In the first part of this book we learned about spark-gap telegraph sets and how the oscillations they set up aredampedand the waves they send out areperiodic. In this and the next chapter we shall find out how vacuum tube telegraph transmitters are made and how they set up oscillations that aresustainedand radiate waves that arecontinuous.
Sending wireless telegraph messages by continuous waves has many features to recommend it as against sending them by periodic waves and among the most important of these are that the transmitter can be: (1) more sharply tuned, (2) it will send signals farther with the same amount of power, and (3) it is noiseless in operation. The disadvantageous features are that: (1) a battery current is not satisfactory, (2) its circuits are somewhat more complicated, and (3) the oscillator tubes burn out occasionally. There is, however, a growing tendency among amateurs to use continuous wave transmitters and they are certainly more up-to-date and interesting than spark gap sets.
Now there are two practical ways by which continuous waves can be set up for sending either telegraphic signals or telephonic speech and music and these are with: (a) anoscillation arc lamp, and (b) avacuum tube oscillator. The oscillation arc was the earliest known way of setting up sustained oscillations, and it is now largely used for commercial high power, long distance work. But since the vacuum tube has been developed to a high degree of efficiency and is the scheme that is now in vogue for amateur stations we shall confine our efforts here to explaining the apparatus necessary and how to wire the various parts together to produce several sizes of vacuum tube telegraph transmitters.
Sources of Current for Telegraph Transmitting Sets.--Differing from a spark-gap transmitter you cannot get any appreciable results with a low voltage battery current to start with. For a purely experimental vacuum tube telegraph transmitter you can use enoughBbatteries to operate it but the current strength of these drops so fact when they are in use, that they are not at all satisfactory for the work.
You can, however, use 110 volt direct current from a lighting circuit as your initial source of power to energize the plate of the vacuum tube oscillator of your experimental transmitter. Where you have a 110 voltdirect currentlighting service in your home and you want a higher voltage for your plate, you will then have to use a motor-generator set and this costs money. If you have 110 voltalternating currentlighting service at hand your troubles are over so far as cost is concerned for you can step it up to any voltage you want with a power transformer. In this chapter will be shown how to use a direct current for your source of initial power and in the next chapter how to use an alternating current for the initial power.
An Experimental Continuous Wave Telegraph Transmitter.--You will remember that inChapter XVwe learned how the heterodyne receiver works and that in the separate heterodyne receiving set the second vacuum tube is used solely to set up oscillations. Now while this extra tube is used as a generator of oscillations these are, of course, very weak and hence a detector tube cannot be used to generate oscillations that are useful for other purposes than heterodyne receptors and measurements.
There is a vacuum tube amplifier [Footnote: This is theradiationUV-201, made by the Radio Corporation of America, Woolworth Bldg., New York City.] made that will stand a plate potential of 100 volts, and this can be used as a generator of oscillations by energizing it with a 110 volt direct current from your lighting service. Or in a pinch you can use five standardBbatteries to develop the plate voltage, but these will soon run down. But whatever you do, never use a current from a lighting circuit on a tube of any kind that has a rated plate potential of less than 100 volts.
The Apparatus You Need.--For this experimental continuous wave telegraph transmitter get the following pieces of apparatus: (1) onesingle coil tuner with three clips; (2) one.002 mfd. fixed condenser; (3) three.001 mfd. condensers; (4) oneadjustable grid leak; (5) onehot-wire ammeter; (6) onebuzzer; (7) onedry cell; (8) onetelegraph key; (9) one100 volt plate vacuum tube amplifier; (10) one6 volt storage battery; (11) onerheostat; (12) oneoscillation choke coil; (13) onepanel cut-outwith asingle-throw, double-pole switch, and a pair offuse socketson it.
