How to Design and Erect a Successful Aerial

Radio Record, Volume I, Issue 27, 20 January 1928, Page 3

How to Design and Erect a Successful Aerial

The Theory of Electro-Magnetic Induction Clearly Explained

By

M.I.R.E.

In this article, the Theory of Electro- Magnetic Induction, or "Lines of Force," as applied to the receiving system, is set forth by. M. 1. R. E. in an exceptionally able and lucid manner, enabling the most uninitiated experi ménters to grasp its principles without effort. Readers will do well to thoroughly digest the theoretical side of the subject, which is intended to throw light upon the practical d emonstrations which are to follow. Stress is given to the little appreciated fact that a lar ge aerial is not a vital necessity to multi-valve set owners.

Undoubtedly one of the most vexed subjects amongst those iuteregted in ‘radio is the question of the erection of an aerial, and how to design it, in such a way that maximum efficiency will he obtained. lirst. of all, Iet it be stated, that if an aerial is to be erected in a flat, open paddock, where the land is of the average, or better, ‘farming variety, then providing certain well-known principles are followed, the clectrical receptivity or efficiency miay be predicted to within narrow limits of accuracy. The conditious alter imumediately, however, just as soon as the ideal or simple conditions give place to more complicated circumstances, such as the presence of buildings, trees, ete. It should be distinctly under: stood that the following remarks | are strictly applicable to the re: ception of signals within the broadcast waye-band, which ex: tends from 220 metres to 580 metres, and particularly with respect to wavelengths between 300 and 450 metres, these being the most frequently used in New ZeaJand. POINTS FOR CONSIDERATION, The main consideration in a flat, open space is to have two wooden masts as high as possible. A "T? type aerial should be used, i¢., with the downleads from the centre, coming J straight down to the receiyer, which should be installed in a Inilding haying as little metal as. possible in its construction. Vrom the receiver to the carth connection should be as short as possible. The carth connection preferred iscone which makes contact to a water pipe forming part of a town Water supply. Stripped of technicalities, this represents an ideal arrangement. Unfortunately, it can seldom be put into practice. In order to enlighten the inquirer as to why the aboye is ideal, it is first necessary to consider briefly the theory of operation of an aerial in absorbing energy in the form of ether waves. In order to avoid complicating the explanation, a bricf statement of what | happens will be made, without detailing why it happens. , "LINES OF FORCE." If a magnet is laid on a table and a piece of paper is laid over it, and. then small pieces of iron (iron filings,

for instance) are sprinkled on the paper, the filings will arrange themselves in a regular manner on the paper in such a way as to represent lines drawn fron one ‘pole’ to the other. If the magnet is of the same shape as a horsesloc, for instance, the ‘poles" are represented by the two ends, one being known as the "south" and the other the "nerth.’? The lines taken up and indicated by the orderly lines of iron filings, reach in semi-circles from one pole to the other across the gap, the semi-circles reaching out and becoming greater in Jength as well as radius, as they lie further away from the magnet. These lines are known as "magnetic lines of foree,’"? aud are always found in close proximity to a inagnet. Now, if a wire (for instance, a piece of copper wire, such as is commonly used in electrical circnits) is caused to move through the magnetic lines of foree in such a way as to ‘cut’? them, then an clectric current will be set up in that wire. ‘This is the basis of the cleetric generator. This action is exactly analagous to what takes place with a wireless aerial, At (he transmitter, power: ful currents are set up in the aerial, thus creating electric lines of force which extend round about the trans: mitting aerial and take up positions cxtending between aerial and earth. By yirtue of the oscillat: ing currents taiing place in the aerial, the lines of force build up and collapse with tremendous rapid: ity, in step with the oscillations. At a wavelength of 360 metres, which is practically the wavelength of 38YA, the frequency at which this action takes place is exactly one million times per second. ‘The result of this disturbance in the electrical (or, perhaps, magnetic) medium, which has been called the ‘‘aether,’’ created by this rapid creation and dissipation of the lites of force, is to send out waves which radiate outwards in all directions, with the aerial as a centre. ‘Ihe

