Making Wireless Telephone Transmitting Sets

The Radio Amateur's Hand Book, by A. Frederick Collins is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here: [LINK TO TABLE OF LINK]. Chapter XVIII: Wireless Telephone Transmitting Sets with Direct and Alternating Currents

XVIII. WIRELESS TELEPHONE TRANSMITTING SETS WITH DIRECT AND ALTERNATING CURRENTS

In time past, the most difficult of all electrical apparatus for the amateur to make, install and work was the wireless telephone. This was because it required a direct current of not less than 500 volts to set up the sustained oscillations and all ordinary direct current for lighting purposes is usually generated at a potential of 110 volts.

Now as you know it is easy to step-up a 110 volt alternating current to any voltage you wish with a power transformer but until within comparatively recent years an alternating current could not be used for the production of sustained oscillations for the very good reason that the state of the art had not advanced that far. In the new order of things these difficulties have all but vanished and while a wireless telephone transmitter still requires a high voltage direct current to operate it this is easily obtained from 110 volt source of alternating current by means of vacuum tube rectifiers.

The pulsating direct currents are then passed through a filtering reactance coil, called a reactor, and one or more condensers, and these smooth them out until they approximate a continuous direct current. The latter is then made to flow through a vacuum tube oscillator when it is converted into high frequency oscillations and these are varied, or modulated, as it is called, by a microphone transmitter such as is used for ordinary wire telephony. The energy of these sustained modulated oscillations is then radiated into space from the aerial in the form of electric waves.

The distance that can be covered with a wireless telephone transmitter is about one-fourth as great as that of a wireless telegraph transmitter having the same input of initial current, but it is long enough to satisfy the most enthusiastic amateur. For instance with a wireless telephone transmitter where an amplifier tube is used to set up the oscillations and which is made for a plate potential of 100 volts, distances up to 10 or 15 miles can be covered.

With a single 5 watt oscillator tube energized by a direct current of 350 volts from either a motor-generator or from a power transformer (after it has been rectified and smoothed out) speech and music can be transmitted to upwards of 25 miles. Where two 5 watt tubes connected in parallel are used wireless telephone messages can be transmitted to distances of 40 or 50 miles. Further, a single 50 watt oscillator tube will send to distances of 50 to 100 miles while two of these tubes in parallel will send from 100 to 200 miles. Finally, where four or five oscillator tubes are connected in parallel proportionately greater distances can be covered.

A Short Distance Wireless Telephone Transmitting Set-With 110 Volt Direct Lighting Current.--For this very simple, short distance wireless telephone transmitting set you need the same apparatus as that described and pictured in the beginning of Chapter XVI for a Short Distance C. W. Telegraph Transmitter, except that you use a microphone transmitter instead of a telegraph key. If you have a 110 volt direct lighting current in your home you can put up this short distance set for very little money and it will be well worth your while to do so.

The Apparatus You Need.--For this set you require: (1) one tuning coil as shown at A and B in Fig. 75; (2) one aerial ammeter as shown at C in Fig. 75; (3) one aerial condenser as shown at C in Fig. 75; (4) one grid, blocking and protective condenser as shown at D in Fig. 75; (5) one grid leak as shown at C in Fig. 77; (6) one vacuum tube amplifier which is used as an oscillator; (7) one 6 volt storage battery; (8) one rheostat as shown at I in Fig. 75; (9) one oscillation choke coil; (10) one panel cut-out as shown at K in Fig. 75 and an ordinary microphone transmitter.

The Microphone Transmitter.--The best kind of a microphone to use with this and other telephone transmitting sets is a Western Electric No. 284-W. [Footnote: Made by the Western Electric Company, Chicago, Ill.] This is known as a solid back transmitter and is the standard commercial type used on all long distance Bell telephone lines. It articulates sharply and distinctly and there are no current variations to distort the wave form of the voice and it will not buzz or sizzle. It is shown in Fig. 84 and costs $2.00. Any other good microphone transmitter can be used if desired.