The Tuning Coil.--You can either make this tuning coil or buy one. To make it get two disks of wood 3/4-inch thick and 5 inches in diameter and four strips of hard wood, or better, hard rubber or composition strips, such asbakelite, 1/2-inch thick, 1 inch wide and 5-3/4 inches long, and screw them to the disks as shown atAinFig. 75. Now wrap on this form about 25 turns of No. 8 or 10, Brown and Sharpe gauge, bare copper wire with a space of 1/8-inch between each turn. Get three of the smallest size terminal clips, seeB, and clip them on to the different turns, when your tuning coil is ready for use. You can buy a coil of this kind for $4.00 or $5.00.
The Condensers.--For the aerial series condenser get one that has a capacitance of .002 mfd. and that will stand a potential of 3,000 volts. [Footnote: The U C-1014Faradoncondenser made by the Radio Corporation of America will serve the purpose.] It is shown atC. The other three condensers, seeD, are also of the fixed type and may have a capacitance of .001 mfd.; [Footnote: List No. 266; fixed receiving condenser, sold by the Manhattan Electrical Supply Co.] the blocking condenser should preferably have a capacitance of 1/2 a mfd. In these condensers the leaves of the sheet metal are embedded in composition. The aerial condenser will cost you $2.00 and the others 75 cents each.
(A) Fig. 75.--Apparatus for Experimental C. W. Telegraph Transmitter.Fig. 75.--Apparatus for Experimental C. W. Telegraph Transmitter.
The Aerial Ammeter.--This instrument is also called ahot-wireammeter because the oscillating currents flowing through a piece of wire heat it according to their current strength and as the wire contracts and expands it moves a needle over a scale. The ammeter is connected in the aerial wire system, either in the aerial side or the ground side--the latter place is usually the most convenient. When you tune the transmitter so that the ammeter shows the largest amount of current surging in the aerial wire system you can consider that the oscillation circuits are in tune. A hot-wire ammeter reading to 2.5 amperes will serve your needs, it costs $6.00 and is shown atEinFig. 75.
The Buzzer and Dry Cell.--While a heterodyne, or beat, receptor can receive continuous wave telegraph signals an ordinary crystal or vacuum tube detector receiving set cannot receive them unless they are broken up into trains either at the sending station or at the receiving station, and it is considered the better practice to do this at the former rather than at the latter station. For this small transmitter you can use an ordinary buzzer as shown atF. A dry cell or two must be used to energize the buzzer. You can get one for about 75 cents.
The Telegraph Key.--Any kind of a telegraph key will serve to break up the trains of sustained oscillations into dots and dashes. The key shown atGis mounted on a composition base and is the cheapest key made, costing $1.50.
The Vacuum Tube Oscillator.--As explained before you can use any amplifying tube that is made for a plate potential of 100 volts. The current required for heating the filament is about 1 ampere at 6 volts. A porcelain socket should be used for this tube as it is the best insulating material for the purpose. An amplifier tube of this type is shown atHand costs $6.50.
The Storage Battery.--A storage battery is used to heat the filament of the tube, just as it is with a detector tube, and it can be of any make or capacity as long as it will develop 6 volts. The cheapest 6 volt storage battery on the market has a 20 to 40 ampere-hour capacity and sells for $13.00.
The Battery Rheostat.--As with the receptors a rheostat is needed to regulate the current that heats the filament. A rheostat of this kind is shown atIand is listed at $1.25.
The Oscillation Choke Coil.--This coil is connected in between the oscillation circuits and the source of current which feeds the oscillator tube to keep the oscillations set up by the latter from surging back into the service wires where they would break down the insulation. You can make an oscillation choke coil by winding say 100 turns of No. 28 Brown and Sharpe gauge double cotton covered magnet wire on a cardboard cylinder 2 inches in diameter and 2-1/2 inches long.
Transmitter Connectors.--For connecting up the different pieces of apparatus of the transmitter it is a good scheme to usecopper braid; this is made of braided copper wire in three sizes and sells for 7,15 and 20 cents a foot respectively. A piece of it is pictured atJ.
