Letters of a Radio-Engineer to His Son Part 19

These are coils in which the turns are wound over each other but in such a way as to avoid in large part the "capacity effects" which usually accompany such winding. You can try winding them for yourself but I doubt if the experience has much value until you have gone farther in the study of the mathematical theory of radio than this series of letters will carry you.

TABLE III Circuit of Fig. 112 Number Inductance in Wave length with antenna of of turns. mil-henries. 0.0001 mf. 0.0002 mf.

14 0.04 120 170 20 0.07 160 220 28 0.12 210 290 36 0.18 250 360 44 0.25 300 420 56 0.38 370 520 75 0.60 460 650

In the secondary circuit there is only one capacity, that of the variable condenser. If it has a range of values from about 0.00005 mf.

to 0.0005 mf. your coil of 60 turns and 0.42 mf. permits a range of wave-lengths from 270 to 860 m. Using half the coil the range is 150 to 480 m. With the larger condenser the ranges are respectively 270 to 1220 and 270 to 670. For longer wave-lengths load with inductance. Four times the inductance will tune to double these wave-lengths.

[Footnote 11: If you can afford to buy, or if you can borrow, ammeters and voltmeters of the proper range you should take the characteristic yourself.]

LETTER 22

HIGH-POWERED RADIO-TELEPHONE TRANSMITTERS

MY DEAR EXPERIMENTER:

This letter is to summarize the operations which must be performed in radio-telephone transmission and reception; and also to describe the circuit of an important commercial system.

To transmit speech by radio three operations are necessary. First, there must be generated a high-frequency alternating current; second, this current must be modulated, that is, varied in intensity in accordance with the human voice; and third, the modulated current must be supplied to an antenna. For efficient operation, of course, the antenna must be tuned to the frequency which is to be transmitted. There is also a fourth operation which is usually performed and that is amplification.

Wherever the electrical effect is smaller than desired, or required for satisfactory transmission, vacuum tubes are used as amplifiers. Of this I shall give you an ill.u.s.tration later.

Three operations are also essential in receiving. First, an antenna must be so arranged and tuned as to receive energy from the distant transmitting station. There is then in the receiving antenna a current similar in wave form to that in the transmitting antenna. Second, the speech significance of this current must be detected, that is, the modulated current must be demodulated. A current is then obtained which has the same wave form as the human voice which was the cause of the modulation at the distant station. The third operation is performed by a telephone receiver which makes the molecules of air in its neighborhood move back and forth in accordance with the detected current. As you already know a fourth operation may be carried on by amplifiers which give on their output sides currents of greater strength but of the same forms as they receive at their input terminals.

In transmitting and in receiving equipment two or more of these operations may be performed by the same vacuum tube as you will remember from our discussion of the regenerative circuit for receiving. For example, also, in any receiving set the vacuum tube which detects is usually amplifying. In the regenerative circuit for receiving continuous waves by the heterodyne method the vacuum tube functions as a generator of high-frequency current and as a detector of the variations in current which occur because the locally-generated current does not keep in step with that generated at the transmitting station.

Another example of a vacuum tube performing simultaneously two different functions is ill.u.s.trated in Fig. 120 which shows a simple radio-telephone transmitter. The single tube performs in itself both the generation of the radio-frequency current and its modulation in accordance with the output of the carbon-b.u.t.ton transmitter. This audion is in a feed-back circuit, the oscillation frequency of which depends upon the condenser _C_ and the inductance _L_. The voice drives the diaphragm of the transmitter and thus varies the resistance of the carbon b.u.t.ton. This varies the current from the battery, _B_{a}_, through the primary, _T_{1}_, of the transformer _T_. The result is a varying voltage applied to the grid by the secondary _T_{2}_. The oscillating current in the plate circuit of the audion varies accordingly because it is dependent upon the grid voltage. The condenser _C_{r}_ offers a low impedance to the radio-frequency current to which the winding _T_{2}_ of audio-frequency transformer offers too much.

[Ill.u.s.tration: Fig 120]

In this case the tube is both generator and "modulator." In some cases these operations are separately performed by different tubes. This was true of the transmitting set used in 1915 when the engineers of the Bell Telephone System talked by radio from Arlington, near Was.h.i.+ngton, D. C., to Paris and Honolulu. I shall not draw out completely the circuit of their apparatus but I shall describe it by using little squares to represent the parts responsible for each of the several operations.

