CAE EASY COURSE in HOME PAM - americanradiohistory.com€¦ · cae easy course in home pam maj. gekqe0. 0. squier, editor-in-chief lessonietun n g and what it means 13,e john v. l.hogan

CAE EASY COURSEin HOME PAMMAJ. GEKqE0. 0. SQUIER, EDITOR-IN-CHIEF

LEssoNIETUN N G AND WHAT IT MEANS13,e JOHN V. L.HOGAN

FORMER PRESIDENT OF INSTITUTE OF RADIC ENGINEERS

ONE OF THE FALLOWING SET OF SEVEN LESSONSI. A GUIDE FOR LISTENERS IN. 2. RADIO SIMPLY EXPLAINED. 3. TUNINGAND WHAT IT MEANS. 4. 1HE ALADDIN'S LAMP OF RADIO. 5. BRINGINGTHE MUSIC TO THE EAR. 6. HOW TO MAKE YOUR L'WN PARTS. 7. INSTALL-ING THE HONE EET.

Pubas-hed by MART I N H . PAYand REVI EVicy'REV I DNS CO..New York

The EASY COURSEIN HOME RADIO

EDITED BY

MAJOR GENERALGEORGE 0 SOUIERCHIEF OF THE SIGNAL CORPS U.S.A.

LESSON THREETuning and What It Means

By John V. L. HoganConsulting Engineer, New York; Past President and Fellow,

Institute of Radio Engineers; Member AmericanInstitute of Electrical Engineers.

Published by Martin H. Ray andThe Review of Reviews Co., New York

Printed in the United States of America

Copyright, 1922, byMARTIN H. RAY and THE REVIEW OF REVIEWS Co.,

New York

All rights reserved, including that of translati:n into foreign languages,including the Scandinavian.

Phot

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LESSON THREE

What We All Know About Tuning

TUNING in radio circuits seems a mysterious process.There is nothing supernatural about the operation,

however, and we can easily get a clear idea of it by com-paring its principles and its effects with other occurrencesin our daily lives. We are all more or less familiar withthe tuning of musical instruments ; we know that when aplayer tightens a string of his violin, that string producesa note higher in pitch than it did before it was tightened.We have noticed that the longer and heavier strings of aharp or piano give off the lower musical tones. I f wewish to tune a string to make a certain sound, say that ofmiddle C on the piano keyboard, we will usually begin bygetting its length and weight about right and then adjustits tightness until we have tuned it to the exact pitch wedesire.

The Musical Scale

What is it that makes the notes of a piano have differ-ent pitches? What do we mean by high-pitched and low-pitched tones ? The thing about a sound wave which tellsus its pitch is simply the rapidity at which it vibrates. Ifthe sound vibration is very rapid, it will produce thesensation of a high-pitched tone; if it vibrates compara-

3 5

6 Tuning and What It Means

tively few times per second, the tone will be low in pitchor frequency. The range of frequencies ordinarily usedin musical compositions extends upward from about 32vibrations per second, which is the third C below middleC on the piano. The highest note commonly heard is thefourth C above middle C, which has a frequency of 4,096vibrations per second.

If we wish to produce a note of any particular pitch,say that of middle C (which vibrates 256 times per sec-ond) we need only to set up a stretched string or someother vibrating body, so that it may oscillate at this par-ticular rate.

Sympathetic Vibration

Did you ever make the experiment of sitting before apiano, and, while holding down the "loud" pedal, singingor whistling some single note? Pressing the pedal lifts adamper from the strings, and leaves them free to vibrate.Whatever note you sing, you will find that the correspond-ing string of the piano will take it up and continue tosound that particular tone for a moment of ter you stopsinging. By uttering in quick succession two or threeloud but short tones, you can get the piano to respond to acomplete chord.

What happens in this demonstration of sympatheticvibration is that your vocal cords produce air vibrationsof the frequency corresponding with the note you sing.These air vibrations travel outward from your mouth as asound wave, which spreads in all directions through theroom somewhat in the form of an imaginary but huge andever-expanding soap bubble. The air -wave strikes against

3

In receiving sets employing electron tubes m

ost of the tuningis done w

ith variable condensers of this type.For the re-

ception of continuous waves, sharp tuning

is required; forthis purpose a variable series condenser m

ay be used in theprim

arycircuit, and a sm

all variable condenser called a"V

ernier" condenser, connected in parallel with

it.

7


all the strings of the piano; some one of those strings istuned to the same pitch (the same frequency of vibration)as the sound wave. This particular string is the only onethat can swing back and forth in unison with the sound -wave, and, as we shall see later, is the one which can most

Fey .1A child's swing; if the length is eighteen feet, it will take about

five seconds to travel from A to B and back to A.

easily be made to move at that individual frequency orpitch. The sound -wave striking this string, and the suc-cessive impulses in the sound -wave coming along at justthe right rate to hit the string exactly in step with its own

3

Tuning and What It Means 9

vibrations, combine to make the string itself swing back

and forth vigorously. When the sound -wave stops, be-cause you stop singing, the string is going strongly ; itsoscillations cannot stop at once, and while they are dyingaway they produce new sound -waves of the original pitchwhich travel outward from the string and are heard byyou as a reproduction of the tone you sang.

The Natural Frequency

Let us see how some of these same effects occur in

other vibrating systems. Instead of considering soundwaves, with their frequencies of from sixteen vibrationsper second upward to thousands of oscillationsond, let us study the action of a child's swing hangingfrom the limb of a tree. If the swing, as shown in Fig. 1,is eighteen feet long, it will travel from one high positionto the other and back again in about five seconds. Thistime will be required to complete an entire double -swingfrom right to left and back, regardless of the distancethrough which the seat moves; if it is "swinging low" itwill move comparatively slowly, and if "swinging high" itwill automatically go fast enough to make each half -trip

in two and one-half seconds.The time required for any vibrating or swinging body

to make a full or double swing is called its time period, orsimply its period. The double -vibration itself is called acycle of oscillation. The frequency of vibration is thenumber of cycles or complete vibrations that the systemcompletes in one unit of time. In the case of sound wavesthe vibrations are so rapid that we measure the number in

3

A modern crystal detector set, with a range of about 40miles under good conditions.

