The design of a radio altimeter using frequency modulation method

Inl

33810

OF A RADIO ALTZUVE 18150 ?R3QUW5OY MODUA 1 RNMURD

lew-Lians ft

8.8., ffassaehbusetts lasittet Of 1*0

ogoy

gabaittod In PartIAl Palfilmuent of the Requirements for the Degree of

"Aa OF BSCIANC3

U84&MXBF.TTS UIiTUTY& W1 TISCIOLU

Signatur*8

of

Authors-Departauts of Iletrica1 Vaginawtig

Aeronautical Engine . rig Octol# 193T Signature# of Pref essors

I& Cbarge Or R*4, arch

sign~ature of Chairisan of Demtent Comitte* *a Gradmate Studeonts___________

MA&sWcjuSrns INSTITUTE0 OF TECHNOLory L

FEB J1~

Nl IT

Liraries

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October 1, 1937

Professor Geourbe W. Swett, eeretary of the FaOulty,

Massachusetta Institate of Technology, Canbridge, massachuastts.

Dear

51rg

We hereby submit the enclosed thesis entitled

aThe Design of a Rao Altimeter Using ?rqgaency

Eodul&aLin MethodN in partial fulfill ant off the

re.-quiresents for the degree of Master of Science free the assachmsetts Institut, of ?echnoloey.

Respectfully yours,

Ieu-Lians Wu

the

authors wish to take this op;o4tmnty to ezpress their gratitee and deepappqreUtaes for thb lvauable assistaas and suggestices rendere b Pret. E. L. " eles, Prof. C. 1. Draper aAd Prof. W. . larrow; als to Vv'.

Azold Peterson for the persission to copy his hi oscillatr' desli.

To Mr. W. f. Cook of th 4eusavtleal Departt, we fee Lndebted for his coastat help a making the ezpewletal

appaatua.

To . Y J. . Liu, we express our deepest appeiatie for the discussions ea the theor

r

fial dei.lp

I. L. W.

8. L.L.

TABLE OF CONTENTS Page Title 1... ...

3

Letter of Transmittal ...

U

Acknowledgemnt ... * Summary ... e**eBOC* 6 Ittredwation.. ...

8

Review of Absolute Altimeters ... 11

I Mechanical

... 11

II Sonic ... 12

III Electrical ... 14

Preeaminary Design and Experimeats ... Theory e... 50

General Considerations ...

56

The Transmitter *...

58

The Receivers

...

42

The Frequency

Moter

...-... 46

The Frequency Modulation Systen .... o, The Parabolic Reflectors ... 60

Description of Experimeats ... 62

Experimental Results ... ConelAsion ... .... ... 66

I General Principle ... 73

II Discussion of the Theory ... 77

III Seleetion of The Parameters ... 87

IV Discussions of The Final Deai&n ... 96

V

Coaclusions ... 9...

98

Appendiees ... ... 99

Photographs ... 100

Tables

.04

1...--.--..

Bibiography

... ... 108

To

desisa

a praetl al radio altiaet~r

rar neasur

l*g the

absolute

at~tu6o

or

an airplas. above the

gwomdw

Anw? a

@atefui

stwi

of

SeW prop""d absolau

alti.

Seters,

a brIef

survey

alaSsiis

them Into three prinelpad

YOU) 6sia

meoeale

NNW

4

5

*ad 9 "we

@V@ ee4 as to

Wbeir sAvaatag"

*ad

415.

4wea~s.

eola attewa

has bee.

"it

to the

kropese

ptb*Lmepgl

of

radio a~lees

~-&lm-qi-ef

~e~i#-.am~te.'.

Ser tffeset qrsta

et rawiQ altte

have bee.

melewal

u

ty.

Ow

fereat baste preLspla.

A

the""

based

as thM beat twaelsf

ieet

am

etfleet

raoie wavais tvva

lftumeay u~iaLatloa

a

ad

eet4.a

or

abeue4i

ALsOMMaAse

therewslayloD

Whs

thewry

t"e

*s~

t as

ujtw*4aUe

*wqueaey

short-mve.

tesasltat~SeaYeles) set

& woSiVtFr as valasafe.

quemmy inodulater

a4

frqueaq

aster

have heas

des"Lbed&

ftwe'rismuts

e&"

ap

w~avse"

were Illustrated with phat.

by this theory. As a conclusica, the preliminary design was not mnoessful.