The Panel Cut-Out.--This is used to connect the cord of the 110-volt lamp socket with the transmitter. It consists of a pair ofplug cutouts and a single-throw, double-poleswitch mounted on a porcelain base as shown atK. In some localities it is necessary to place these in an iron box to conform to the requirements of the fire underwriters.
Connecting Up the Transmitting Apparatus.--The way the various pieces of apparatus are connected together is shown in the wiring diagram.Fig. 76. Begin by connecting one post of the ammeter with the wire that leads to the aerial and the other post of it to one end of the tuning coil; connect clip1to one terminal of the .002 mfd. 3,000 volt aerial condenser and the other post of this with the ground.
Fig. 76--Experimental C.W. Telegraph Transmitter
Now connect the end of the tuning coil that leads to the ammeter with one end of the .001 mfd. grid condenser and the other end of this with the grid of the vacuum tube. Connect the telegraph key, the buzzer and the dry cell in series and then shunt them around the grid condenser. Next connect the plate of the tube with one end of the .001 mfd. blocking condenser and the other end of this with the clip2on the tuning coil.
Connect one end of the filament with the + or positive electrode of the storage battery, the - or negative electrode of this with one post of the rheostat and the other post of the latter with the other end of the filament; then connect clip3with the + or positive side of the storage battery. This done connect one end of the choke coil to the conductor that leads to the plate and connect the other end of the choke coil to one of the taps of the switch on the panel cut-out. Connect the + or positive electrode of the storage battery to the other switch tap and between the switch and the choke coil connect the protective condenser across the 110 volt feed wires. Finally connect the lamp cord from the socket to the plug fuse taps when your experimental continuous wave telegraph transmitter is ready to use.
A 100 Mile C. W. Telegraph Transmitter.--Here is a continuous wave telegraph transmitter that will cover distances up to 100 miles that you can rely on. It is built on exactly the same lines as the experimental transmitter just described, but instead of using a 100 volt plate amplifier as a makeshift generator of oscillations it employs a vacuum tube made especially for setting up oscillations and instead of having a low plate voltage it is energized with 350 volts.
The Apparatus You Need.--For this transmitter you require: (1) oneoscillation transformer; (2) onehot-wire ammeter; (3) oneaerial series condenser; (4) onegrid leak resistance; (5) onechopper; (6) onekey circuit choke coil; (7) one5 watt vacuum tube oscillator; (8) one6 volt storage battery; (9) onebattery rheostat; (10) onebattery voltmeter; (11) oneblocking condenser; (12) onepower circuit choke coil, and (13) onemotor-generator.
The Oscillation Transformer.--The tuning coil, oroscillation transformeras this one is called, is a conductively coupled tuner--that is, the primary and secondary coils form one continuous coil instead of two separate coils. This tuner is made up of 25 turns of thin copper strip, 3/8 inch wide and with its edges rounded, and this is secured to a wood base as shown atAinFig. 77. It is fitted with one fixed tap and three clips to each of which a length of copper braid is attached. It has a diameter of 6-1/4 inches, a height of 7-7/8 inches and a length of 9-3/8 inches, and it costs $11.00.
Fig. 77.--Apparatus of 100 Mile C. W. Telegraph Transmitter.
The Aerial Condenser.--This condenser is made up of three fixed condensers of different capacitances, namely .0003, .0004 and .0005 mfd., and these are made to stand a potential of 7500 volts. The condenser is therefore adjustable and, as you will see from the pictureB, it has one terminal wire at one end and three terminal wires at the other end so that one, two or three condensers can be used in series with the aerial. A condenser of this kind costs $5.40.
The Aerial Ammeter.--This is the same kind of a hot-wire ammeter already described in connection with the experimental set, but it reads to 5 amperes.
The Grid and Blocking Condensers.--Each of these is a fixed condenser of .002 mfd. capacitance and is rated to stand 3,000 volts. It is made like the aerial condenser but has only two terminals. It costs $2.00.