In

The sim

plest typeof

vacuum -tube

detector set has ahandle for tuningin and beside

it a"tickler" knob forsharpening the tun-ing.

The second

knob shown (that

next to the lamp -

liketube)

merely

controls the currentsupplied to the fil-

ament.


one second, and speak of the frequency in cycles persecond; for our swing which has a period of five seconds,it is more convenient to measure the frequency perminute.

Since there are sixty seconds in one minute and sincethe swing uses five seconds to make a complete swing orcycle, it is quite evident that the swing will describe twelveround -trips in one minute. Thus, its frequency is twelvecycles per minute. This is the frequency that the swing

will take if started and left to itself, and consequently it iscalled the free or natural frequency of the swing. Of

course, we can force the swing to vibrate at any other ratewe desire if we will hold on to it and move it by mainstrength; in that way we can force it to move back andforth only five or six times per minute, or we can speedit up to go even twenty times per minute. To do this, how-ever, we will have to work hard and actually drive theswing at every point of its motion; for if we leave it alonefor an instant it will begin to regain its natural frequencyof twelve per minute.

On the other hand, we can keep the swing in motion atits natural frequency simply by pushing it slightly eachtime it passes us. Very little effort is required to keep itgoing at this rate, for we are taking advantage of itsnatural tendency to go back and forth at its own fre-quency. This natural frequency of vibration is the rate ofswinging which is most easily attained by the system.

The middle C string of the piano has a natural fre-quency of 256 cycles per second. When an air -wave ofthis same frequency strikes the tuned string, the wave -impulses are able to set the string swinging back and forthstrongly. It is a case very much like that of the swing,

3

W m

.D

ubilier,w

ho invented thepressed

mica

condenser which

is now used

widely for both

transmitting and

receivingradio

comm

unication.

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poly

gona

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ames

atth

eri

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aves

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for when the driving impulses pushing the swing are timedto the rate of twelve per minute (which is the natural fre-quency of our swing), it is easy to make the seat movehigher and higher with each oscillation.

Changing the Natural FrequencyWe have seen that we can change the natural frequency

of a musical string, or tune it, to give off any desired note,merely by changing its size or its tightness. Let us nowfind out how the natural frequency of a mechanical systemcan be controlled.

The simplest slowly -oscillating mechanical arrangementwhich is capable of easy and flexible adjustment is proba-bly the "twisting dumb -bell" or torsion pendulum shownin Fig. 2. This is nothing more than a vertical rod or stiffwire, clamped firmly at the upper end to prevent it fromturning at that point, suspending a horizontal sectionwhich carries a weight at each end. The bottom part re-sembles a dumb -bell ; for convenience in experiment itshould be provided with interchangeable weights of severaldifferent sizes.

If we set up such a torsion pendulum with a rather thinvertical twist -rod and moderately heavy weights, it willhave a fairly slow motion or low natural frequency. Toset it going, you twist the dumb -bell around in a horizontalplane as far as it will move easily, and let go. The dumb-bell then swings back and forth, the weights moving in ahorizontal circular path, first in, one direction and then inthe other, the motion gradually becoming less and lessuntil the weighted arm finally comes to rest in its initial orundisturbed position. We may measure the time requiredfor the weights to swing from one extreme position to the

3


A

. ,

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-- Twist rod

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.N\szs, Y1/Yeig. hi

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rt. "yr. 2

A twisting Dumb -bell or Torsion Pendulum; the weights swingback and forth between A and B as indicated by the dotted lines.

other and back again. This time is, of course, the periodof the pendulum. The number of double swings or com-plete cycles that it makes in one second or in one minute isthe natural frequency per second or per minute. A typical

3

High jrcquciicy transformers and condensers for theAlexanderson alternator of the great station at Port

Jefferson, L. I.

17

(i Tuning and What It Means

twisting dumb -bell of this sort will swing through a fullcycle in one second; therefore its free or natural fre-quency is one per second or sixty per minute.

Suppose that we have such a torsion pendulum, with anatural frequency of sixty cycles per minute. If we takethe twist rod between thumb and finger, about half -waydown from the supporting clamp, and "roll" it slightly, wecan move the swinging weights a short distance along theircircular path. If, having given the pendulum such afeeble impulse, we allow the weights to swing back, theirmomentum will carry them past the position of rest andpart away around in the opposite direction. The springi-ness of the twist -rod will soon arrest this motion, however,and the weights will once more swing in the direction inwhich we started them. We may now "roll" the rod be-tween thumb and finger again, as before, and the secondimpulse will carry the weights a little farther away fromthe rest or neutral position. By impulsing the system oncein each swing we can build up an oscillation of considerablesize, in which the weights swing with a good deal ofmotion. But to do this, the driving force must be appliedat the right time in each oscillation; in other words, itsfrequency must agree with the natural frequency of thedriven system. If we desire the dumb -bell to oscillate atany other rate, we must either force it to do so by mainstrength (keeping hold of the swinging weights through-out their movements and not permitting them to oscillatefreely) or we must change the natural frequency of thependulum to the new value which we desire.

How can we change the natural frequency of such atwisting pendulum ? What can we do to it that will cor-

3

Photo by Keystone

This antenna tower, one hundred feet high zoos constructed by twoamateurs in ,Salt Lake City. With the antenna they have pickedup radio music as far east as Schenectady, and as far west as

Oakland.19


respond with changing the size or the tension of a pianostring, and so change its rate of vibration?

