Theory and Construction of Frame Antennae

Radio Record, Volume III, Issue 22, 13 December 1929, Page 25

Theory and Construction of Frame Antennae

Concluded from Last Week

By

CATHODE

AST. week’s instalment was concluded with a description of the method of. using an aerial and a ground with a loop antenna to increase the signal. strength. This is one of the easiest and most commonly used methods, but probably. the best method of increasing the volume of sound produced from this arrangemerit is, as with ordinary receivers, the use of regeneration, or the feeding back of signals intgajhe grid circuit of the detector val In an ordinary receiver this is brought about by very, many methods. The usual is a swinging coil, a continuation of the grid coil, controlled by either a series or a parallel variable condenser, or a fixed condenser and a means of. varying. the voltage on the plate.of the detector. These methods can in general be ap‘plied ‘to a receiver. using a loop. Tabulating the-methods we get :- 1 Loop antenna applied in parallel to the grid coil of a normal receiver and regeneration applied by the usual method. . .'A series coil as in diagram 1, . Reinartz method of a tapped inductance and a series condenser. ‘y (Diagram 2.) "s ooh In view of the wide application of teaction to loop antenna these methods must be elaborated a little. 1. Parallel coil. Where it is desired to use a frame antenna with an existing receiver without in any way altering the. wiring, probably the most convenient arrangement is that of employing the loop in parallel to the grid coil of the detector or radio frequency valve. This method: merely consists of attaching one end of the loop to the aerial terminal and one to the earth. Reaction is applied in the usual method. This system enables the experimenter to make a simple preliminary test to determine what measure of efficiency the loop antenna will DPossess. There is no necessity to disturb the wiring of the receiver. It should be noted in this case, however, that since the coil and the frame are in ‘parallel the effective inductance of the two will be less than either singly, so that the total inductance 1 & L2° L equals. ---__ "L1i+ L2

where Li and ‘L2 are the inductances of the: grid coil and.the frame antenna respectively. It is apparent then to obtain maximum efficiency that both the grid coil and the loop must have a greater inductance than usual, 4 in other words, both coils must be enlarged.. : ‘2, The method of obtaining the reaction by placing the coil in series with the frame is shown in the diagYam. The size of the coil between the loop and the. filament ‘return: need only be 20 or 25. turns; while the reaction coil should. beas:small as possible consistent with getting oscillation control over the whole tuning

band of the receiver. It should be noticed, however, that if this coil is too large it’ will not be possible to tune up to the higher frequencies, unless also the size of the loop is reduced. Since, however, the application. of the above method will require a certain alteration of the receiver it will be worth making further alterations in order to further increase the efficiency. Where there is so little damping the control of reaction becomes a matter requiring a little added skill and the use of the swinging coil becomes rather clumsy. and in some ways difficult to control. The method known as throttle control gives wonderful ease of adjustment though it means the introduction of two extra components. A high frequency choke must be introduced between the swinging coil and the ’phones. From the side of this nearest the coil a connection must be taken to the fixed vanes of the .00025

moving condenser. The moving vanes are connected with A +. In practice it is found that smooth and progressive control of reaction is given by this method, for it has less effect on the tuning than has the method employed with swinging coil. It is free from any possibility of any echanical backlash such as may exist in a moying coil holder. 8. The most widely used method: is that known ‘as the Reinartz system. Assuming’ that the frame consists ‘of twelve to fourteen: turns on a 2ft. side, a further three to five turns should be wound on for reaction, The direction of winding must be the same’as for the frame. The actual number of

turns depends upon the method of winding employed, and the valves and "B" battery used. The extra turns are . employed on the "A" battery side. A series condenser characteristic of the Reinartz circuit is employed between the plate and the extra turns. The value of this condenser need not even have a greater value than .0001 in view of the low damping of the frame. Construction of a Loop. ROM 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 distance between the two upright portions of the winding, the greater will be the difference in, voltage reduced. It is, therefore, an advantage to keep the Size of the frame large than small. In building a loop for a receiver the safest method is to use an.excess of wire to begin with. After a loop is wound a low frequency broadcasting station should be tuned in. If the dial setting of the loop tuning condenser is much too low, wire should be removed from the loop. Take off half a turn at a time. The inductance of the loop and the maximum capacity of a tuning condenser used with it determine the lowest frequency to which the combination will be resonant. Ideal conditions cannot exist and it is necessary to make certain compromises. The smaller the condenser the better the results, but a very small condenser cannot adequately cover the broadcast band. Furthermore the distributed

capacity of the loop forms a much larger proportion of the whole capacity, of the circuit when the variable cons denser is a small one, and this dise tributed capacity does not enter to the variable portion which alters the resonance to different frequencies. It will be found that a tuning condenser of .00025 mfd. capacity is slightly too small for loop work in many cases, With a earefully constructed loop, having the wires well spaced, a ..00035 condenser, will generally cover the entire range. A .0005 condenser will be still easier to tune and the signal power will be reduced only slightly. yy

