Breaking into the Amateur Game

Radio Record, Volume IV, Issue 26, 9 January 1931, Page 7

Breaking into the Amateur Game

Part []]-The Transmitter, Power Supply and Antenna

By

Q.R.

L.

‘N the last: article.we discussed the requirements of the amateur operator’s examination ; this included a: description of a low-priced. transmitter suitable for a beginner. The simplicity and cheapness of it will probably surprise the reader, but though using no more power than a large receiver, it is nevertheless a transmitter capable of sending signals considerable distances.- Most amateurs commence transmitting with just such apparatus, and invariably increase it as they gain experience in its operation. This transmitter is. mainly for Morse work. In general, a transmitter may be divided into three parts-the transmitting circuit, the power supply, and the antenna, each of which will be dealt with in turn. The Transmitting Circuit. ‘THERE are several different types ; of transmitting circuits, with difnumbers of valves, but the great majority of amateurs use only a single valve, which is simply an oscillator, or generator of radio-frequency eurrent, aS in the transmitter described. Now with an oscillator, as with a regenerative receiver, there are only two fundmental circuits, which are those using inductive coupling feedback from the plate to the grid to cause oscillation, and those using capacitive coupling feed-back. A. circuit consists simply of a means of varying this feed-back, of varying the frequency of the wave, and of resonantly coupling the antenna circuit to the oscillator. The chief factor in a circuit is to get the largest output for a given input, other things remaining equal, but in this respect, however, there is no circuit. particularly better . than the others, as each will give the same performance, if correctly ad-' -_ which is important. "efhe circuit shown here is-simple and popular, and is just as efficient as any othey’ when handled correctly. It is known..as the Hartley circuit and employs the inductive coupling method. of plate to grid feed-back. The plate in. ductance (PI) is-virtually divided into two by the tapping from the filament (FT), making the smaller portion in the grid circuit and the larger in the plate circuit, and induction between these two portions maintains the. feedback and oscillation. The amount of this feed-back will be varied by the number of turns in.both the grid and plate parts of the coil, thus by chanzgdog the point of tapping of the filament * clip.. For best results it should be’ on. about the fourth ‘turn from the grid end of the coil. eee As with a‘receiver, the frequency of the wave will be varied by means: vi | the variable condenser C, across the | plate coil The wavelength span with

the coil shown is from 70 to 100 metres approx., so as to get the 75-85 metres amateur band in comfortably. For the other bands a proportional number of turns in the coil would be used-e.g., for the 40-metre band, seven turns. it is in this condenser-coil circuit that the heaviest R.F. currents flow; these being much greater than those in a receiver, hence the necessity for heavy conductors in the inductances, etec., as later described. The antenna circuit is coupled inductively to the oscillator through the antenna coil (AI), resonance being obtained by tuning with the antenna condenser C, and indicated by the lighting of the bulb B, as in the absorption wavemeter described in the last article. Making the Transmitter. OW for the individual parts of the transmitter. Firstly, all are mounted, as shown in the plan diagram, on ‘a wooden baseboard, about 14in, x 8in.,, which should preferably be shellaced. The layout of the parts need not be strictly adhered to, but as all leads are conveniently short, it is hard to better. For the low power input being used, the valve V may be any receiving type such as UX-201A, 112A, or 171A. The power amplifying valves give a little more output than the 201A, but the latter is entirely satisfactory for this transmitter If the input is later increased, however, larger transmitting valves will be necessary, its size being directly dependent on the input used. An ordinary UX type socket with terminals arranged as in the plan holds the valve. The two variable condensers, -O, dre simply good receiving type condensers of a .0005 mfd. capacity, with any kind of dial (not necessarily slow motion), and mounted on short strips of ebonite screwed to the front edge of the base-