wayes reaching the receiving acrial have a couverse eftect, because im ‘"‘eutting’’ across the acrial wire they represent lines of force and induce electrical currents to flow in the aerial system. ‘To all intents and purposes, therefore, the currents sct up in the receiving aerial system represents electrical energy transferred trom the transmitter through the aetlhier. PRINCIPLES APPLIED, To apply these principles to the case of the receiving system cited at the beginning of this discussion it is only necessary to point out that the more lines cut (ignoring the time factor) or the longer the length of the lines cut means more energy induced, or, in other words, stronger signils. As the wave is advancing vertically, the higher the aerial the more this effect is taken adyantage of. Long length of flat top portion of aerial is inconsequent so far as signal strength is coucerned when considering the pick-up of the aerial, The length of the acrial mmst be kept within certain limits purely for purposes of ensuring that, in combination with the tuning arrangements of the receiver, it will function efficiently within the broadcast waye-band so far as tuning is concerned. There is a great deal of misunderstanding on this point, but, broadly speaking, there are the correct relationships between height and leugth of aerial systems. ELECTRICAL AND GEOMETRICAL HEIGHT. The reason it was recomiusnded that wooden masts be used is purely a theoretical one, and that is that the geometrical height of the acrial is not the electrical height, In the case of the ideal ‘T-shaped aerial the electrical height would approximate two-thirds of the geometrical height. If metal masts are used and these are connected to the earth, then the earth is brought so much nearer the aerial at each end, and although the geonietrical height remains the same, the electrical

height is reduced somewiat. A building with a metal roof erected under an acrial materially reduces the electrical height, just as the funnels of a steamer do to a ship’s acrial system, DETERMINING FACTORS OF CURRENT FLOW. The question of a good earth connection introduces a subject which is commen to the question of size of wire used in an acrial, and also the question of tite use of one or more wires in the Gveriiead system, Suppose water were leing forced through a pipe. To get double the water through the pipe in a given time, one of two things would Lave to be done, either the pressure would haye to be doubled or the size of pipe be made twice as large. Therefore, in determining how much water is pessed through the pipe there are two controlling factors, pressure and pipe resistance, In the aerial system (or any electrical system for that matter) there are the same factors determining current flow. With a given height of aerial so inany lines of force are cut and this represents a certain pressure ayailalle to induce currents to flow in the acrial system. The amount set up is dependent on the resistance offered to its passage in the circuit. Too sinall conductors or a bad connection to carth represents resistance which will limit aerial currents, and therefore signal strength. Several conductors in parallel in the overhead system and downleads are therefore the oretically an advantage. The theoretically ideal aerial described is of the T type. Undoubtedly the most popular type is the L type, and that, one that has the downleads coming down at one end, from a purely practical point of view, this is quite all right, but the reason the T aerial is put forward as superior is because it is abso: lutely undirectional in its receptivity.

That is it receives equally well north, south, cast, or west, whereas the Iy type has directive effects in the direc tion in which it is pointing and towards the end from which the dowzleads are brought down Thus an serial pointing north and south and with downleads at the sonthern end will receive better from a southerly direction than any other point of the cempass. If reception is to be carried out from a definite direction then it is obviously an adyantage to erect an aerial in this fashion, but it is a ques tionable arrangement where reception is being carried ont from all directions, In a continued discussion on thia subject it is proposed to show that the main considerations in this discussion haye becn more of a theoretical na ture in order to demonstrate principles end in order to throw more light on the discussion of practical points to follow. LARGE AERIAL NOT A NECESSITY In conclusion and in addition to the last paragraph it is necessary to point out that a large acrial is not a vital necessity where multi-valve receivers are in use. One of the big things searing prospective purchasers from investing in radio is the idea that masts and wire must be plastered all rcund the house in order to instal radio. This is not so. A yery modest affair will give all the results desired even with distant reception and the use of, an indoor aerial or loop will give complete satisfaction to most peo ple who object to visible outside wires. So far as obtaining perfect ree sults from an aerial are concerned, it is an obvious but very little aps preciated point that if another valve is added to a set, the ampli« fication represented by that valve will more than offset minor, or even major inefficiencies in the aerial system. Hence the individual who makes his property look like a full-rigged battle. ship and uses a 4-valve set . might just as well use a 5-valve set and be less ambitions with his aerial, Very large aerials are not therefore a necege sity, though they may be desirable.