A 200 to 500 Mile C. W. Telegraph Transmitting Set.--Distances of from 200 to 500 miles can be successfully covered with a telegraph transmitter using two, three or four 5 watt oscillator tubes in parallel. The apparatus needed is identical with that used for the 100 mile transmitter just described. The tubes are connected in parallel as shown in the wiring diagram in Fig. 83.
A 500 to 1,000 Mile C. W. Telegraph Transmitting Set.--With the apparatus described for the above set and a single 50 watt oscillator tube a distance of upwards of 500 miles can be covered, while with two 50 watt oscillator tubes in parallel you can cover a distance of 1,000 miles without difficulty, and nearly 2,000 miles have been covered with this set.
The Apparatus Required.--All of the apparatus for this C. W. telegraph transmitting set is the same as that described for the 100 and 200 mile sets but you will need: (1) one or two 50 watt oscillator tubes with sockets; (2) one key condenser that has a capacitance of 1 mfd., and a rated potential of 1,750 volts; (3) one 0 to 500 milli-ammeter; (4) one aerial ammeter reading to 5 amperes, and (5) an A. C. power transformer for one or two 50 watt tubes.
| Photograph unavailable |
| Broadcasting Government Reports by Wireless from Washington. This shows Mr. Gale at work with his set in the Post Office Department. |
The Alternating Current Power Transformer.--This power transformer is made exactly like the one described in connection with the preceding 100 mile transmitter and pictured in Fig. 81, but it is considerably larger. Like the smaller one, however, it is made to work with a 50 to 60 cycle current at 102.5 to 115 volts and, hence, can be used with any A. C. lighting current.
It has an input of 750 volts and the high voltage secondary coil which energizes the plate has an output of 450 watts and develops 1,500 to 3,000 volts. The low voltage secondary coil which heats the filament develops 10.5 volts. This transformer will supply current for one or two 50-watt oscillator tubes and it costs about $40.00.
Connecting Up the Apparatus.--Where a single oscillator tube is used the parts are connected as shown in Fig. 82, and where two tubes are connected in parallel the various pieces of apparatus are wired together as shown in Fig. 83. The only difference between the 5 watt tube transmitter and the 50 watt tube transmitter is in the size of the apparatus with one exception; where one or two 50 watt tubes are used a second condenser of large capacitance (1 mfd.) is placed in the grid circuit and the telegraph key is shunted around it as shown in the diagram Fig. 83.
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.
The three foregoing chapters explained in detail the design and construction of (1) two kinds of C. W. telegraph transmitters, and (2) two kinds of wireless telephone transmitters, the difference between them being whether they used (A) a direct current, or (B) an alternating current as the initial source of energy. Of course there are other differences between those of like types as, for instance, the apparatus and connections used (a) in the key circuits, and (b) in the microphone circuits. But in all of the transmitters described of whatever type or kind the same fundamental device is used for setting up sustained oscillations and this is the vacuum tube.
The Operation of the Vacuum Tube Oscillator.--The operation of the vacuum tube in producing sustained oscillations depends on (1) the action of the tube as a valve in setting up the oscillations in the first place and (2) the action of the grid in amplifying the oscillations thus set up, both of which we explained in Chapter XIV. In that chapter it was also pointed out that a very small change in the grid potential causes a corresponding and larger change in the amount of current flowing from the plate to the filament; and that if a vacuum tube is used for the production of oscillations the initial source of current must have a high voltage, in fact the higher the plate voltage the more powerful will be the oscillations.
To understand how oscillations are set up by a vacuum tube when a direct current is applied to it, take a look at the simple circuits shown in Fig. 94. Now when you close the switch the voltage from the battery charges the condenser and keeps it charged until you open it again; the instant you do this the condenser discharges through the circuit which includes it and the inductance coil, and the discharge of a condenser is always oscillatory.
Where an oscillator tube is included in the circuits as shown at A and B in Fig. 94, the grid takes the place of the switch and any slight change in the voltage of either the grid or the plate is sufficient to start a train of oscillations going. As these oscillations surge through the tube the positive parts of them flow from the plate to the filament and these carry more of the direct current with them.
To make a tube set up powerful oscillations then, it is only necessary that an oscillation circuit shall be provided which will feed part of the oscillations set up by the tube back to the grid circuit and when this is done the oscillations will keep on being amplified until the tube reaches the limit of its output.
