THE WIRELESS WORLD

Otago Daily Times, Issue 21579, 26 February 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.—Bss K.C., 853 metres. 2KG, Sydney.—66s K.C., 422 metres. 3AU, Melbourne,—62o K.G., 484 metres 3LO, Melbourne.—B3o K.C., 371 metres SCL, Adelaide.—73o K.G., 395 metres. 4QG Brisbane.—76o K.C., 385 metres. IYA, Auckland.—B7o K.C., 333 metres Every day except Monday. 2YA Wellington,—72o K.C.. 420 metres Every day except Wednesday. 3YA, Christchurch.—9Bo K.G., 306 metres Every day except Tuesday. 4 YA, Dunedin.—6so K.C.. n 463 metres Every (day except Thursday. 4ZM, Dunedin —IOBO K.C., 277.8 metres Tuesdays and Saturdays. 4ZO, Dunedin.—loßo K.C.. 277.8 metres Monday and Friday. 4ZB, Dunedin.—loßo K.C., 277.8 metres Wednesday, Thursday, and Sunday morning.

ANSWERS TO CORRESPONDENTS. G. M. O. G., Dunedin. —The tapping for the aerial will be brought in three turns lower than for .00035 cpndcnser. The usual value for a neutralising condenser is about .00005. Just aft your dealer for. a neutralising, condenser and you will get the right thing. Cone,” 1 Wakari. —It does not harm the set to have it working without a speaker. It all depends on how your set is wired whether the last valve is cut out when you use the first stage jack. In most outfits the first jack cuts out the last valve. If your circuit is of the popular kit set type, then it will be cut out when the first stage is used.

“ Loud-speaker,” Dunedin. —There is little that can be done short of discohnecting the two speakers when not in use. It is a well-known fact that loud-speakers can work backwards, and -instead of producing sound, they can turn sound into electrical vibrations, being heard faintly on - another speaker. Little should be heard, however, unless someone is speaking within a foot or two of the loudspeaker. The process, of course, is reversible. Speech in the kitchen can be in the front rooiji, A CONDENSER HINT.

If a power valve is left fully operating while the control electrode is disconnected from the grid jnas battery, it means that the valve is left “ up in the air ” and the excessive anode current 'which flows in these circumstances may damage the filament by reducing its emission, 'and ruin a perfectly good and expensive power valve. On the other hand, it is naturally a great convenience to be able to adjust the grid bias with the received alive, ns the effects of changes made can then most readily be judged. If condensers of sufficiently large capacity are connected across the grid battery, as is often done in the case of H.T. batteries, then the danger of harm arising by shifting the tappings with the receiver alive is very much minimised. These condensers should be of good make and of large capacity, so as to retain their charge long enough to prevent damage. Of course, it is obvious that the use of condensers only gives you a reasonable time in which to make the grid bias changes, and it is assumed that you aye not going to leave the battery disconnected indefinitely. \ ; GANGING PITFALL. The question of the proximity of metal screens and coils is very important in all cases of H.F. coils, and is particularly important where you are using ganged condensers. If, for instance, you have two ganged condensers and two coils, the condensers having equal capacities and the coils having eqaal inductances, you I may find that whereas the arrangement tunes properly at one part of -the wavelength range it goes out of balance at the other end of the range. Quite possibly you will find that this is due to the too close proximity of one of the condensers to one of the coils, or to some other accidental condition where the presence of a metal plate is upsetting the inductance of one of the coils. ■ This is always a point td look out for, especially if you find, that it is impossible to keep your ganged circuits in balance over any reasonable range of wavelengths. When you consider the .close and compact arrangement of the average modern receiver with all its shields and metal parts, it is obvious that a lot of thought_ has been necessary in order to get the shields and the components all in the right relationship to one another without causing trouble of the kind dis'cussed above. NEW AUSTRALIAN STATION. The new broadcast station . of the Catholic Broadcasting Co., which was opened on Christmas Eve, is notable for many new features. The studio, which is situated at Australia House, Carrington street, Sydney, was personally designed by Mr E. T. Fisk, managing director of A.W.A., and for the first time the sthdio is utilising a new invention of Mr Fisk’s whereby the vibration period can be altered at will, and the studio thus used for all classes of broadcast items. The ( ( larger broadcasting organisations throughout the world have recently designed and are building a multiplicity of studios each specially treated acoustically to a set degree of damping, but in the case of the studio 2SM, Sydney, the same results, dan be got with the one studio. The control equipment of the studio is the latest of its kind, and was delivered /to the studio a few days ago fuora the radio-electric works of I A.W.A. It incorporates all the most up-to-date features required to provide a broadcast service of the highest order. The artistic side of the studio has not been lost sight of by Mr Fisk, and the colour scheme has been very carefully planned and executed. A particular featurd of the studio is the layout of the ■ offices, the reception room, library, and control rooms, all of which are adjacent to the studio. Another innovation of the studio is the installation of the Fisk sound-proof windows, an invention by Mr Fisk which eliminates street noises while at the same time permitting ample ventilation. The transmitter for 2SM was designed by Amalgamated Wireless, and is installed at the A.W.A. radio centre, Pennant Hills. 2SM is the highestpowered B class station in Australia, as it has 1000 watts in 'the aerial, and is operated from an aerial 350 ft in height. / !, NEW WIRELESS BEACON AT . CROYDON. A new 'type of wireless directional beacon station is to be erected at the London air port, Croydon, by the Marconi Company to the order of the Air Ministry. Known as The Marconi visual type course indicator, 1 the new station represents the latest means of assisting aerial navigation by wireless, and it will act as an automatic guide to aircraft approaching and leaving the aerodrome on the Continental route. By watching a signal dial, controlled j by wireless from the beacon station on the ground, the pilot of an aeroplane fitted with a special light-weight beacon receiver will be able to see immediately whether he is on his correct course or whether he has deviated to the right or to the left. The dial, which is connected with the receiver, has a black background on which appear two vertical white lines. If the aircraft deviates to either side of the correct course the white line on that side of the dial increases in length, and the white line on the other side decreases. The instrument thus gives a direct indication of course, and is as simple to read as the familiar speedometer or oil pressure dials. No tuning or adjustments are required, as the beacon , transmitter and the automatic receiver operate on a fixed wavelength. The course indicator transmitting station emits, from an aerial system consisting of two loops arranged at an angle, a “ beam ” emission on each side of the air route, so that an aircraft travelling On the correct route receives the signals-of the two wireless beams at equal strength. On the dial of the instrument in the cockpit the two white lines, which are in fact the ends of a pair of rapidly vibrating reeds, are equal in length—that is, the reeds are both vibrating at the same amplitude. If, however, the aircraft deviates from the correct route, one beam is received at, greater strength, and ah the two beams are modulated at different frequencies, the reed which is tuned to the frequency of the stronger signal vibrates more strongly. The visual line depicted by the end of this reed on the dial thus becomes longer, while tue vibrations of the other reed weaken, and the corresponding visual line shortens. In this way the pilot is immediately warned of his deviation and of the direction in which he should steer for its correction.

