THE WIRELESS WORLD

Otago Daily Times, Issue 21643, 13 May 1932, Page 2

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.—3ss K. 0., 353 mt-lres. 2FG. Sydney.—66s K.C., 422 metres. 3AR. Melbourne.—62o K.G., 484 metres3LO. Melbourne.—B3o K.G., 371 metres SGL. Adelaide. —730 K.C., 895 metres 4QG Brisbane. —760 K.C., 385 metres. IVA, Auckland.—B7o K.C., 333 metres Every day except Monday. 2YA Wellington.—72o K.C., 420 metres Every day except Wednesday. 3YA, Christchurch. —980 K.C., 306 metres Every day excepl Tuesday. 4Y a, Dunedin.—oso K.C., 403 metres Every day excepl Thursday. 4ZM, Dunedin —IOBO K.G., 277.8 metres Tuesdays and Saturdays. 4ZO. Dunedin.—loßo K.C.. 277.8 metres Monday and Friday. 4ZB. Dunedin.—-1080 K.C., 277.8 metres Wednesday, Thursday, and Sunday morning ANSWERS TO CORRESPONDENTS. “ Longwave,” Owaka. —Output valves of the battery type, when supplied with low-tension current from the A.C. mains through a step-down transformer, may normally be operated with a grid bias that is slightly in excess of that usually recommended. This is due to the fact that this particular method of filament heating, the “point of departure.” is, in effect, the centre point of the filament, and not its negative end. “Valve,” Temnka.— (1) The choice of a valve would depend on the nature of the filament, grid, and plate supply arrangements. It might be advisable for you to ascertain these details and consult the agents for the speaker. Unless higher plate voltages are available, you may possibly not be able to make the change. (2) The process you mention consists in the elimination of the higher notes, and is, therefore, detrimental to quality, although it can do no physical harm to the receiver. “A and B,” Dunedin.—With a batteryoperated set you should be careful not to use more high-tension current than you need,-for the biggest expense in running such a set is in the provision and renewal of high-tension batteries. One of the most important ways of economising in hightension current is to See that the last valve in the set is properly gridbiased; and a good rule is to give it as much negative bias as it will stand without causing distortion. The stronger the negative bias, the less is the amount of current that this valve consumes. See if you cannot increase the bias of your last valve without spoiling reproduction. ’ GRID DAMPING. There are a good many amateurs who still have a leaning to anode-bend detection, notwithstanding that the ordinary grid-leak detector arrangement can now be operated in such a way as to involve! really very little distortion indeed. One reason for the continued popularity of the anode-bend detection system is that, since the grid is negatively biased, so that there is no grid current flowing through the tuning coil in the grid circuit of the detector, the damping of the grid is avoided. The effect of this is to improve selectivity and, as most readers know perfectly well, this may in some cases make all the difference to the ability of the set to cut out a particular station. When using the anode-bend detector a fairly high impedance should be used in the anode current. Incidentally, if the resistance feed method is used, it is often quite satisfactory to use an L.P. transformer of only comparatively low primary impedance, operating with an anode-bend detector. CHOICE OF LOUD-SPEAKER. The apparent sensitivity of most magnetic speakers is high because of the fact that the moving system is tuned to a frequency between 150 and 350 cycles. This, rise in the middle of the range, in which the fundamental frequencies' of speech . occur,, produces unnatural or “ yappy ” reproduction. It does, however, increase the apparent sensitivity. Recourse is had to the same design practice in the smallest and least expensive electrodynamic types where high apparent sensitivity at the cost of fidelity is wanted. The clear reproduction of the low frequencies, while highly desirable for high-grade reproduction, does have one disadvantage. It has been found in the cheaper type of building construction which is now prevalent that the sound transmission through the thin walls is sufficient at the low-frequency end to introduce the problem of “ cross-talk ’ between rooms. In high grade installations the trend therefore, appears to he toward the use of a speaker in a small attractive cabinet as a part of the room furniture. This also takes care of the problem of baffling the speaker and greatly simplifies the installation of the speaker where the building is already constructed.

AUTOMATIC TUNING

A new method of shortwave radio telephony, demonstrated at Trappes, France, is considered one of the most interesting experiments in transmission in recent years. Experts who gathered at the radio station of Le Materiel lelephonique witnessed a demonstration recently by which the transmitting station automatically kept the receiving station tuned in. The method, which is called the single side-band system, is based on the discovery that only_ a part of the ordinary radio wave _is essential _ lor perfect reception, provided the receiver is kept exactly tuned with the transmitting station. The maintaining of, this special tuning at 18,000,000 vibrations per second baffled radio research men until recently. The secret was found in the system of sending out a special wave, called a pilot wave, for the sole surpose of keeping the receiver in tune. Engineers found that the pilot wave could actually keep both ends of the communication m tunc. A radio circuit was established between the Trappes laboratories and the laboratories of the Spanish National Telephone Company at Madrid, and the radio men reported that it was satisfactory. The new method is reported to be of great advantage, as it permits a station to operate with as little as onesixth of the power necessary to transmit and keep in tune under the present systems. The new system also eliminates much of the distortion now found so troublesome in radio communications. This is made possible by the elimination of the greater part of the radio wave. In addition to this it permits the operation of nearly twice as many radio stations without interference. RECEPTION FROM AMERICA. If the proposed radio changes m America eventuate, reception this winter will be chaotic, at least as far as early evening is concerned. There are at present 15 stations in America using a power of 50,000 watts, 6 of 25,000, one of 15,000, 15 of 10,000, and 30 of 5000. Canada has one of 10,000 and 12 of 5000, while Mexico has one of 75,000, one of 10,000 and three of 5000. This makes a total of over 75 stations on the Continent of America using 5000 watts and over. There are about 700 more stations of lesser power. At present a number of stations are increasing their power to the limit allowed by the Federal Radio Commission at Washington. When one realises that all our New Zealand stations, to say nothing of the Australian, have high-powered stations in America on approximate frequencies, is it any wonder that reception here suffers? (asks a writer in a northern paper). The comparatively high amplification required to bring in our weak stations also brings in power.nl distant ones. Great Britain suffered the same way when the Continental stations increased their powers, although they have a much greater range of frequencies for broadcasting than Australia and Now Zealand have. The building of super-power stations in England and Scotland solved their difficulties, at the same time making for cheaper radio enjoyment, as only two or three valves are required to give the necessary amplification. It is too much to expect the YA stations to be increased in power as much as that.' The time element, of course, prevents a great deal of interference in the same way that Australian stations do not greatly disturb us in the summer months, but most American stations do a lot of testing and give special early morning programmes, which can easily be a great source of annoyance to us.