WIRELESS NEWS

Press, Volume LXVIII, Issue 20494, 12 March 1932, Page 9

WIRELESS NEWS

(By "MAX.") ' One hqars very little of television - these days, though there are stations .which transmit regularly in some parts of; the world. Looking at three reproductions of televised-filins of a footballer running, the differences between the three examples of transmission were most noticeable. The most striking feature was the fact that if the reader • .wero not aware that they were televised 'pictures, he would probably think that the best of tho three was merely a : poor snapshot'. There was no mistaking . the fact that a footballer was repre- « -sented. The second picture was more blurred and the third was very much so, and the outline was very irregular, the result of insufficient impulses per second. Some excellent televising can be. done, but these systems are not commercially suitable, the cost being prohibitive. However, there is no question but, that great progress is being made, and the time will soon come when we shall all see. by wireless, receivers being marketed for the purpose. # # Last Sunday the Iladio Emergency Corps throughout Kew Zealand held a field day. The system adopted was for two portable stations to bo taken out, the most distant ono being considered to bo a light emergency one, and fairly close to this was the support station, which maintained contact with a district station and with the advance station, passing on messages between the two. Tho district station was a fixed station of usual power capable of connecting with the central station operated by Mr W. Ashbridge, ZL2GP, of Wellington. Other districts and subdistricts had their groups of three stations also, and messages had to be passed from one outpost or advance station via the chain, to other outpost stations.. Altogether some one hundred operators were taking part. Prom the results held in Christchurch some most useful lessons havo been learned and when the executive here roceivcs complete reports from the supervisor, Mr Ashbridge, any necessary improvements considered desirable will be put into force. Probably local field days will be held monthly, the outpost , stations going further afield and making tests in country which is..likely .to representwhat will be covered in actual emergency. Amongst the ideas for study is the one for simple two letter groups to cover short complete messages similar to, what. Tyill be used if in action in the . field. This will speed up the handling and transfer of stereotyped messages considerably. ♦ * The first type of wireless valve -wa3 invented by Dr. Fleming, and consisted ;o£ ( simply a bright, emitter filamont and an; anode. Used' as a plain rectifier it was no more efficient than a good crys- i tal, tho circuit being one where positive half-cycles of signal energy caused current to flow by making the anode positive whilst negative half-cycles showed no current. The rectified currents wore passed through a grid-lealj; and condenser combination connected to tho filament side of the circuit and with th&-'Ueadphoi*es between this unit ;and the filament. The writer has used a three electrode valve in a similar manner by joining grid and anode together. This was used some years ago in a reflex circuit and gavo quite good results for experimental purposes. Tho advantages of "diode" detection are being advocated by at .least one leading radio engineer. The grid condenser and 1 leak method, power grid detection and bend detection all have their disadvantages in the ordinary three electrodo and screen grid valves, the bad points being too technical for mention in this column, but including effects upon selectivity, high and low tone empliflcation, etc. Every detector valve has two functions which arc inseparI ably bound together in' these methods, •hamely to rectify the high frequency impulses and also to amplify. If these two features can be separately performed by using two valves, the first to rectify only, better tonal quality will result. Circuits have been developed for this purpose, but a new idea seems to give greater promise of obtaining all. the advantages and no disadvantages. The latest suggestion is to leave the - plate unused nnd utilise the grid alone. A tuned circuit is joined to the grid and filament, a special size of grid condenser acting as a block to direct current when tho grid is positive. From the grid side of this condenser runs an H.F. choke connecting to si resistance which completes the circuit back to the cathode. . This it-si stance is shunted by a condenser nnd the rectified impulses are passed via voltages variations across the resistance, to the grid of the next valve. Better tona,l quality results and distortion and other ill-effects are reduced. The objection is that an extra valve is necessary, and the advantages arc not so great when reaction can be used or is desirable, which is not often the case, however. # # According to "Wireless World," tho Council of the Institution of Electrical Engineers have made the eleventh auard of the Faraday Medal to Sir Oliver Lodfe&. This medal is awarded not more than once in any year for notable Achievements in electrical science. # # "Broak-in," the official of tho N.Z.A.R.T., will contain articles on short wave receiver design by Mr R. A. McLennan, in the next two issues. The other evening Mr McLennan demonstrated to the writer a combination containing several excellent features, the most noticeable being the cut down of background noises and the clarity of tho received signal though the latter was naturally not so strong as before. As was pointed out as well-as proved aurally, even a weak signal is easier to copy if there are no disturbing noises in the background. The main points of interest 'in the forthcoming articles will be published in these columns. The March copy of the "Amateurs' Journal" contained an article for students on the amplification properties of the valve, and also dealt with conductance and mutual conduct- i ance in a manner which makes the sub- )' .icct easy to follow.: It is understood j that other subjects will be dealt with j by the same writer in subsequent issues. » * "Radio News" for March, which is J just to hand, describes a new valve called tho "triple-twin tube." Most of us are familiar with the Loewe valves made in two types, one being a two stage H.F., and the other a three stage (detector and two resistance-capacity : coupled units in one glass envelope). The new valve contains two sets of elements in one bulb, the cathode of the first section being directly joined to the* grid of tho second section, and is the only inside connexion except that tho filaments are in parallel. . It is claimed

Notes From Far and Near

that one unit valve will deliver twelve times the power of a type 245, anft three and a half times the output of a penthode. Special circuits of a comparatively simple nature are employed to produce these results, which, if they live up to the statements, bid fair to become standard. A point which will puzzle the technically inclined until they peruse the article is that the grid or the second section draws grid current on positive half cycles of grid swing, but no distortion occurs. Connexions are for a standard five-prong socket, and in appearance the tube is similar to the usual type or ordinary valve, A maximum plate voltage of 250 is employed to obtain the power given.