Design of High-Frequency Chokes

Radio Record, Volume III, Issue 22, 13 December 1929, Page 28

Design of High-Frequency Chokes

An Article to Fill a Definite Need

By

CATHODE

HE H.F. choke is a device which offers a high impedance to radio frequency currents without: introducing appreciable D.O, resistance. The explanation generally given is that the H.I’, choke is essentially an inductance, anil that its impedance is therefore proportional to the frequency or inversely proportional to the wave-length.applied. This explanation, however, only approaches the’truth in the case of a parallel feed circuit, where the seéelfcapacity of the choke is.absorbed into the tuning capacity, and the impedance or reactance presented by the choke is that of its inductance alone. The impedance presented by a, typical commer: cial choke under these circumstances and at different frequencies is shown diagramatically in the upper curve of Fig. i (reproduced from "The Efficiency of Parallel Feed," "Radio Record," October 4, 1929, to which the reader is referred for a fuller explanaion on this point). At radio frequencies the association of capacity with inductance results in resonance. When it is realised that the capacity across a choke is raised by , 10 or 20 micro-microfarads immediately it is inserted in a receiver circuit there éan be no longer any doubt that ‘every H.F. choke resonates at some welldefined wave-length. The lower curve of Fig 1 is typical! ef the majority of H.F. chokes on the market to-day, and clearly proves that the H.F. choke must be treated as a tuned circuit. The resonant frequency of this choke is about 2500 metres, and it will be seen that although the impedance to H.F. is very high at long wave-lengths, it falls off to 25,000 ohnis at 500 metres. This falling off is serious if the choke is to be used for coupling the H.F. valves in a portable set, as in Fig. 2, but in capacity-con- . trolled reaction circuits is fortunately

not so important since the impedance of _ the reaction condenser algo falls as the wave-length is reduced, and is able to compensate for the falling-off in H.F. current available at the plate. ‘ From Vig 3 it is obvious that a choke is a parallel resonant circuit. In other words any H.I’. current which flows is divided between the capacity. branch and the inductive branch with its resistance, and the currents in the two branches may be widely different. The current is greatest in the capacity branch at wave-lengths below resonance, but above it the greater current transfers to the inductive branch. Most chokes are worked at wave-lengths below resonance, so that the self-capacity does the choking and not the high inductance marked on the carton! In Fig 3 the anode-filament capacity Caf is in parallel with the choke and merely tends to raise its natural reson‘ance, but the anode-grid capacity Cag serves to transfer H.F. energy back to the grid. The phase of the H.¥F. voltage returned to the grid in this way will depend on whether the choke is being worked on the capacitative of the inductance side of resonance. If the choke is worked below the resonant wave-length the reaction effect is negative and the valve cannot oscillate; above the resonant wave-length the reaction effect is positive, and the valve will oscillate without any coupling between the adone and grid circuits other than the valve capacity Cag. Thus .the choke used in drawing the lower curve of Fig. 1 would cause oscillation above a wave-length of 2500 metres, but would be satisfactory at

any less wave-length. A point which is immediately apparent is that. the reactance is highest at wave-lengths approaching resonance; consequently it naturally occurs to one to try the effect of reducing the inductance of the choke

so as to work it nearer to its resonant point. In practice there proves to be a defi‘nite advantage in so reducing the inductance, although it is always necessary to be careful not to reduce the inductance so far that resonance occurs within the broadcast band. When the choke is- wired into cireuit, its selfcapacity is augmented by the valve capacities, and any other stray capacities, so that even thougli the choke itself were resonant at some point within the broadcast band, in any practical receiver the resonant point would be

moved to a ionger wave-length which might be outside the band, thus permitting satisfactory operation. This point is well illustrated in Figs. 4 and 5, in which are given the resonance curves of a number of representative chokes of various induetances, firstly without any added capacity (in Fig. 4), and secondly with an added shunt capacity of 15 .micremicrofarads such as would be intreduced by incorporating any of. the chokes in a practical receiver (Tig. 5).

Note that curve 1, relating to a choke’ of 10 millihenries, resonates, without. any shunt capacity, between 800 and 350 metres; this choke would at first sight appear likely to prove unsatisfactory, but, when an added shunt capacity of 15 mmfd. is introduced ~ (Fig. 5), as would almost inevitably be the case in practice, the resonaut point is moved to a higher wavelength which, as it happens, is outside the broadcast band. Thus this choke ,of 10 millihenries would almost certainly be nq gctly satisfactory in practice; in c is the most satisfactory of the several chokes whose curves are given in Figs, 4and 5. At the same time, an inductance of 10 millihenries does not offer much margin of safety (as regards avoiding resonance within the broadcast band) and the writer prefers to adopt an inductance of about 15 or 20 millihenries, just in case the incidental capacities should be lower than antici- . pated in any particular instance. It seéms appropriate, now that the theory of high-frequency choke design has been dealt with more or less coipletely, to summarise the principles -involved and the requirements entailed thereby, subsequently giving practical designs to meet these requirements. (1). Firstly, then, for a parallel few choke, we require a choke having a very high inductance indeed; but fortunately self-capacity is of only minor importance, as it is absorbed into the tuning capacity. . (2) Secondly, for an untuned highfrequency coupling (Fig. 2) we require a choke haying maximum reactance over the broadcast band-that is to say, we, require a choke whose resonant point, when in cireuit, is round about 600 to 700 metres. A choke of about 15 or 20 millihenries will best fit this requirement, especiaily since the less amount of wire entailed by a fairly low inductance enables us to keep the sélf-capacity down without taking any particular precautions. (8) Thirdly, for use in the plate citcuit of a detector valve to give re; action effects. Such a choke as is ouilined in paragraph (2) above suitable, but the fact that inefficiency