board, a whole panel being superfluous. It will be seen in the plan diagram that connections to the moving plates of the condensers are made to the metal frames at either side, thus simplifying wiring. For this purpose most condensers have convenient screws and nuts, which may. be used as terminals, holding the frame together. Next come the plate and antenna inductances. They consist of about 3-16 in. diameter soft copper tubing, wound in 3in. diameter coils, with approx. 3-16in. spacing between them. Once wound, the coils are self-supporting and need no former, thus avoiding dielectric losses. There are fourteen turns in the plate coil, requiring 12 feet of tubing and 5 turns in the antenna coil, requiring 4 feet. For winding the coils, a 3in. diamater former, preferably wooden, is used, and the tubing wound on with the turns touching. The former is then displaced with, and the turns pulled apart, until the desired spacing of 3-16in. is obtained. On the baseboard the coils are mounted on two tin, diameter glass rods (GR), 24in. apart, running the length of and lin. above the baseboard, and held in place by two wooden blocks with suitable grooves. A simple means of sending " ’phone" with the transmitter is shown in Fig. I. From the grid end of the plate coil a similar coil of about three turns is coupled about jin. away, and an ordinary carbon telephone microphone connected to its ends. This system of modulation is not very efficient. It is known as "loop" modulation. The antenna coupling, or amount of RF energy transferred from the oscillator to the antenna, is varied by sliding the antenna coil along the glass rods to or from the. plate. coil,.

and the distance between the two should not be lesS than 2in, or the tube may be put out of oscillation, The purpose of the radio frequency choke (RFC) is to prevent the RE currents generated from leaking away into the power supply, but to allow the D.C. to pass. The choke is a coil of 150 turns wound on a half-inch wooden dowel with about 30 g. dec. wire, and mounted with small clips. C2 is the plate blocking of condenser, which allows the R.F. current to pass, but blocks the D.C. It may be any receiver type fixed condenser of capacity .002 mfd., but as the whole plate voltage is across it, it should have good insulation, say 500 volts test. The grid condenser (C8) is for the purpose of passing R.F. voltages on to the grid, causing on it a charge which is eventually allowed to leak off by means of the grid leak to the filament. For this transmitter the grid leak may be any 5000 to 10,000-ohm resistance, and the grid condenser a .00025 mfd. fixed receiver type. In the antenna circuit the bulk B is an ordinary 6-volt torch bulb in a suitable socket, with a small shorting switch, which is closed after the antenna has been tuned, as later described. No metres have been included in the. transmitter, as for the power used they are not absolutely necessary, and would add considerably to- the initial cost. If the constructor does not mind this, however, suitable metres would be a filament voltmeter, a plate milliammetre, reading "0-100 mills., and an r.f. ammeter (0.1 amp.), the latter to take the place of the torch bulb. The whole of the transmitter must be firmly constructed so that the parts and leads, especially the inductances, cannot vibrate, as this would cause the signal to "wobble." For the leads between the condensers and coils, heavy rubber covered fiex should be used, with strong clips to connect to the ends of the coils, so that here losses through resistance will be minimised. The remainder of the wiring may be done with 14g. copper wire. | No terminals are used, as the power wiring to the filament, choke and coll-tap-ping is run direct underneath the baseboard with insulated cable. The key may be mounted anywhere convenient on the opérating table or bench. The Power Supply. 'THERD is nothing more to the oscillator portion of the transmitter, so we may go on to the next part, the power supply. This in turn is divided-into the filament supply, and the plate supply. For the filament, the same 6-volt battery as is in use in the receiver may be used, with a D.P.D.T. switch to, change it over from the receiver to the transmitter, and. vice versa. Alternating eur-

