NEWS AND NOTES

Radio Record, Volume I, Issue 21, 9 December 1927, Page 8

NEWS AND NOTES

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It is always better to use. wire stays | and wire halyards for radio aeriais, Rope contracts in wet weather, and stretches in dry weather. Galvanised wire lasts better than unprepared rope, It has been calculated that even the best receiving aerial does not receive more than about one-trillionth of the power radiated by a broadcast sta. tion, dven in the vicinity. An aerial connected directly to earth is capable of receiving radio waves of a wave-length about four times its length of wire. Thus a single wire 100 feet in length can receive waves of a wave-length of 400 feet, without the addition of the usual tuning inductances in a wireless set, which increase or decrease the wave-length at will, A metre is equivalent to 3,28 feet, or, roughly, 8 feet 34 inches. The broadcast waves picked up by your aerial travel only along the surface exterior of the aerial wire, and do not penetrate to the core of the wire. ‘This phenomenon is known as *‘skin effect." An aerial which is badly corroded subjects the radio waves to a certain amount of resistance, In some home-made "B" elim inators, especially when the transformers and chokes are wound by the constructor, there is apt to be a lot of heat generated. never enclose a home-made eliminator in an ait-tight cabinet. If vou must use a cabinet, provide an ample number of holes in its sides to allow good ventilation. If a "B" eliminator transformer is allowed to heat up, it may burn out the windings and blow the fuses, Ventilation will keep the heat at a minimum and often prevent such an occurrence, Though radio waves can pass through bricks, mortar, concrete, and wood, metais have the same effect upon them as mitfrors have upon light, deflecting them temporarily from their natural course, The best way to connect the eatth wire to a waterpipe is to clean the surface of the pipe thoroughly by scraping, filing, or sandpapering. The end of the earth wire should then be cleaned the same way, and wrapped two or three times around the waterpipe. It is then essential that the wire be firmly soldered to the waterpipe. Valves should not be burned too brightly or their lives will be shortened, Turn on the rheostats only so far as the point at which best repro- duction is obtained. ‘To turn the rheostats further than this is to shorten the lives of the valves without any benefit. By operating the filament of a valve at 95 per cent. of its rated voltage, its life will be doubled, An outdoor aerial will become heayily oxidised within the space of a year. This oxidation reduces the over-all efficiency of the aerial system; and should therefore be cleaned off every six months or so, Here is a simple and effective way of doing it. Purchase a shilling’s worth of muriatic acid, and mix it with three parts of water in a large earthenware dish. Coil the aerial wire into a toll and place it in the dish. Leave it there for about an hour aua stir the solution three or four times every fifteen minutes or so, Next prepare a strong solution of soda and water in another large dish and place the roll of wire in this to neutralise the acid. Let the wire remain in this solution for about ten minutes; then take it out and scrub off any remaining scales of oxidation with an old tooth You will find the wire to be in the same clean condition as when purchased. An indoor loop aerial to tune to the wavelengths of the New Zealand and Australian broadcast stations can be made with a diameter of 4 feet to carry seven turns of wire spaced half an inch apart and to be operated with a .0005 mfd. variable condenser in parallel, High resistance headphones are sensitive because they are wound with a very large uumber of turns of wire, and not as a natural result of their high ohmic resistance. This latter is simply the result of their requiring to be wound with fine wire to pack the numerous turns into a small space. and is no advantage in itself, The more ambitious of cxperimenters will find it interesting and instructive to test out crystals of different minerals for sensitivity. While thousands of crystals and combinations of crystals have been tried out, a new combination may accidentally he discovered that will bring results well worth the trouble, Now, jou hoys, get busy! The "earth" wire should be kept as short as possible, otherwise, not only is the resistance of the aerial-earth circuit as a whole increased, but useless inductance is added to the Ccirenit, necessitating the use of a smaller number of turns in the tuning, inductance, without the compensating advantage of any additional energy collected by the earth wire. ~

