THE WIRELESS WORLD

Otago Daily Times, Issue 20271, 2 December 1927, Page 5

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 ol each week for insertion on the following Friday.

ANSWERS TO CORRESPONDENTS. “ Beginner,” Becks. —For 1.0005 variable condenser use 12 fixed and 11 moving plates, with a spacing of l-Bin; 1.00025 condenser will require sis fixed and five moving plates, with the same spacing. Regret the delay in replying to your query. “J.li.” Dunedin.—Station SLEW is Burbank, California. It has a power of 250 watts, and operates on a wave-length of 229 metres.

“ J.T.,” Palmerston.—l regret I cannot specify any particular firm. There are, however, several reputable radio firms in Dunedin who would doubtless advise you. , “ D.F.W.,” Green Island. —I should think you are taking the B battery voltage reading with a faulty voltometer. This is a mistake, as the amount of current necessary to operate them is often quite sufficient to operate a receiver for a number of hours, besides giving a faulty reading. “ Inquirer,” St. Clair.—The howling in your audio-frequency valves might be cured by separating all wiring as far as possible to reduce inductive effects. Place the transformers further apart, and arrange their wirings at right angles n possible. Also by reversing the connections to on or more of the primary windings of the audio transformers, and also experiment with connections from the iron cores of the transformers, either to earth or to the positive of the high-tension bat- “ Accumulator,” Gamaru writes: -(1) In mixing red read and letharge for both plus and minus plates, what specific gravity of acid would you recommend. (2) After the paste has been pressed into the grids and allowed to dry, what s.g. of acid would be correct to put into cells for charging? (3) Would a 60-watt lamp through a chemical rectifier be suitable for charging a battery of 120 volts with plates sin by Jin?—(l) Dilute sulphuric acid not stronger than 1190. A small quantity of glycerine may also be used; it will held the paste to bind the grids. (2) It is advisable to place them in weak acid first. After the: have been formed on the weaker acid, new acid of 1240 s.g. should be used. (3) A 60-watt lamp will be necessary with a home-made accumulator.

MB MARCUSE TO BROADCAST. Advice has been received that Mr G. Marcuse will in future be transmitting on a wave length of 32.5 meters on Sunday and Tuesday mornings at 6 o clock, and on Sunday evenings between 6 and o o’clock, Greenwich mean time.

RATTLING IN CONE SPEAKERS. Many complain of rattling in cone speakers when a voice reaches a high pitch or more volume is turned on. this is not an uncommon failing in some cone speakers, and usually denotes that the current in the speaker is too heavy, if you are not using an output transformer or filter between the output of the set and the speaker, you should insert one. Such a device prevents the heavy direct current from the power source from entering the speaker windings, while allowing the pulsating currents of the reproduced signals to go through the speaker. Any cone speaker will soon deteriorate if heavy “ B “ power is used on the speaker without the transformer or filter being inserted in between.

RADIO “NEWSPAPERS.” Russia has its own radio “newspaper.” This newspaper is heard, not read. It has a complete news bulletin of 11 sections, the material being condensed. It has replaced manv of the printed newspapers, and is available to many millions daily The service is limited to 5000 words a day, so careful sifting of material is necessary. It has its own editor and staff of reporters. The 11 sections are news from abroad, from various republics, from Moscow, scientific and technical, books, theatres, workmen’s clubs, letters of criti cism, answers to correspondents, sport,folk songs. The real value of this type of transmission is rather doubtful, as it was very marked in England during the last strike, when news was broadcast that most of the people who walked to tl\p various depots where the news service was written out and pasted up for the benefit of nonlisteners were people who owned sets, but had an unsatisfied feeling with the spoken word, and desired to see and not to hear the news.

LIGHT AND CRYSTAL DETECTORS. The effect of light on crystal detectors is very curious, though not. more so *than the characteristic detecting action of the crystal itself, which has never been satisfactorily eplained. Most crystals will work better if strong light, such as sun light focussed by means of a lens, be allowed to fall upon them. The light from a tungsten lamp may similarly be used. Galena behaves in this way, and so do most samples of pyrites. Carborundum and telurium mixtures, on the other hand, do not seem to be affected by light. There is here a very interesting field for experiment by the enthusiastic amateur.

ADVANCE IN TELEVISION. John L. Baird, television inventor, of England, has closed hi s experimental transmitter, and is building a much larger station with a four-kilowatt output at Purley, in Surrey. The first work of the new station will be experimental, but Mr Baird is hopeful that actual televisor pro grammes will be presented to the public ere long, according to reports from London. Alteration in the apparatus have overcame the necessity of using two wave lengths, and now the pictures can be sent out on one wave. Receiving outposts are being installed at Dublin, Belfast, Liverpool, and Manchester, with the object of testing television over long distances Hitherto the longest distance over which television transmissions have been received is 130 miles.

A USEFUL SAFETY VALVE. Those who are bothered by the thunderous crashes of nearby and powerful signals will find some degree of relief through the simple expedient of inserting a vari able high resistance between antenna and ground binding posts of the receiving set, thus by-passing the nearby or high voltage signals more or less to the ground, according to the resistance setting, and bringing the signal strength down to within the limitations of the receiver. When the usual signals are tuned in, the weaker signals, having less voltage, are not by-passed so readily through the high resistance, it is claimed, and therefore retain most of their power. This is not a wave trap or tuning scheme, it will be noted, but just a sort of safety valve to prevent the roof from being blown off the house when extra loud signals are being received.