rent at a suitable voltage may also be used on the filament, provided that a., means of obtaining ,the electrical eentre of the filament (where the alternating of the voltage has. no effect) by means of resistances, is used, as shown in fig. II. The condensers are for the purpose of bypassing r.f. currents. This apparatus eould be mounted at the back of the baseboard where the tapping comes through. , Now the strength of the transmitter’s signals and the distances coverable depend greatly on the voltage of the plate power supply, or on the amount of power input to the valve. With a voltage of 100 to 200, all New Zealand will be easily covered, and occasionally Australia, on the higher amateur wave ‘ bands, and with from 300 volts upwards nearly all the world may be worked on the shorter . wave-lengths. This transmitter works well with from 100 to 300 volts on the plate of the valve, and is not designed for use with over 3800, as receiving valves do not stand up for long to higher voltages, and heavier coils would be required. The plate supply may be obtained either from receiver B batteries or from the A.C. mains, suitably rectified and filtered, as in a B eliminator; the oscillator plate current does not normally exceed 20 milliamps. It is most important that if rectified alternating current is used (R.A.C.), it should be well smoothed, as the pureness of the signal’s note depends largely on this. As explained in the last article, a rough, harsh note is caused by using unsmoothed R.A.C., while a pure musical one covering less territory of the band results fiom using direct current or well-smoothed R.A.C. These types of ‘notes are respectively termed R.A.C. and D.C. notes. If batteries are not available, a good way of using the A.C. mains for power supply is by means of a chemical rectifier, which for low voltage is both cheap and simple. The parts for such a rectifier may be obtained for a’ few shillings, and with

a suitable. filter, such as four 2 mfd. condensers, it should give a DC note. There is a voltage drop of about 20 per cent. through a chemical rectifier, so a transformer delivering 400 volts with a centre-tap should be used, a suitable one being described in tie ‘Radio Listeners’ Guide." It may also have a filament winding on it to supply current at 6 volts for the filament of the valve. About eight jars in each "leg" of the rectifier should be used, as shown in fig. III, with lead and aiuminium electrodes in a saturated borax solution,.so that there are about 2} square inches of each electrode in the solution in each jar. The rectifier must be "formed" in the usual way by placing an ordinary 230-volt lamp in series ‘with it, and running the current until the lamp becomes dim. It is worth while noting: that the regulations prohibit the use of "slop" reetifiers unless a transformer is used between them and the mains. A full-wave rectifying valve would be less trouble to hook up than the chemical rectifier, but would cost more and need more filter, also the latter rectifier requires little attention once installed properly. To conclude the power supply discussion, suitable power switches should be connected so that the transmitter may be turned on or off quickly. The Antenna Supply. HE last, but not least, part of the ‘transmitter is the antenna, or radiator. So that waves of the desired frequency will be most easily radiated by them, antennas must be of definite overall lengths, the best being half. the length of the waves being radiated, and they must be "fed" from the oscillator through the antenna coil at one of several definite points in their lengths. Thus for frequencies of 3500 to 4000 kilocycles (85 to 75 metres), which is the amateur band in which the oscillator is tuned, the length of the antenna should be 132 feet, or about 40 metres (1 metre equals 3} feet). To most easily obtain this length, what 1s often termed an antenna-counterpoise system may be used, as shown in the accompanying diagrams, the _ total length of the antenna and counterpoise being in each case 132 feet, and feeding taking place at either of two suitable places-in the centre, or quarterlength from the lower end. As the latter type is the better radiator, it should be used if sufficient length in the antenna (99 feet) can be obtained. This is the simplest type of antenna for the beginner, though there are other types, employing different feed sys’.ms, and enabling the whole antenna to be strung well up in the clear-the disadvantage of the type described is that the radiator has to be brought into the station. However, it will

work quite well until the beginner becomes "wise" to the various systems. The antenna and counterpoise may be ordinary receiving aerial wire, preferably enamelled, and the insulation at the ends and where they enter the station should be good. It must be noted that the lengths shown are those from the far insulators right to the antenna coil, and they should be made exact to within one foot. The antenna should be strung as high as possible, 30 feet being satisfactory, and the counterpoise need not be run directly underneath the antenna. It may be any suitable distanc.. not much beyond eight feet, above the ground. As already mentioned, either system A (66 feet in both the antenna and counterpoise) or B (99 feet in the antenna and 33 in the counterpoise) may be used, but the latter is preferable if the lemgths can be suitably worked in. The same antenna may also be used for receiving, a 8.P.D.T. switch being arranged to throw it over to either transmitter or receiver. : We have now covered all the ground considered necessary in this article and if any reader requires further information it will be given on application to the writer, addressed to the "Record" office... In the next article there will be a description of the tuning of the transmitter and general operating practice.