Make Your Aerial Fully Efficient

Radio Record, Volume III, Issue 36, 21 March 1930, Page 30

Make Your Aerial Fully Efficient

An account of the types of aerials — and hints in choosing and ereciing

|F one were to remark to every second owner of a wireless >| installation, "You are losing } strength because your aerial ties | is not as efficient as it might i be," he would be decidedly un- _ popular, but nevertheless substantially correct. Many, of course, will reply, "I am satisfied, and do not propose to interfere with my installation." To them all we can say is do not read a word more of this talk, but concentrate the programmes of the local station. We all know the purpose of an aerial -to collect energy from the transmitting station. This energy is very weak, thousands of times weaker than that for the smallest radio battery, and effort is made to-collect as much -as possible. The more that is ‘collected the more can be amplified by the wireless:set, hence it follows that the more efficient the system the greater the volume.

The Importance of Height. HE earth is not a-good conductor of wireless waves, with the result that the strongest waves are not near the ground. This brings us to the first requirement of the ideal aerial-height. In every set there is a coil and a condenser that are used to bring the set. into "reasonance" with the incoming; waves. The waves with which the set: resonates are those received. It is this fact that makes separation of stations: possible. The condenser is a combination of fixed and moving plates electrically disconnected. This is shown in Fig. 1, designated "tuning capacity." A moment’s thought will disclose the fact that the-aerial and the earth are in reality one great condenser, and being "in parallel" with the tuning condenser, the total capacity will be that of both. If the aerial to earth capacity is great, the tuning of the aerial system will be affected, and the set is affected. Especially is this of importa EP IT ES TE TE

ance where the modern single control receiver is used, where, contrary to general belief, a more efficient derial system is necessary than -vith the older two-dial receivers. The capacity of an elevated aerial might be considered to be roughly .0002 mfds. but a low one may approach .0004, a value sometimes greater than the tuning capacity. Signals will pass through a large eapacity, and not through the detector. If the aerial is near an earthed object such as a tree, roof, damp wall, etc., the

capacity to earth will be increased, and there will be losses which will be more apparent in wet weather. We have arrived at the first asset of a good aerial-it. must be high and clear of all earthed objects. Height is probably the most important consideration in erecting an aerial system. Unfortunately it is generally the most difficult to obtain, for the erec-

tion of tall masts is not as simple as it may appear. The optimum height is 40 feet. Beyond this the.signal strength does not increase in proportion to the increased height. Below this there is a rapid decrease in strength for every foot descended, and tuning becomes flatter. The reasons have been made clear in the text referring to the condensers. Aerial Length. HAVING decided "that the aerial is to be no less than 40 feet high, let us examine the next important considera-tion-length. This depends upon the type of receiver to be used, bearing in mind.that the greater the length up. to three times the height the stronger the signals, With the large sets using several stages of radio amplification, the aerial should be. short, otherwise the capacity to earth will be increased and the aerial circuit thrown out of resonance, The losses from this factor result in a flatter signal (unselective) and a weaker one through not enabling the set to resonate. The accompanying table gives some indication of the length for the different types of sets. With a crystal or a small valve set where only the local station is desired, a long aerial should be employed, where possible-100 to 150 feet. Three valves using reaction will find | 100 to 120 feet quite sufficient, but the

wave trap will be needed if the local station is to be passed. " [F dio Four valves with one stage amplification, 80 to 100 feet. |... Five and more valves will find 60 to 75 feet ample. : These figures include the lead-in, According to a recent authority, the most effective aerial is one with a lead and aerial in the relation of 7 to 3. Roughly the mast is twice as high as the aerial is long. A fifty-foot mast and a 25-foot aerial make a good combination. Directional Effect. RECENT experiments have shown that for the multi-valve set directional effect is not important, and only when the set is so small that the relation of height to length is 1 to 8 can the effect of direction be appreciated. That is to say, that with a 40-feet mast the aerial should be 120 feet, but as we have already shown such a system ca} not be successfully employed on.the multi-valve’ receiver. A directional aerial is of most value to the owner of the crystal or small valve set that is capabje of receiving only the local sta- ° tion, The free end should point away from the transmitting station. This naturally refers only to the inverted L aerial, which is the most. successful., If conditions do not allow its erection, the T type with the lead-in coming from the centre is used. It is important, ‘however, to see that the lead comes from the exact centre.

