may be encountered from flight noises, but these have been minimised by special anti-noise microphones, which are sensitive to voice and exclude most extraneous sounds. The power supply for the transmitter may be derived directly from the aeroplane motor by gearing a generator ,to it, or, indirectly, by fan drive in the air stream; or from a storage battery through a suitable dynamotor. The last method will furnish full power on the ground or on the water, as well as in flight, for a limited time unless the battery is kept charged by another generator. The wind-driven generator, with selfregulating fan, gives the most flexible instalation, and will furnish power as long as the piano is in flight. Should it be suitably inst I led in the propeller slipstream, the wind-driven generator can also be used on the ground for testing or emergency. A type of current supply eminently suitable for emergency use is the handdriven radio generator. This device furnished to up to 50 watts of combined electrical energy for plate and filament supply. It has been found that one man is ! not able to exceed this power for any length of time, and, in view of this, such hand-driven generators lend themselves to high frequency emergency transmitters, which may, regardless of their lower power, reach out to a considerable distance.

In conclusion, one will see the possibilities of aircaft radio, and appreciate the difficulties to be overcome be fore it is perfected. It is a specialised branch of the radio science, and requires men thoroughly trained both in aircraft and radio to further its developments. Although there are many difficulties, it is possible to'Overcome most of these with proper designs of installations. One hopes . that, in the near future, aircraft radio will become more popular. It is inevitable that this will come, and then aviation will have another great aid to safety. One docs not understand why more use is not made of radio, when, undoubtedly, it will help the safety of flying, especially in long distance and commercial flights.—‘Listener In.’ ACOUSTIC PROPERTIES. Acoustic properties i na concert hall or theatre can make or spoil a performance, but it is not generally known that the acoustics of the rbom in which radio sets are to be placed have a great deal to do with the kind of results that are to be expected. The results of a good receiver, placed in a position where it has to work at an acoustical disadvantage by either the position in the room or the placing of the furniture, may be entirely spoiled. Dead spots or places where waves from different directions meet and neutralise each other, and echo, due to tho bouncing back and forth from wall to wall, nr from ceiling to floor, of sound waves, must bo avoided. To prove that these acoustical faults exist, try talking or playing a musical instrument in an unfurnished house.or room. The echo is due to the fact that there are no objects, furniture, draperies, rugs, or curtains to absorb the sound. To assure the absence of echo, a room should be well furnished. Rugs, draperies, curtains, and table covers help, but paper or cloth, walls are best. Otherwise a rough-coated wall is better than a smooth one, and a dull, dark iinish better than a glossy, light one. Beamed ceilings' and brick fireplaces are assets, while tile is a liability. There should be>no piano or stringed instrument in the same room as the radio. The strings of such instruments, repiesenting the frequencies of the sounds emanating from the loud speaker, will vibrate, distorting the sound from the radio. If the loud speaker is placed near a piano and the radio suddenly turned off, one may hear the piano strings quite clearly. If a piano must be in the same room its soft pedal should bo pressed down. The loud speaker should face a curtain or other absorbing material. Since waves go out from the back of the speaker as well, it would not do harm to have a tapestry on the wall behind the cone. But pictures that are liable to rattle in answer to their natural frequencies should be avoided. STRAY SPARKS. Last Sunday's concert from 4YA was quite a welcome change from the nature of concerts we have been given lately. The relays on line nights that none' of us want to hear for the reason that vve can attend the concerts in person or tho gramophone recitals when concerts have been cancelled, have come in for a considerable amount of comment lately, and, therefore. the performance of the Woolston Band from tho studio was decidedly more attractive than previous Sunday music. It was doubly enjoyable, beeause we could not otherwise hear the hand (unless wo attended tho quickstep competition this afternoon) and also because it was new to the majority of us. The 10 o’clock closing on Sunday instead of 9.15 was also what the listeners wanted. It is to be hoped that now the corner has been turned in this latter direction, at any rate, 4YA will not turn round and come back. * « « * The A.B.C. has commenced a new system of announcing, and the various Items are now weaved into a story. When Paul Jeagle’s band appeared at OLO recently a symphony in bine was worked out, each item containing tho word “ blue,’.’ such as ‘My Blue Heaven,’ etc An Australian magazine comments that tho new system certainly adds a spice of freshment to the programmes, and adds that, provided the announcer is not called upon in painting too much atmosphere, it should be greatly appreciated by listener’s.

Summer is generally a dull period for rtidio. Now that the winter months are approaching listeners _ are beginning to look to their radio receivers, and one or two dealers in town report a briskfer business now than a month or two ago. The a.c. set is becoming increasingly popular, and is fast replacing xne old type of battery receiver. At the present rate of progress there should be a big demand for radio this winter. > • • • In co-operation with the, civic commissioners and the Professional Musicians’ Union, the A.B.C. is conducting a series of orchestral concerts in which unemployed professional musicians are taking part. If the public support is sufficient the concerts will continue for some time. « * * ♦ Rebroadcasting by the B'stations in Dunedin is occasionally attempted sometimes with great success and sometimes with poor results, which is only to be expected. Last Sunday ZM treated listeners to a short rebroadcast of the American station, KGY, and reports to hand indicate that this was one of the clearest rebroadcasts yet carried out. The speaking was very distinct. * * * • In the in America a wireless listener is termed a “ dialer.” * * • * Something new in the way of compliments reached the 8.8. C. recently, when the matron of a Liverpool hospital referred to the words of a woman patient who once said: “ We do so like the musical programmes. They send us to sleep.” , * * * * The synchronisation of clocks by wireless is a natural development upon the older and widely used method of synchronisation by land line, but iip to the present few attempts appear to have been made to use wireless for this purpose. An interesting device is, however, in use at the home of a London clock department manager. He has applied his invention to the “ Pul-symetic ” system of electric clocks manufactured by his firm Eight electric clocks are at present controlled by the device, which operates on the well-known six-dot Greenwich time signal broadcast from Daventry A four valve receiver is employed with a small aerial permanently turned to SXX. The set is switched on automatically by one of the impulse clocks some seconds before the 10.30 a.m. time signal. The last dot impulse, in addition to effecting synchronisation, automatically switches off the valves, while an electrical switch automatically earths the aerial except for the brief period when the valves are lit. The installation has operated without any attention whatever for the last six months, and the maximum error recorded has not exceeded one second fast or slow.