Finl nDalans

Ire. the experiene4 l&rned in the preliminary 4esiga,

a aew theory founded as the standing wave effect, using OS

asetially the same apparatus as in the preliminary desi6a,

wae d*eloped and thercumhy dcussed. heOretical gharw

aetOristic curves as well as control-ing desta evnstants

were draea. It was founa that by this new theory, both the

*perilental results ootained in the preliminary design and

the SuCegs fully experimeitaIl results publighed by Mr. "tu

were satisfactoiLy explained aau theoretical predictions

agreed remarkably.

Iu enelusica, a revised final desiga based e the mew

theory

was

presented. It eas reomameded that further

Ox-pealmeatal confirmatios of the new theory will lead to a

Ua the 'rts~l

JU440at

Of a

Skitlled

pilotL Is tet.

epto4 by bad

wathbo

~~t

e the

oweeary

eemtinum"

-mi-yI4e or

tbv

aititude or m ia"

&n.Ve tho 6"N"

1.vTO Aks~t aepand Iap~b ULatwiuat seauwma.

As

an lablicator, tb. ganoraiA4 ua*

simple and JUMh

braetria alitimeter

suffers tkte fol4owift, defeat* la jl .

(I)

it

eu~Lter. tim elbvatics above a&.0v

9o "t

tba

asls t AUtLtd *at a SIwpjaa awo

hM gpome *Urfa"e

9"1

l

_{e~vatica of the groamd ab*Ve$ 6" level a&e b&*M*}

(a)

the

o4~lftyT IfrtVow altlAwpter Loasot SmISOtive

eoowh fer

a*nfo laiang "4ratlte Ia bed wISIb"It4.

(a)

fte

Iaxift sI'satlceo

iserolA type altimeter

elsa Ives

yowl... ~zwrea*

pu'tloI&ry st low altit&ndea

Therefore

th*

nec~saity of an absolat. aItiweter wbich

give*

the

altiude direatly above the guamd &nd I"*peadet

of the teirala beneath has delves

mny

ezperlsninters Le In#*

vestigate the vita"I probimas

?r~ a areful situd1y of the paist pub iat

ices# it was

fowl that thar* ewe prinaipaluy tbree saaerai types or nip.

solute altinsterw; afsmeh. sebaialD sf-ic and *leiaJ.o

probles lies an the improvement of the last two possibilitiess (a) the gsnic altiaeter, utizlaing reflection phenomena of sound waves.

(b) The radiL aitimeter, utilizing reflection of radio

waves.

Up to the moment, a number of investigators have been suowesstul la produeing practically working Lnstrua*ts of the seale type, yet very few of the esperimental results frea the. proposed radio altimeters ea show satisfactory cesequuaeoe

In the airplene.

esewver, the possiblty for the radio type altiueter in Ub fature to overea the present disadvantage encountered In the saet type, namely, the bulkiness, the short range of praetical applieation, "nd probably the ese weight, Is very grat through the rapid evelopmaet of ultra-shert wave tech.

atgn atilised 1a radio altimeters.

tt is the purpe.. of this thesis to derive a most

satis-featery theory for the radio type using frequency modulation and to desigA an altimeter based em this theory se that It may overeone the present disadvantages involved Ia the sonic type. EvidAetly, the fate of the radio type altimeter depends upn further investlgaties of the skillful applicatica *f

the developed theory so that a practicable apparatus am be worked eat.

aEVlE

OF ABS 0L 1 ALTIMET R

While few

practicable saa sa isfaetory absolate ati. noters have bte demonstrated, a, laooratories aW experl. seaters have engaged in their developseat. The means of attaei were as diverse as the number of researah workers. Videly dtaferent statements have been reported as to the success at their mdels. This article will review briefly acme of the moat iaportant nethods an results obtained at preenat.

In general, there are three principal types ea absolute altimeters# naaely, mechanical, snie and eleetrical.

