ON THE AIR

Hawera Star, Volume XLVI, 11 September 1926, Page 8

ON THE AIR

NOTES FOR LISTENEKS-IN.

(Conducted by “Wave-Length.”) In order that this column may render the maximum of service to wireless enthusiasts, “WaveLength” invites correspondence from fellow listener's. Accounts of reception, new stations heard, items especially appreciated and the like, will make interesting and often helpful reading for others. Address “Wave-Length,” c/o Hawera Star, Hawera. Advice has been received that 3LO Melbourne, will put out, a special pro gramme for Xew Zealand listeners, the date of which will be announced in the near future Located at Southbrook, Canterbury Mr .T. Williamson uses his superhet t< good effect. To date he has logged 2-1 American broadcasters, all at goo-1 strength on the loud speaker. Aus Italian and Xew Zealand stations conn in without any aerial. It has been announced that 3LO Melbourne, is to move into it nev. studio on the top floor of the Cam bridge Building, rt is anticipated that the new studio will have a marked improvement in the quality of the transmission.

W EEK-EXT) .PR OGR AMM E S

4QG —At 0.30 o’clock to-night (Xew Zealand time), Brisbane will relay a musical programme from the Gaiety Theatre, followed, at 10.30 p.m.. by an hour’s dance music from Lennon s ballroom. To-morrow, both morning and eveniii.j- the complete service will be relayed" from All Saints Church of England (12 30 and 8.45 p.m., Xew Zealand time). In the afternoon -at 4.45 and in the evening, after church (10 p.m.) concerts by the Excelsior and Greater Brisbane Municipal Conceit bands will be broadcast. °TK —To-night Wellington will relay from the Town Hall the final demonstration concert -of winners in the Competitions Festival. To-morrow night a broadcast of the evening service from the Terrace Congregation -Church will be followed by a Municipal Band recital.

THE THEORY OF TUNING

Many amateurs are puzzled to know how it is that though, there may be a lar-T- number of wireless stations in one neighbourhood, all operating at the same time it is possible for the transmission from one. of these stations to be received without any sign ot the other transmissions being heard, even though the signals being received may be much loss powerful than those from other stations also transmitting. It is generally known that the medium by which this selection of a given station m exclusion of all other stations is accomplished is known as tuning, but just how tuning permits this selection to be made is rather a mystery to many. A good idea of what actually happens can be obtained by considering what takes- place when two children are playing on a swing. One of the children ii’et-s on to the swing, and the other begins to push on the sndng to set it in motion. At first the swing will move very little, but in about a minute it will have “picked up” a very great amplitude of motion. The individual impulses impressed on the swing were relatively weak, and one of them could have'made very little impression on it. But a long succession of pushes was able to store up so much energy in the swing, which is merely a form of pendulum, that it was able to swing over a wide arc. Now, the mere communication of pushes to the swing would not in itself be sufficient to set it in strong motion. The important thing is that the pushes were timed and imparted at just the right moment. A swing of a given length lias a certain definite period of “swing” or, as it is more commonly termed, vibration, and irrespective "of the weight on the seat, or the amplitude over which it is swinging this period of vibration is constant. It is. in fact, on this principle that a clock is controlled by a the impulses had been given at the wrong moment, for instance, if the swing had been pushed as it was moving towards the person imparting the pushes, instead of just as it was moving away from him, strong vibration would never have been built up. In fact, the pushes must be “tuned” in period to correspond with the natural period of the swins;. The action can die demonstrated quite easily by making a pendulum with a weight and a piece of string. It can be readily shown that the rate at which the pendulum will vibrate will be determined by its length, the number of vibrations a minute being increased by reducing the length of the string, and reduced by increasing this length. It can also be shown that properly timed but very feeble impulses will set the pendulum into a state of strong vibration, while strong but improperly timed impulses will not have any effect in setting it ihto a state of harmonic motion.

Those illustrations provide an exact parallel with what happens when, a wireless message is transmitted and tuned. The sending of a wireless message consists essentially of transmitting electric impulses from one point’ to another. In tin; transmitting aerial are surging, vibrating, or oscillating electric currents, of a very definite period. The period, or the number of surges a second, depends on the wave length of transmission, or, more correctly, the wave length of transmission depends on the number of surges. The more electric surges there are in. the aerial in a second the shorter is the wave length of transmission. For instance, when 3LO is working on its wave length of 371 metres currents are surging up and down the aerial about 809,000 times a second. To produce the longer 484metre wave length used by 3AR, how ever, the number of electric vibrations a second in the aerial is only about 020,000. 'Plie number of surges in the aerial a second is the same as the number of waves produced in a second, so that, at the receiver each wave that arrives can be regarded as an individual impulse. Now', the receiver is like the swing of the pendulum. If if has impressed upon it a series of properly timed electric impulses a series of electric surges of corresponding frequency can bo set up in it. Hut. if it is subjected to a series of impulses to which it is not timed, or, as it is known, tuned, then no appreciable currents will be generated in the receiving circuits, even though the impulses to which the system is subjected are quite powerful.