RADIO

Southland Times, Issue 20147, 6 April 1927, Page 14

RADIO

NEWS AND NOTES (By “Reflex.”) DECEIVING CONDITIONS. During the last few weeks reception has been quite satisfactory locally. Except for a few evenings when static has been prevalent there has been ample variety from Australian and New Zealand stations from which the listener may choose his evening’s entertainment. Auckland and Christchurch have of course been the most popular but Dunedin has also put satisfactory transmissions on the air. The Wellingtno station continues with its mediocre transmissions but an improvement can be looked for in this direction in the very near future. Speaking from 3YA on a recent evening the Prime Minister (Rt. Hon. J. G. Coates) said that when in England he had the opportunity of viewing the equipment for the new 5000-watt broadcasting station to be erected there. “I have every confidence,” he said, “that when it is completed New Zealand will have a station secodd to none in the Southern Hemisphere.” Such statements are indeed highly gratifying to Maoriland listeners, who have had reason from time to time to complain of the poor treatment they were receiving. About forty men are now at work on the buildings at Wellington and the station should be in full swing before the winter has far advanced.

The American stations are once more making themselves heard in the evenings, provided static is not furious, and an exceptionally good winter can confidently be looked forward to. The number of receiving licenses issued by the Post and Telegraph officials is increasing month by month and with this increase further improvements can be made in the services from the New Zealand stations as the result of the additional revenue. HOWLING. Readers will be indebted to Mr. R. G. Meldrum for the useful hints given through the correspondence columns of the Southland Times in connection with the howling valve. Owing to the absence of “Reflex” the matter has been left in abeyance until the present titme. The attitude of “Listener-in” cannot be altogether commended. The step he proposes should be made —that the P. and T. Department should refuse to grant a license to the owner until his set is converted into a non-radiating set—is very drastic and likely to bar many younger enthusiasts from following up the game. It is often the simplest thing which is the cause of a great deal of the trouble. For example, leakage across the base of the valve sockets often causes peculiar • whistling sounds, which can be remedied iby carefully cleaning them. Another very i important point (often neglected) is to ' keep the vanes of the variable condenser (free from dust. Faulty windings in the i headphones, transformers, or loud speaker, ■ or a few broken strands of the loud speaker • cord, may cause high and shrill whistles. I Reaction used in excess, or improperly and j unskilfully used, causes much unnecessary annoyance. So much has been said in previous * supplements about this subject that it is only proposed here to give a fairly good and reliable test for oscillation. Touch the aerial terminal with the finger; if a click is heard in the ’phones, both when the terminal is touched and when the finger is removed, then the set may reasonably be supposed to be oscillating. This can be remedied by reducing the coupling of the reaction coil or by reducing the filament current of plate voltage. Just a little experimenting will often ensure smooth working. To those who are really beginners building their own sets on the unit system, perhaps a few’ words of advice may prove helpful and prevent any disappointment and overhauling of the set when finished. I A golden rule is to keep all H.T. and , L. T. wires well apart. \ I All wires to plate and grid connections i should be as nearly at right angles as ‘ possible. (Take all connections the shortest route.) The aerial wire should be away from the earth terminal; and, incidentally, both of these should be isolated from all battery leads. Probably the most frequent cause of howling is* imperfect amplification, rundown A and B batteries, etc. An unsuitable valve for the grid leak will often cause howling, and a little time spent in finding the most suitable one is not wasted. Badly-soldered joints from the amplifier to the battery are yet another cause. The H.T. battery’ should also be suspected. Keep it perfectly clean. Howls may also be due to dirty wander-plugs. CIRCUIT NOISES. Every set owner naturally begins to look for static disturbances with the advent of warm weiwher, but when the familiar crackle is ushered in with the first approach of winter the only deduction he can draw is that atmospheric disturbances are not purely a function of heat and moisture after all. On the following morning he asks his nearest radio neighbour whether or not he was troubled with static on the previous evening. This fan who has purchased a new battery vigorously denies that he had any’ trouble, and describes several new stations heard by him as “clear as a bell.” Strange thing this static business, particularly’ when two men live within a 50 yard radius and use exactly the same type of radio receivers. There follows a few nights more of the same sort of grief, and then our party of the first part buckles down to a serious investigation of the problem. On testing his B battery simply as a matter of routine he discovers that it is way down and buys a new one. Further, he charges his A battery to the standard full charge mark of ■ 12.75, and when all of these operations i have been completed he tunes in again. Gone is the static, the scratching, the crashes, the rattles and the grating that was formerly assumed to be static. —Sun. AERIAL EFFICIENCY. At long last listeners are beginning to realise that the aerial has something to do with reception and a lot to do with the performance of the set they have made or purchased. The time has passed when a wire 100 feet long was considered the acme of prefection. The length of the aerial is an important factor. A long aerial tends to absorb energy from the set and prevent oscillation. A long aerial is, in effect, an absorption or loading circuit. It will not permit tuning of the modern set to very low wave lengths.

For shorter wave lengths a short aerial is required. A short aerial also lends pep to any set. It may be made too short and the radio frequency tubes are made to oscillate. On sets thoretically constructed so as not to oscillate the short aerial raises havoc.

The height of the aerial depends upon surrounding objects. If possible get above them say of 20 or 30 feet. In wide open spaces there is no particular advantage in going much over 40 feet above the ground, although increased height does add some to selectivity and signal strength from weak stations.

If selectivity is to be obtained, a counterpoise under the aerial and close to the ground seems preferable. In case a ground is used a good ground is essential. This can only be found by experiment.