Loops with a few turns have @ greater range of tuning than those with many turns. The problem is to cover the whole band of frequencies with a condenser small enough to en-

a , gure good results. With such a combination not only must the tuning condenser be of small value But the dis-. tributed capacity of the loop must likewise be kept at a minimum. By using a large condenser and a small loop the change of capacity in the condenser between minimum .and maximum settings is great enough to avoid trouble in tuning, but the signal ‘will not be as strong. The length of wire in a loop has no direct bearing on the wavelength to be tuned. The frequency depends entirely upon the inductance of the loop and its distributed capacity. A loop, like any other coil, has an inductance which is desirable, and. a distributed capacity which is undesirable. Inductance is increased by using more turns, greater length in each turn and greater spacing between each turn. Distributed capacity is reduced by using fewer turns’ and more spacing between the turns. It will be seen that these requirements oppose one another, and it-is necessary to design a-loop which will satisfy both. There is more or less a. critical spacing beyond which additional spacing does not greatly reduce the distributed capacity. For a loop only 2ft. square the gain with spacing greater than 1-8jn. becomes less noticeable. For 2 loop 4ft. square this critical spacing is somewhat less than jin. and so the spacing increases with the increase

in size of the loop. The following table shows the number of turns required on box loops of various dimensions, when used .with tuning condensers from: .00025 to .0005 mfd. The ‘loops are considered as being square. ‘Rectangular loops having the same area as a given square will be identical. In the case of a spiral loop the dimensions apply to the average turn, N a frame 2ft. square 500 metres would tune in on 12 turns, which would require about 98. feet of wire; 800 metres on 9 turns, or about 74 feet ; 180 metres on 6 turns or 50 feet. Lower wavelengths -would require fewer turns. Many taps are not actually required, because the parallel tuning condenser in the aerial circuit of the receiver gives a wide range. Twelve turns will be ample for any broadcast reception, and it is a good idea to take a tap at the-centre or sixth turn, and one at the ninth. One system of tapping is to merely scrape away the insulation at desired points, and make connection by means of a clip attached to the lead. . Dead ends are not desirable in frame aerials, so if short-wave is to be worked it would be a good: plan to bring out the ends of both halves 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. ‘ Two laths one

inch by half an inch will be required, 3 feet’ and 3. feet Ginches in length respectively. The shorter one is fixed by its centre at right angles to the longer at a distance ‘18 inches from one end of the latter by "halving." Further security is obtained by fastening on a 4-inch square of 8-ply or 3-8in,

wood, as shown in the diagram, 20, or 22 s.w.g. enamelled copper wire is the best to use. Before putting the cross pieces 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 if 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 will be obtained by drilling holes 1-8in. apart and threading the wire through which is rather a tedious operation. Brass (not iron) tacks should be used on one edge 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 short pieces of wood may be. serewed to the*upright as shown in diagram. Ends of. wires may be se cured by passing through a hole drilled in the strips, or may be connected to

terminals on a small 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 toj a baseboard and allows of the frame being rotated with ease. If a calibration scale is .réquired for directionfinding, the top flange ofthe spool , should be left intact, and upon it cardboard seale 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 ‘eorinection with crystal sets, and an outside aerial will give better volume than a loop, but loses the advantages | of the frame. aerial’s selectivity. A . 5 SESTETEEUEETRRARECESRERRERESREREEEREREREREEEEEEEETE

Turns Required for Rectangular. ‘Loops. Length of side in Inches-Square. Loop. or ;

Condenser Capacity in Mfds. 00025 .00035 .0005 001 Spacing 4-inch Between Turns 10x10 12x12 14x14 16x16 18x18 20x20 25x25 30x30 35x35 100 ‘144 196 256 824 400 625 900 225 -- — — 2 228 «2.21. .17 15) Fras -~ — 2 20 18. 4 44 WwW" — 21 18 16 1. 1B lis «10 9 16 i3 i 10 9 9 7 6 6

Area of Rectangular Loop.

10x10 12x12 14xi4 16x16 18x18 20x20 25x25 30x30 35x35 100 144 196 256 824 400... 625 900 1225 — -_ — -- 20 17 15 .00025 -- — .00035 --- — = = 28 20 1 14 Ww .0005 — -~ 2% 2 18 -46 18 «i 10 22 001 17 14 12 11 10 868 7 6 TTT T ETT ERTP CUTTER ECU LELEEECUELEEEEEECELECELECOPEETEPIPUPPLEEEDLIGEUTEPLELL DETER LEE EDO EET EEE EL

Spacing ½-inch Between Turns.