£I echaicals

(A) basic principlea

To time the fall of an objiet from a airplane to the gr und.im

(3) Metheos used&

smoke pots or torpedoes Dave been the sisales employed and a stop wateh or slailr device does the timina. Recently, a siaple, saal radio oscillator Las been constructea La a

bomb shape and the impact of the &rid by th bomb after re-leased sechambally stops the transmitting. This short iatere val of OsillLia is In4lated in a radio detector i the

plane.

(C) Disadvantagess

grouma in order to observe the burst.

8. The airereft engine has to be extremeLy quiet so

that the sound o. torpedoes exploding aen be heard by the pilot.. 5. The correct tining of all bomb types is subjet4d to variatic of weather ounditions such as wind velocity, air density and pressure.

4. Not eoneaoically practicabLe.

1. Not practical for easeedingly low or hI~h altitude. (D) Puture Possibilitie&I

The scheae has a liited appioaticn (not suitable ia a city) sad appears to have no great possibilities worthy of fature developments.

U15Son1c6

The preseat knowledge concerning the developmeat of

senie altimeters is best described in Teeanicaa Ntes, 39g GI,

N.A.C.A.

(A) Prineiples

the Aethod of Measuring the absclute altitude Is based on the rate of travel of somd -waves. A sendermreolver

com-binaties able to produce aeoustie signals ad definitely to

detest the corresponding e*bes Is instaleG In the plane. The time interval between the sonding of signals an the receiving of echoes indicates the altitude.

(B) Ietbods and Emperimental Resultas

*xperiaatal

results

up

to date give the

following SeacusionsS

. A inbewr of seale altimeters have bees operated

matistfoUrily qp to a a.a aLtitsee of a0 feet Under

airplane ewuislag *oaditicas.

5. For iAding fl1ght in airplanes, the maximns

aI-titue will be double that for crusiagf eeditioas. In

al.-ships, it is approximately thre times hgher.

U. The amimum altitwe seasurable is abct tea feet.

4. At preest, at least five o e eal seate atlmeters

are em the market with a ainiatanstallation weight of 60

pouads ad maxima Useful altitude of 00 feet.

(C) Disadvatage ad Difficult&e.

. maimum altitude measurable and ainiaan weight of iastaliation limit the practical use ti airsbips and large

troasport pianes.

U. The maximma operating altitude is limited by the

engine noises on the aireraft.

3. Urors due to buoldity and temperature effects e

st uad velocity and errers from aircraft speed ate iberly to

be

acre setrious than high frequency avs uset Is radio al.

timters.

(O

Ntmre

peasibiliiess

etwlthsteaimg the present difleulties eAeUbm.

troed in

sQ&iU

altimeters, the future possibility of the

method Is very great. The Pblished experimental results are such more anwmrwos tham those employing radio waves. The rapid improveMents in this type sow a tend~emy of immediate future success.

IIElodtrical

The elctric&l art has offereda rich field for

lnvestlgators

to find the soluties for an absolute altimter. Nearly

all

aodels Involve the adapttte of

ultra-bigh-freqsacy currents. I& general, It ga" be Classifie4d into the wtilization Of capaitty effect of radio frequemny eurreats and the reflection p1-114M of rauto waves.

(A) C&PacitV Altimeterst (hefrenao a)

1. Principles

The capactty of an electrieel aondest1a6 system changes witb any alteratios in the relative poeition e the parts ia the system.

B. Experimental Resultas

Qee of the successful types called Gum Alti-aeter SheM4 that satisfactory performanoes are limited to 100 feet Or less, and the sensitivity is very poor above tais

5. Disadvantas

the maal range of indicatica attained is of the order of 100 feet which is too low for most purposes.

4. fNture possibiAtiess

By applying the recent developstint in uatra-igh-frequeny, tobique, the oapacity effect can be inreased to such an extent that practical applicaica may be possible.

(3) Rladio Altimetersi

Nearly all radio altimters utilise the refleeting pheanmana of the eletro-magnetic tave. At present, only a few experimental res.ultz have been published. The metods and apparatuo employed show that the experimental stage has

just

-began and for future commercial practice, r~eent radio altintor desoig is lagging behind that of the s"ic typ. Iogver in the laat three. yeaxs, an*ay patents in this field show that strictly confidential investigations are being carried on in the United States as well as in foreijia mc.uatries. Fellwlng is a list of the advantages of radio altimeters that ar eeno-sidered suprior to those of the sonic type.

