RADIO AND ITS RECEIVERS

Sun (Auckland), Volume III, Issue 687, 12 June 1929, Page 14

RADIO AND ITS RECEIVERS

Conducted fcr THE SUN by

R. F. HAYCOCK.

RADIO ON AIRPLANES IMPROVED APPARATUS NEW TYPE OF AERIAL Steady, although perhaps not very rapid, progress is being made ill the adaptation of wireless and receivers for use on aircraft in flight. That communication over very long distances may be maintained by relatively simple apparatus attached to airplanes was first demonstrated in the flight from America of the Southern Cross, when its signals were audible in Australia and New Zealand from the time it left America. Striking though the results obtained with this installation were, it also demonstrated the need for several definite improvements. Of these, the ! most important was a method of keep- j ing the wave-length steady. The | signals from the Southern Cross, although always loud, were often very difficult to read owing to the rapid slight changes in wave-length which occurred throughout each transmission. Hitherto it has been the practice to employ a trailing wire as the aerial for wireless sets on airplanes. Until recently a fairly long wire has been necessary for effective communication, and-the only way the required length could be obtained was to suspend the aerial from below the body of the machine with a weight attached to the end to keep the wire tight. This arrangement has a number of serious disadvantages. In the first place, to avoid the danger of fouling trees and telegraph wires, the aerial must be reeled in before the machine approaches the ground. Even when flying at high altitudes the presence of a trailing wire on the machine introduces an element of danger, because in bad weather, when the machine tends to “bump” and pitch the trailing wire may foul some portion of its body. Such cases have actually occurred, and the danger is considerably increased if the fouling takes place during actual transmission. Then there is a danger of a spark from the electrically charged wire setting fire to the machine. The loose trailing wire Is exceedingly undesirable from a technical point of view, because it contributes almost more than all other factors put together to unsteady signalling. It is impossible to keep the trailing wire “still” with reference to the body of the machine. It swings to and fro, and round in a circular path in such a manner that its position relative to the airplane is constantly changing, and its electrical characteristics change correspondingly. It is this fact that makes stable transmission with a trailing aerial impossible. Important progress in eliminating this trouble has recently been made in experiments on machines belonging to the Imperial Airways in Australia. The trailing wire has been replaced with a pair of tightly stretched wires, one of which runs from the tip of each wing to the tail. Until the development of short-wave sets, such an arrangement would have been impracticable, as the length of wire available would have been insufficient for effective transmission. This disability does not apply, however, when the wave-length of transmission is sufficiently short. The use of this form of aerial has greatly increased the reliability of airplane sets. The arrangement, however, has several disadvantages. The rush of air past the tightly stretched wires while the machine is in flight sets them in vibration, and a certain amount of motion takes place. Moreover, the vibration of the wires is transmitted to the machine itself, and increases the strain on the wings and body. Finally, in case of breakage, the wires would probably foul the controls, and render the airplane unmanageable. To overcome these difficulties, experiments are being made with wires inside the wings of the machine itself. Such an arrangement could not, of course, be employed on all-metal airplanes, but with the fabric and wood structure it is quite practicable, and unless the machine is badly damaged, it provides an aerial which is entirely protected, and which is as permanent as a ship’s aerial. NIGHT FLYING Wireless amplifiers have been applied recently in an interesting manner to make night flying safe. A difficulty formerly encountered was that of operating landing-ground lights in the event of the unexpected arrival of a machine, or of switching on landing lights at. emergency landing grounds at which no staff was employed when a forced landing became necessar3 r . This problem has now been solved by means of the wireless amplifier. The airplane must be fitted with a siren, which will produce a note of a predetermined pitch, usually considerably higher than the note made by the airplane’s motors and propeller. A form of microphone so “tuned” that it will respond only to a note having the same pitch as that of the siren is installed at the airdrome, and by means of an appropriate valve amplifier is connected to an electrical relay that operates the landing-ground lights. When a pilot wishes to turn on the lights he simply turns on the siren. The noise is picked up by the microphone, and after amplification switches on the landing lights. WHEN SET GOES DEAD Experience lias shown that in 95 , per cent, of the cases where the set goes dead after having been in use, defective, loose or broken accessories or connections are found. The valves, batteries, power equipment and their connections, as well as as the speaker and the aerial and ground connections, should always be carefully checked over by a service man who has the esesntiai test instruments.

HIGH BROADCAST FEES FEWER CHURCH SERVICES COMPANY’S DIFFICULTIES ! Church broadcasts have been reduced in number. To the Auckland Presbytery yesterday, the Rev. W. D. Morrison-Sutherland reported that | , difficulty over the number of services ! broadcast came from the Post Office. ; The broadcasting company, lie said, j wanted more speeches by ministers, j The obstacle was in the charges ) made by the Post Office. For suburban churches, these amounted to about £3-1 a year. He thought it would help the company if the Presbytery stated that services at certain suburban churches should be broadcast, and that the charges imposed on the company by the Post Office were too high. The company had made it plain that it would be willing to broadcast from j j only two Presbyterian churches in! the city, instead of three, from two! I Anglican churches and one Methodist. | j Broadcasting was being done from j St. David’s, St. Andrew’s and St. j James’s, and the question was which j of these was to be the church sup- j pressed. One church could solve the problem by paying expenses, but this might not be accepted by the company. The Rev. D. C. Herron remarked that the question apparently involved a payment of £3 5s quarterly, and he thought that the three churches would perhaps be able to share the amount. On Mr. Herron’s motion, it was agreed that the Presbytery did not wish to lose one of the three city services for the sake of establishing a service in the suburbs. Members nevertheless stated their desire that suburban broadcasting be established. CHOOSING A SET QUALITIES TO SEEK There are five paramount features that should he sought In airy receiver by the prospective purchaser. Each feature should he present in a fair degree, but should not be overemphasised if this leads to a sacrifice of any other major point. Sensitivity.—The set must be sufficiently sensitive to give satisfactory loud-speaker operation from all local stations and some of the more powerful distant ones as well. Selectivity.—The receiver should tune sufficiently sharp so that any of the nearer stations can be chosen without interference from the rest, and some of the more powerful distant stations be selected through the locals. Fidelity.—The receiver and loudspeaker should give, enjoyable, accurate reproductions of the programmes, thus enabling the listener to get a fair illusion of being present at the performance. Convenience.—Tuning to all of the nearby stations with satisfactory volume from a single tuning control is desirable, though not essential. Convenience also implies that the receiver shall be thoroughly reliable under service conditions while'requiring a minimum of repairs and replacements. Appearance.—The appearance of the receiver is entirely a matter to please the individual prospective purchaser. BATTERY HINTS If your battery leads have been in use for 12 months or more, it is certainly time they were overhauled if your set is to remain free from crackling noises. Great progress has been made in late years in the design of “mass” accumulator plates for use when the rate of discharge is very low relative to the capacity of the cell. Special plates of the above type—- ' generally known as “mass” type plates —can be left for very long periods in a semi or nearly fully discharged condition without any fear of sulphation.