is of small moment in a reaction choke dispenses with the need for earetul construction; the most compact design will be most satisfactory, for littie a room is usually available for a re-’ action choke. (4) Fourthly, the experimenter who insists on using one choke for all purposes confronts -us with the difficult problem of providing a ‘choke of high inductance (to cater for parallel-feed circuits) whose reactance over the broadcast bend shall yet be high

en | iad enough to permit of amplification with , an untuned coupling. The design of ‘this can at best be but a compromise. as in the case of the chokes commercially .available. Dealing with the construction of these four types of chokes in turn :- , a) Firstly, in regard to the choke fay narallel feed. we are not faced with

the necessity of reducing self-capacity to thé lowest possible figure. Therefore it is suggested that the constructor should adopt a simple slot winding with its attendant ease of construction. For making the winding: former, three discs of hard rubber will be required, two of two-inch diameter . and. o1ieeighth inch thickness, the other of halfinch diameter and three-sixteenths-inch thickness; one of the larger discs may well have three "wings" left on it when potting it out with thé fretsaw or cop-

ing saw, two of the "wings" being fitted with terminals or soldering lugs to which the ends of the winding are connected, the remaining wing being utilised for mounting by means of an angle bracket. The three discs are then bolted together by means of a small brass bolt through their centres; the small disc is of course placed in .the middle, and the slot thus formed accommodates the winding; the illustfation of such a former in Fig. 6 will make matters clear. In this instance a winding of about 200 millihenries should be quite large enough, and this will require some 2500 turns of either 40 S.W.G. double silk covered or 38 S.W.G. enamelled wire (the enamelled wire is the cheaper). If

the dimensions given are adhered Lo, and ove of the recomimended wires used. it will be unnecessary to count the turns; just wind the slot full. About one ounce of wire will be sufficient. If the constructor so desires, . he may adopt a subdivided type of winding as described in paragraph (4) below, but there is no particular advantage in so doing if the choke is to be used in a parallel feed circuit. , (2) Secondly, for a coupling choke for ‘a portable receiver or other receiver with a stage of untuned highfrequency amplification the same type of construction is entirely suitable. The diameter of the large discs, however, is reduced to one and one-half inches-while the thickness of the smaller is reduced to one-eighth of an inch. On this smaller former, then, 900 turns of either 38 8.W.G. douhle silk covered or 36 S.W.G. enamelled svire should be wound. Here again, if the dimensions given are followed, winding the slot full of one of the recommended wires will. dispense with the necessity of counting the turns. One ounce of wire will be sufficient to wind two of these chokes.

(8) Thirdly, for a reaction choke in the plate circuit of a detettor valve, such a choke as is described in paragraph (2) above will be very suitable and efficient. Where compactness is a consideration, however, the diameter of the larger dises may "be reduced to one inch, the winding then being made with finer wire, says, No, 40 S.W.G. enamelled, of which the required number of turns will just fill the slot.

(4) Fourthly, for a general-purpose choke, some subdivision of windings is necessary, in order to gain the necesgary high inductance for a parallel feed circuit, without having so high a self capacity to render the choke useless for coupling an untuned highfrequency stage. Such a choke may well be wound on a piece of half-inch ebonite tubing, such as is commonly used for leads-in, this having a number of one and one half-inch discs of one-eighth inch thickness. (drilled with a half-inch hole in the centre) forced over it in the manner illustrated in Fig. 7, and well secured with shellac

varnish. Bach slot is then filled with 88 S.W.G. double silk or 36 S.W.G. enamelled wire, all the slots being, of course, wound in the same direction. Some slight advantage may be gained by placing two discs between each wound slot and leaving an air space

between them, but this will make the _ choke rather long unless the dises aré made of one-sixteenth inch formica, which by the way can be obtained from. Johns, Ltd., Auckland. For that matte. there would be no particular objection to reducing the number of wound slots to four instead of the six shown in the diagram, as the induct-

ance would still be sufficiently high for all practical purposes, Where compactness is a consideration, the diameter of the discs may be reduced to one inch, and the slots wound full with 40 S.W.G. enamelled wire. AS an alternative method of construction of a former for this type of choke, a number of discs of one-eighth inch hard rubber, alternatively half-inch and one or one and one-half inches in diameter, may be bolted together by means of a length of thing threaded brass rod through thier centres. The wire is then wound in the resulting slots in the same manner as has just been described. ‘ For short-wave work any of the foregoing chokes will be found reasonably efficient. Where a receiver is to be used exclusively for short-wave work, however, better results will be obtained by the use of a straightforward single layer winding of 200 or 300 turns of

fine enamélled wire (38 or 40 S.W.G.) on a length of half-inch or three-quar~ ter-inch ebonite tubing. The effect of using a. short-wave choke in series with an ordinary choke has been tried for use in an all-wave receiver, but not with very promising results. In order to make the shortwave choke operate separately on short waves it was found necessary to cole nect a tiny condenser from "the junction of the two chokes to earth in the manner illustrated in Fig. 8, and this condenser, being in parallel with the long-wave choke, naturally changed the resonant frequency of the latter component and impaired its performance to some. extent. However, some e¢Xperimenters may think the idea worth playing with, notwithstanding the complication introduced by its use.