1 Radio waves are not disturbed by airplaa

engiae selse.

S. Errors due to weather conditica, sach as wiA velocity, atmospherie temperature, hamidity, etc. are

likely to be less serious than in sonic attimeters.

5. Radiation power of the eleotro-magnetie

wave can be obtained with uuch higher effetiency and greatw eemvenienee thaa that of sound waves; C with

suitable output of the oeillator, a useh higher altitude, of several thousand meters cam be measred.

4. The power supply of nearly all raao altimeters

comes from the same installatioa which is required by all

modern airplanes equipped with two way radie ceemmieations.

Consequently, the extra installation weight encountered in radio-altimeters is maost likely to be much easi.r to. reduce to a practicakole quantity.

5. Through the development of wltra-short. wave,

the spce. required for reflecters and antennae is going to be such less bulky than the meshanical parts of sonic altimeters.

From the above cinsiderations, the future possibility of a final solution of the developtnt of radio altimeters is

more wospecti e and optfibstic than that of soaic altimeters

although the present available experimental information

re-garding this type is sare,*

ed

A didst of the suggest radio altimeters wil show that twe basie principles have been tollowed, namely,

1. The cyclie cbange in the phase pattern of a standing

wave after reflectic.

2. The time interval taken by the radio wave to reflect back fr a am object.

In order to have a thorough understanding et the theory adopted in the design of a radio altimeter using frequency

aodulatioa, a careful review of the suggeste and pLatented nethods basing an the above principles is very helpfl.

l.

Theories and methods suI

ssoed

for bwil1ina rad_* altimtera utilizIna th~e crain shae

shange

as ~a inica~ion

Consider a train of waves movia to the right while a reflocted wave m:ves to the left. Interference wil. take place, and the resultant displemaLnt of the mediam at a given point and tiae will be the sam of the InA iviAuais 1 p'laceents.

In fig. 1., the positions of the waves at successive

intervals Of

1/8

period are plotted.

I$ eas be soon that there are always points of ser displaeement N at intervals of balf a wave length.

Half way between these points, at L, the Waves will always mst in the sase phase &ad the aislacement will be

a

Maaimi

the former poititas are *a!led aedes, the latter

loops or anti-odea. Disturbanues of this sart are called staiding waves.

Free the abve comsidewationa, It is elAeat that the tranwatted and the refleeted radi ainals (f wsae length 7? will give a standing wave pattern as showa la f(4.

It ea& be seea that a receiver at an altitude erraes.

po4 to eatl-mrnde. sa.e will give a maximm curreat ldUatiem Wiile at mode saae will give a ma1 Osteent indiatLon.

Ihis stanudg wave phrnemeas is utilsed is

m

ways as sa alJtitde seasuremsst.

A

A general method as described I& Refereea* .

a. Pr iciples

ftanding raai waves ar prodaeed by reflectica fres any eoAduetIve refleetlag object whes radio freqaeay

ergy is radiated

(reM

the airplas. The modes Saa leepS

of the standing waves predmOAd by the Wefleet.d eA dire*t waves is detested by a reeniver 40 the plane, thus giving a& iadioatim of the altitude of the plase from the grOem.

b. Disadvantagess

It is obviems that it suh a system Is to be effeetive, it must utilise Wave egths of the Oder Of a"* peters or acre, beeasse any attempt by the pilot to .*eet the

mew of aedes or leops of the standiag wave *hich be has passed while aseeadlng

or

deseending resuLts LA comfusios, die

to the olse spaeing of the said modes or loops.

The see of MW moters requires a bulky aatena systee and In addittaa the avaiUable frequencies in this portIca of the frequeny spectrum are eongested, so that interference free other stations may cause erroneous readings.

Jeg

Utboas developed by E. P. W. Alexandersoa (Referenes 4, S, 7, a, 9)

a. Principles

If the time Interval in units is measured equal to the time of one oscillation or eycle of the antomna current, the timeu interval of the eahe is equal to the number of wave lengths the refleted wave has travelled on Its way from the antenna to the grcsmg and baeh agaia. If this distasee is varied by an amount which Is a fractica of a waye.-length, this variaties wIll sualfest itself Ia a variation of phase of the returning i7ave relatively to the phase of the transmitted wave. If the distaace is varied by an amount of several

wave lengths, the phase of the returning wave will go through the cerrespoding anmber of eyelie changes of phase. Thus it we have means for asertaining the phase .of the retwring we

waveand are able to ecmt the numo er of eyc.ie changes, we are thereby able to make absolute seasurements of the height over the groud.