SHORT-WAVE FADING. A peculiarity which often occurs in the reception of snort-wave telephony, especially from stations at a distance, is the phenomenon of fading. On some nights this may not be noticeable, the signal strength being quite steady. At other times the signals will vary from practical inaudibility up to full telephone strength within a short space of time, the signal strength continuously rising and falling This phenomenon may lead those who are not acquainted with it to suspect a fault in tfie set or its accessories, whereas, of course, no alterations made there can effect a cure. It will be as well, how ever, to make sure that there is no fault at the receiving end giving rise to a somewhat similar effect. Owing to the sharp tuning necessary on the short waves, very

slight variations in the inductance of capacity of the tuned circuits will suffice to upset the tuning. Swinging of the aerial wire will give a very fair imitation of the “ fading ” phenomenon. This effect will be particularly noticeable if the aerial runs close to surrounding objects such as trees or buildings, signals appearing to “come and go” as the aerial swings Backwards and forwards. This fault can usually be distinguished from the true fading effect, because the aerial swing will normally produce much quicker and more regular variations in signal strength. A simple test may also be applied if the aerial is suspected. A strong signal should be tuned in, and the aerial disconnected from the set. By slight retuning it will be possible to hear the station still, though, of course, more weakly. If the signals are now steady, while connecting thj aerial again produces the swing, uhe aerial is obviously to blame. It will be noticed, if the aerial wire is swinging, that all signals are more or less equally affected, whereas the fading phenomenon is much more noticeable on the transmissions from more distant sources.

LOUD SPEAKERS ON CRYSTAL. Probably there are but few listeners who realise that a crystal receiver is capable of working a small loud speaker at sufficient strength to entertain a room full of people (writes “0. P.,” in the Radio Record). That this means greater enjoyment than the usual “taking turns with the ’phones ” goes without question. This applies to practically all crystal receivers within two miles or so of the Auckland and Christchurch stations, and within five to 10 miles of the new 2YA Wellington station. The most important part of any crystal set is the aerial. It must be as long and as high as possible. Listeners should always remember that signal strength is directly proportional to the height of their aerials. Also the heavier the wire (in gauge) the better. The Post Office makes no restrictions whatever on height or length of aerials. One hundred feet should be used at the least. Two hundred feet or even more would be ideal, especially if stranded copper wire is used. Next "in importance is the earth connection. It is essential that this be the best possible. It is not sufficient uo merely twist the wire a few times round the water pipe or the tap. If the water pipe is used, the earth .wire (preferably thick insulated wire) should be tightly soldered to it. Care must be taken to see that the water pipe actually goes into the ground within a short distance of where the earth wire is connected to it. Otherwise it would be better to solder a wire to a pipe three or four feet long, and then drive it into the ground just outside. In this case keep the ground always damp by tipping water over it occasionally. Where possible take your lead-in off the highest end of your aerial and always have everything well insulated. Any well made home-constructed set or anv standard crystal receiver should be capable of working the loud speaker. Of course a big loud speaker suitable for a multi-valve set must not be expected to give satisfactory results at any distance from the broadcasting station. There are plenty of small loud speakers suitable for the purpose which can be obtained for a very slight outlay In the writer’s opinion the ideal receiver is one which has a fixed crystal contact (commonly called a permanent detector) with a potentiometer and a small li volt battery to bring the crystal up to its most sensitive condition. Carborundum for preference as this is generally recognised as one of the most stable detectors known. Another point—when buying crystals, the dealer will probably offer specimens at graded prices. Some crystals give as much volume as some valves, the dealer has tested them, and. the price is an indication of their quality. Take the best! However,' your own crystal set should do the work. Give it a trial.

RADIO IN THE COUNTRY. Considerable attention is being given of late to the conditions under whicn broadcast service is obtained in country districts, and to the measures desirable for the improvement of the services. it is only right that special attention should be given to the subject, because the, city and suburban listeners are well catered for. The service in country districts—say over 100 miles or more from the city—cannot be considered satisfactory, if we assess a good service as one reliably received, and “ reliably received ' means that the speech and music, or the ‘ signal ” as it is termed technically is louder than the noises which affect the receivers. Those noises may be our old friend atmospherics or may be th? compratively newer acquaintance—radiation and induction energy from the post office telephone apparatus used in exchanges, or interfering energy from electric power systems. These noises or noise energy may be sufficiently strong to affect the quality of broadcast music received. The noise is as great or, greater than the signal. The aim of broadcast engineers is to ensure that a safe ratio between the two will be maintained. This signal to noise ratio should be such that the signal will always b e greater than the noise. The ideal condition is that when the signal is as much louder than the noise as phonograph music is louder than the needle scratch. Such a condition is obtainable only in receivers located near the broadcasting station. In country districts it is impossible with ordinary conditions of atmospherics in summer. _ We may assume that the noise “ level or strength throughout the Dominion is uniformly 10 units. Tlie signal strength near the broadcast station may be 100 units; at a distance of 20 miles away ( t may be 50 units; and gradually falling away until at 100 miles it is only 10. At that point it has a moderate chance only of giving some sort of good service, but further on when, say, it is down to _5, then we know it is down below the noise level—the noise is twice as strong as the signal with consequent and well known annoyance to listeners. No efforts by the owner of the receiver can improve the conditions considerably. There are many methods of attempting to cut out atmos pherics, but they all result in cutting down the signal strength as well. The only real remedy is to bring the signal strength up. This can only be done bv increasing the power of the transmitting station greatlv or by a ‘ local ” retransmitting or rebroadcastiug station. As discussed recently, the more satisfactory method is the latter one. These country stations—whether we call them relay stations, retransmitting stations, rebroadcasting stations, or simultaneous broadcasting stations doesn’t matter—are urgeritlv reauired. The last named definition is perhaps the best, because the purpose of the stations is to broadcast on their own respective wave lengths the programme that is broadcast by the main station. The main station is connected bv a telephone circuit to the country station and simultaneously the programme is broadcast bv the main and the country station.