The Choice of a Receiving Aerial

Radio Record, Volume II, Issue 22, 14 December 1928, Page 8

The Choice of a Receiving Aerial

gt) some a discussion on the type of aerial they shall use is futile-they have ix: nothing but Hobson's ay Yas, Choice. But the average -_ me jistener has a_ choice, -even if only one out of two. say, indoor and outdoor, and to this section those few remarks are addressed. With conditions so variable it is impossible to lay down hard and fast rules about aerials for wireless reception. One has always to consider the type of set he is using, his locality, and his proximity to the broadcasting station. The first problem, however, that confronts fhe intending licensee is, "Shall I erect an aerial or use an inside antenna?" (antenna being a more ‘suitable. term for inside aerial). Great things are claimed for indoor antenna, but except in exceptional cases they are inferior to a good outside aerial. Quite frequently a letter comes from @ correspondent: "My set works better from an indoor... aerial, than an outdoor. No difference is noticed when I connect up: my aerial." Reeently such.a letfer was answered by an Enelish paper by a pithy statement: "Then there is something wrong with your set or your aerial." A long technical discussion of why this should be so is uncalled for here. The reader, anxious to get on with his aerial, wants facts, so considering that the greater majority of readers are going to erect an aerial they will be considered first. The loop will be considered in a future issue,

Kind of Wire. : A NOVICE intends to erect an verial, His first problem is, What kind of wire shall he use-feunc-ing Wire, strunded wire, bare, covered or enamelled, or plain copper wire?

The first type can be readily dismissed, for compared with copper, galranised iron is not a good conductor. The main problem lies in the selection among the copper wires. Hlectricity travels along the outside of each wire. or strand, so that the greater the sur-

fuce the better the conductor, this would: place the stranded before the plain. Unprotected stranded wire will corrode and tend to cover in the extra surface provided by the stranded wire. In this way it looses its efficiency. Then the wire should be protected. If each wire in the cable were protected from its neighbour, the’ collecting surface would be at 2 maximum, so that wire with each strand insulated would be the best. This would he rather expensive so that a compromise is made and each strand evamelled. Summing up, it seems that the best wire is stranded and enamelled say 7/22. The Length and Height of the Aerial. HE next consideration is length. This depends on two factors: (a) type of set to be used; (b) location of receiver. A. Type of set to be used. If a short-wave set is to be operated a short high aerial should be used, but we will not dwell on that point, for most beginners use something more simple than the short-wave receiver. The aerial for the broadcast receiver depends on the number of valves. In general the longer the’ aerial, the greater the signal strength, the more the noise and the flatter the tuning will be. With wv crystal or a small yalve set where only the local station is desired, uw long aerial should be employed, where

possible, say 100 to 150 feet, and even 200 feet would not be too great. Three valves using reaction will find 100 to 120 feet quite sufficient. _ Four valves with one stage of radio amplification 80 to 100 feet. Vive-valvers will find 60 to Td feet ample, while six valves and more require nothing longer than 40 to 50 feet. In each case the lead-in is added to the length of the actual aerial. Country listeners will no doubt have: little difficulty in securing these lengths, but the city owner will probably have trouble. His best plan is to.get as near as possible to the length given that is providing he is not troubled with interference. B. Loecality.-The city owner will find that a long aerial will collect more noise from power-lines, ete., than signals, so that he may have to shorten the serial considerably. If selectivity is required a short aerial, say, 45 to 50 feet, employed with a wavetrap, should be the optimum length for the city dweller. TNO reduce static two aerials might well be employed, a short, low one and a high, long one, with a changeover switch. When static is bad it is futile to try and listen to distant stations, the local is bad enough, especially if the receiver is any distance from it. . SEATIC, as was stated in last week’s article, decreases rapidly: as the earth is neared, hence the endeavour to work from underground antenna. Signal strength decreases, .though not to the same extent as does the static. Hence to overcome the difficulty use a lower, shorter aerial on. the local station in times when this interference is bad. By a low aerial is meant 10 to 15 feet from the ground. Type of Aerial. (THERE are. several types of aerial, the inverted L, T, multiple wire hoop and cage are the best known. The pros. and cons. of the L and T were fully considered in our issue of November 2, 1928, and space will not allow of reiteration. Readers will do well to turn back to this number, as directional effect is fully considered-and it is important. . Multiple wires are effective -: separated by six feet and separated by hard wood supports. Likewise the hoop aerial, where the wires are separated by hoops of insulator substance. Another type was suggested by a correspondent in our issue of Decem-