A dircot measurement of the phase of the reflected wave In relatin to the transmitted wave is difficult. Nwever, it

Was diswred 1W A*exars that the refleeted wave modife the freqawy fe the origna

& wave.

The ae 1& frqsy is dependeat m the st"ath as well as the phae of the eftleted

wave.

the sae fe these e*anes of fregeamy is explalm as followe

A* OsIllater will os*ilUate at its natur&l period osly wheM tb reSterlt foree. WkiIh A" @aaa54 1n the esellater itself are the oSly Oes thetmexist. UhSm the Oseilate is ated

Go

by

foW0.

ftR

o0t sie, these fOeweS ay a4d to 4w

sabteset free the Sabereat restorwg tore*. It is, however, the resultaat restoring foree obIk deteranes the actual perid

of the o Ullater. 2bs it the forcta coing from utsUie Is Ia phase with the Iabereat restering fore* ad inrease It# the

seillator will ellate with * bigbr frequemey, and Wise versa This eyelle variattA6 at ftreuemey was atilised bf

Alama rses is asaseng at~iud be agtheds se$t

The dtail of the metheds use are best desorlbed ja

referomees 5, 1, 6 la geaai, the eyelie variatios of frequenya of the seinlater duo to the effeet of varyiag altitude is

masured by beeting with emother eseillater of oanstant freqmey a4 the beat freque.ay .bteimed is seaarue4 bf a frequmey Seter I terms of *yel*I varyltg eurreats.

*. Results ObtaIaed

up to an altitude of 4000 feet with readings at each 1W0 feet as a step. A comparisoa with the readings obtained from

barometric altimeter shows fairly goed agreemenut.

4. Difficulties and disadvantages.

4.1 Special attention ana expert interpretation are necessaryo

4.2 Not direct indication.

4.- Too many apparatus needed so that excessive

weight and bulA prevent its being used as practical altiweter.

d.4 The long trailag antenna used is inconvenient.

5

ja Methods suggested by P. 0. Patterson (References 6 and 1I.)

a. Principles

This is an improved application of alexandarsents method. In the latter, a heterodyne beat note due to

vary-ing frequencies (which is due to varyvary-ing altitudes) and

other oscillations of constant frequency has been used. This beat note is then supplied to a frequency discriminating circait and rectified and caused to control the indicating

device.

Bowever, Ly PNtterson's improvement, this indication

of altitude is produeed witUhat resorting to any use of

the heterodyme beat principle thereby avoiding the necessity for an indbpendent source of oscillations and other equipment necebsry to the se %f that principle.

AMth r methd L# to have the eaillationas enItea la the Laten to be eaued to vary in awplitude as the eraft Changes ia altit e. The aaplitude of these variatios being

utialeatly great to pemit their translatica dl~etly late iowications of altitude,

b. methods used$

The above principle Is accoaplishe by adjustig the frequency of the osaillater to be slghtly differeat fro& the natural frequency of the antenna eircuLt. Thb the antemaa

circialt and the oscillator eireuit form two eopled Indepea-deat eireuits. As the altitude varios the antenma circuit bears a varying natural period. Throuh the couplinj betwe

the main escillator lircuit and the antenna cireuit, the an.-tenna eirouit gives an e:uivalent varying lead in thfaain usaillator circuit so that tke osUilltica aaplitude aa4 I& tura, the plate current of the oesillater is a fumctica of al. titude.

0. Eperimetal

Rbta

This result is deaeribed in retfereseo G. Itgives a warning device at ON feet and IOW feet of altituse with the ooaplete lnstallatia w.ight of 6 1/2 pounas.

d. Disadvantagess

1.~ Diseoatinwusa inaAcation. 2. Long trailing antenna.

WLUW(Mt SiViLJ4 r*atty at OtMeV ittkJAU41

SMUtho" by4 To Js Boraor (Aeferewc lzk.)