The Key Circuit Apparatus.--This consists of: (1) thegrid leak; (2) thechopper; (3) thechoke coil, and (4) thekey. The grid leak is connected in the lead from the grid to the aerial to keep the voltage on the grid at the right potential. It has a resistance of 5000 ohms with a mid-tap at 2500 ohms as shown atC. It costs $2.00.
The chopper is simply a rotary interrupter driven by a small motor. It comprises a wheel of insulating material in which 30 or more metal segments are set in an insulating disk as shown atD. A metal contact called a brush is fixed on either side of the wheel. It costs about $7.00 and the motor to drive it is extra. The choke coil is wound up of about 250 turns of No. 30 Brown and Sharpe gauge cotton covered magnet wire on a spool which has a diameter of 2 inches and a length of 3-1/4 inches.
The 5 Watt Oscillator Vacuum Tube.--This tube is made like the amplifier tube described for use with the preceding experimental transmitter, but it is larger, has a more perfect vacuum, and will stand a plate potential of 350 volts while the plate current is .045 ampere. The filament takes a current of a little more than 2 amperes at 7.5 volts. A standard 4-tap base is used with it. The tube costs $8.00 and the porcelain base is $1.00 extra. It is shown atE.
The Storage Battery and Rheostat.--This must be a 5-cell battery so that it will develop 10 volts. A storage battery of any capacity can be used but the lowest priced one costs about $22.00. The rheostat for regulating the battery current is the same as that used in the preceding experimental transmitter.
The Filament Voltmeter.--To get the best results it is necessary that the voltage of the current which heats the filament be kept at the same value all of the time. For this transmitter a direct current voltmeter reading from 0 to 15 volts is used. It is shown atFand costs $7.50.
The Oscillation Choke Coil.--This is made exactly like the one described in connection with the experimental transmitter.
The Motor-Generator Set.--Where you have only a 110 or a 220 volt direct current available as a source of power you need amotor-generatorto change it to 350 volts, and this is an expensive piece of apparatus. It consists of a single armature core with a motor winding and a generator winding on it and each of these has its own commutator. Where the low voltage current flows into one of the windings it drives its as a motor and this in turn generates the higher voltage current in the other winding. Get a 100 watt 350 volt motor-generator; it is shown atFand costs about $75.00.
The Panel Cut-Out.--This switch and fuse block is the same as that used in the experimental set.
The Protective Condenser.--This is a fixed condenser having a capacitance of 1 mfd. and will stand 750 volts. It costs $2.00.
Connecting Up the Transmitting Apparatus.--From all that has gone before you have seen that each piece of apparatus is fitted with terminal, wires, taps or binding posts. To connect up the parts of this transmitter it is only necessary to make the connections as shown in the wiring diagramFig. 78.
Fig. 78.--5 to 50 Watt C. W. Telegraph Transmitter. (With Single Oscillation Tube.)
A 200 Mile C. W. Telegraph Transmitter.--To make a continuous wave telegraph transmitter that will cover distances up to 200 miles all you have to do is to use two 5 watt vacuum tubes inparallel, all of the rest of the apparatus being exactly the same. Connecting the oscillator tubes up in parallel means that the two filaments are connected across the leads of the storage battery, the two grids on the same lead that goes to the aerial and the two plates on the same lead that goes to the positive pole of the generator. Where two or more oscillator tubes are used only one storage battery is needed, but each filament must have its own rheostat. The wiring diagramFig. 79shows how the two tubes are connected up in parallel.
Fig. 79.--200 Mile C.W. Telegraph Transmitter (With Two Tubes in Parallel.)