Connecting Up the Apparatus.--Begin by connecting the leading-in wire with one of the terminals of the microphone transmitter, as shown in the wiring diagram Fig. 85, and the other terminal of this to one end of the tuning coil. Now connect clip 1 of the tuning coil to one of the posts of the hot-wire ammeter, the other post of this to one end of aerial condenser and, finally, the other end of the latter with the water pipe or other ground. The microphone can be connected in the ground wire and the ammeter in the aerial wire and the results will be practically the same.

Next connect one end of the grid condenser to the post of the tuning coil that makes connection with the microphone and the other end to the grid of the tube, and then shunt the grid leak around the condenser. Connect the + or positive electrode of the storage battery with one terminal of the filament of the vacuum tube, the other terminal of the filament with one post of the rheostat and the other post of this with the - or negative electrode of the battery. This done, connect clip 2 of the tuning coil to the + or positive electrode of the battery and bring a lead from it to one of the switch taps of the panel cut-out.

Now connect clip 3 of the tuning coil with one end of the blocking condenser, the other end of this with one terminal of the choke coil and the other terminal of the latter with the other switch tap of the cut-out. Connect the protective condenser across the direct current feed wires between the panel cut-out and the choke coil. Finally connect the ends of a lamp cord to the fuse socket taps of the cut-out, and connect the other ends to a lamp plug and screw it into the lamp socket of the feed wires. Screw in a pair of 5 ampere fuse plugs, close the switch and you are ready to tune the transmitter and talk to your friends.

A 25 to 50 Mile Wireless Telephone Transmitter--With Direct Current Motor Generator.--Where you have to start with 110 or 220 volt direct current and you want to transmit to a distance of 25 miles or more you will have to install a motor-generator. To make this transmitter you will need exactly the same apparatus as that described and pictured for the 100 Mile C. W. Telegraph Transmitting Set in Chapter XVI, except that you must substitute a microphone transmitter and a telephone induction coil, or a microphone transformer, or still better, a magnetic modulator, for the telegraph key and chopper.

The Apparatus You Need.--To reiterate; the pieces of apparatus you need are: (1) one aerial ammeter as shown at E in Fig. 75; (2) one tuning coil as shown at A in Fig. 77; (3) one aerial condenser as shown at B in Fig. 77; (4) one grid leak as shown at C in Fig. 77; (5) one grid, blocking and protective condenser; (6) one 5 watt oscillator tube as shown at E in Fig. 77; (7) one rheostat as shown at I in Fig. 75; (8) one 10 volt (5 cell) storage battery; (9) one choke coil; (10) one panel cut-out as shown at K in Fig. 75, and (11) a motor-generator having an input of 110 or 220 volts and an output of 350 volts.

In addition to the above apparatus you will need: (12) a microphone transmitter as shown in Fig. 84; (13) a battery of four dry cells or a 6 volt storage battery, and either (14) a telephone induction coil as shown in Fig. 86; (15) a microphone transformer as shown in Fig. 87; or a magnetic modulator as shown in Fig. 88. All of these parts have been described, as said above, in Chapter XVI, except the microphone modulators.

The Telephone Induction Coil.--This is a little induction coil that transforms the 6-volt battery current after it has flowed through and been modulated by the microphone transmitter into alternating currents that have a potential of 1,000 volts of more. It consists of a primary coil of No. 20 B. and S. gauge cotton covered magnet wire wound on a core of soft iron wires while around the primary coil is wound a secondary coil of No. 30 magnet wire. Get a standard telephone induction coil that has a resistance of 500 or 750 ohms and this will cost you a couple of dollars.

The Microphone Transformer.--This device is built on exactly the same principle as the telephone induction coil just described but it is more effective because it is designed especially for modulating the oscillations set up by vacuum tube transmitters. As with the telephone induction coil, the microphone transmitter is connected in series with the primary coil and a 6 volt dry or storage battery.

In the better makes of microphone transformer, there is a third winding, called a side tone coil, to which a headphone can be connected so that the operator who is speaking into the microphone can listen-in and so learn if his transmitter is working up to standard.

The Magnetic Modulator.--This is a small closed iron core transformer of peculiar design and having a primary and a secondary coil wound on it. This device is used to control the variations of the oscillating currents that are set up by the oscillator tube. It is made in three sizes and for the transmitter here described you want the smallest size, which has an output of 1/2 to 1-1/2 amperes. It costs about $10.00.