This is

n~o more than aa Improved

way

of A.tectlat the altitude by measuring Ct~k relative phase betiwea La traa-nitted and reflectedi waves.

The phase abift

by

refiectliag a le~t 4av.

(IbOO

meter)

Is

aecospliuied thr~' gh the~ md~1im of a carrier wav* of ultra..

bik-frequeacy so tbat the sum*e pbase wmamuresamt *&a bo rvache4 withcut tht. uisavantaes 4 usin4 bul*.y aateuria aad undesirable iateirferewo.

be Ntkkoa susoste4.O

fte ultra-highkrequowy is aoduiateo or

wbLledi at a comparative~ly law freqiieacy, sarnL as U(Mi tc,,, 4 the altitude ij-dicati~u

is obtained by measurLm

tbo V""s

difforen betvwen telc cat and wtieoted MOOi to* acaqwumits

of the u~tr&-Jwibtwr~gemoy *eirgy.

As deserited Ina reteremee

I** phase abifteErsO bah*IMots, cuuplturs, etc* are U&I

C. RzperiuetaJ. kOsilts

Noexperileutal results have beat published or

are

available.

to

Disadvantages#

I* Altkwu%,h the *UltituI inl'.cat.1aIs be lie~ved

go Toe

sueb

apparatuas

r;

.4uireQ

so

t~t

*m.S&AiY

weigut is aee.sawyo

laA

tbA4 ae

.iiby

Ll

keaJAehifi4 (Ie*reuo

Lro.)

Altkhc .

It

Is

4ecribvd kW bpenshie

that bU

nettogd of deerminlai

the

altitwie Is beo

as th pwia.ipl.

fT

aiaurift

tth* tice

I.*y

tf

the

ref~te*t U,.,

with

respect to

thet ditect ways la

actuality It is pwtm..wilr

tcou&W

c the pb& .J*-shi

tUMi

rather than tim d.1ay as caa

be man by a oXc,** examilat~ca or tMe svgestet *thoa*

~aSbbhl~

C4~ hU

UOt the ISOM 'beat tre1

1 Fq Isa A

incrrect saise., is method is no nae

ZMa a wPt*tica

of the ptnlla

ho

ve

already bema 4.v.)op.A*

Sim*e the astods sitgostod beta aumuaese

alytkw sam as the autbors, a moro 4etaU.4d t18soma4e

go~ publisk*4 experimentl results are ava

b~l.-do Advataes aM ODiadamntaiewe

See te eaclusiao

~.Tkoowies ;;md

lethets

sugsted tor

bt A-if Radio

Atinoet.s atUilait the time Interwal take% by ths radio wave

an n

It 1s a well kaowa fact that electro-mgnetle waves are

traVelling at tb velocity of ight, i.e. g x L0 u/4.

Referriu to fl.₆ ,lt

R 0 Eeight of airplane above groAnd Li Seters. D a idteane between transuitter and reeiver

in eters.

* a Velocity of light in a/see.

Then the time taken ky the diaret wave to reach the

receiver is

aad the tie taAen y thbO reflected wave is tb-

28/a see.

The time lagging of refleeted wave by the dizret -ave is ta t2 W , a(2R-Swa

am** eithew the time laterval taxe by the "flet ted ave, toe the ts e la"gin of the refleeted wave behinA the direot wave is a Usea"r fuastio of the altwitue an sa be

StiUsed as an indicatica of aititude provided aUitable seaas

at

wtdh

interval is extreelr

==U=

0..

at g 80.

*

ilea -

(3S00e)/(az

OSs) S

z W

s-*ee.

so that aoy attempt of measuring it by direct method vill be a fail*r sine the acat soasitive tiining lastruweat

is

5 at

preseat, Ltmited to I

x

wsee. Only Sadiroot means can be used.