In effect, he has only to "steer towards the shorter line ” until the reeds are again balanced, in order to regain his course. The vibration of the reeds is scarcely observable on the dial of the instrument, the visual effect being that of white lines, the relative length of which give the required guidance. OUTPUT VALVES. Of the various forms of coupling between the output valve and the speaker the simplest is that in which the speaker is connected in the plate circuit of the output valve, but this means that its windings must carry the full plate current of the valve.\ A choke-filter circuit or a one-to-one transform’er is to be preferred, because the speaker winding is saved from what is probably a large direct current, and, moreover, in the first ease, by connecting the return lead to the filament or cathode the speech currents aVe diverted'from the battery or power supply circuits, where they ■ may cause trouble. As far as impedance relationship between valve and speaker is concerned there is no difference between the two methods, and if a moving iron type of speaker is used the relationship is generally correct. When a moving coil speaker is used the 'problem needs more attention. The coil i& usually a few turns of relatively stout wire, and its impedance is generally in the vicinity of 10 ohms. It is fairly constant at all frequencies. The connection of such a coil in the plate circuit would result, firstly, in the permanent displacement of the coil by the steady plate current and, secondly, in the transfer of very little signal energy, because of the great disparity Between the coil and valve impedance. It must be remembered that the impedance of the speaker should be about 1.7 times that of the valve for maximum transfer of undistorted energy. A transformer is, therefore, a necessity because by its assistance the effective impedance of the speaker may be brought to any desired value. The impedance of the speaker and transformer combination is then, in effect, multiplied by the square of the ratio of the transformer. For example, a transformer of 10 to 1 ratio makes a 10 ohm winding look like 1000 ohms to the valve. Conversely, if it is decided that a speaker load of 4000 ohms is necessary, and'the coil is 10 ohms, the transformer ratio is found by dividing the required load by' the actual load and taking the square root.. In this case a transformer of 20 to 1 ratio is necessary. With triodes the transformer ratio is . not particularly critical, but when pentodes are used the matter must be given* attention. .The a.c. resistance or impedance is of no value in this case. Some makers give figures for ■what is. called the optimum load for the valve. ' As a rule this ■ is about 10,000 ohms, and this figure is used for obtaining, the transformer yatio. Therefore, if the speaker coil impedance is 8 ohms the ratio is obtained by dividing 10,000 by 8 and taking the square root, in this case giving a ratio of a little over 35 to 1. Although the pentode tends to accentuate the high notes because of its desire to maintaip a constant output current, a Corrector circuit is theoretically not required with a moving coil speaker, but one is generally fitted either as a tone control or permanently. A resistance of 20,000 ohms and a condenser of .02 mfd acrosis the primary will supply the necessary effect if required. With the moving iron type of speaker the pentode, because of the property just mentioned, presents peculiar problems, since such a speaker has high impedance at high frequencies. A good scheme here is to use a step-down transformer of about 2 to 1, and a device to limit the effective impedance of the speaker a/ high frequencies. This may'be done by bridging the speaker by a 5000 ohm resistance and a .05mfd condenser. If the former is variable the combination provides a tone control. Parallel or pushpull stages are matched in the same way, but it is necessary to remember that, with two parallel valves, the effective impedance is half that of a single valve; In push-pull the impedance is twice that of the single valve, so that in parallel pushpull it is brought back to the value of the single valve. ■