A 500 Mile C. W. Telegraph Transmitter.--For sending to distances of over 200 miles and up to 500 miles you can use either: (1) three or four 5 watt oscillator tubes in parallel as described above, or (2) one 50 watt oscillator tube. Much of the apparatus for a 50 watt tube set is exactly the same as that used for the 5 watt sets. Some of the parts, however, must be proportionately larger though the design all the way through remains the same.
The Apparatus and Connections.--The aerial series condenser, the blocking condenser, the grid condenser, the telegraph key, the chopper, the choke coil in the key circuit, the filament voltmeter and the protective condenser in the power circuit are identical with those described for the 5 watt transmitting set.
The 50 Watt Vacuum Tube Oscillator.--This is the size of tube generally used by amateurs for long distance continuous wave telegraphy. A single tube will develop 2 to 3 amperes in your aerial. The filament takes a 10 volt current and a plate potential of 1,000 volts is needed. One of these tubes is shown inFig. 80and the cost is $30.00. A tube socket to fit it costs $2.50 extra.
Fig. 80.--50 Watt Oscillator Vacuum Tube.
The Aerial Ammeter.--This should read to 5 amperes and the cost is $6.25.
The Grid Leak Resistance.--It has the same resistance, namely 5,000 ohms as the one used with the 5 watt tube transmitter, but it is a little larger. It is listed at $1.65.
The Oscillation Choke Coil.--The choke coil in the power circuit is made of about 260 turns of No. 30 B. & S. cotton covered magnet wire wound on a spool 2-1/4 inches in diameter and 3-1/4 inches long.
The Filament Rheostat.--This is made to take care of a 10 volt current and it costs $10.00.
The Filament Storage Battery.--This must develop 12 volts and one having an output of 40 ampere-hours costs about $25.00.
The Protective Condenser.--This condenser has a capacitance of 1 mfd. and costs $2.00.
The Motor-Generator.--Where you use one 50 watt oscillator tube you will need a motor-generator that develops a plate potential of 1000 volts and has an output of 200 watts. This machine will stand you about $100.00.
The different pieces of apparatus for this set are connected up exactly the same as shown in the wiring diagram inFig. 78.
A 1000 Mile C. W. Telegraph Transmitter.--All of the parts of this transmitting set are the same as for the 500 mile transmitter just described except the motor generator and while this develops the same plate potential,i.e., 1,000 volts, it must have an output of 500 watts; it will cost you in the neighborhood of $175.00. For this long distance transmitter you use two 50 watt oscillator tubes in parallel and all of the parts are connected together exactly the same as for the 200 mile transmitter shown in the wiring diagram inFig. 79.
Within the last few years alternating current has largely taken the place of direct current for light, heat and power purposes in and around towns and cities and if you have alternating current service in your home you can install a long distance continuous wave telegraph transmitter with very little trouble and at a comparatively small expense.
A 100 Mile C. W. Telegraph Transmitting Set.--The principal pieces of apparatus for this transmitter are the same as those used for the100 Mile Continuous Wave Telegraph Transmitting Setdescribed and pictured in the preceding chapter which used direct current, except that analternating current power transformeris employed instead of the more costlymotor-generator.
The Apparatus Required.--The various pieces of apparatus you will need for this transmitting set are: (1) onehot-wire ammeterfor the aerial as shown atEinFig. 75, but which reads to 5 amperes instead of to 2.5 amperes; (2) onetuning coilas shown atAinFig. 77; (3) one aerial condenser as shown atBinFig. 77; (4) onegrid leakas shown atCinFig. 77; (5) onetelegraph keyas shown atGinFig. 75; (6) onegrid condenser, made like the aerial condenser but having only two terminals; (7) one5 watt oscillator tubeas shown atEinFig. 77; (8) one.002 mfd. 3,000 volt by-pass condenser, made like the aerial and grid condensers; (9) one pair ofchoke coilsfor the high voltage secondary circuit; (10) onemilli-ammeter; (11) one A. C.power transformer; (12) onerheostatas shown atIinFig. 75, and (13) onepanel cut-outas shown atKinFig. 75.