I.1

Sethods

smggested by c. 3. Terry. (Reference 1i.)

a., ?rlacipies

ralo ftvqueaey sigualling system whreia

a

high freque.ay is generated interaittently and seans out lapes.. frea a transitter, which ixp@Laeo strike the earth

fre

*hioh they are retleted baak to a reeiver. The reseiver

is operative e-ly ew aImpase is ast

by

the' tramaittor. A indicater sooperating with the ree.iver end transmitter 1s

aetuated ely by refieeted esre to Ininte the time the weVe

used in travellung from the trManittea to the earth and tow

turSang t4 the receAver.

b. :ethods foeds

A very high frequensy is keyed ky a seemd osillIater at a eonlderably lower frequeSoy so that Ay a few eyeles of high frtqueny energy at a time are radiated. The tramitter at the sane time cbarges a condenser whieb has arossing its terminals a suitable resistaaee. This resistanee, when the transmitter is Ia space, discharges the condenser

Is provided with meas coatrolled by the low frequency oacil-lator for readering it inoperative when transitter Is oper-ative thereby preventing the transmitted energy fro& directly

affeting the receiver. The receiver readered operative by the refloeted wave furthr Ascages the condensed unit. Consequeitly, when a transaitted wave is refLected fr o the groad and reaches the receiver, the coadensr will have a

charge d6ependat an the 1ength of time between the cutting off of the tranmitter and the reception of the reflected wave at

the receiver. the average value of the *bare an this

een-denser Is utilisxed is an indicati.n of altitude. Several

modiflid methcdas have been maticned in reference 13, although the nature Is just the same.

a. ExpeWImental Results

go published results are available.

d. Disadvantages.

Although direct and coationaus indiestica of altitude any be obtainable, it is believod that the excessive units required as described in referen=e 13, vill give an e*x

cessive wieght as mell as balk.

g

Dethod used by S. MatsUo. (iteference 14, 15.) a. Principle.

The principle iavolves the time interval, thereby the beat frequency of the tr ansaitted waves on their

path to the dateetor as gewerated b7 frequency modulation

of the oscillator.

This idea was developed and ewplcyed in the preliminary

design of this thesis without firs, knowing that it has been

aeationed and used by Matsuo. Iowever, frou a *ore detailed

dissuasion later, it shows that tAis theory will involve a frequency modulation which is also utilUad by Er. Aspurschied as phase shifting effeetg and it is the belief ofthe authors

that the steading wave offct is acre prominent than the beat

frequency (unless special aiacriaminating method is used), 8e that the successful results obtaind by Matsuo might be

due to standing wave rather than beat frequency effect. b. Method use4

An altram-high frequenay oaciliator Qf 600 mega-cyeles (wave-leagth a 60 an.) has ben used. 1t was frequency Modulated to , 0,19X108 ecles /se. ad to1x108

cycles/see. (two separate experiments). The beat frquencies after being rectified by the detector were convertoo into

nieronres. Both the oseiliator ana receiver were of the

Bo.. type.

e. Exp rimental Results obtained.

do

Mvatass

?bw

An icet~Io of altituto to *matcvs w4 limear* Th4v &uthors bf .. U3^1 this pawTU~..4W~ liao or

4I,*'v.Lpastut

vU

pave LUG way tV.wa4 tbo tt&uV* OctIta Ot

ftoe a Utor dismusim

Is thi s~e ol&

JLI

fowA %M44

PNLIMINAR DESIG AND EXPERIMEnTS

(a) Development of the Tebory.

It ts a fact that during the perioA of the

praliminary design, the authors have neither seen the

experinental results published by Natso as in references 14 and 15 aor the idea pateated by Espmasohied as in

reference 16.

from the suggestions of Prof. Bowles, a basic theory for the design of a frequency modulation type radio

altimeter was established. It is very much the same as the

one mentioned and used by Matsu enly more detailed

di.-cussioms have been cared out.

(b) General Principle.

The systen used oasiste of a trasaitter and a reeiver earried in an airplane. The frequwcy of the

transmitter is allowed to vary linearly fro* tine to time within a few percentage of the carrier. This can be

illustrated by means of Fig. 2(a) and Fig. 2(b).

At any instant ta, the frequeney of the direct

wave is at fa while the reflected vave arriving at the receiver is of the frequency f#, sine it was radiated

Reace a beat fre ueney of Af da to the direct and reflected waves can be detected in2 the receiver.

Since At O(Sh,

and af a t for linear frequency modulatioa,

it is evident, that A ft(2h.

Therefore, the beat frequency ef can be &ade to

(c) Discusaien.

The system is shown diagram atically in Pig. Sa

Let the ecastrical path taxen by the radio %*90s be as shown in Fig. Yb and distances are measured in =eters.