ber 7. It consists of a single high mast, with aerial wires radiating like the ribs of an open umbrella. They are insulated from the top of the mast and well above the earth. As the lead-in is attached to each the effective height is the distance of the aerial wire from the nearest earthed object. The aerial would, if not raised to considerable height, act as a very low aerial. However, it would be very suited to congested city areas. Erecting the Aerial. HAVING now decided the dimensions of the aerial, the next procedure is to select masts, "What kind of masts shall I use?’ Two factors should be-born in mind-neatness and efficiency. A beginner will usually glance round his section to find whether there are any trees, or chimneys, handy to attach the wire. This may be all right,

put there are a few points to be borne in mind. The effective height of an aerial is its distance from the nearest earthed object. Thus.an aerial passing over a roof would in reality be only a low aerial. The remedy is to connect the aerial by means of a support, insulated at both ends to the chimney. This should be long enough to clear the roof. To the farther insulator, attach the aerial wire. For this reason, too, the actual collecting aerial wire should be some distance from the mast. Six feet is none too much. Where aerials are slung between two hills or two similar objects the actual aerial should commence well out from the support. (a) Trees as aerial masts. An article has already appeared in a previous issue concerning trees as aerials, but a word of warning repeated would not

be amiss. Trees form good aerials, especially if high, providing the collecting wire is well away from the tree, say, twenty or thirty feet, at the least, and that the supports are attached to a branch not likely to sway. A swaying aerial gives an unpleasant, rising and falling effect. Wooden masts. A good mast can be made from 4 x 2 and 3.x 2 timper. For the first twenty feet. use 4 x », then taper to the 3 x 2, but provide at least two sets of stays, one set of four at the very top, and the next set at the join. If these are insulated, by egg insulaters near the mast, and near the ground, so much the better. A diagram is given, showing how a good base can be made, allowing the aerial to be collapsed merely by removing the bolt, and slackening the . stays. (b) Steel or galvanised iron pipe masts These are usually the neatest and easiest to erect, but should be well provided with stays. Commence with, say, 2 water pipe of 3 inches diameter, and gradually reduce till the top few feet is 1 inch diameter. The collecting wire should be well away from the mast, and the support well insulated from the mast. The stays, too, should be insulated from mast and ground. (c) Tead-in, This should be soldered either to the end, or to the middle of the collecting wire, and should be heavily insulated when nearing any earthed object. It is inadvisable to drill a hole in the wall to pass the lead in to the set, rather procure a fiat strip of copper, well insulated, and pass it under a window. Similarly with the earth wire. From the above summary, it will be evident that the best aerial is a single wire, well insulated from nearby objects, and supported by lofty masts. Facilities to keep the aerial taut should be provided, as shown,

Various Hints. T is unwise to share an aerial, as the more powerful set is almost certain to rob the weaker of the waves. A gompromise is to break the wire by a series of insulators, and to run a lead from each half. " In congested areas, it is wise to erect aerials at right angles to one another. ‘This will minimise interference by radiation. For the same reason, aerials should run at right angles to the power lines, even if it means a shorter aerial. The effect will be noticed, especially if a sensitive receivyer is used. There is little danger from lightning, but in order that the reader may not be the one in a thousand whose house is destroyed, and who loses all insurance, purchase and install a lightning arrester. Some firms are selling these, very cheaply, at present, and all receivers must be provided with one.

Further extracts from the Fire Underwriters’ Rules are :- 1. Aerial. (a) Aerial or counterpoise or part thereof outside of buildings together with any stay wire in connection therewith shall be so located in respect of aerial conductors of either power or telephone lines that contact by swinging, sagging or breaking is impossible. ( b) Aerial and counterpoise outside (c) buildings and any stay wire in connection therewith shall be so located in respect of aerial power conductors that a person cannot make simultaneous contact with them. When the span does not exceed 100 feet the size of the aerial

(d) Supporting structures for ae (c) feet the aerial wire shall not b wire shall have a cross sectiong) area of not less than 0.0045 s in. (7/.029"") or No. 14 S.W.G Where the span exceeds 10 less than 0.0070 sq. in. (7/.036") ials shall be of ample strengtl and securely fixed in position b? staying or otherwise. ' Indoor aerials must not twisted round .or attached ft any electric wiring or fittings or to gas fititngs.