METAL MASTS. A striking feature of broadcasting development in New Zealand has been the limited field for the use of metal masts for supporting aerials. Apparently there is some prejudice against the use of metal masts on the ground that they are likely to impair signal strength. As a matter of fact a metal mast is no more likely to reduce the strength of wireless signals than a wooden mast fitted with a sWel halyard, or a wooden mast stayed with wires. Although, in some cases wooden masts are cheaper and more easily obtained than metal masts, many listeners who go* to the expense of purchasing wooden masts have enough lengths of old water-pipe lying about their homes to make excellent supports for their aerials.

Ordinary lin. water-pipe, if properly stayed, is quite strong enough to make a mast 30ft. or 40ft. in height, which is as high as the average broadcasting aerial should be. The chief difficulty in using a pipe mast is the tendency of the pipe to bend while the mast is being erected. By making full use of the guy wires while the mast is being raised into position, however, this tendency can be corrected easily. Sufficient assistance should be available for all the guy wires to be manned. In attaching the guy wires to a pipe mast the guys should be split by insulators placed in the wires as close to the mast as possible. POWER VALVES. T

Results from a great many first-class radio receivers are often marred because the owners of the sets try to force them to deliver an output volume which considerably exceeds the output which the receiver is capable of handling without distortion. In most cases this distortion is due entirely to the overloading of the final valve of the receiver. Although most ordinary receiving valves are capable of giving a great deal of volume when used in the final stage of the amplifier, greatly improved results will almost invariably be obtained by providing a special power amplifying valve in the last stage. This applies particularly to dry battery operated receivers in which such valves as the .06ampere consumption type are used. To obtain satisfactory volume with these valves a certain amount of forcing at the expense of clarity is essential. Ihis applies also to ordinary valves of the .1-ampere and .25ampere classes when the signals are as loud as those obtained in Auckland when listening to IYA.

The use of a power valve in the last stage of amplification will not as is frequently supposed, give louder signals than the ordinary valve, but will certainly give louder undistorted signals. The general clarity will be considerably improved and bass notes, so frequently absent in receiving sets, will be heard. With a power valve a grid-biassing battery, a component too frequently omitted from the broadcast receiver, is essential. It is slightly more expensive to operate than an ordinary receiving valve. The filament current required is usually higher and so is the drain on the B-battery. For irMamce, the writer uses a most excellent English valve having a filament current of I-ampere at 4 volts. The plate current is 14 milliamperes with 100 volts on the plate, as compared with 6 milliamperes drawn from the B-battery by an ordinary valve. This current drain is, however, of comparatively little importance in these days of heavy duty B-batteries and B-battery eliminators. BEAM TESTS. Preliminary reports on the last official tests of the Australian-England beam wireless service indicate that the operation of the service has been even more successful than was expected when it was decided to replace the high-power long-wave system for Empire communication with the beam system, writes “Thermion” in the New Zealand Herald. The tests have demon-

strated thtat a wireless system operating over a distance of almost 12,000 miles is capable of attaining considerably higher speeds than the most perfect system of land line working over very much shorter distances.

So far, the maximum speed obtained with the beam system has been 325 words a minute, which is almost double the speed considered highly satisfactory on trunk telegraph services of average length. The

actual transmitting and receiving equipment associated with the beam system is capable of a maximum speed of 400 words a minute, and although the wireless section of the service has not yet been capable of the associated apparatus, there is every reason to expect that developments in the next few months will make the capacity of the service the speed at which the signalling apparatus can be worked, and not the speed at which the wireless equipment itself will operate. In fact, this seems so certain that attempts are already being made to design automatic transmitting and receiving equipment capable of a speed of 1000 words a minute.

Another striking feature of the tests is the extremely long period daily for which communication could be maintained. Although the specifications for the service prescribe a period of communication of only seven hours daily, the tests have shown that signals can be received throughout the 24 hours. For a period of three or four hours about mid-day, however, fading occurred which made commercial operation unsatisfactory. The short period of each day in which signalling is interrupted by fading is likely to vary considerably at different times of the year. BURNED OUT VALVES. The chief cause of burned out tubes is a weak filament unless the tube has been abused, states a writer in the Sun. Seldom, however, does such a tube last any length of time at all. The weakness has been with it from the beginning. More frequently they burn out after an accidental passing of the B battery through the filament. This is possible froc crossing grid and plate wires, it minus B connects to positive A and there is no condenser in the grid circuit. The only other possibility is crossing the plate wires with the filament wires.

It is also entirely possible to burn out a filament properly connected by putting the plate voltage too high. This is not apt to happen with the standard tungsten filament tubes, but may with coated filament tubes, that is, those tubes having barium or strontium oxides on their filament to reduce thte temperature of filament necessary for high emission or electrons. Particularly if such tubes have some degree of gas in them, the phenomenon known as ionization takes place. In all tubes there is some gas and there is gas in the metal of the parts within the evacuated space* Too high a plate voltage causes the gas to ionize. The posititve parts of the ionized gas travel toward the filament, the negative parts go toward the plate. The bombardment of the filament heats it, usually in a weak spot, and the emulative effect of heating and bombardment results in a break. It may even result in a broken tube.

Luckily this does not happen often. It is more frequent in experimental work where several hundred volts are available than in must sets where not over 90 to 120 volts are needed. Fortunately, too most of the storage battery tubes available are of a type which do not ionize at any reasonable voltage. The one exception is the soft detector tube still used for its sensitivity, but this is always used at a very low voltage and ionization results in a very apparent blue glow and complete cessation of the tube operation. After a short rest such a tube will operate as well as ever.