How to Design and Erect a Successful Aerial

Radio Record, Volume I, Issue 28, 27 January 1928, Page 3

How to Design and Erect a Successful Aerial

Practical application of principles and theories

By

M.[?].R.

E.

N last week’s issue of this 6©coluinn a discussion was commenced on the question of the design of aerial systems, and it was al shown that certain factors of design were theoretically ideal. In this issue the question of the practical application of the principles discussed will be treated. Briefly, the ideal ar--Yalgement was shown to be one in. which the aerial was as high as possible from a geometrical point of view in order that the electrical height (which is less than the former) should be as great a value as possible. ‘I'he length of ilat-top portion was shown to be a function of tuning rather than receptivity and the sensitivity of I, aud I’ designs of aerials insofar as their directional properties are concerned, were dealt with, the ‘I’ acrialbeing shown to be uni-directional in receptivity, while the L aerial was found to be directional in a line from the down-lead end,

AN OBVIOUS COURSE. Now these considerations cre theoretical, especially in the case of the height, because the question where Space is limited is, what is the best arrangement pernissible in terms of the limitations, | Very few people are inclined to go in fer elaborate arYangements, especially when, as was stressed in our last issue, an elaborate aerial and a 4-valve set, and a modest aerial and a 5-yalye set, will give the same results, There might be a temptation to erect a forty-foot mast, but if a thirty-foot tree is available there is an obvious course to pursue-an extra valve is cheaper than a mast. SOLVING THE PROBLEM. Either in town or.country the problem of aerial erection is best solved by selecting the two highest points on the property, such that the distance between them plus the length of the down lead does not exceed 150 feet. As far as possible, the whole length of the wire should be in the same horizontal direction, that is to say, if the flat=a

top portion of the aerial is erected. north and south, the lead-down and inwires should come straight down and not point in any other direction, east and west for instance, especially if the down-leads are of a length comparable to that of the flat-iop, Again it is not good practice to bring the down-leads in an aerial of the design back at an acute angle with respect .to the horizontal portion, but the leads should be taken from the flat-top portion either at the centre (thus miaking a T aerial) or else at such a position as to enable the leads to be dropped vertically to the lead-in tube. . There are all manner of wierd notions in existence regarding the necessity of preserving symmetry of design of aerials in order to obtain balanced flow of currents, but although there js some method in the madness due to interesting phenomena called ‘reflections,’ these notions shoald be forgotten, ‘the preservation of symmetry is neither here nor there when it comes to practical results.

AVOID ACUTE ANGLES, Before leaving the question, however, it would be desirable to clear up definitely another point much debated amougst laymen (aud amongst many people who ought to know better as well, incidentally) and that is, that the down-leads should always be brought down from the higher end of the flattop, where the flat-top portion is not horizontal (that is, parallel with the ground). Remember the necessity to avoid an acute angle of down-lead to flattop, sa that if such a cendition is going to result it would probably be bettér to use a_ wire as yertical as possible with the bottom end connected to the Jeads-in tube and receiver. When an acute angle occurs, although increased, effective, or electrical height is forthcoming, the ‘actual receptivity of the aerial system is reduced, because of the fact that the currents picked up in the downleads, due to ether waves, tend to counter those picked up in the flat-top. If the angle of downlead to flat-top lies between 0 and 80 degrees this will happen to an increasing degree, as the angle becomes more acute. Thus two Wires joined at one end (i.e., in series), _

and running parallel to one another, should be entirely nou-receptive, aud this happeus in practice. MULTIPLE WIRE AERIALS, Reverting to the question of the resistance of an aerial and the necessity to keep any losses at an absolute minimum, it was mentioned in our last issue that it is theoretically desirable to have a number of wires joined in parallel in both the flat-top portion, as well as the down-leads, in order to keep down resistance, and thereby ayoid loss. of signal strength. Here, again, practice decides that a single wire is sufficient for reception, at any rate. As a matter of fact, the reason that mniltiple wire aerials came into being was because ships used them in order to be able to transmit efficiently on 600 metres wavelength. The average height and distance hetween masts of ships called for a standard design of tuning arrangement on a ship’s set, and in order to meet this the number of wires in an aerial Was varied according to mast dimensions. Invariably "men-o’-war used many wires in ‘sqnirrelceage’’ form to cope with the high powers of their transinitters.