How the Apparatus Is Connected Up.--The different pieces of apparatus are connected together in exactly the same way as the 100 Mile C. W. Telegraph Set in Chapter XVI except that the microphone transmitter and microphone modulator (whichever kind you use) is substituted for the telegraph key and chopper.

Now there are three different ways that the microphone and its modulator can be connected in circuit. Two of the best ways are shown at A and B in Fig. 89. In the first way the secondary terminals of the modulator are shunted around the grid leak in the grid circuit as at A, and in the second the secondary terminals are connected in the aerial as at B. Where an induction coil or a microphone transformer is used they are shunted around a condenser, but this is not necessary with the magnetic modulator. Where a second tube is used as in Fig. 90 then the microphone and its modulator are connected with the grid circuit and clip 3 of the tuning coil.

A 50 to 100 Mile Wireless Telephone Transmitter--With Direct Current Motor Generator.--As the initial source of current available is taken to be a 110 or 220 volt direct current a motor-generator having an output of 350 volts must be used as before. The only difference between this transmitter and the preceding one is that: (1) two 5 watt tubes are used, the first serving as an oscillator and the second as a modulator; (2) an oscillation choke coil is used in the plate circuit; (3) a reactance coil or reactor, is used in the plate circuit; and (4) a reactor is used in the grid circuit.

The Oscillation Choke Coil.--You can make this choke coil by winding about 275 turns of No. 28 B. and S. gauge cotton covered magnet wire on a spool 2 inches in diameter and 4 inches long. Give it a good coat of shellac varnish and let it dry thoroughly.

The Plate and Grid Circuit Reactance Coils.--Where a single tube is used as an oscillator and a second tube is employed as a modulator, a reactor, which is a coil of wire wound on an iron core, is used in the plate circuit to keep the high voltage direct current of the motor-generator the same at all times. Likewise the grid circuit reactor is used to keep the voltage of the grid at a constant value. These reactors are made alike and a picture of one of them is shown in Fig. 91 and each one will cost you $5.75.

Connecting up the Apparatus.--All of the different pieces of apparatus are connected up as shown in Fig. 89. One of the ends of the secondary of the induction coil, or the microphone transformer, or the magnetic modulator is connected to the grid circuit and the other end to clip 3 of the tuning coil.

A 100 to 200 Mile Wireless Telephone Transmitter--With Direct Current Motor Generator.--By using the same connections shown in the wiring diagrams in Fig. 89 and a single 50 watt oscillator tube your transmitter will then have a range of 100 miles or so, while if you connect up the apparatus as shown in Fig. 90 and use two 50 watt tubes you can work up to 200 miles. Much of the apparatus for a 50 watt oscillator set where either one or two tubes are used is of the same size and design as that just described for the 5 watt oscillator sets, but, as in the C. W. telegraph sets, some of the parts must be proportionately larger. The required parts are (1) the 50 watt tube; (2) the grid leak resistance; (3) the filament rheostat; (4) the filament storage battery; and (5) the magnetic modulator. All of these parts, except the latter, are described in detail under the heading of a 500 Mile C. W. Telegraph Transmitting Set in Chapter XVI, and are also pictured in that chapter.

It is not advisable to use an induction coil for the modulator for this set, but use, instead, either a telephone transformer, or better, a magnetic modulator of the second size which has an output of from 1-1/2 to 3-1/2 amperes. The magnetic modulator is described and pictured in this chapter.

A 50 to 100 Mile Wireless Telephone Transmitting Set--With 110 Volt Alternating Current.--If you have a 110 volt [Footnote: Alternating current for lighting purposes ranges from 102.5 volts to 115 volts, so we take the median and call it 110 volts.] alternating current available you can use it for the initial source of energy for your wireless telephone transmitter. The chief difference between a wireless telephone transmitting set that uses an alternating current and one that uses a direct current is that: (1) a power transformer is used for stepping up the voltage instead of a motor-generator, and (2) a vacuum tube rectifier must be used to convert the alternating current into direct current.