It should be almost self-evident that if the twist rod ismade more flexible, or more flimsy, it will not have somuch spring or restoring force and consequently it willnot be able to swing back the weights so fast when theyare displaced from the position of rest by turning them inone direction. Therefore the time period may belengthened, and the natural frequency correspondinglydecreased, by increasing the flimsiness of the torsion rod.If the rod is left without change, but heavier weights areput on the swinging arms, the restoring force of the rodwill be insufficient to move the larger weights as rapidly asbefore. Consequently, increasing the mass of the pendulumweights in a twisting dumb -bell system will also reducethe frequency of natural oscillation. By a proper choiceof flimsiness and mass we can make the natural frequencyof the pendulum any value we desire within the practicallimits of the structure. It is interesting to note that whenwe change the tension of a vibrating musical string wechange the restoring force, and that alterations in the sizeof the wire change its mass for a given length thus thecontrol of the natural vibrating frequency of a piano wireis much like that of the torsion pendulum.

Tuning and Resonance

We have seen that in order to drive a vibrating systemmost effectively, the frequency of the driving impulsesmust be practically the same as the natural frequency ofthe driven system. This was true of the musical string,

3

© Underwood & Underwood

Tuning in-a process that consists in much more than turninga knob.

21


the child's swing, and the twisting dumb -bell. It is equallytrue of all other systems, mechanical or electrical, whichare capable of natural or free oscillations and which conse-quently have natural frequencies of vibration. The agree-ment or practical identity of the frequency of the appliedforce and the system which it is intended to operate iswhat we call resonance or syntony. When the vibratingsystem has its natural frequency adjusted to agree withthe frequency of the driving force, it is said to be tuned tothat frequency.

In this connection we must not overlook the fact thatany oscillating system may be forced to vibrate at fre-quencies which are different from its natural or resonantrate. But, as in the case of the swing, such non -resonantvibrations must be maintained by the use of excessivelylarge power (by main strength, as we said) since they donot take advantage of the tendency of the system to swingback and forth at its natural frequency. These non -resonant vibrations are called forced vibrations, to dis-tinguish them from the natural or free vibrations of thesystem.

Electrical Vibrations

One might well ask what the preceding discussion ofmusic and pendulums has to do with electricity and par-ticularly with radio. The answer is simple; in a word,everything. All the phenomena that we have consideredfrom the mechanical viewpoint are reproduced in electricalcircuits and are used in radio communication. We havemerely to work out the electrical counterparts of thevarious mechanical effects to understand the basis of

3

The largest radio transmitting coil in the world-that of theLafayette Station vicar Bordeaux.

23

In the early stages of the 7011", 01,,c,'17ii's found it difficult to receiveradio telegraph signals the Ic/itlione headsets because of theroar of the engine. Hence this instrument zcas used. The observerlooked through a pair of glasses and SOW dots and dashes of thecode in the form of flashes longer or shorter duration, a filamentor string being electrically heated. Later this ingenious instrumentteas displaced by telephone sets so constructed that the observer

could hear easily.

24


tuning and resonance in radio circuits. Although theoscillations in an electric circuit are not visible, as theyare in the case of a pendulum, they nevertheless occur inthe same way and are subject to the same sort of controlas to natural frequency, intensity of vibration, etc.

Let us first get a general idea of an electric circuit withelectric current flowing in it. Suppose that the positiveend of a battery is connected by a piece of wire with acurrent -indicating meter, that the meter is connected with

Current Resistance-.meter unit

Battery+ ( -

'1111

f<"9. 3A simple battery circuit; direct current Mows through the wires

in the direction indicated by the arrows.

one end of a resistance unit, and that the other terminalof the resistance is wired back to the negative side of thebattery, as illustrated in Fig. 3. The battery is somethinglike a pump for electric current; it produces between itsterminals an electric pressure which tends to cause a flowof electric current when the terminals are connected by awire or other conductor. The positive end of the battery,indicated by a plus sign, is the high-pressure end ; the dif-ference in electrical pressure or potential between the bat -

3

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tery terminals is measured in volts, and the higher thevoltage the higher the electrical pressure.

In the circuit that we have imagined, a path for current -flow from one end of the battery to the other is providedthrough the resistance unit and meter. The battery pres-sure will force a current to pass through the circuit; theamount of current will be registered in amperes, and willdepend upon the pressure of the battery and the resistanceof the circuit. The resistance is measured in ohms, whichis merely that property of a wire which holds back electriccurrent. In this circuit the current flows only in one direc-tion, from high to low potential (Or plus to minus ends ofthe battery, according to the conventional expressionadopted many years ago) ; it is therefore called a uni-directional or direct current.

Suppose that the battery were connected into the circuitwith its terminals reversed. The current would still flowfrom high potential or positive to low potential or negative,but its path through the wire would be in the reverseddirection. If the battery in such a circuit were regularlyand rapidly turned end for end, being connected with thewires each time, it is not hard to see that the current in thecircuit would flow first in one direction and then in theother direction. In other words, the current would nolonger be unidirectional or direct, but would change itsdirection of flow at regular intervals, going first one wayand then the other. Such a regularly reversing flow iscalled an alternating current.

Alternating currents are not produced in practice byreversing the terminals of a battery, but instead by meansof an alternating current dynamo or alternator. Thisdynamo machine automatically sets up electric potentials

3

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rate

c n

r -

rent

s w

ith a

fre

-,,

gurn

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f th

ou-

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ds o

f cy

cles

per

seco

nd. T

his

is th

e al

tern

ator

of th

e gr

eat

Laf

ayet

te,

s t a

-lio

n bu

ilt b

y th

eA

mer

ican

arm

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Fra

nce

dur-

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the

Tea

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hest

atio

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one

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.


across its terminals, first in one direction and then in theother. The fluctuating potentials cause correspondingalternating currents in any electrical circuit connectedfrom one of the alternator terminals to the other, such,for example, as that of Fig. 4.

It is not hard to see that electric currents which flowfirst in one direction and then in the other, that is,alternating currents which swing back and forth in a cir-cuit, must have something in common with mechanical

fig.4Alternating current circuit: alien the left-hand terminal of thealternator is positive the current flows as indicated by the solidarrows. The dotted arrows show the direction of current flow

when the right-hand terminal is positive.

oscillations. They are evidently electrical vibrations, justas the swings of a pendulum are mechanical vibrations.The electrical swings, of course, require a definite lengthof time to complete one double -oscillation or cycle ; thislength of time is, as before, called the period. The num-ber of complete electrical cycles of alternating current inone second is, as in the mechanical case, the frequency incycles per second.