If an accumulator is rated at, say, 60 ampere hours capacity it will deliver a current of one ampere for 60 hours, or 2 amperes for 30 hours, or 4 amperes for 15 hours, It is advisable, however, never to perinit a wet battery to become more than three quarters discharged, if a long life is desired from the battery. It is not advisable to attempt to mse more than two stages of transformer audio amplification in a receiving set, as the inherent reaction effects in the circuit become more pronounced, and produce a tendency to self-oscillation at audible frequencies. This selfoscillation creates the sound known as ‘thowling.’’ The query is often put as to whether using two crystal detectors simultaneously on one set is better than one crystal. Little or no advantage is gained by employing two crystal detectors, for, although more complete rectification might be obtained, the added resistance in the circnit would cut down the current flowing. It is the effective height of an aerial that counts, and not always the height above ground. If an aerial is 15 feet above the galvanised irom voof of a two-storied lofty residence the ef- fective height of the aerial would be only 15 feet, and not 35 or 45 feet, which is its height above the ground. The aerial’s effective height is only its height above the objects oyer which it passes. Jacks frequently occasion trouble by reason of one of the contact leaves or springs failing to make contact, or being so closely spaced that a spark discharge takes place. The little points on the leaves of a jack will corrode through even a small amount of gas fumes reaching them. Sea or salt air will also cause corrosion. These points should be cleaned gently with the finest gluss-paper occasionally.

. If you operate a crystal detector { keep your fingers off the mineral. Handling the crystal ‘leaves a greasy surface, spoils its sensitivity, and it is more apt to collect dust, Always use tweezers to lift.a crystal, . _A well-known fan remarked at the mectitg of the Wellington Kadio Society the other evening: ‘I am_ satisfied that nearly ail the interference from howling valves one hears at night originaies in home built sets." The Melbourne ‘‘I,istener In" states: "The majority of the reports of reception of inter-States with wrystal receivers are freak receptions put up under conditions where results would have been astonishing on a valve re- . ceiver. as well as a crystal. The majority of these reports are aiso from country listeners who are not troubled with interference and are able to put up aerials of length up to hundreds of feet. They hold the advantage over the suburban listener, who lias to limit the size of jhis aerial so that he can separate the local stations.’

-_- habbit elle ‘a Bm 2s a ee ee oe ee OR Hfome-builders should) always temember that space is the best insulator for the high frequency curreuts used in radio. Keep the wires well apart and avoid trouble iu the set, To form a good "earth where a waterpipe is not available, a couple of kerosene or benzine tins filled with a mixture of finely-crushed coke and calcium chloride should be buried two or three feet below the surface of thie ground after they have been soldered together and the "earth’’ wire also soldered to one of them. ‘The ground should afterwards be kept moist by pouring water upon it. Some listeners wonder why the enamel on the aerial wire does not prevent the broadcast waves from reaching and travelling along thie aerial, Electro-magnetic waves pass freely through ail insulating substances, but once they strike an aerial they create an oscillating current in the aerial, which is then restrained by insulation. A gas-pipe should never be used for an "earth." Owing to the number of "red-leaded" joints which usually occur in the average household gas-pipes the conductivity is poor. A more important objection is the risk of fire owing to the possibility of a heavy static charge induced in the aerial by a passing thundercloud. Rust forming on nickel may be readily removed by first greasing the atlicle, and, after a few days, rubbing it with a rag whicii has been soaked in ammonia. If the rust does not come off readily under the treatment, add a few drops of hydrochloric acid to the ammonia and rub the object well, Then rinse with water, Home-builders should keep the wires off the base board as much as possibile. Run each lead direct and short. Don’t try to see how many faucy square corners you can make. [iliciency is the main rule in witing up a set. If a particular wire can be made extremely short but does not fool neat, do mot mind the appearance; just make the wire as short as possible, —