First there was a vacuum tube oscillator which generated a small current of the desired frequency. Then there was a telephone transmitter which made variations in a direct-current flowing through the primary of a transformer. The e. m. f. from the secondary of this transformer and the e. m. f. from the radio-frequency oscillator were both impressed upon the grid of an audion which acted as a modulator. The output of this audion was a radio-frequency current modulated by the voice. The output was amplified by a two-stage audion amplifier and supplied through a coupling coil to the large antenna of the U. S. Navy Station at Arlington. Fig. 121 shows the system.

[Ill.u.s.tration: Fig 121]

The audion amplifiers each consisted of a number of tubes operating in parallel. When tubes are operated in parallel they are connected as shown in Fig. 122 so that the same e. m. f. is impressed on all the grids and the same plate-battery voltage on all the plates. As the grids vary in voltage there is a corresponding variation of current in the plate circuit of each tube. The total change of the current in the plate-battery circuit is, then, the sum of the changes in all the plate-filament circuits of the tubes. This scheme of connections gives a result equivalent to that of a single tube with a correspondingly larger plate and filament.

[Ill.u.s.tration: Fig 122]

Parallel connection is necessary because a single tube would be overheated in delivering to the antenna the desired amount of power. You remember that when the audion is operated as an amplifier the resistance to which it supplies current is made equal to its own internal resistance of _R_{p}_. That means that there is in the plate circuit just as much resistance inside the tube as outside. Hence there is the same amount of work done each second in forcing the current through the tube as through the antenna circuit, if that is what the tube supplies. "Work per second" is power; the plate battery is spending energy in the tube at the same rate as it is supplying it to the antenna where it is useful for radiation.

[Ill.u.s.tration: Pl. XI.--Broadcasting Equipment, Developed by the American Telephone and Telegraph Company and the Western Electric Company.]

All the energy expended in the tube appears as heat. It is due to the blows which the electrons strike against the plate when they are drawn across from the filament. These impacts set into more rapid motion the molecules of the plate; and the temperature of the tube rises. There is a limit to the amount the temperature can rise without destroying the tube. For that reason the heat produced inside it must not exceed a certain limit depending upon the design of the tube and the method of cooling it as it is operated. In the Arlington experiments, which I mentioned a moment ago, the tubes were cooled by blowing air on them from fans.

We can find the power expended in the plate circuit of a tube by multiplying the number of volts in its battery by the number of amperes which flows. Suppose the battery is 250 volts and the current 0.02 amperes, then the power is 5 watts. The "watt" is the unit for measuring power. Tubes are rated by the number of watts which can be safely expended in them. You might ask, when you buy an audion, what is a safe rating for it. The question will not be an important one, however, unless you are to set up a transmitting set since a detector is usually operated with such small plate-voltage as not to have expended in it an amount of power dangerous to its life.

In recent transmitting sets the tubes are used in parallel for the reasons I have just told, but a different method of modulation is used.

The generation of the radio-frequency current is by large-powered tubes which are operated with high voltages in their plate circuits. The output of these oscillators is supplied to the antenna. The intensity of the oscillations of the current in these tubes is controlled by changing the voltage applied in their plate circuits. You can see from Fig. 123 that if the plate voltage is changed the strength of the alternating current is changed accordingly. It is the method used in changing the voltage which is particularly interesting.

[Ill.u.s.tration: Fig 123]

The high voltages which are used in the plate circuits of these high-powered audions are obtained from generators instead of batteries.

You remember from Letter 20 that an e. m. f. is induced in a coil when the coil and a magnet are suddenly changed in their positions, one being turned with reference to the other. A generator is a machine for turning a coil so that a magnet is always inducing an e. m. f. in it. It is formed by an armature carrying coils and by strong electromagnets. The machine can be driven by a steam or gas engine, by a water wheel, or by an electric motor. Generators are designed either to give steady streams of electrons, that is for d-c currents, or to act as alternators.

[Ill.u.s.tration: Fig 124]

Suppose we have, as shown in Fig. 124, a d-c generator supplying current to a vacuum tube oscillator. The current from the generator pa.s.ses through an iron-cored choke coil, marked _L_{a}_ in the figure.

Between this coil and the plate circuit we connect across the line a telephone transmitter. To make a system which will work efficiently we shall have to suppose that this transmitter has a high resistance, say about the same as the internal resistance, _R_{p}_, of the tube and also that it can carry as large a current.