3

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l Du

-bi

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he tu

be c

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.


Everyday electric alternating current used for houselighting has a frequency of 60 cycles per second, or a timeperiod of 1/60 second. This is a much faster vibrationthan either the swing or the pendulum we have considered,but is right in the musical frequency range and correspondsto a tone about two octaves below middle C.

Free Electrical Oscillations

The alternating currents we have just considered are notfree or natural electrical vibrations, for their frequency ofalternation is determined by the frequency of the voltagesdeveloped by the dynamo machine. The current that flowsthrough the circuit connected to this alternator is forcedto follow the voltage which sets it up, both as to directionand amount of flow, and hence as to frequency. It is, trueenough, an electrical vibration, but it is a forced vibration.The dynamo voltage forces the current to flow in a givendirection, and with a given intensity, by main strength.

There is another sort of alternating current generatingsystem, however, in which the electrical oscillations arenot forced. If we connect an electrical condenser in cir-cuit with a coil of wire, which has the electrical propertycalled inductance, we make a system which is capable offreely oscillating electrically. If such a circuit is providedwith a battery and a charging switch, as shown in Fig. 5,as well as with a discharging switch, it may be used for thepurpose of generating alternating currents. The workingof such an arrangement, complicated as it may appear atfirst sight, is not hard to understand if one considers eachportion of the system separately.

3

V

MWABLE LOOP 0,0.5PEA,ERHORN

Loop which when turned indicates the direction from which wavesare coming. Such a loop constitutes what is called a radio -compass.By its means ships can find their way through fogs and starless

nights.

32

Some coils used in radio receiving sets have more than one layer.fere Tee have a coupling coil with a type of winding resembling

This tyPe of coil is variously called "lattice wound,"alular," "basket :cot nd," "honeycomb." In this type a winding

oizvii inductance occupies a small ,r space than if a single-Imv., chiding Were used, and the distributing capacity of the.cimling is small. The type doesn't give very satisfactory results

on zauve lengths less thai 2,500 meters.


In the first place, let us look at the battery, the chargingswitch and the condenser. The electrical condenser ismerely a pair of electrical conductors of somewhat largesurface arranged so that their faces are close to, but not incontact with each other. Two sheets of tinfoil placed onopposite sides of a sheet of thin glass will make an excel-lent condenser for many purposes; two plates of coppereach about two feet square, hung freely in the air abouthalf an inch apart, give us another useful form. The glassin the first type and the air between the plates in the secondarrangement, serve to keep the two plates electricallyapart so that current cannot flow directly from one to theother. Yet the two comparatively large surfaces areallowed to act upon each other so as to develop the out-standing property of an electrical condenser, namely,capacitance.

This capacitance (formerly called capacity) measuresthe electrical size of a condenser, and shows the quantityof electricity which may be stored in a condenser at anygiven voltage or electrical pressure. A condenser is toelectricity about what a toy balloon is to gas ; one cancharge a condenser with electricity just as one can blowup a rubber bag with gas. Electricity is stored in acharged condenser ; gas is stored in an inflated balloon.If a gas -bag is filled from a pump producing some certaindefinite pressure, it will be possible to store a certainamount of gas in the balloon, and no more ; the restoringforce produced by the stretched walls of the hag will atsome point be equal to the force of the pump, and no moregas will go into the balloon. The amount of gas that canbe stored at some particular pressure will measure the sizeof the balloon for storage purposes. In the same way, if

3

Photo by B

ii),avof

Standai

Lis

This m

odel was built to show

how the radio com

pass which is dependent on a loop (in the foreground)

can help ships to find their way through fogs.

The system

illustrated is that developed by the Bureau

of Standards.


a condenser is charged at a definite electrical pressure orvoltage, it will hold just so much electricity and no more;the restoring force developed in the condenser will preventmore electricity from entering as soon as the force be-comes as great as the charging pressure. Thus, the quan-tity of electricity that a condenser will hold at some definitevoltage will measure the size or .capacitance of the con-denser for storage purposes. In general, the capacitanceis larger the greater the oppositelyexposed surfaces of the

+SEIM.

gatie-;,

fi:y.A condenser-coil or oscillatory

circuit, provided with chargingbattery and switch in addition to a dischargingswitch. Oscillationsare produced in the inductance coil circuit shown in heavy lines.

condenser plates and the closer the separation betweenthem.We should now be able to see that if the chargingswitch between the battery and the condenser in Fig. 5is closed, a certain amount of electricity will flow into thecondenser from the battery. This amount will be greaterthe larger the capacitance of the condenser and the higherthe potential or voltage of the battery. For the condenserto charge in this way, however, the discharging switch3

K.1

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also shown in Fig. 5 must be open, so that current cannotflow around the condenser through the wires leading to theinductance coil shown.

Having charged the condenser, we may open the charg-ing switch. This leaves the condenser filled with elec-tricity. The upper plate (Fig. 5), which was connectedto the positive terminal of the battery, is of high potential;the lower plate is of low potential; the potential differenceof the two plates is the same as the voltage of the charg-ing battery. In fact, this charged condenser is just like abattery provided with only a limited quantity of electric-ity; it can, by reason of its charged voltage, set up a cur-rent in any electrical circuit connected across its two platesor terminals, but the amount of current which will flow islimited by the amount of charge that the capacitance ofthe condenser permitted it to draw from the chargingbattery.

Suppose now that we close the discharging switch. Thisact provides a direct electrical circuit from one side ofthe condenser through the inductance coil to the otherside, as shown in Fig. 5. The electricity stored in thecondenser must discharge through this circuit as a currentfrom the high potential plate to the low potential plate,passing through the inductance coil.