A good, long, well-insulated indoor aerial is better for long-distance reception than an indoor loop aerial. Nei ther of these, however, can compare: with even a poor: outdoor aerial. A first-rate aerial should be about 40: feet above all obects and about 145 feet in length, including the lead-in. Tins is a fairly long aerial and not likely to be selective, but it will serve splendidly for reaching out for the Australian broadcast: stations. Radio valves are a. delicate mechanism, Handle them gently and do not jar them by dumping your receiving set down on the table. A little care and with nightly use your valves will give efficient reception for upwards of a year, and even two years. If you must use an indoor aerial wound around the walls of your room see that the aerial is enamelled or otherwise insulated, There are heavy leakages when an uninsulated wire is. used for this purpose. Keep all batteries out of the sun, for a high temperature: is bad for all types of batteries, wet or dry. If your batteries are near a window and the sunshine falls on them, cover them up with a newspaper, or, better still, an old: cloth. If 250,000,000 atoms of topper or gold were lined up in a row, like marbles, just tonching each other, the row would be only an inch in length. But an electron, which is a smali atom of electricity, has a diameter of probably only one hundred-thousandth of that of an atom. Resistance is the opposition to the flow of an electric current through a conducting medium. All metals have more or less electrical resistance. Copper is used universally for both electrical’ and radio work on account of its low resistance, comparatively low cost and ready supply. If a T shaped aerial is used the leadin should be attached to the very centre of the aerial, otherwise the aerial will consist of two unequal parts, each with its own wave-length. In the latter case anything like sharp tuning will be impossible, since the two sets of oscillations will fight each other. When a battery is said to be suffering from sulphation, it means that the plates have become coated in places with an insoluble deposit (basic lead sulphate) which reduces their active surfaces. Sulphation is caused throughs neglect; a battery should not be left long in a discharged condition, but it should be recharged as soon as possible at the correct charging rate. If the natural wavelength of an aerial is 400 metres and it is desired to receive waves of about 30) metres, the natural wavelength of the aerial has to be artificially reduced by the use of a small series condenser, which appreciably lessens the efficiency. In practice the best possible resnits are obtained when the natural wavelength of the aerial is from two-thirds to three-quarters the wavelength which it is desired to receive. Some home-builders like to put lettering on the front panels of their sets. The surface of the panel should first be cleaned with wood alcoho} to remove anv film of grease. The lettering can then be put on the panel with a steel pen and draftsmen’s white ink. When the lettering is dry it should be covered with a protective coat of transparent varnish, using a fine camel’s hair brush. A rag damped in wood alcohol will remove the lettering at any time. If an audio amplifying valve persists in howling, try attaching a wire from the negative terminal of the "A" battery to the earth post of the set. If the howling continues, try earthing the iron core of the audio transformers by attaching a wire to the core and bringing it to the earth terminai of the set. Use insulated wire for this purpose to avoid the risk of a shortcircuit. The audio frequency transformers should be placed about four inches apart, and mounted with the windings at right angles to each other. Make the grid and plate wires as short as possible, and keep them apart. See that no wires are running too close or parallel, ‘Try connecting the cores of the audio frequency transformers to the ground binding post. . Many of the noises in a radio set can be traced to the batteries. Because these noises resemble static it is seldom that any other source for them is thought of. Poor or dirty battery connections cause more ‘‘static" than any other one fault. ‘tA’? battery connections should be made by means of a clip which can be made to grip the terminals of the battery. It is preferable that the ‘B’’ battery connections also be made by means of 4 smajler clip than used on the storage battery; these clips should have a very strong gripping power. Never use oldescraps of wire to connect up the "A" or the "B" battery to the set, or either the input or output circuits of the "B" eliminator. If a short-circuit should occur, you may find some damaged apparatus, a badly run-down battery, or even a fire started by the heat generated at the shortcircuit. Buy good, heavily-insulated wire and, if a break in the insulation should occur, replace the entire wire. Do not attempt to splice in another short piece, as you will probably only | run into more trouble.