Of the current which comes from the generator about one-half goes to the tube and the rest to the transmitter. If the resistance of the transmitter is increased it can't take as much current. The coil, _L_{a}_, however, because of its inductance, tends to keep the same amount of current flowing through itself. For just an instant then the current in _L_{a}_ keeps steady even though the transmitter doesn't take its share. The result is more current for the oscillating tube. On the other hand if the transmitter takes more current, because its resistance is decreased, the choke coil, _L_{a}_, will momentarily tend to keep the current steady so that what the transmitter takes must be at the expense of the oscillating tube.

That's one way of looking at what happens. We know, however, from Fig.

123 that to get an increase in the amplitude of the current in the oscillating tube we must apply an increased voltage to its plate circuit. That is what really happens when the transmitter increases in resistance and so doesn't take its full share of the current. The reason is this: When the transmitter resistance is increased the current in the transmitter decreases. Just for a moment it looks as though the current in _L_{a}_ is going to decrease. That's the way it looks to the electrons; and you know what electrons do in an inductive circuit when they think they shall have to stop. They induce each other to keep on for a moment. For a moment they act just as if there was some extra e. m. f. which was acting to keep them going. We say, therefore, that there is an extra e. m. f., and we call this an e. m.

f. of self-induction. All this time there has been active on the plate circuit of the tube the e. m. f. of the generator. To this there is added at the instant when the transmitter resistance increases, the e.

m. f. of self-induction in the coil, _L_{a}_ and so the total e. m. f.

applied to the tube is momentarily increased. This increased e. m. f., of course, results in an increased amplitude for the alternating current which the oscillator is supplying to the transmitting antenna.

When the transmitter resistance is decreased, and a larger current should flow through the choke coil, the electrons are asked to speed up in going through the coil. At first they object and during that instant they express their objection by an e. m. f. of self-induction which opposes the generator voltage. For an instant, then, the voltage of the oscillating tube is lowered and its alternating-current output is smaller.

[Ill.u.s.tration: Fig 125]

For the purpose of bringing about such threatened changes in current, and hence such e. m. f.'s of self-induction, the carbon transmitter is not suitable because it has too small a resistance and too small a current carrying ability. The plate circuit of a vacuum tube will serve admirably. You know from the audion characteristic that without changing the plate voltage we can, by applying a voltage to the grid, change the current through the plate circuit. Now if it was a wire resistance with which we were dealing and we should be able to obtain a change in current without changing the voltage acting on this wire we would say that we had changed the resistance. We can say, therefore, that the internal resistance of the plate circuit of a vacuum tube can be changed by what we do to the grid.

In Fig. 125 I have subst.i.tuted the plate circuit of an audion for the transmitter of Fig. 124 and arranged to vary its resistance by changing the potential of the grid. This we do by impressing upon the grid the e.

m. f. developed in the secondary of a transformer, to the primary of which is connected a battery and a carbon transmitter. The current through the primary varies in accordance with the sounds spoken into the transmitter. And for all the reasons which we have just finished studying there are similar variations in the output current of the oscillating tube in the transmitting set of Fig. 125.

In this latter figure you will notice a small air-core coil, _L_{R}_, between the oscillator and the modulator tube. This coil has a small inductance but it is enough to offer a large impedance to radio-frequency currents. The result is, it does not let the alternating currents of the oscillating tube flow into the modulator. These currents are confined to their own circuit, where they are useful in establis.h.i.+ng similar currents in the antenna. On the other hand, the coil _L_{R}_ doesn't seriously impede low-frequency currents and therefore it does not prevent variations in the current which are at audio-frequency. It does not interfere with the changes in current which accompany the variations in the resistance of the plate circuit of the modulator.

That is, it has too little impedance to act like _L_{a}_ and so it permits the modulator to vary the output of the oscillator.

[Ill.u.s.tration: Fig 126]

The oscillating circuit of Fig. 125 includes part of the antenna. It differs also from the others I have shown in the manner in which grid and plate circuits are coupled. I'll explain by Fig. 126.

The transmitting set which I have just described involves many of the principles of the most modern sets. If you understand its operation you can probably reason out for yourself any of the other sets of which you will hear from time to time.

LETTER 23

AMPLIFICATION AT INTERMEDIATE FREQUENCIES

DEAR SON:

In the matter of receiving I have already covered all the important principles. There is one more system, however, which you will need to know. This is spoken of either as the "super-heterodyne" or as the "intermediate-frequency amplification" method of reception.