We come now to what is probably the most interestingthing about condenser -coil circuits of this sort. If thecircuit is a good one, that is, if it has not such high re-sistance that electricity will not flow freely through it ;

the effect of the coil will cause the current to keep ongoing after the condenser is fully discharged. The cur-rent in passing through such a coil will gain a sort ofmomentum ; it will not die away when the condenser

3

In this receiving transformer, one prim

ary switch has thirty-four turns betw

een points, and the other switch has tw

oturns betw

eenpoints.

The secondary sw

itch adjusts by groups ofturns,

finetuning

is accomplished by a variable

condenser.T

he coupling between the prim

ary and the secondary is loosened by pulling the secondary out of the primary.

The sw

itches of the transformer are adjusted until

the approximate w

ave lengthrange of

thestation w

hichit

isdesired

toreceive,

isreached.

The secondary

circuit is then turned by a variable condenser to the station which it

is desired to receive.

.1nother view of the British Government's ball condenser with oneof its hemispheres partially turned.

40

collapsi7dc loop -antenna t S"knocked - down"

position.

41

4? Tuning and What It Means

voltage has fallen to zero, but will overshoot and pile upon the condenser plates in the opposite direction of charge.By reason of this peculiar action, the condenser will againbecome charged, but in the opposite sense; the upper platewill now be negative and the lower plate positive. How-ever, some electricity will have been lost in passing throughthe circuit, and the new charge will be of somewhat smallerpotential than was the initial charge. Otherwise the con-ditions will be as they were when the charging switch wasopened and the discharging switch closed ; the new re-versed charge in the condenser will discharge through thecoil in the opposite direction, and will again overshootenough to charge the condenser a third time. Now thepolarity will be as it was originally ; the upper plate willbe positive and the lower negative, but the amount of elec-tricity in the condenser will, of course, be still less.

It is easy to see that the co-operative action of the coiland the condenser will cause, for a single initial charge, along series of discharges which alternate in direction andwhich gradually die away in intensity. Thus there is pro-duced in the condenser -coil circuit an oscillatory discharge,an alternating current which persists for many completecycles but which finally dies away and which cannot there-after be renewed until the condenser is again chargedfrom the prime source (in this instance the battery). Thenumber of complete oscillations produced by such a dis-charge may easily be several hundred, before the currentfalls off to small values.

What is it about an ordinary coil of wire that will causea current through it to show a sort of inertia or mo-mentum effect? Why does the condenser discharge cur-rent, passing through the coil, keep on going after the

2

The British. Government during the year introduced thisingenious ball condenser. It consists of three concentrichemispheres which can be rotated to vary the capacity.Once adjusted the capacity is permanent; no sag is

possible.

43

This is a Variometer of the usual type used in ordinaryradio practise.

44

The col-lapsibleloop an-tenna ex -t e0d cfor eon -

C C tionwith t h er c c riving

set.

45


condenser voltage has become zero, so as to charge thecondenser again in the opposite direction ? The answer toboth of these questions lies in the property of inductance,which is possessed by any conducting wire, but which ap-pears in more concentrated or exaggerated form when thewire is coiled up into a spiral. This electrical property ofinductance is analogous to the mechanical property ofmass; it gives rise to inertia or the tendency to keep thingsas they are. Mass in a pendulum causes the weight toswing past the position of rest, so as to travel part wayagainst the restoring force and prepare for the next swing.Inductance in an electrical circuit causes the current topersist beyond the condition of zero voltage, so as partlyto charge the condenser for the succeeding discharge.Any coil of wire possesses inductance; the more turns,the closer the turns to each other, and the greater the areaenclosed by each turn, the greater the inductance of thecoil.

Natural Electrical Frequency

When a condenser -coil circuit such as that shown inFig. 5 produces electrical oscillations or alternating cur-rents of gradually decreasing intensity, what governs thefrequency of these oscillations? Reasoning that the in-ductance of the circuit corresponds with the mass of thetorsion -pendulum weights, and that the capacitance of theelectric example is comparable to the flimsiness of thespring in the mechanical case (since the condenser is whatsupplies the restoring force in the electric circuit), onemight conclude that the frequency of vibration would de-pend upon the amounts of inductance and capacitance

3

"Siatic" ist h e

bugbear of radio.In the receiver itsounds like fryingfat.

It is causedby lightning andother electric dis-charges.

General

Squieris

hereshow

n experim

entingw

itha

"staticelim

-inator."

('tutu by Bureau of S

tandards

48 Tuning and What it Means

which are effective in the electric circuit. This conclusionwould be exactly right; the analogy is close and complete.We may make the natural frequency of a condenser -coilcircuit almost what we will, merely by changing the size ofthe condenser and of the coil. Since the natural electricaldischarge of the condenser through the coil is always ofthe free frequency of the circuit, by changing the in-ductance and the capacity, or either of them, we may varythe frequency of the oscillations generated by such acircuit.

The arithmetical rule for finding the natural frequencyis simply to multiply the inductance in henrys (the practicalunit) by the capacitance in farads, take the square root ofthe product, multiply this root by 6.28 and find thereciprocal of the isnatural frequency of a circuit having the effectivecapacitance and inductance used for the calculation. Thepractical value of such a rule is that it enables us to findthe natural frequency when we know the size of the coiland the condenser, and that it shows us that the frequencybecomes higher and higher as the size of the coil and ofthe condenser are decreased.

Although it is easy to build vibrating electrical circuitshaving natural frequencies within the range of musicaltones, those used in radio signaling ordinarily have muchmore rapid rates of vibration. Whereas the musical scaleruns from about 16 to about 4,000 cycles per second, andwhile frequencies even lower and higher than these areaudible and hence come within the range of so-called "audiofrequencies," the radio frequencies are usually consideredto begin at about 10,000 cycles per second and run on upto several millions per second. By proper choice of coils

3

Instead of using am

icrophonetrans-

mitter in place of

the reproducer of aphonograph an ex-tension

ma y

b ebuilt on the horn oft h e

phonograph.T

hisextension

isconnected

with

atelephone transm

it-ter, as here show

n.

I hobo by Burcan of S

tandards


and condensers it is not difficult to set up oscillating elec-trical circuits with natural frequencies even as high asthese. Thus we have found a way to produce free elec-trical oscillations with these enormously high frequenciesthat are used in radio.

