The design and construction of H.F. chokes (Coil Design and Construction)

In the ordinary receiver the H.F. Choke can never be better than a compromise -- even for one waveband a properly designed choke can have its maximum efficiency at only one frequency although the drop in response is gradual enough for working all over the band and one choke will suffice for the medium and long wavebands--and the cause of this is easily seen when the theory of the choke is understood.

The purpose of the H.F. choke is to allow low frequency signals to pass on to further circuits after separation from their H.F. carrier whilst blocking the H.F. and by-passing it to earth (as in the case of the ordinary triode detector); to prevent H.F. from entering, the high tension supply; to pass H.F. from radio frequency amplifiers to following circuits with as little loss as possible, as it Fig. 2, together with other less frequently used applications. A high resistance will oppose a flow of H.F. and is sometimes used in the anode circuits of radio frequency amplifiers and detectors, but an H.F. choke wilt perform the same function without the high voltage drop which must of necessity appear across such a resistance.

The action of the choke depends upon the fact that if a radio frequency signal is applied to a long wire whose electrical length is of the wavelength of the signal applied a standing voltage wave is, set up on the wire or, in other words, the wire behaves as if it had very high impedance at one end and very low impedance at the other. If one end of the wire is earthed (via a condenser) therefore, the other end of the wire opposes the passage of H.F. currents of the resonant frequency.

Thus all that is required is to wrap the length of wire into a, more convenient form, and it will be realised that a high number of turns will be necessary for the lower frequencies. The characteristics of the choke are not greatly changed by coiling the wire.

For the medium and long wavebands it is not economical to construct H.F. chokes but should it be desired to do so a convenient method is to slot an ebonite rod 1" in diameter with six equally spaced rings each 1" deep and 1 " wide, winding 500 turns of No. 34 S.W.G. enameled copper wire in each slot (Fig. 7g). The windings, of course, are carried from slot to slot and are all in the same direction of rotation. This gives 3,000 turns of wire, the two ends being anchored to soldering tags or terminals threaded into the ebonite former.

When a set is to receive medium, long and short waves a short wave choke should be wired into circuit before each medium and long wave choke-that is nearer the valve (tube) in question-the chokes being in series. Short wave chokes are simple to calculate by using the wavelength ruling; for example a choke for wavelengths around 50 meters would have 12.5 meters (41 feet) of No. 34 S.W.G. enameled or silk covered wire wound on to a narrow paxolin former in three or four small banks, the banks being spaced to cut down the self-capacity. For Ultra Short Wave work the wire becomes very short and is then best wound on to a glass tube of suitable diameter in a single spaced winding.

Transmitter and oscillator chokes for amateur equipment may be made in the same way with any necessary allowance in the wire gauge to suit the probably heavier currents flowing.

Chokes are better un-impregnated, and if protection is thought to be necessary for the 3.000 turn choke it is best provided by winding cellophane tape round the former or by cementing a sheet of celluloid over the wire.

The only test necessary for the choke is a simple continuity check.

In all circuits the choke should not be allowed to approach other wiring in the same valve (tube) circuit, particularly if it is an anode choke as is usually the case. The bypass condenser lead should go directly to earth by the shortest route, and in Ultra Short Wave equipment it is beneficial to run all bypass leads to one main connecting point on the chassis.

Medium and long wave chokes can be screened if desired if the precautions observed in screening coils are noted.