The Operation of C. W. Telegraph Transmitters With Direct Current--Short Distance C. W. Transmitter.--In the transmitter shown in the wiring diagram in Fig. 76 the positive part of the 110 volt direct current is carried down from the lamp socket through one side of the panel cut-out, thence through the choke coil and to the plate of the oscillator tube, when the latter is charged to the positive sign. The negative part of the 110 volt direct current then flows down the other wire to the filament so that there is a difference of potential between the plate and the filament of 110 volts. Now when the 6-volt battery current is switched on the filament is heated to brilliancy, and the electrons thrown off by it form a conducting path between it and the plate; the 110 volt current then flows from the latter to the former.
Now follow the wiring from the plate over to the blocking condenser, thence to clip 3 of the tuning coil, through the turns of the latter to clip 2 and over to the filament and, when the latter is heated, you have a closed oscillation circuit. The oscillations surging in the latter set up other and like oscillations in the tuning coil between the end of which is connected with the grid, the aerial and the clip 2, and these surge through the circuit formed by this portion of the coil, the grid condenser and the filament; this is the amplifying circuit and it corresponds to the regenerative circuit of a receiving set.
When oscillations are set up in it the grid is alternately charged to the positive and negative signs. These reversals of voltage set up stronger and ever stronger oscillations in the plate circuit as before explained. Not only do the oscillations surge in the closed circuits but they run to and fro on the aerial wire when their energy is radiated in the form of electric waves. The oscillations are varied by means of the telegraph key which is placed in the grid circuit as shown in Fig. 76.
The Operation of the Key Circuit.--The effect in a C. W. transmitter when a telegraph key is connected in series with a buzzer and a battery and these are shunted around the condenser in the grid circuit, is to rapidly change the wave form of the sustained oscillations, and hence, the length of the waves that are sent out. While no sound can be heard in the headphones at the receiving station so long as the points of the key are not in contact, when they are in contact the oscillations are modulated and sounds are heard in the headphones that correspond to the frequency of the buzzer in the key circuit.
The Operation of C. W. Telegraph Transmitters with Direct Current.--The chief differences between the long distance sets which use a direct current, i.e., those described in Chapter XVI, and the short distance transmitting sets are that the former use: (1) a motor-generator set for changing the low voltage direct current into high voltage direct current, and (2) a chopper in the key circuit. The way the motor-generator changes the low- into high-voltage current has been explained in Chapter XVI.
The chopper interrupts the oscillations surging through the grid circuit at a frequency that the ear can hear, that is to say, about 800 to 1,000 times per second. When the key is open, of course, the sustained oscillations set up in the circuits will send out continuous waves but when the key is closed these oscillations are broken up and then they send out discontinuous waves. If a heterodyne receiving set, see Chapter XV, is being used at the other end you can dispense with the chopper and the key circuit needed is very much simplified. The operation of key circuits of the latter kind will be described presently.
The Operation of C. W. Telegraph Transmitters with Alternating Current--With a Single Oscillator Tube.--Where an oscillator tube telegraph transmitter is operated by a 110 volt alternating current as the initial source of energy, a buzzer, chopper or other interruptor is not needed in the key circuit. This is because oscillations are set up only when the plate is energized with the positive part of the alternating current and this produces an intermittent musical tone in the headphones. Hence this kind of a sending set is called a tone transmitter.
Since oscillations are set up only by the positive part or voltage of an alternating current it is clear that, as a matter of fact, this kind of a transmitter does not send out continuous waves and therefore it is not a C. W. transmitter. This is graphically shown by the curve of the wave form of the alternating current and the oscillations that are set up by the positive part of it in Fig. 95. Whenever the positive half of the alternating current energizes the plate then oscillations are set up by the tube and, conversely, when the negative half of the current charges the plate no oscillations are produced.
You will also observe that the oscillations set up by the positive part of the current are not of constant amplitude but start at zero the instant the positive part begins to energize the plate and they keep on increasing in amplitude as the current rises in voltage until the latter reaches its maximum; then as it gradually drops again to zero the oscillations decrease proportionately in amplitude with it.
Heating the Filament with Alternating Current.--Where an alternating current power transformer is used to develop the necessary plate voltage a second secondary coil is generally provided for heating the filament of the oscillation tube. This is better than a direct current for it adds to the life of the filament. When you use an alternating current to heat the filament keep it at the same voltage rather than at the same amperage (current strength). To do this you need only to use a voltmeter across the filament terminals instead of an ammeter in series with it; then regulate the voltage of the filament with a rheostat.