The Radio Frequency Generator

The discharge of a condenser through a coil in an oscilla-tory or resonant circuit is not the only way we have toproduce these radio frequency currents, however. Inspite of the fact that the ordinary power alternator gen-erates alternating current of only 25 or 60 cycles per sec-ond, it has been found practicable to increase the speed ofspecial dynamos to the extent that they can directly pro-duce alternating currents of 100,000 or even 200,000cycles per second. Such high frequencies are entirelysuitable for use in radio transmission. The electrical cur-rents made by such generators are forced oscillations ; theyare constant in intensity, and do not die away and requirerenewal charges ever so often. In addition to thealternator of radio frequency, which is clue primarily toProfessor R. A. Fessenden and in its present-day detailto E. F. W. Alexanderson and others, there have beendiscovered several ways to keep a condenser -coil circuitoperating constantly and to prevent its oscillations fromdecreasing in intensity. Among these is the Poulsen arc,which takes the place of the discharge switch in Fig. 5,and the Armstrong regenerative circuit for the vacuumtube, which is described by Professor Morecroft in thepresent series of pamphlets. All of these generators pro -

3

To the left is the standard Leyden jar which was longused in radio; to the right is the modern Dubilier micacondenser which has taken the place of the old Leydenjar throughout the world. The old Leyden jar wasfragile. Sometimes it was broken by a salvo on a battle-ship with the result that the entire wireless set wasrendered useless. It was also responsible for what arecalled brush charges, which generated ozone gas, andinduced headaches. All these objections are overcome

by the modern mica condenser.51


duce in effect a smooth alternating current of controllableradio frequency. With the alternator, the frequency ischanged by altering the speed of the machine; with thearc and vacuum tube, the frequency is controlled by vary-ing the capacitance or inductance of the oscillating circuit.

Coupled Radio Circuits

Let us now consider a circuit such as that shown in Fig.6. Here a generator of radio frequency currents is shown

Cod

Par/atlei'Condenser

fiy 6 Current'meter

How a high -frequency dynanto may be arranged with a two -coiltransformer to produce radio currents in a coupled condenser -coil

circuit.

at the left, connected to an inductance coil marked A.The alternating voltages produced by the generator willforce alternating current of corresponding frequencythrough coil A. The magnetic field around this coil willinduce voltages of the same frequency in coil B, which isconnected in series with a current indicating meter and acondenser whose capacitance may be varied. What will

3

A Leyden jar consists of a thin glass vessel coated inside and outwith metal foil. Connection is established between the two coatingsto charge and discharge the jar. When it was discovered that theelectricity is connected by the coatings, condensers were made inthe limm of glass plates, as here shown. These are called parallelplate condensers and consist of alternating layers of dielectric

(glass in this instance) and conducting material (metal foil).

53

Wires leading in from aerial to large outdoor tuning coilat the Port Jefferson station.

54

Large

tuningcoil in the A

r-lington.

Station(W

ashington).T

o theleft

a,,,sm

alltransm

it -'nting set.

1:J1,1er:coot/


happen in this second circuit, containing coil B and thecondenser?

It should be evident that we have here an electrical casewhich may be compared with the torsion pendulum drivenby rolling the twist -rod between the thumb and finger.The condenser -coil circuit at the right is an electrical sys-tem having its own natural frequency, which frequencywe may control by changing the size of the condenser.This system is impulsed periodically by the alternatingvoltages induced upon the coil B from the forced elec-trical vibrations or currents flowing from the generatorthrough coil A. We should expect that it would be dif-ficult to produce a current in the circuit of coil B unlessits natural frequency agreed with the frequency of thedriving or applied electrical forces, and this is exactly thecase. The natural frequency of the coil B circuit may bevaried by changing the capacitance of the condenser; asthis frebquency is brought more and more exactly intoagreement or resonance with the frequency of the alter-nator, the current in the coil B circuit will increase to amaximum value.

Thus we see that to produce the greatest current in asecond circuit associated with or coupled to another inwhich an alternating current is flowing, we must adjustthe natural frequency of the second circuit to agree withthe frequency of the driving current. This is calledtuning the second circuit.

The Antenna or Aerial CircuitSo far we have considered only coil and condenser cir-

cuits, in connection with their natural frequencies. Never-theless, what we have learned may be applied to any elec-

3

A Navy loose coupled tuning coil used in portable or packsets.

57


trical circuit which possesses the properties of inductanceand capacitance, whether or not it contains either coilsor condensers. For instance, instead of the inductiveeffect being produced by a coil of several turns, it is pos-sible to supply the requisite inductance by means of asingle turn loop enclosing a considerable area. Instead ofusing a two -plate or multiple -plate condenser, the neces-sary capacitance may be found in a network of wires sup-ported above the earth and connected to it through the cir-cuit. Clearly, such an elevated group of wires will takethe part of one condenser plate while the surface of theearth (which is a good electrical conductor) will act as theother plate of the condenser. The fact that wires are

Radio frequencyd/ternator

forth --r-

Aerialwires/

e-71471/7g CO/

K=>B"

k.=>

-Current meter

fty. 7As shown here, an elevated antenna wire system and earth con-nection may be substituted for the condenser of a resonant circuit.

3

The outdoor coil

for tuning the an-tenna of the great.,trans -A

tlantic sta-"'tion at Port Jef

ferson, L. I.


hung quite high above the ground will necessarily reducethe capacitance of the system, but this will largely be com-pensated for by the great extent of the aerial wires (whichmay be several hundred feet in length) as compared withthe size of ordinary condenser plates.