To-day we see the majority _ of merchant ships with only one wire for transmission and reception, owing to new and more flexibie designs of tuning arrangements. Here, again, the addition of some amplification in the shape of an extra yalve will make one wire aerial perform with the efficiency oi a four-wire aerial of a greater height, A VEXED QUESTION, The question of the insulation of an aerial is a vexed one. Why should an aerial wire be left bare, and: vet kept insulated from masts, stays, etc.? Why does an aerial wire still work if the wire is covered: all over with insulation? It is unnecessary to retrace the ground covered in last week’s discussion, but it was pointed out that ether wares, in "cutting across the Wires of an aerial, set up electrical | currents, which represent the energy to work the receiver. '" eo seas.

Now, aether waves are capable of penetrating all insuiators without hindrance, but when they strike substances which are conductors they give up their energy in the form of currents, Hence an aerial made of insulated wire is no different to a bare wire, becanse the aether waves ignore the insulation completely. However, it is necessary to preserve the currents induced in the wire intact, in crder to make full use of them, and the aerial is, therefore, well insulated to prevent their escape to earth, except through the receiver. Aerials are very often seen with the flat-top portion composed of bare wire, and the down-leads of insulated wire, the impression being that the flat-top portion of the aerial does the picking"up, and the down-leads are for the purpose of carrying the results into the receiver. Tit view of the preceding explanations, it will be seen that this idea is very wide of the mark, as the down-leads are effective pick-ups, and form part cf the active system of the aerial, PAWNSHOP ADVERTISEMENT. The practice has grown of fixing a metal ball to an aerial with the object of sometimes increasing the pickup of the aerial, reduction of static, reduction of interference, ete. The only actual effect is to make an aerial look like an adyertisemeut for a pawnshop. The metal ball inereases the capacity slightly, has no material effect in reducing the resistance, its effect on static and interference is nil, and dif- J ference in pick-up is only noticeable Where the ball is fixed to the top of a mast (without any flat-top) and a Wire is brought down to the receiver. With or without the ball, slightly increaséal results will be noticeable with a local transmitting station when the ball is in circuit. There are many people who will recount wonderful results with these gadgets. The politest way of disposing of such claims is to say that they do not fit in with observed results measured with scientific instruments. actors governing reception are so varied and legion in numLer that it is a very difficult thing to say detinitely just what improvement

may take place due to some alteration of aerial system, The attual improvement, if any is noted, may be due to something quite different, Without complicated measuring instruments it is impossible to make comparison of results, because ideas of signal strength cannot be carried in the head even for five minutes to justify a statement of other: than very definite altera: tions, Reverting to the directiye characteristics of Ty aerials, unless the flat-top portion is at least ten times the height of the downleads there is no marked result. ‘To get really definite results a wire 160 to 150 feet long about a foot or two above the earth is necessary. The Beveridge aerial used by the Transatlantic receiving stations on the "American seaboard have a flat-top of over ten miles at a height of 80 feet! These are used on wavelengths approxiating 14,000 metres. Directional ef. fects may be often noticed with stand. ard designs of I, aerials, but the results are invariably lue to either chielding or reradiation from close-by objects, such as buildings, trees, wire fences, cite. An aerial 100 feet lone and 26 feet high would show no appreciable dircetive results if erected in an opeth paddock. This is not to sav that it is inadvisable to erect an aerial east and West with the downlands from the Western end if Australian reception is to be concentrated. ‘The advantage Sained is worth voing after, because Every little bit helps. Here, again, an extra valve will be worth half a dozen directional aerials}! Summed up, then, an aerial should be as high as possible, have no acute angles of down lead with respect to ilat-tops, have a short lead from lea:l-in tube to set, and set to earth connection, and the resistance shouid be as low as pose sible. In practice, however, a compromise will usually have to be effected by making the best of the advantages offering in the shape of trees, etc., but the easiest method is to do the best possible and save worry by bringing a six-valve set in place of the proposed five.