The Choke Coils.--Each of these is made by winding about 100 turns of No. 28, Brown and Sharpe gauge, cotton covered magnet wire on a spool 2 inches in diameter and 2-1/2 inches long, when it will have an inductance of about 0.5millihenry[Footnote: A millihenry is 1/1000th part of a henry.] at 1,000 cycles.
The Milli-ammeter.--This is an alternating current ammeter and reads from 0 to 250milliamperes; [Footnote: Amilliampereis the 1/1000th part of an ampere.] and is used for measuring the secondary current that energizes the plate of the oscillator tube. It looks like the aerial ammeter and costs about $7.50.
The A. C. Power Transformer.--Differing from the motor generator set the power transformer has no moving parts. For this transmitting set you need a transformer that has an input of 325 volts. It is made to work on a 50 to 60 cycle current at 102.5 to 115 volts, which is the range of voltage of the ordinary alternating lighting current. This adjustment for voltage is made by means of taps brought out from the primary coil to a rotary switch.
The high voltage secondary coil which energizes the plate has an output of 175 watts and develops a potential of from 350 to 1,100 volts. The low voltage secondary coil which heats the filament has an output of 175 watts and develops 7.5 volts. This transformer, which is shown inFig. 81, is large enough to take care of from one to four 5 watt oscillator tubes. It weighs about 15 pounds and sells for $25.00.
Fig. 81.--Alternation Current Power Transformer. (For C. W. Telegraphy and Wireless Telephony.)
Connecting Up the Apparatus.--The wiring diagramFig. 82shows clearly how all of the connections are made. It will be observed that a storage battery is not needed as the secondary coil of the transformer supplies the current to heat the filament of the oscillator. The filament voltmeter is connected across the filament secondary coil terminals, while the plate milli-ammeter is connected to the mid-taps of the plate secondary coil and the filament secondary coil.
Fig. 82. Wiring Diagram for 200 to 500 Mile C.W. Telegraph Transmitting Set. (With Alternating Current)
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 inFig. 83.
Fig. 83.--Wiring Diagram for 500 to 1000 Mile C. W. Telegraph Transmitter.
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 two50 watt oscillator tubes with sockets;(2) onekeycondenserthat has a capacitance of 1 mfd., and a rated potential of 1,750 volts; (3) one0 to 500 milli-ammeter; (4) oneaerial ammeterreading to 5 amperes, and (5) anA. C. power transformerfor one or two 50 watt tubes.
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 inFig. 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 inFig. 82, and where two tubes are connected in parallel the various pieces of apparatus are wired together as shown inFig. 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 diagramFig. 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 adirect currentof 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 tostep-upa 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 ofvacuum tube rectifiers.
The pulsating direct currents are then passed through a filtering reactance coil, called areactor, 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 arevaried, ormodulated, as it is called, by amicrophone transmittersuch 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 ofChapter XVIfor aShort Distance C. W. Telegraph Transmitter, except that you use amicrophone transmitterinstead of atelegraph 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) onetuning coilas shown atAandBinFig. 75; (2) oneaerial ammeteras shown atCinFig. 75; (3) oneaerial condenseras shown atCinFig. 75; (4) onegrid, blocking and protective condenseras shown atDinFig. 75; (5) onegrid leakas shown atCinFig. 77; (6) onevacuum tube amplifierwhich is used as anoscillator; (7) one6 volt storage battery; (8) onerheostatas shown atIinFig. 75; (9) oneoscillation choke coil; (10) onepanel cut-outas shown atKinFig. 75and an ordinarymicrophone transmitter.
TheMicrophone Transmitter.--The best kind of a microphone to use with this and other telephone transmitting sets is aWestern 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 inFig. 84and costs $2.00. Any other good microphone transmitter can be used if desired.
Fig. 84.--Standard Microphone Transmitter.