With this in mind, we may re -draw Fig. 6 in the formof Fig. 7, where the condenser connected to coil B hasbeen replaced by an aerial and ground corresponding tothe upper and lower plates of the condenser. Because ofthe capacitance of the aerial with respect to the ground,taken in connection with the inductance of the coil B, sucha system will have a definite natural electrical frequencyjust as had the condenser -coil circuit. However, thecapacitance is now fixed by the size of the particular aerialemployed, and so in order to vary the natural frequencyof the circuit it is necessary to make the inductance of thecoil B adjustable. This is indicated by the arrow -tippedconnection, which may be moved from turn to turn untilthe best value of inductance is found. When the in-ductance coil is made adjustable in this way, it is usuallycalled a tuning coil. By changing the amount of in-ductance, the antenna circuit may be brought into reso-nance with the frequency of the driving alternator, asalready explained in the case of the condenser circuit, soas to secure the greatest possible current flow at radio fre-quency between aerial and ground.

Creating Radio Waves

Whenever rapidly -alternating or radio frequency cur-rents flow in an aerial -and -ground system such as that

3

How the antennae are connected with the great LafayetteStation in France.

61

()) Tuning and `hat It Means

shown in Fig. 7, they produce radio waves in space aroundthe aerial. These waves shoot off in all directions fromthe aerial, and pass outward over the surface of the earthat the speed of 186,000 miles per second. The waves areelectro-magnetic disturbances of the all-pervading mediumof transmission that we call the ether of space, and, exceptfor their frequency, are similar to light and heat wavesthat travel through this same ether. The frequency of theradio waves is exactly the frequency of the currents thatproduce them; if the radio frequency alternator of Fig. 7is operating at 100,000 cycles per second, the currents inthe antenna or aerial circuit will be of 100,000 cycles persecond ; the radiated waves will also be of this same radiofrequency.

imagine the radio waves to spread outward fromthe sending station in ever-increasing hemispheres, some-what as indicated in Figs. 8 and 9. Fig. 8 shows roughly aside view of the antenna and the waves leaving it inopposite directions, as indicated by the arrows. Fig. 9 is

lend/

.--- - forth'

I

f(9This illustration shows how the radio-waves are imagined to pass

out over the earth's surface from a sending aerial.3

Duplex radiotelephone trans-mitter radio unit,showing tuninginductance (coilat the lop) and',.cave changer(handle on pa-nel) for tuningto different wavelengths. By "du-plex telephony"is meant tele-phony thatmakes it possibleto talk and listenwith the same

apparatus.

63


\ \ \

Looking down from directly above a radio station. The waves arefound to spread outward in all directions, as suggested by the

arrows and ever -widening circles.

a plan representing the way in which the waves spreadout in all directions from a radio station. At the tre-mendous speed with which they travel, radio waves willcover nearly 200 miles in 1/1000 of a second.

3

Model

builtby

t h cB

ureauof

Standardsto

il-lustrate

how an

electric cir cu itcan producew

aves ofIt i g h

frequency, which

waves are

cc-ccivcd by anothercircuit

tunedto

resonance.T

h ebox -like

casingson the table fromw

hichthe

polesprotrude

upare

Dubilior m

ica con-densers.

Photo by B

ureau of Standards

Tuning and What It Means

Intercepting Radio Waves

The radio wave travelling outward through the etheris a combined magnetic and electric disturbance thatgradually grows weaker as it passes farther and. fartherfrom the sending station. So long as it retains enoughstrength to be recognizable, however, it will induce electricpressures or voltages in any conductor which it strikes.Just as a water wave will float a bit of wood up and downas it passes by, so will a radio wave set up electricalmotions or currents in any circuit through which radio-frequency currents can readily flow. This is the propertyupon which we rely to receive radio messages. Antennawires are thrust upward into the air and provided withground connections; in them feeble currents are pro-duced by the passing radio waves, and these currents areused to operate the radio receiving instruments. The aerialconductors may be of any form whatever; a single wiredropped from a flag -pole, a T-shaped group of wires, oran enlarged umbrella -like structure will operate satisfac-torily. Major -General G. 0. Squier has discovered thatliving trees possess sufficient electrical conductivity tointercept and bring down strong enough radio frequencyimpulses for practical reception of messages by wireless.Even a coil or loop of wire within a building, formingwhat is known as a loop aerial, will pick up enough powerto operate the most sensitive modern radio receivers.

What can we do to get the greatest amount of currentin our receiving aerial systems, so as to make their effectsas large as possible? The answer to this follows fromwhat has gone before. Once more we have an electricalcircuit which is impulsed by a radio frequency electrical

3

Photo by Bureau of Standards

Here a large resonance -wave coil -antenna (Rolling Pintype) is shown, mounted for transmitting.

67


force, the force in this case being in the arriving wave.We have learned that to secure the greatest response tosuch an alternating force, we must tune the driven sys-tem. In our present problem, then, we must provide thereceiving aerial system with capacitance and inductance insuch proportion that its natural frequency will be the sameas the frequency of the wave which we desire to receive.The receiving aerial already supplies us with the necessarycapacitance ; consequently we need only to insert theproper amount of inductance as a tuning coil between theaerial and the ground connection, as indicated in Fig. 10.By adjusting this coil, the aerial system may be tuned sothat its natural frequency will be the same as the fre-quency of the arriving waves; under this condition we willsecure the largest possible radio frequency current in theaerial for a given voltage induced by the waves. The re-ceiving instrument used may be connected directly to thistuning coil, or, as suggested in Fig. 10, another tunedcondenser -coil circuit may be coupled to it and the con-verter and receiver (to be described in a later pamphlet)connected to that circuit.

Interference

When the natural frequency of the receiver agrees withthe frequency of the received wave, the greatest current isgenerated in the receiving system by that wave. Otherwaves from other transmitters, at different frequencies,will produce less than the greatest or maximum resonantcurrent; hence their effects on the receiver will be less-

3

Photo by B

urcau of Standards

Here w

e sec the "Rolling Pin" antenna-a com

pactcylindrical m

ulti -turn coil-with

amplifier for receiving.


ened, and we have a simple means (in the tuning of radiocircuits) to reduce "interference," as the unwanted signalsheard from disturbing transmitters are called.