The Apparatus You Need.--For this telephone transmitting set you need: (1) one aerial ammeter; (2) one tuning coil; (3) one telephone modulator; (4) one aerial series condenser; (5) one 4 cell dry battery or a 6 volt storage battery; (6) one microphone transmitter; (7) one battery switch; (8) one grid condenser; (9) one grid leak; (10) two 5 watt oscillator tubes with sockets; (11) one blocking condenser; (12) one oscillation choke coil; (13) two filter condensers; (14) one filter reactance coil; (15) an alternating current power transformer, and (16) two 20 watt rectifier vacuum tubes.

All of the above pieces of apparatus are the same as those described for the 100 Mile C. W. Telegraph Transmitter in Chapter XVII, except: (a) the microphone modulator; (b) the microphone transmitter and (c) the dry or storage battery, all of which are described in this chapter; and the new parts which are: (d) the rectifier vacuum tubes; (e) the filter condensers; and (f) the filter reactance coil; further and finally, the power transformer has a third secondary coil on it and it is this that feeds the alternating current to the rectifier tubes, which in turn converts it into a pulsating direct current.

The Vacuum Tube Rectifier.--This rectifier has two electrodes, that is, it has a filament and a plate like the original vacuum tube detector, The smallest size rectifier tube requires a plate potential of 550 volts which is developed by one of the secondary coils of the power transformer. The filament terminal takes a current of 7.5 volts and this is supplied by another secondary coil of the transformer. This rectifier tube delivers a direct current of 20 watts at 350 volts. It looks exactly like the 5 watt oscillator tube which is pictured at E in Fig. 77. The price is $7.50.

The Filter Condensers.--These condensers are used in connection with the reactance coil to smooth out the pulsating direct current after it has passed through the rectifier tube. They have a capacitance of 1 mfd. and will stand 750 volts. These condensers cost about $2.00 each.

The Filter Reactance Coil.--This reactor which is shown in Fig. 92, has about the same appearance as the power transformer but it is somewhat smaller. It consists of a coil of wire wound on a soft iron core and has a large inductance, hence the capacitance of the filter condensers are proportionately smaller than where a small inductance is used which has been the general practice. The size you require for this set has an output of 160 milliamperes and it will supply current for one to four 5 watt oscillator tubes. This size of reactor costs $11.50.

Connecting Up the Apparatus.--The wiring diagram in Fig. 93 shows how the various pieces of apparatus for this telephone transmitter are connected up. You will observe: (1) that the terminals of the power transformer secondary coil which develops 10 volts are connected to the filaments of the oscillator tubes; (2) that the terminals of the other secondary coil which develops 10 volts are connected with the filaments of the rectifier tubes; (3) that the terminals of the third secondary coil which develops 550 volts are connected with the plates of the rectifier tubes; (4) that the pair of filter condensers are connected in parallel and these are connected to the mid-taps of the two filament secondary coils; (5) that the reactance coil and the third filter condenser are connected together in series and these are shunted across the filter condensers, which are in parallel; and, finally, (6) a lead connects the mid-tap of the 550-volt secondary coil of the power transformer with the connection between the reactor and the third filter condenser.

A 100 to 200 Mile Wireless Telephone Transmitting Set--With 110 Volt Alternating Current.--This telephone transmitter is built up of exactly the same pieces of apparatus and connected up in precisely the same way as the one just described and shown in Fig. 93.

Apparatus Required.--The only differences between this and the preceding transmitter are: (1) the magnetic modulator, if you use one, should have an output of 3-1/2 to 5 amperes; (2) you will need two 50 watt oscillator tubes with sockets; (3) two 150 watt rectifier tubes with sockets; (4) an aerial ammeter that reads to 5 amperes; (5) three 1 mfd. filter condensers in parallel; (6) two filter condensers of 1 mfd. capacitance that will stand 1750 volts; and (6) a 300 milliampere filter reactor.

The apparatus is wired up as shown in Fig. 93.

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Collins, A. Frederick. 2002. The Radio Amateur's Hand Book. Urbana, Illinois: Project Gutenberg. Retrieved April 2022, from https://www.gutenberg.org/files/6934/6934-h/6934-h.htm#chap18

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