Letters of a Radio-Engineer to His Son Part 10

If we want the electron stream in coil _ab_ to have a large guiding effect on the stream in coil _L-C_ we must see that both circuits have the same tune, that is the same natural frequency of oscillation.

[Ill.u.s.tration: Fig 51]

This can be shown very easily by a simple experiment. Suppose we set up our circuit _L-C_ with an ammeter in it, so as to be able to tell how large an electron stream is oscillating in that circuit. Let us also make the condenser a variable one so that we can change the natural frequency or tune of the circuit. Now let's see what happens to the current as we vary this condenser, changing the capacity and thus changing the tune of the circuit. If we use a variable plate condenser it will have a scale on top graduated in degrees and we can note the reading of the ammeter for each position of the movable plates. If we do, we find one position of these plates, that is one setting, corresponding to one value of capacity in the condenser, where the current in the circuit is a maximum. This is the setting of the condenser for which the circuit has the same tune or natural frequency as the circuit _cd_. Sometimes we say that the circuits are now in resonance. We also refer to the curve of values of current and condenser positions as a "tuning curve." Such a curve is shown in Fig. 51.

[Ill.u.s.tration: Fig 52]

That's all there is to tuning--adjusting the capacity and inductance of a circuit until it has the same natural frequency as some other circuit with which we want it to work. We can either adjust the capacity as we just did, or we can adjust the inductance. In that case we use a variable inductance as in Fig. 52.

If we want to be able to tune to any of a large range of frequencies we usually have to take out or put into the circuit a whole lot of mil-henries at a time. When we do we get these mil-henries of inductance from a coil which we call a "loading coil." That's why your friends add a loading coil when they want to tune for the long wave-length stations, that is, those with a low frequency.

When our circuit _L-C_ of Fig. 49 is tuned to the frequency of the oscillator we get in it a maximum current. There is a maximum stream of electrons, and hence a maximum number of them crowded first into one and then into the other plate of the condenser. And so the condenser is charged to a maximum voltage, first in one direction and then in the other.

[Ill.u.s.tration: Fig 53]

Now connect the circuit _L-C_ to the grid of an audion. If the circuit is tuned we'll have the maximum possible voltage applied between grid and filament. In the plate circuit we'll get an increase and then a decrease of current. You know that will happen for I prepared you for this moment by the last page of my ninth letter. I'll tell you more about that current in the plate circuit in a later letter. I am connecting a telephone receiver in the plate circuit, and also a condenser, the latter for a reason to be explained later. The combination appears then as in Fig. 53. That figure shows a C-W transmitter and an audion detector. This is the sort of a detector we would use for radio-telephony, but the transmitter is the sort we would use for radio-telegraphy. We shall make some changes in them later.

[Ill.u.s.tration: Fig 54]

Whenever we start the oscillating current in the transmitter we get an effect in the detector circuit, of which I'll tell you more later. For the moment I am interested in showing you how the transmitter and the detector may be separated by miles and still there will be an effect in the detector circuit every time the key in the transmitter circuit is closed.

This is how we do it. At the sending station, that is, wherever we locate the transmitter, we make a condenser using the earth, or ground, as one plate. We do the same thing at the receiving station where the detector circuit is located. To these condensers we connect inductances and these inductances we couple to our transmitter and receiver as shown in Fig. 54. The upper plate of the condenser in each case is a few horizontal wires. The lower plate is the moist earth of the ground and we arrange to get in contact with that in various ways. One of the simplest methods is to connect to the water pipes of the city water-system.

Now we have our radio transmitting-station and a station for receiving its signals. You remember we can make dots and dashes by the key or switch in the oscillator circuit. When we depress the key we start the oscillator going. That sets up oscillations in the circuit with the inductance and the capacity formed by the antenna. If we want a real-sized stream of electrons up and down this antenna lead (the vertical wire), we must tune that circuit. That is why I have shown a variable inductance in the circuit of the transmitting antenna.

What happens when these electrons surge back and forth between the horizontal wires and the ground, I don't know. I do know, however, that if we tune the antenna circuit at the receiving station there will be a small stream of electrons surging back and forth in that circuit.

Usually scientists explain what happens by saying that the transmitting station sends out waves in the ether and that these waves are received by the antenna system at the distant station. Wherever you put up a receiving station you will get the effect. It will be much smaller, however, the farther the two stations are apart.

I am not going to tell you anything about wave motion in the ether because I don't believe we know enough about the ether to try to explain, but I shall tell you what we mean by "wave length."

Somehow energy, the ability to do work, travels out from the sending antenna in all directions. Wherever you put up your receiving station you get more or less of this energy. Of course, energy is being sent out only while the key is depressed and the oscillator going. This energy travels just as fast as light, that is at the enormous speed of 186,000 miles a second. If you use meters instead of miles the speed is 300,000,000 meters a second.

Now, how far will the energy which is sent out from the antenna travel during the time it takes for one oscillation of the current in the antenna? Suppose the current is oscillating one million times a second.

Then it takes one-millionth of a second for one oscillation. In that time the energy will have traveled away from the antenna one-millionth part of the distance it will travel in a whole second. That is one-millionth of 300 million meters or 300 meters.

The distance which energy will go in the time taken by one oscillation of the source of that energy is the wave length. In the case just given that distance is 300 meters. The wave length, then, of 300 meters corresponds to a frequency of one million. In fact if we divide 300 million meters by the frequency we get the wave length, and that's the same rule as I gave you in the last letter.

In further letters I'll tell you how the audion works as a detector and how we connect a telephone transmitter to the oscillator to make it send out energy with a speech significance instead of a mere dot and dash significance, or signal significance. We shall have to learn quite a little about the telephone itself and about the human voice.