If we use a second tuned circuit coupled to the antennacircuit of our receivers, as in Fig. 10, we gain the selectivepower of two tuned adjustments in succession. In this

Receiving Aerial-%

forth,\

TuningCa/

Fig. /0

Td converterand receiver

A receiving system of this kind combines an aerial tuning coil witha closed resonant circuit so as to produce a highly selectivearrangement.

way it is possible to secure sharp resonant effects, inwhich the desired or tuned incoming waves produce arelatively large current while interfering waves of slightlydifferent frequency set up little or no current. Underthese conditions the selectivity of the receiver is said to

3

General Squier

some years ago

made the dis-

covery that theelectric

wires

that enter ourhom

es act asantennae.H

erehe

isshow

nseated

at an electriclam

p which is

connected with

thereceiving

apparatnson

the table in thebackground.

72 Tuning and What ft Means

be high, or the tuning sharp. The less electrical resistancein any resonant radio circuit, the sharper will be its tuning;a two -circuit or coupled receiver permits control of theeffective resistance conditions, and, although more difficultto adjust, is capable of greater selectivity or discriminationbetween desired and unwanted signals of slightly differentfrequency.

There is another kind of interference, however, whichdoes not arise from other radio stations transmitting wavesof definite frequencies differing from that which it is de-sired to receive. This is "atmospheric" or "static" inter-ference, produced by electrical discharges in nature, bylightning storms, etc. Static interference does not appearto have a definite wave frequency, and hence it cannot betuned out by simple resonant methods. Fortunately, thesenatural disturbances do not persist all year long nor at alltimes of day. Neither are they powerful enough to causetrouble in normal broadcast radio reception, except underextreme conditions or when the incoming signals are weak.There is less static interference, in Northern latitudes, inWinter than in Summer ; frequently there is less in theearly evening than during the day, even in Summer. Allof this favors broadcast radiotelephone operations.

Radio Frequency and Wavelength

In this pamphlet we have considered mainly thefrequency of the radio waves, because that is the charac-teristic most closely associated with tuning effects. Thereis another term much used in describing radio waves, how-ever, viz. wavelength. The wavelength is simply the dis-

3

General Squicr

is here shown,

holding in hishands w

hat iscalled a "R

oll-ing Pin" typeof antenna be-cause

ofits

characteristicshape.

Photo by Bureau of Standards


tance from crest to crest of the radio wave we imagine inthe ether; it is usually measured in meters (one meterequals 39.37 inches) instead of in the more commonEnglish unit of feet. The wavelength is intimately con-nected with the wave frequency; either constant of aradio wave may be found from the other by dividing intothe wave velocity factor of 300,000,000 meters per sec-ond. A wave of frequency 833,000 cycles per second hasa length of 360 meters. A 1,000 meter wave has a fre-quency of 300,000 cycles per second. Other correspond-ing values are given in the following table:

Wave Frequency10,000 cycles per second

CorrespondingWavelength

30,000 meters20,000 CI id id 15,000 "30,000 ,, ,, ,, 10,00050,000 ,,

loopoo

,, ,, 6,0003,000 "

150,000 CC IC 11C

2,000 CII

200,000 ,, ,,1,500 ,,

300,000 CC id di1,000 CC

400,000 4C CC

750 ,,

500,000 ,, ,, ,, 600 if

600,000 ,, if " 500 ,,

750,000 ,, iC IC400 ,,

833,000 " " 360 ,,

L000poo "1,500,000

IC di 300200

4C

ii

2,000,000 ,, " 150 ,,

3,000,000 ,, 1i al 100 id

3

Major G

eneralS

miler has discover-

ed that any electriclight w

ire is an an-tenna.

Sim

ply con-nect

yourreceiver

with

alam

p -socket,through the m

ediumof

asuitable

con-denser and you hearjust as w

ell as you'w

ould with

anan-

tenna.T

he General

is here shown seated

at a lamp w

ith which

theproper

connec-tions have been m

adew

ith the set on thetable behind him

.

Photo by International

76. Tuning and What It Means

There is, of course, a multitude of wavelengths and fre-quencies between the limits of the above table, where onlythose in round figures are given. Each different wavefrequency provides a channel for communication by radio,but even under ideal practical conditions the closest ad-jacent frequencies cannot be used for simultaneousoperation without setting up interference. With goodmodern apparatus, however, independent and non -inter-fering signaling can be carried on upon frequencies differ-ing by 10,000 cycles or more, so long as the difference inintensity at the receiving station (where resonant selectionmust, of course, be accomplished) is not too great.

Survey of the Tuned System

Tuning is evidently an essential throughout the entireradio sending and receiving system. At every point weare dealing with alternating electric forces, and always wedesire to produce the maximum effects consistent with thevoltage and power available. Consequently we tune thetransmitting antenna to coincide in frequency with thegenerator, and we tune the receiving antenna to coincidewith the wave frequency. From the beginning to the endof the radio circuits the working frequency remains thesame in ordinary practice; if our generator delivers a fre-quency of 833,000 cycles per second, this also will be thefrequency of the sending antenna current and of the radiowaves it produces. The receiving aerial will be tuned to833,000 cycles, and the currents flowing in it will havethat frequency. If it were not for the principle of reso-nance, as exemplified by the timing of pushes to agree

3

Tuning and What it Means 77

with the natural rate of the child's swing, radio in itsmodern forms would be a practical impossibility.

You have now seen how a stream of waves can be pro-duced at a radio sending station, how the waves willspread outward for miles in every direction, and how theireffects may be intercepted at any point within range of thetransmitter. In order to understand how this system isused for the transmission of messages either by the dotsand dashes of the Morse code (in wireless telegraphy) orby the spoken word (in radio telephony) you need onlyto grasp the idea of how the signals themselves are im-pressed upon and carried by the radio waves, and how, atthe receiving station, they are first separated from theradio frequency currents which the waves produce andthen applied to a responding instrument. This generaltopic of signaling is the subject of the next pamphlet.

Documents

CAE EASY COURSE in HOME PAM - americanradiohistory.com€¦ · cae easy course in home pam maj. gekqe0. 0. squier, editor-in-chief lessonietun n g and what it means 13,e john v. l.hogan