Frame Aerials and their Uses

Radio Record, Volume I, Issue 41, 27 April 1928, Page 13

Frame Aerials and their Uses

SIMPLE CONSTRUCTION FOR EXPERIMENTERS

N ANY inquiries have been received regarding the use and efficiency of loop or frame aerials, and it ig im response to these that this aiticle is written. oo It should be stated as a that the sensitivity of an, average frame aerial compared to that of an outdoor aerial of ordinary proportions 1s from one-sixth to one-tenth, sv that where heavy volume is sought, a franic is out of the question with aul ordinary set, except at very short dislances from the transmitter, In ths ordinary way, a frame aerial is more frequently used with a suner-heterodyne type of receiver, which is both sensitive and selective, the latte: quality beinz further augmented by the use ef the frame or loop. Yet even with an crdinary set of three or more yalreas, good results may be obtained over tong distances, using headpliones, by anyone of an experimental turn of mind. A few vears ago the writer and others have on one or two favourable occasions been able to pick un KGO (California} on one and two valves, using a smaii loop, but such reception appezrs to be immossible at the present time owing to the congestion of the ether or some «ther reason. 1 spite of its limitations, however, ; the frame has some compensations, and the lack of sensitivity is made up for to some extent by the gteat reduction of interference and the clearnes of reception owing te the silent background. Tuniny, ‘s very fine, and will seem particularly so to those. Who have not had experience on shortwave, PRAME aeria:s a2 made according to several different patterns, but the most selective is that in which the wires all lie in the saine plane. ‘I'he wiuding is connec.cd across the aerial and terminals of the set, the aerial tuning condenser tien being in parallel with it. The reason for its selectivity is because waves from a transmitting station reach one side of the frame hefore the other, it the pline of the wires lies in the path of the waves When the waves strike the first wire of a turn they induce vollage in a certain direction; when they strike the further side they induce yoltage in the cpposite direction. These two voltayes would exactly balance each cther, except for the fact that the waves strike ene side before the othcr, and on this account the voltuges on the two sides are unequal, and the simatl difference only traverses the windings. The fact of there being several wires supplies a difference of voltage tor each turn and these differences add together, giying the total effect. ‘Ihe waves strike the top and bottom wires equally, so no difference of potuntial is available from them. In like manner, when the plane of the frame is turned across the direction of the waves the latter strike more aud Jnore equally until the position js at right angles, when there is equal eifect upon all Wires, and consequently no voltage difference to traverse the windings, "[HUS property is mate use of in finding the direction from which Waves are coming, and it is quite easy to distinguish the difference on strong signals between maximum and minimum settings, the fraine being parallel With or at right-angles to the incoming waves. Failing any known data as a guide, this test wil] give a line of direction along which the wayes mav be travelling in cither one direction or the other. ‘To determine the direction of origin upon this line, ‘wo bearings are usually taken by moying the frame and receiver after the first one, to some distance either side of the line that has been found. Such new bearing will converge upon the original line, thus indicating very clearly the direction from which the waves emanate. This is the method adopted by the British Post Office for tracking-up offending oscillators, all the apparatus being installed in a specially-bvilt miotor-van. Two vans are often entployed, each one taking a different bearirg simuitaneously, THIS method also constitutes the radio compass, by which means bearings may be found by a ship or its bearings communicated to it from shore. The directional effect of frame aerials is much more marked on short than upon long wavelengths, so for compass work short waves are used. Some years ago there was established a radio beacon of limited range working upon a wavelength of seven metres. ESSENTIAL POINTS. QE point in using a frame aerial is . that any circuit whatever may be employed, including any that would be illegal on an outside aerial. In the construction of a frame aerial attention

must be paid to making it as ‘‘low-loss"’ as possible, especially if for use at high frequencies (short-wave) where its eMficiency will be the greatest if design and insulation are good, Essential points for successful operation are good audio amplificatio:1, a good B battery, silent backgrouné, and full control of reaction. All oscillation backlash must be eliminated so that the detector glides imperceptibly into and out of oscillation. CONCTRUCTION. frROM the foregoing remarks it will be seen that the amount of energy dealt with by the frame is very small, and it is clear that the greater the distance between the two upright portions of the winding, the greater will: be the difference in voltage induced. It is therefore an advantage to keep the size of the frame large rather than small, vet within reasonable limits, to suit the rooms of a house. A convenient and fairly effective size is one having two-foot sides, which has a diagonal of nearly three feet. N a frame of these dimensions 590 metres would tune in on i2 turns, which would require about 95 feet of wire; 800 metres ou 9 turus, or about 74 feet; 180 metres on 6 turus or 50 feet. Lower wavelength would rejuire fewer turns. Many taps are uct actually required, becatse the par licl tuning condenser in the aerial circuit of the receiver’ gives a wide range. Twelve turns will be ample for any broadcast reception, aud it is a good idea to take a tap at the centre or sixth turn, and one at the ninth. One system of Tapping 18 to mereiy serape away the insulation at desired points,

and make connection by means of a clip attached to tlie lead. Deal encis are not desirable in frame aerials, so if short-wave is te be worked it would be a good plan to bring out the ends of both halyes separately at the sixth turn, and connect together to put the whole in series, or use only one winding of six turns, further reducing it by clip connections as mentioned. No doubt a mwimber of enthusiasts will be Pleased to add to ther er ycriv es be experimenting with the frame, and m order to bring the opportunity within the reach of the greatest number, the simplest possible way of constructing the frame will be described. Jn some frame acriat cirer ‘ts rea tion is obtained by including some of the turns in the plate circuit. TWO laths one inch by helf an inch ~ will be required, 3 feet and 3 feet 6 inches in length respectively, Ihe sliorter one is fixed by ils centre at right angles to the longer at a distance of 18 inches from one end of the latter by "halving." LVurther security is obtained by tastening on a 4-inch square of 3-ply or } wood, as shown in the diagram. Twenties or 22's enamelled copper wire is the best to use. Before putting the cross-picces together they must be slotted to take the wire. ‘The slots are made with a saw, the slots, sloping diagonally, can be sawn in two strips at one operation, and uf held in a vice with a waste strip outside each side, there will be no fear of breaking out the small pieces of wood between the slots, which are jin. apart. Just the same effect gwill be obtained by Grilling holes jin. apart, and threading the wire through, which is rathcr a tedious operation. Brass (not iron) tacks sould be used on one cdge of the strips as a makeshift idea. It is important that the wood used be of a good solid kind and thoroughly dry, and after cutting the slots, should be well shellaced, getting the shellac well into the cuts. To obtain stability of the cross-piece, two shoit pieces of wood may be screwed to the upright as shown at AA. Ends of wires may be secured by passing through a ‘hole drilled in the strips, or may be connected to terminals on # smell square of ebonite as shown. The lower extremity of the upright is rounded to fit the centre of an old wire spool, from which one flange may be removed, This is screwed to a bascboard and allows of the frame being rotated with ease, If a calibration scale is required for direction-finding, the top flange of the spool should be left iutact, and upon it a cardboard scale marked in degrees can be fixed. A pointer of wire or a large needle is then attached to the upright. No useful work can be done with loop aerials in connection with crystal sets, and an inside aerial will give better volume than a loop, but loses the advantages of the frame aerial’s sclectivity.