THE WIRELESS WORLD

Otago Daily Times, Issue 21733, 26 August 1932, Page 4

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 2UL. Sydney.—Bss K.O. 353 met res. 2FC, Sydney.—oos K.C., 422 metres. 3AU Melbourne—o2o K.C.. 484 metres 3LO Melbourne. —830 K C.. 371 metres SCL. Adelaide. —730 K.C.. 305 metres 4QG Brisbane.—7(iO K.C., 385 metres. IVA. Auckland —B7O 'K.C.. 333 metres Every day except Monday 2YA Wellington —72 UK. 0.. 420 metres Every day. 1 3YA. Christchurch UBo U.C.. KOfl metres Every day except Tuesday. 4VA. Dunedin.—oso K.C., 403 metres Every day except Thursday. 4ZM, Dunedin —IOBO K.C.. ,277.8 metres Tuesdays and Saturdays. 4ZO. Dunedin. —IOBO K.C.. 277.8 metres Monday and Friday. 4ZB. Dunedin,—loßo K.C.. 277.8 metres Wednesday. Thursday, and Sunday morning

. ANSWERS TO CORRESPONDENTS T R! F., Otekaike.— (1) Customs Dept.. Bond street, Dunedin. (2) Under good conditions, yes. (3) Messrs Whitcombo and Tombs, Ltd., Princes street, Dunedin.

“Homebuilt,” Mataura.—From what you say it would appear that yon have gone astray in assembling the set. Go over your valve connections, see that the voltages on the screening grids of tjie valves are correct, that the valves are properly decoupled, and that your choke is O.K.

“ Zen.,” Omarama. —The usual formula can be used, but without an oscillator it is virtually impossible to secure accuracy. One hundred and eighty turns on a lin former are approximately correct for the lower frequency, and about 70 for the higher. It is impossible to make exact determinations.

“ Superhet,” Dunedin—When a superheterodyne receiver works normally at the lower end of ,the wavelength scale, but fails altogether to receive signals at wavelengths greater than, say, 450 metres, you have almost certain evidence that the emission of the oscillator valve has fallen off. The effect is one that is fairly common, and is due'to the fact that with the usual fixed inductive relationship between grid and plate coils, the effective coupling falls off progressively as the wavelength is increased. Unless the valve is in good condition self-oscillation may stop suddenly when ,a critical wavelength is reached.

NOVEL RADIO ITEM. , The Radio Bellringers and Xylophone Quintet, who will be heard oyer the air /from 4YA to-morrow evening, are probably the only party of bellringers in New Zealand to-day, and additional, interest will he lent to their performance by the fact that the bells which they use , were the property of a troupe of bellringers who toured New Zealand nearly 30 years ago. Although some 20 years ago bell music was a popular form of entertainment, it is doubtful if the present generation of music-lovers have any idea of the delightful harmonies that can be produced by bells properly played; to-morrow s performance, therefore, _ should provide a unique event in the history of radio music in the Dominion. , SIDE-BAND JAMMING. Side-band jamming, as heard in a loudspeaker, is not unlike the noise made by a chorus of grasshoppers, but the cause of the noise is not always realised, writes Noel Ashbridge, chief engineer to the 8.8. C., in Popular Wireless. Let us assume that we have two stations separated by nine kilocycles, each modulating with frequencies up to 9000 cycles on either side of their respective carrier waves, and assume that the receiver is capable of giving good response up to 9000 cycles. One will hear, first of all, the heterodyne between the carriers of the two stations, but this is really not a serious difficulty, because it can be cut out by a filter without interfering very much with the quality. However, we also have the sidehands of the two stations heterodynim; together, in addition to one side-band of the unwanted carrier heterodying with the carrier of the wanted station. The latter is the cause of what I have described as the grasshopper noise, in other words, sideband jamming. The heterodyning of the side-bands with each other does not seem to contribute much to this noise, because the volume of interference is usually about the same, whether the wanted station is modulating or not. COIL WINDINGS. Some set owners have probably removed the covers from the tuning coils and have noticed that the winding is divided into two portions, in that near the top end about' 12 turns, more or less, are not close to the main number. The reason for tins lies in the fact that copper wire, when being drawn through the steel die plates down to the required gauge, varies. With a new die the wire is the minimum diameter, but this increases as the holes in the plate wear slightly. It is therefore necessary to allow for a length of coil winding, for a given number of turns, sufficient to allow for wire which .is a fraction over the correct diameter, ihe rule is then to drill starting and finishing holes at exact positions evenly spaced. Then if a thin wire happens to ho used, some of the turns are pushed up to one end. The number shifted depends upon the result of a test against a standard coil kept for the purpose. If the inductance is 100 high.' more turns are separated from the main lot until the test shows the correct tuning value. The closer the turns are to each other the greater i« (he inductance for a given number of turns, and so the finer, wire must he treated by separating some of the turns slightly, but still being kept within definite points. TRANSFORMER TAPPINGS. It sometimes happens that none of tinprimary tappings on a power transformer correspond exactly with the rated voltage, of the mains supply available to the user. In such cases there is a natural uncertainty as to which terminal connection should lie made. Taking the practical case of a transformer with provision for inputs of 200, 220, 230, and 240 volts, and a supply of 210 volts, it will bo seen that the 200 and 220 volt tappings differ by a similar amount from the supply voltage. It should be remembered that, if connection be made to the 200 volt terminal, it is to he expected that all the secondary voltage outputs will be slightly high, and so, to bo on the safe side, it is usual to employ the 220 volt terminal. This decision ~will certainly be right if it is known that (ho mains voltage is generally rather over than under its rated value. If, however. it is found that the anode current of all the valves seem to be low (anode current measurements are the only pertinent ones in such eases that are practicable for the amateur with ordinary apparatus), a change to the 200 volt terminal would be indicated. -