The Operation of C. W. Telegraph Transmitters with Alternating Current--With Two Oscillator Tubes.--By using two oscillator tubes and connecting them up with the power transformer and oscillating circuits as shown in the wiring diagram in Fig. 83 the plates are positively energized alternately with every reversal of the current and, consequently, there is no time period between the ending of the oscillations set up by one tube and the beginning of the oscillations set up by the other tube. In other words these oscillations are sustained but as in the case of those of a single tube, their amplitude rises and falls. This kind of a set is called a full wave rectification transmitter.
The waves radiated by this transmitter can be received by either a crystal detector or a plain vacuum-tube detector but the heterodyne receptor will give you better results than either of the foregoing types.
The Operation of Wireless Telephone Transmitters with Direct Current--Short Distance Transmitter.--The operation of this short distance wireless telephone transmitter, a wiring diagram of which is shown in Fig. 85 is exactly the same as that of the Direct Current Short Distance C. W. Telegraph Transmitter already explained in this chapter. The only difference in the operation of these sets is the substitution of the microphone transmitter for the telegraph key.
The Microphone Transmitter.--The microphone transmitter that is used to vary, or modulate, the sustained oscillations set up by the oscillator tube and circuits is shown in Fig. 84. By referring to the diagram at A in this figure you will readily understand how it operates. When you speak into the mouthpiece the sound waves, which are waves in the air, impinge upon the diaphragm and these set it into vibration--that is, they make it move to and fro.
When the diaphragm moves toward the back of the transmitter it forces the carbon granules that are in the cup closer together; this lowers their resistance and allows more current from the battery to flow through them; when the pressure of the air waves is removed from the diaphragm it springs back toward the mouth-piece and the carbon granules loosen up when the resistance offered by them is increased and less current can flow through them. Where the oscillation current in the aerial wire is small the transmitter can be connected directly in series with the latter when the former will surge through it. As you speak into the microphone transmitter its resistance is varied and the current strength of the oscillations is varied accordingly.
The Operation of Wireless Telephone Transmitters with Direct Current--Long Distance Transmitters.--In the wireless telephone transmitters for long distance work which were shown and described in the preceding chapter a battery is used to energize the microphone transmitter, and these two elements are connected in series with a microphone modulator. This latter device may be either (1) a telephone induction coil, (2) a microphone transformer, or (3) a magnetic modulator; the first two of these devices step-up the voltage of the battery current and the amplified voltage thus developed is impressed on the oscillations that surge through the closed oscillation circuit or the aerial wire system according to the place where you connect it. The third device works on a different principle and this will be described a little farther along.
The Operation of Microphone Modulators--The Induction Coil.--This device is really a miniature transformer, see A in Fig. 86, and its purpose is to change the 6 volt direct current that flows through the microphone into 100 volts alternating current; in turn, this is impressed on the oscillations that are surging in either (1) the grid circuit as shown at A in Fig. 89, and in Fig. 90, (2) the aerial wire system, as shown at B in Fig. 89 and Fig. 93. When the current from the battery flows through the primary coil it magnetizes the soft iron core and as the microphone varies the strength of the current the high voltage alternating currents set up in the secondary coil of the induction coil are likewise varied, when they are impressed upon and modulate the oscillating currents.
The Microphone Transformer.--This is an induction coil that is designed especially for wireless telephone modulation. The iron core of this transformer is also of the open magnetic circuit type, see A in Fig. 87, and the ratio of the turns [Footnote: See Chapter VI] of the primary and the secondary coil is such that when the secondary current is impressed upon either the grid circuit or the aerial wire system it controls the oscillations flowing through it with the greatest efficiency.
The Magnetic Modulator.--This piece of apparatus is also called a magnetic amplifier. The iron core is formed of very thin plates, or laminations as they are called, and this permits high-frequency oscillations to surge in a coil wound on it. In this transformer, see A in Fig. 88, the current flowing through the microphone varies the magnetic permeability of the soft iron core by the magnetic saturation of the latter. Since the microphone current is absolutely distinct from the oscillating currents surging through the coil of the transformer a very small direct current flowing through a coil on the latter will vary or modulate very large oscillating currents surging through the former. It is shown connected in the aerial wire system at A in Fig. 88, and in Fig. 93.
Operation of the Vacuum Tube as a Modulator.--Where a microphone modulator of the induction coil or microphone transformer type is connected in the grid circuit or aerial wire system the modulation is not very effective, but by using a second tube as a modulator, as shown in Fig. 90, an efficient degree of modulation can be had. Now there are two methods by which a vacuum tube can be used as a modulator and these are: (1) by the absorption of the energy of the current set up by the oscillator tube, and (2) by varying the direct current that energizes the plate of the oscillator tube.