LETTER 14

WHY AND HOW TO USE A DETECTOR

DEAR SON:

In the last letter we got far enough to sketch, in Fig. 54, a radio transmitting station and a receiving station. We should never, however, use just this combination because the transmitting station is intended to send telegraph signals and the receiving station is best suited to receiving telephonic transmission. But let us see what happens.

[Ill.u.s.tration: Fig 54]

When the key in the plate circuit of the audion at the sending station is depressed an alternating current is started. This induces an alternating current in the neighboring antenna circuit. If this antenna circuit, which is formed by a coil and a condenser, is tuned to the frequency of oscillations which are being produced in the audion circuit then there is a maximum current induced in the antenna.

As soon as this starts the antenna starts to send out energy in all directions, or "radiate" energy as we say. How this energy, or ability to do work, gets across s.p.a.ce we don't know. However it may be, it does get to the receiving station. It only takes a small fraction of a second before the antenna at the receiving station starts to receive energy, because energy travels at the rate of 186,000 miles a second.

The energy which is received does its work in making the electrons in that antenna oscillate back and forth. If the receiving antenna is tuned to the frequency which the sending station is producing, then the electrons in the receiving antenna oscillate back and forth most widely and there is a maximum current in this circuit.

The oscillations of the electrons in the receiving antenna induce similar oscillations in the tuned circuit which is coupled to it. This circuit also is tuned to the frequency which the distant oscillator is producing and so in it we have the maximum oscillation of the electrons.

The condenser in that circuit charges and discharges alternately.

The grid of the receiving audion always has the same voltage as the condenser to which it is connected and so it becomes alternately positive and negative. This state of affairs starts almost as soon as the key at the sending station is depressed and continues as long as it is held down.

Now what happens inside the audion? As the grid becomes more and more positive the current in the plate circuit increases. When the grid no longer grows more positive but rather becomes less and less positive the current in the plate circuit decreases. As the grid becomes of zero voltage and then negative, that is as the grid "reverses its polarity,"

the plate current continues to decrease. When the grid stops growing more negative and starts to become less so, the plate current stops decreasing and starts to increase.

All this you know, for you have followed through such a cycle of changes before. You know also how we can use the audion characteristic to tell us what sort of changes take place in the plate current when the grid voltage changes. The plate current increases and decreases alternately, becoming greater and less than it would be if the grid were not interfering. These variations in its intensity take place very rapidly, that is with whatever high frequency the sending station operates. What happens to the plate current on the average?

The plate current, you remember, is a stream of electrons from the filament to the plate (on the inside of the tube), and from the plate back through the B-battery to the filament (on the outside of the tube).

The grid alternately a.s.sists and opposes that stream. When it a.s.sists, the electrons in the plate circuit are moved at a faster rate. When the grid becomes negative and opposes the plate the stream of electrons is at a slower rate. The stream is always going in the same direction but it varies in its rate depending upon the changes in grid potential.

[Ill.u.s.tration: Fig 55]

When the grid is positive, that is for half a cycle of the alternating grid-voltage, the stream is larger than it would be if the plate current depended only on the B-battery. For the other half of a cycle it is less. The question I am raising is this: Do more electrons move around the plate circuit if there is a signal coming in than when there is no incoming signal? To answer this we must look at the audion characteristic of our particular tube and this characteristic must have been taken with the same B-battery as we use when we try to receive the signals.

There are just three possible answers to this question. The first answer is: "No, there is a smaller number of electrons pa.s.sing through the plate circuit each second if the grid is being affected by an incoming signal." The second is: "The signal doesn't make any difference in the total number of electrons which move each second from filament to plate." And the third answer is: "Yes, there is a greater total number each second."

[Ill.u.s.tration: Fig 56]

Any one of the three answers may be right. It all depends on the characteristic of the tube as we are operating it, and that depends not only upon the type and design of tube but also upon what voltages we are using in our batteries. Suppose the variations in the voltage of the grid are as represented in Fig. 55, and that the characteristic of the tube is as shown in the same figure. Then obviously the first answer is correct. You can see for yourself that when the grid becomes positive the current in the plate circuit can't increase much anyway. For the other half of the cycle, that is, while the grid is negative, the current in the plate is very much decreased. The decrease in one half-cycle is larger than the increase during the other half-cycle, so that on the average the current is less when the signal is coming in.

The dotted line shows the average current.

Suppose that we take the same tube and use a B-battery of lower voltage.

The characteristic will have the same shape but there will not be as much current unless the grid helps, so that the characteristic will be like that of Fig. 56. This characteristic crosses the axis of zero volts at a smaller number of mil-amperes than does the other because the B-batteries can't pull as hard as they did in the other case.

[Ill.u.s.tration: Fig 57]

You can see the result. When the grid becomes positive it helps and increases the plate current. When it becomes negative it opposes and decreases the plate current. But the increase just balances the decrease, so that on the average the current is unchanged, as shown by the dotted line.

On the other hand, if we use a still smaller voltage of B-battery we get a characteristic which shows a still smaller current when the grid is at zero potential. For this case, as shown in Fig. 57, the plate current is larger on the average when there is an incoming signal.

If we want to know whether or not there is any incoming signal we will not use the tube in the second condition, that of Fig. 56, because it won't tell us anything. On the other hand why use the tube under the first conditions where we need a large plate battery? If we can get the same result, that is an indication when the other station is signalling, by using a small battery let's do it that way for batteries cost money.

For that reason we shall confine ourselves to the study of what takes place under the conditions of Fig. 57.

We now know that when a signal is being sent by the distant station the current in the plate circuit of our audion at the receiving station is greater, on the average. We are ready to see what effect this has on the telephone receiver. And to do this requires a little study of how the telephone receiver works and why.