EVOLUTION OF CRYSTAL SETS. A noticeable feature of New Zealand development, ways the Coverage Report, lias been a steady evolution from the crystal receiver—once familiar in tbe vicinity of broadcast stations —to more elaborate and efficient types. In the early days of New Zealand broadcasting the percentage of crystal sets was approximately 50, whereas to-day it has fallen

to about 14 per cent. This may be attributed largely to a natural desire to reach out beyond the local station and to participate in the., variety afforded by other stations in New Zealand, as well as in countries such as Australia and America, which, under favourable circumstances and during certain hours, are well received in New Zealand. From a coverage point of view, it is of interest that the evolution of the high amplification high selectivity receiver and its growing popularity, tend to nullify to a considerable extent, the idcifl held by different countries during the early stages of broadcast development, that reception should be made universally available to crystal sets. On the other hand it must be admitted that the provision of a satisfactory signal-to-noise ratio—where such has not previously been enjoyed—stimulates the entry of new listeners into the .broadcast circle, because of the utility under such improved conditions of lowpowered, low-priced sets. Listener density is thus increased as a direct result of the increased field strength. Experience, however, indicates that a fair proportion of such new listeners will eventually change over to more powerful sets, capable of -tuning in at will to signals from more distant centres. This general tendency is a somewhat significant one in view of the inherent difficulties—both technical and financial—of covering a country such as New Zealand—with its 100,000 square miles of territory and its scattered population of I.soo,ooo—with broadcast signals capable of actuating the simplest types of receivers with which the broadcasting art made its debut. I BY-PASS CONDENSERS. A modern set contains a large number of condensers of various types and sizes. It is not uncommon to find even in the less elaborate types of circuit from 15 to 20 of these units. Many of them are used for by-passing purposes, and in these locations they act with an alternating current as a pure resistance acts with a direct current. In other words, they provide a more or less free path for such currents as it is desired to deflect from the circuit. The property of a condenser which determines rate of flow of alternating current through it is known as “ reactance,” and this decreases as the frequency increases. It is measured in ohms, and the exact figure may be determined by dividing 1.000.000 by 6.28 times the product of the frequency and the capacity in mfd. Thus a condenser of 3m fd has a resistance slightly over a thousand ohms at 50 cycles, and a i reactance of only about one-twelfth of- an ohm at 600 kilocycles (500 metres). Thus we can see the reason for providing large by-pass condensers in the audio frequency part of a set where, for example, a 4mfd condenser will have a reactance of 800 ohms to a current of 50 cycles, while a small condenser may be used when r.f.

currents flow, a .Olmfd condenser, for example. being equivalent to a little more than 25 ohms at 500 metres. Sometimes the selection of a by-pass condenser is a matter of compromise, a fair example being the by-pass condenser from the plate of a detector ■ valve to earth. Here uts object is to provide a virtual short circuit, for high frequency currents which, having been rectified by the detector, are no longer required in the circuit. Therefore, it would seem that as large a capacity as possible should be chosen, for .then its impedance would be low, but if its value is too high the higher notes of the speech or music which have to be passed on to succeeding stages of the/receiver would be likely to pass through it to earth, thus avoiding the loud speaker. These factors have to be balanced as far as possible. For instance, a .OOOSmfd condenser p tiers an impedance of about 450 ohms .'to a 500 metre signal, and about 00,000 ohms to a 5000 cycle note, which is about the highest frequency that can beconveniently handled by broadcasting stations or receivers. This value would probably be satisfactory, although if the detector valve lias a high impedance a smaller capacity (.00025mfd or so) is better. It is, however, not always sufficient in high frequency circuits merely to determine a suitable size, for, although the value may be apparently satisfactory it may bring trouble in such places as the by-pass from the screening grid and plate circuits of r.f. amplifiers because of its inductive properties, due to the fact that in some condensers the electrodes are metal hrd, d rolled up between waxed paper. Naturally the high frequency currents passing through the condenser if they come in at one end of the foil have to traverse the whole winding before the condenser is charged. In effect, they are fed into a coil, and. as is well known, a coil offers considerable impedance to a high frequency current, so that condensers of this type are likely to fail_ in the purpose for which they were originally intended. Any trouble here may be ‘avoided by the use of -what is known as the “ non-in-ductive” condenser, in the design of which not only capacity is considered, but also the residual inductance and resistance, so that by a proper choice of the relative amounts of each, it is possible to obtain a component having a low total impedance ’• at broadcast frequencies. Another important group of by-pass condensers are those which are placed round the resistance by means of which automatic grid bias is obtained. In the high frequency circuits a value of about .Imfd is usually sufficient. Larger values may be used, but too small a value may reduce amplification. In the output valve, where a great variety of frequencies are to be handled, the value of the component may have an effect upon the relative amplification of low and high notes.