THE WIRELESS WORLD

Otago Daily Times, Issue 21431, 4 September 1931, 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 nfatter should reach this office' by Tuesday of each week for insertion" on the following Friday. Metres.

ANSWERS TO CORRESPONDENTS. “Meter,” Ravensbourne. — (1) You can procure what you require at .Messrs Kempthorfte, Prosser, and Go., Ltd., Stafford street. (2) The current taken by the voltmeter is so small that the voltage drop through the regulating resistance is inappreciable. Hence the apparently high reading. Connect the charger to the battery, put it tinder load, and then measure the output voltage. E. M. S., Clyde.—Assuming that your batteries are 0.K., I should say that the trouble lies in your .aerial. See that the aerial lead is not touching the platelead, as this would result in capacity feed-back. Plate and grid leads too • close together will; also cause feed-back. Try an outdoor aerial. 0. R. M‘C., Dunedin.—ln its usually accepted sense, “power output” means the amount of audio frequency or signal energy which can be handed on to the loud-speaker. Plate dissipation, anode voltage, or similar terms refer only to the, output of d.c. energy consumed in the plate circuit (i.e., plate amps, by volts). This is always greater than power output. For example, a valve consuming eight watts in the anode circuit may only be able to transfer signal energy to the extent of 1.5 watt without distortion. “ Coil,” Ranfurly,—ln spacing out the components for. a, set have particular regard to the coils, because it is in them that the constructor moat frequently goes wrong, Remethber that every coil (even the so-called “ fieldless ” coils) when in operation is surrounded by a field of force, and "that consequently interaction between, one coil and another ia taking place. ’ The extent of this interaction (or " coupling”) depends upon the distance apart of tne. colls, and also upon, their directions dn : relation to one another—whether: their,, axes are in line, parallel, or at right angles. Naturally, the further coils the less coupling, for given 'pbsitions of the coils will be a 1 minimum,, whilst, if the axes 'are in line, the l coupling will probably be a maximum. •

" Quality,” Hillgrove.—Primary, 1610 turns 28 gauge . (centre tapped); secondary, 117 turns 18 gauge. fhe primary impedance would be 7600 ohms. The core you suggest would be right. STATION 4ZL. Station 4ZL has been carrying out a series,, of testa recently, and _ it is expected that: this popular; station will be on the , air in a week ;or; 10- days, TELEPHONY FROM SCOW. The first veeeel on the New Zealand coast to'be fitted with a wireless telephony apparatus is the auxiliary scow Echo, which trades between Blenheim and ' Wellington. , The vessel has been allotted the call-sign ZMJD, and her transmitter has a daylight range of about 100 miles. It usee three valves—oscillator, modulator, and speech amplifier —and current is derived from , the ship's lighting set, which operates a dynamotor supplying plate current at 500 volts. The output is .40 watts and the wavelength’ used is 198 metres or 1515 kilocycles. This is. just off the broadcast -band, so that the Echo can be heard talking by most listendrs-in. The other vessels of Messrs Eckford and Company's fleet are to be similarly equipped. The firm has the distinction of having introduced the first full motor , ship to New Zealand waters, and' is how'pioneering the way with the first ships fitted with telephony. . The equipment makes provision for the emergency use of Morse code signals on the continuous wave system. ’ INVENTOR OP MICROPHONE. Several English newspapers ' recently published articles about David,'Hughes, the inventor of the microphone, who was born in London 100 yearsyago. In ,1877 he invented the microphone^'and the e.arly models of the Hughes microphone can be seen in the Science ; Museum : at South Kensington, From 1879 to -1886 Hughes concentrated, on experiments in wireless, and he eventually succeeded in transmitting over a distance of 1500 feet. These early researches were dismissed as more or less unimportant by the president of the Royal Society,, and Hughes discontinued hia experimente. But eight years later the German, scientist Hei'tz discovered the electro-magnetic wave, which had been mathematically discovered by Clerk Maxwell. Hughes was undoubtedly a true radio pioneer, worthy to rank with Lodge,' Hertz, Fleming, Marconi, and others. SCREEN GRID AND GRID BIAS. The matter, of grid current in amplifying valves is an important one. So many screen-grid high-frequency stages are used without grid bias that it is necessary when possible to avoid valves which pass a fair grid current when the grid bias is zero. Some valves pass much more than others at this point, and you would therefore expect ' the performance to suffer with a valve current as compared with another passing a lesser current. There are considerable ditfer-j ences between the grid current characteristics of valves, and these are reflected to an ■ extent in the _ other characteristics. If you just plug in one valve, note the results, and then, try another valve and obtain different results; ' grid current may be playing a part. It has been noted that a valve having a heavy grid current at zero bias tends to broaden the tuning. and to “ deaden ” the circuit. When grid bias is used, such as negative .9 volt, the results are not likely to be so different. In fact, provided there is no grid current at this bins, the valve having most at zero bias will probably provide the strongest signals. CONDENSER SPECIFICATIONS. Condensers used for radio work have to have a certain safety margin, that is, they have to be able to stand up to an actual test voltage considerably higher than the voltage at which they are meant to operate in ordinary working conditions. The margin of safety differs a good deal according to the type of condenser and the * kind of voltage with which it is to be used, and a good deal upon the quality of the condenser and the repute of the manufacturer. Clearly, a reliable firm will take every precaution to avoid the possibility of any of their condensers breaking down in use, whilst a cheapjack condenser, such as some of the foreign ones which find their way.upon the market, may be made only “ to sell,” and with little or no' regard for its reliability in service. In regard to condensers, almost more than any other component in the set. you should be careful to go in for reliability, because in many positions the very purpose of the condenser is , to safeguard some other component, such as a valve or loudspeaker, or, indeed, even to safeguard the user of the set, which is yourself! LACK OF SENSITIVITY. Often a receiver will perform fairly satisfactorily, but will lack the sensitivity or the selectivity that might be expected. Generally the fault is due to the, bad design or careless construction of the tuned circuits or the throwing of excessive loads upon such circuits. In the grid circuit of an r.f. stage there are three possible sources of loss, viz., the