The first of these two methods is not used because it absorbs the energy of the oscillating current produced by the tube and it is therefore wasteful. The second method is an efficient one, as the direct current is varied before it passes into the oscillator tube. This is sufficient reason for describing only the second method. The voltage of the grid of the modulator tube is varied by the secondary coil of the induction coil or microphone transformer, above described. In this way the modulator tube acts like a variable resistance but it amplifies the variations impressed on the oscillations set up by the oscillator tube. As the magnetic modulator does the same thing a vacuum tube used as a modulator is not needed where the former is employed. For this reason a magnetic modulator is the cheapest in the long run.
The Operation of Wireless Telephone Transmitters with Alternating Current.--Where an initial alternating current is used for wireless telephony, the current must be rectified first and then smoothed out before passing into the oscillator tube to be converted into oscillations. Further so that the oscillations will be sustained, two oscillator tubes must be used, and, finally, in order that the oscillations may not vary in amplitude the alternating current must be first changed into direct current by a pair of rectifier vacuum tubes, as shown in Fig. 93. When this is done the plates will be positively charged alternately with every reversal of the current in which case there will be no break in the continuity of the oscillations set up and therefore in the waves that are sent out.
The Operation of Rectifier Vacuum Tubes.--The vacuum tube rectifier is simply a two electrode vacuum tube. The way in which it changes a commercial alternating current into pulsating direct current is the same as that in which a two electrode vacuum tube detector changes an oscillating current into pulsating direct currents and this has been explained in detail under the heading of The Operation of a Two Electrode Vacuum Tube Detector in Chapter XII. In the C. W. Telegraph Transmitting Sets described in Chapter XVII, the oscillator tubes act as rectifiers as well as oscillators but for wireless telephony the alternating current must be rectified first so that a continuous direct current will result.
The Operation of Reactors and Condensers.--A reactor is a single coil of wire wound on an iron core, see Fig. 90 and A in Fig. 91, and it should preferably have a large inductance. The reactor for the plate and grid circuit of a wireless telephone transmitter where one or more tubes are used as modulators as shown in the wiring diagram in Fig. 90, and the filter reactor shown in Fig. 92, operate in the same way.
When an alternating current flows through a coil of wire the reversals of the current set up a counter electromotive force in it which opposes, that is reacts, on the current, and the higher the frequency of the current the greater will be the reactance. When the positive half of an alternating current is made to flow through a large resistance the current is smoothed out but at the same time a large amount of its energy is used up in producing heat.
But when the positive half of an alternating current is made to flow through a large inductance it acts like a large resistance as before and likewise smooths out the current, but none of its energy is wasted in heat and so a coil having a large inductance, which is called an inductive reactance, or just reactor for short, is used to smooth out, or filter, the alternating current after it has been changed into a pulsating direct current by the rectifier tubes.
A condenser also has a reactance effect on an alternating current but different from an induction coil the lower the frequency the greater will be the reactance. For this reason both a filter reactor and filter condensers are used to smooth out the pulsating direct currents.
In the chapters on Receptors you have been told how to build up high-grade sets. But there are thousands of boys, and, probably, not a few men, who cannot afford to invest $25.00, more or less, in a receiving set and would like to experiment in a small way.
The following set is inexpensive, and with this cheap, little portable receptor you can get the Morse code from stations a hundred miles distant and messages and music from broadcasting stations if you do not live too far away from them. All you need for this set are: (1) a crystal detector, (2) a tuning coil and (3) an earphone. You can make a crystal detector out of a couple of binding posts, a bit of galena and a piece of brass wire, or, better, you can buy one all ready to use for 50 cents.
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| Wireless Receptor, the size of a Safety Match Box. A Youthful Genius in the person of Kenneth R. Hinman, Who is only twelve years old, has made a Wireless Receiving Set that fits neatly into a Safety Match Box. With this Instrument and a Pair of Ordinary Receivers, He is able to catch not only Code Messages but the regular Broadcasting Programs from Stations Twenty and Thirty Miles Distant. |
The Crystal Detector.--This is known as the Rasco baby detector and it is made and sold by the Radio Specialty Company, 96 Park Place, New York City. It is shown in Fig. 96. The base is made of black composition and on it is mounted a standard in which a rod slides and on one end of this there is fixed a hard rubber adjusting knob while the other end carries a thin piece of phosphor-bronze wire, called a cat-whisker To secure the galena crystal in the cup you simply unscrew the knurled cap, place it in the cavity of the post and screw the cap back on again. The free end of the cat-whisker wire is then adjusted so that it will rest lightly on the exposed part of the galena.