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 diagramFig. 85, and the other terminal of this to one end of the tuning coil. Now connectclip 1of 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.
Fig. 85.--Wiring Diagram of Short Distance Wireless Telephone Set. (Microphone in Aerial Wire.)
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 + orpositiveelectrode 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 - ornegativeelectrode of the battery. This done, connectclip 2of the tuning coil to the + orpositiveelectrode of the battery and bring a lead from it to one of the switch taps of the panel cut-out.
Now connectclip 3of 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 amperefuse 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 amotor-generator. To make this transmitter you will need exactly the same apparatus as that described and pictured for the100 Mile C. W. Telegraph Transmitting SetinChapter XVI, except that you must substitute amicrophone transmitterand atelephone induction coil, or amicrophone transformer, or still better, amagnetic modulator, for the telegraph key and chopper.
The Apparatus You Need.--To reiterate; the pieces of apparatus you need are: (1) oneaerial ammeteras shown atEinFig. 75; (2) onetuning coilas shown atAinFig. 77; (3) oneaerial condenseras shown atBinFig. 77; (4) onegrid leakas shown atCinFig. 77; (5) onegrid, blockingandprotective condenser; (6) one5 watt oscillator tubeas shown atEinFig. 77; (7) onerheostatas shown atIinFig. 75; (8) one10 volt (5 cell) storage battery; (9) onechoke coil; (10) onepanel cut-outas shown atKinFig. 75, and (11) amotor-generatorhaving an input of 110 or 220 volts and an output of 350 volts.
In addition to the above apparatus you will need: (12) amicrophone transmitteras shown inFig. 84; (13) a battery of four dry cells or a 6 volt storage battery, and either (14) atelephone induction coilas shown inFig. 86; (15) amicrophone transformeras shown inFig. 87; or amagnetic modulatoras shown inFig. 88. All of these parts have been described, as said above, inChapter XVI, except the microphone modulators.
Fig. 86.--Telephone Induction Coil. (Used with Microphone Transmitter.)Fig. 87.--Microphone Transformer. (Used with Microphone Transmitter.)Fig. 88.--Magnetic Modulator. (Used with Microphone Transmitter.)
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 ofNo. 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 ofNo. 30magnet wire. Get astandard telephone induction coilthat 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 aside tonecoil, 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 the100 Mile C. W. Telegraph SetinChapter XVIexcept 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 atAandBinFig. 89. In the first way the secondary terminals of the modulator are shunted around the grid leak in the grid circuit as atA, and in the second the secondary terminals are connected in the aerial as atB. 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 inFig. 90then the microphone and its modulator are connected with the grid circuit andclip 3of the tuning coil.
Fig. 89.--Wiring Diagram of 25 to 50 Mile Wireless Telephone. (Microphone Modulator Shunted Around Grid-Leak Condenser.)(B) Fig. 89.--Microphone Modulator Connected in Aerial Wire.Fig. 90.--Wiring Diagram of 50 to 100 Mile Wireless Telephone Transmitting Set.
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 anoscillatorand the second as amodulator; (2) anoscillation choke coilis used in the plate circuit; (3) areactance coilorreactor, is used in the plate circuit; and (4) areactoris used in the grid circuit.
The Oscillation Choke Coil.--You can make this choke coil by winding about 275 turns ofNo. 28 B. and S. gaugecotton 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, areactor, 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 inFig. 91and each one will cost you $5.75.
Fig. 91.--Plate and Grid Circuit Reactor.
Connecting up the Apparatus.--All of the different pieces of apparatus are connected up as shown inFig. 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 toclip 3of 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 inFig. 89and 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 inFig. 90and 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) the50 watt tube; (2) thegrid leak resistance; (3) thefilament rheostat; (4) thefilament storage battery; and (5) themagnetic modulator. All of these parts, except the latter, are described in detail under the heading of a500 Mile C. W. Telegraph Transmitting SetinChapter 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) apower transformeris used for stepping up the voltage instead of a motor-generator, and (2) avacuum tube rectifiermust be used to convert the alternating current into direct current.