coil, the tuning condenser, and the valve. The reduction o£ any losses occurring in the coil must be beneficial, although it must not be forgotten that this may lead to instability and, in extreme cases, to loss of the high notes of the transmission. The high frequency resistance of a coil depends upon many factors, amongst which are its physical dimensions, type of wire, the nature of the material in the former upon which it is wound, the proximity of other components or shielding material, and the spacing of lends or terminals. As a general rule, the larger a coil is the better it is, but improvements may be effected in other ways, and if it is Hot possible to include a larger coil the question of winding an improved unit of the same size should be considered. Preferably the ratio of length to diameter should be lees than unity, a winding of IJin on a 2in former being a suitable proportion. An increase of size, of course, means an increase in the external field, and hence it may be necessary to extend the screening, if any. The tuning condenser is in parallel with the coil, and thus any loss of energy which takes place in it must be added to that which is wasted in the coil. If the condenser has been made by a manufacturer - with a good reputation it is not likely that its losses will amount to much, but sometimes a definite improvement may be noted by substituting another. The valve and its cap are in a similar position to the tuning condenser, but here little can be done beyond replacing it by one of beam' qua- ay or characteristics. The socket, too. must not be forgotten, and its replacement by one designed to avoid losses as far as possible will always assist in- the general improvement. These points must be considered in every grid circuit, and since the losses there are always much greater than in the primary circuits, the plate circuits may be practically neglected. If the detector operates on the leaky grid principle, it is probable that reaction is provided, in which case there is not the need to investigate fully all possible sources of loss, because regeneration will bring about a reduction in the resistance of this circuit, but the detector plate circuit cannot be entirely neglected. Here there may be reverse reaction through the capacity of tbe valve itself, which has the effect of throwing a load on the grid circuit. Rectification efficiency may be improved by connecting a larger capacity between the plate and filament circuits, but care must be exercised, because too large a value will bring about a reduction in the high-note response. The importance of the points mentioned above may be gauged from the folowing example:—ln a 2|in coil of about 60 turns of No. 20 s.w.g. wire, which is of comparatively heavy gauge for this purpose, the actual resistance of the wire is about a third of an ohm. At a wavelength of 300 metres the r.f. resistance of such a coil would be about 4 ohms, due to the loss ot energy caused by the creation of eddy currents in the turns of the coil itself. In addition to this, there is the loss of energy in the material on which the coil is wound, the tuning condenser, the loading of the valve, and its associated parts, so that ultimately the r.f. resistance may by 10 ohms, or 30 times the d.c. resistance. The grid leak should be placed across the grid condenser, and not from the grid to filament plus or cathode, where it would be in parallel with the coil, especially in a power grid detector, where the leak is probably about 250,000 ohms, and is, therefore, comparable in value with the dynamic resistance of the coil.

•7ZL, Hobart ( . . 535 3AR, Melbourne • • . . 484 4YA, Dunedin 2FC, Sydney . 2YA, Wellington . 420 4QG, Brisbane 3LO, Melbourne .. • • . . 371 2AB, Sydney , 310 2KY, Sydney .. . . 280