Insulation. HIS is where most installations fant. The insulators are frequently not put on correctly, with the result, that they break under strain, and a makeshift is used until the aerial can receive attention, which is usually a.considerable time. Diagram 2 represents a well insulated system, and thifinset shows clearly the method enp f in coupling insulators. Note the’endless halyard passing over the pulley attached to the masthead. If a single halyard is used and the aerial breaks near the mast, or if the rope sticks in the pulley, the mast may have to be lowered (or climbed !). Two or three small insulators are preferable to one large one, and all insulators should be secured in ‘such a way that if one breaks the wires themselves hold and prevent the aerial from falling. At the "house end," the aerial is secured to a distance piece, which may be of aerial wire, and this in turn is secured to a galvanised iron band (say of 14-inch by 1-16-inch section),: bolted round the chimney above a convenient projecting ledge. The distance piece, or spacing wire, keeps the downlead away from the roof-gutter and walls of the house, and, as shown, is insulated from the aerial and from the

chimney, We have already spoken of need of avoiding trees and other eatthed objects. Better to use a short aerial that is clear of these than a long one passing over any of them, Gauge and Type of Wire. A NUMBBHR of readers have written. in asking what advantage enamelled wire has over bare copper wire, when used for an aerial. In districts near the sea, or in the thermal regions, salt spray in the first case and sulphurous gas in the second, have a corroding action upon the copper. ‘This has no effett upon the electrical properties of’ the aerial unless it goes too far and increases the resistance of the wire. When it has reached this stage, the wire will have corroded to such extent that'a breakage will occ revesit this it is advisable to employ enamelled eopper wire. Unlike direct current, radio frequency electricity: passes on the outside. of a conductor. If the outside is insulated, either by ‘corrosion or enamel, the current penetrates further into the centre. Practice has proved 7/22 or similar wire to be the most suitable. Types of Aerial. MBITIOUS but uninformed enthusiasts sometimes erect most elaborate cage and multiple wire aerials. They certainly look imposing and awe the outsider, hut the old hand merely nods -"He’ll learn some day." For his

benefit’ we reproduce a curve showing the effectiveness of a twin wire aerial with varying distances between the wires. It will be seen that if two wires are used they should be at least six, and. preferably eight, feet apart. Hven with this spacing the increase in current obtainable over that on a single wire is not large, being about thirty per cent. in the case quoted. Using the full eight feet, further measurements made with or four wires arranged symmetrically showed that nothing was to be gained by increasing the number of wires in this way. The measurements were carried out with a fairly efficient earth screen, the lowest actual resistance of the aerial circuit being in the last case about 11, ohms. If as may be the case with the average aerial a much higher resistance were associated with the earthing system it is evident that any improvement in the aerial alone

will have less effect on the magnitude of the received current. As an illustration, a single wire of 20 gauge was substituted for two-stranded wires of 8/18 gauge and -there was no perceptable difference in the signal strength though the resistance of the former is about eight times that of the latter. The explanation is that the actual ohmic resistance: of the aerial is only a very small proportion of the total resistance incorporated in the system. To be of effect an elaborate multiwire system must be supported with elaborate screen earths, Cage aerials are used by transmitting stations and there is no advantage to be gained by using them for reception. In fact, the added capacity of the aerial system has a distinct disadvantage when coupled to a receiving set, Summary. AVING now reached a. convenient stopping point, the end of the theoretical considerations, we shall defer the remainder of our chat till next week, when we shall consider the practical aspects in erecting the aerial. However, in conclusion, we shall summarise this week’s article as a refresher. The optimum aerial for the modern set is1. At least forty feet high. 2. 60 feet long. 8. Well insulated and clear of earthed objects. 4, Stranded insulated wire ig preferable. 5. Multiwire aerials are only effective with an elaborate earth ‘system.