The Apparatus You Need.--For this telephone transmitting set you need: (1) oneaerial ammeter; (2) onetuning coil; (3) onetelephone modulator; (4) oneaerial series condenser; (5) one4 cell dry batteryor a 6 volt storage battery; (6) onemicrophone transmitter; (7) onebattery switch; (8) onegrid condenser; (9) onegrid leak; (10) two5 watt oscillator tubes with sockets; (11) oneblocking condenser; (12) oneoscillation choke coil; (13) twofilter condensers; (14) onefilter reactance coil; (15) analternating current power transformer, and (16) two20 watt rectifier vacuum tubes.
All of the above pieces of apparatus are the same as those described for the100 Mile C. W. Telegraph Transmitter in Chapter XVII, except: (a) themicrophone modulator; (b) themicrophone transmitterand (c) thedryorstorage battery, all of which are described in this chapter; and the new parts which are: (d) therectifier vacuum tubes; (e) thefilter condensers; and (f) thefilter reactance coil; further and finally, the power transformer has athirdsecondary 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 atEinFig. 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 inFig. 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.
Fig. 92.--Filter Reactor for Smoothing out Rectified Currents.
Connecting Up the Apparatus.--The wiring diagram inFig. 93shows 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.
Fig 93.--100 to 200 Mile Wireless Telephone Transmitter.
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 inFig. 93.
Apparatus Required.--The only differences between this and the preceding transmitter are: (1) themagnetic modulator, if you use one, should have an output of 3-1/2 to 5 amperes; (2) you will need two50 watt oscillator tubes with sockets; (3) two150 watt rectifier tubes with sockets; (4) anaerial ammeterthat reads to5 amperes; (5) three1 mfd. filter condensersin parallel; (6)two filter condensers of 1 mfd. capacitancethat will stand1750 volts; and (6) a300 milliampere filter reactor.
The apparatus is wired up as shown inFig. 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 thevacuum 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 inChapter 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 inFig. 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.
(A) and (B) Fig. 94. Operation of Vacuum Tube Oscillators.
Where an oscillator tube is included in the circuits as shown atAandBinFig. 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.
(C) Fig. 94.--How a Direct Current Sets up Oscillations.
The Operation of C. W. Telegraph Transmitters With Direct Current--Short Distance C. W. Transmitter.--In the transmitter shown in the wiring diagram inFig. 76the 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 toclip 3of the tuning coil, through the turns of the latter toclip 2and over to the filament and, when the latter is heated, you have aclosed 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 theclip 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 inFig. 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 inChapter 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 inChapter 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, seeChapter 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 atone 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 inFig. 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.
Fig. 95.--Positive Voltage only sets up Oscillations.
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 inFig. 83the 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 afull 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 inFig. 85is exactly the same as that of theDirect Current Short Distance C. W. Telegraph Transmitteralready explained in this chapter. The only difference in the operation of these sets is the substitution of themicrophone transmitterfor 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 inFig. 84. By referring to the diagram atAin this figure you will readily understand how it operates. When you speak into the mouthpiece thesound 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 amicrophone modulator. This latter device may be either (1) atelephone induction coil, (2) amicrophone transformer, or (3) amagnetic 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, seeAinFig. 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 atAinFig. 89, and inFig. 90, (2) the aerial wire system, as shown atBinFig. 89andFig. 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, seeAinFig. 87, and theratioof 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 amagnetic amplifier. The iron core is formed of very thin plates, orlaminationsas they are called, and this permits high-frequency oscillations to surge in a coil wound on it. In this transformer, seeAinFig. 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 atAinFig. 88, and inFig. 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 amodulator, as shown inFig. 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 theabsorptionof the energy of the current set up by the oscillator tube, and (2) byvaryingthe 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 inFig. 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.