SUPER-HETERODYNE

Poverty Bay Herald, Volume LIII, Issue 16233, 6 January 1927, Page 3

SUPER-HETERODYNE

FOR SHORT WAVE SIGNALS

THE RECEIVER DESCRIBED

Receivers used for the reception of short-wave signals generally make use of a regenerative detector followed by one or two stages of audio frequency amplification. In the majority of cases r.f. amplification is not resorted to, tho reason being that the use of r.f. stages becomes very difficult for the high frequencies involved. In the November Radio Broadcast magazine, however, is described a super-hetero-dyne short-wave receiver, efficient,, and easy to handle. This receiver was first, tried out just a year ago and has since boon developed to that stage where it may be duplicated by tho average set constructor 1 without fear of any snags cropping up. It employs two intermediate stages of r.f. amplification.

The super-heterodyne used for tho reception of short waves differs somewhat from that used for the reception of broadcasting, although of course, the general theory is identical.

INCOMING FREQUENC V

Tho super-heterodyne method of reception consists of tuning to the incoming frequency, beating with it another frequency, and then amplifying and detecting the beat note. The actual signal listened to has in it none of the original frequency or the frequency which caused the beat note. In the reception of broadcast programmes or other signals, between 1500 and 550 kc (200 and 500 meters) tho beat note selected is a frequency somewhere between 30 and 40 kilocycles. This relatively high frequency is selected to prevent the introduction of distortion, by elimination of tho side-band frequencies in the intermediate amplifier and filter.

DISTORTION ELEMENT

In the reception of short waves, particularly the reception of c.w., this element of distortion may be disregarded, and such has been the case in this receiver, the assumption being that most of the signals received will be c.w. The ordinary "super" used for broadcast reception has two tunings; first, the loop or antenna circuit; second, the oscillator circuit. This shortwave "super" has only one tuning arrangement, in which is combined both the tuning operations indicated above This method of tuning was selected because of its simplicity and because it makes possiblo the construction of what is practically a single-control set. The intermediate frequency chosen is 22 kilocycles which. Avhilc too low a frequency for good telephone reception when simple tuned circuits arc used, is satisfactory for c.w. or telegraph signals. The selection of this frequency necessitates detuning the set 22 kilocycles from the incoming signal, but at the frequencies corresponding to wave lengths of 100 meters or under, this detuning is of no importance in decreasing signal strength.

THE BEAT NOTE.

The reader will recognise the description above as applying to the "autodyne" or ''self-heterodyne" typo of"' ?< super.'' The beat noto of 22-k'c. is created in the same manner as in the broadcast set but at a lower frequency. For the reception of short-wave telephone signals,, the amplification aud detection of the 22-kc. beat note is in the usual manner. When c.w. signals are to be received, another beat note must be created either by means of another oscillator tube or by a self-heterodyne beat not in the second detector tube. The latter method has been selected, a beat note of 1000 cycles being chosen as the most satisfactory. This detuning of the second detector circuit, while it may appear to be inefficient because of the low intermediate frequency, is not so bad as it seems since the amplification in the intermediate circuit is very great and there is plenty of energy to spare. CONDENSERS SHOWN.

All the tuning is done with one epndenser in the 'grid circuit. The condenser in the plate circuit must be set for each band of frequencies covered; for instance from 709 kc. to 6663 k.c. (40 to 4£>meters), G6G3 kc. to 5996 kc. (45 to 50 meters), etc. This sotting is not critical, the only requirement is that the tube oscillate strongly but not so violently that it blocks. The variable condenser across the secondary in the silter circuit is not considered a major control.

The choke coil consists of- 100 turns wound on a wooden form 1 inch in diameter aud 2 inches long. A-honey-coiub or similar coil of 150 or 250 turns will also serve very nicely. The intermediate transformers must be capable of amplifying the rather low frequency of 22-kc. In this set, those manufactured by the General Radio Company were xised, but there aro probably any number of others which will serve. The coils used in tho antenna, grid, and plate circuits are made by winding bare copper wire of No. Hi gauge over a form on which are placed four narrow strips of celluloid, equally spaced. The wire is spaced with string, and wheu completely wound, the string is removed, and the wire cemented to the strips by means of liquid celluloid. The; construction of this type of coil is familiar to anyone who has followed the development of the short-wave regenerative receiver.

THE FKEQUENCY BAND,

The diameter of the coils is 3 inches for whatever frequency band the coil is designed to cover. In tho radio broadcast article, details are given for making coils for the 40, 50 and 80 meter band. In tho first detector circuit,, for forty meters, the antenna coil has six turns; the grid coil seventeen turns with tap at the fourth turn from tho filament end; and the plate coil three funis. Tho figures given for the coils aro only approximately correct, as tho method of wiring, mounting, etc., all affect the capacity of the coils, aud, in consequence, the number of turns required to cover a given frequency range. Where the operator or constructor has a satisfactory regenerative receiver already in operation, there is no need to change, even though the circuit differs from the onp described. The only requirement is that the primary of thp intermediate transformer bo free "of a capacity shunt greater than 0.00025 mfd.