In Fig. Jb,

ta a lalaum frequeacy of the transmitter. fb a maximum freguaeey of the transmitter.

f = 4f- fa overall frequency

eabane

of the oscillator.

4

, fa , and fa are instantaneous freqeaew1es

correspfndir4 to the instances t. & t1 and tareapsetively.

a _{= slope of the frequeay-time curve a ±df/dt} -!Ay/r/ ),

wher. T a period of the modulation frequenay In see.

let a * the velocity of light.

At the instant t, , the tranamitter radiates a wabe of frequency f, , toward the groand and is refleeted back

to the reeeiver after trvelLing the path S in a time interval of (ta t* ) seoads.

Whenee, -. /a se0... (1)

At the instant ta , i.e. whe the reflected wave arrives at the receiver, the direet wave reaehee the rece-Iver Is at a frequency ta corresponding- to the teat

ts on the curve.

EvIdently to - tia D/a seeW... _(E) By deteeting two waves of different frequencies, one at fe an _{another at fa , a beat frequency current of}

lqp(fh. f.) cycles/second wili present in the receiver. At

a certain altitude h, both 2H and D are fixed, so that the constant beat frequency can be utilized as an indication

of the altitude.

In order to express the beat frequency /$ as a

mathen-atical function of altitude h, a time funtion for the

oscillator fruquency must first be derived. Any attempt

to write the general expression of the frequency-time functioS

la

terus of Fourier$* series. Rowever, the expressia can be Oiaplified if we limit the discussion to a tie intervl of T/I seoends and the time axis is chosen as shown ia Fig. 4.

Beret f = f,, + at ... (3) where, a = slope of the curve a df/dt (f4 -fa)/(T/2)

= (a)/(T/2)

We can write f) f,+ uta,

and f - f, + at.

Hence, - f f. - a(t- t.) -a(t, t.) -(t. - ta) *a(t lo t.- (te* l From equations (1) and (E),

63- a

[((Da)

(D/a)

- (a/a)[tii -DJ ... *...S (4)

If

2H>> D,

SH - D a all iah

Therefore,

,/

=

(ft/a)h ...

(5)

From equation (4),, we aa see that the beat frequency,/I flihietly proportional to the altitude h, only

wheat-a. The path taken by the reflected wave, 2E, in such

longer than the distance between the transmitter and receiver,D. b. The slope of the frequency-time curve, a, is coastant. The first condition can be fulfilled easily In ordinary

ait1tU measurement. For a distanc betwes the te'aminitter

eM teeoiver

of half a meter, an altitude greater thea J Amters WIll eata the reqatse t. to fultiul the soomt nSeMistea, a lineer ret

edatia

system an a stable

aretri"

ems latrAeso that say undesirabIe lastataneoma

ttg

haMe s be ave14e.

It

Is obvies that

the

same beat frequenm wil be btainst at ether altitudes too rer4les at the atga St

the spe Ot the freqOemqy4ie *erve since the beat fre-queasy has Ae aegative sense. Ibis autematieall eztns the diseasi from time

Inteeval

of

1/21

setSMa to eas time Iaterval exeept at polats Ike a, b

ad

in

FIg. Me where the rate of frequeny variatioa is equal to Sere.

eo, at lnstanees t a "b t t b there ns so beat fretuae*y ia the deteeter.

Prem the above disssicas the fellowing theoretteal erves (Yig. ga a" 8b) ar dsam.

Pt. the above diaeussions, a frequeny sedalatioa type radio altiseter may coasist of the following e smas4.

a. A very stable asillater of any easwomet

frOMeeay.

b. A resolver Ot high seasitivlty but of

breO"

slest t*ity ssee the trequency deviates a few per Oesat free the

c.

A frequency modulating system giving a linear rate of change of frequency havbng a charackeristic as

shown in Fig. 3b.

d. An efficient directive reflector for the

trans-mittor.

0. A frequency meter independent of ampIitud*. A linear frequency-current chaar .cberistic of the frequency meter is preferabie, because the aititude is

The

fransaitters

(referenees

18, 19,

20, 21, and 2.) (a) Selection of Frequency.

Froa the consideration that convienent reflester design of hisb directivity can only be easily accomplished with transmitter of ultra-high freqeney, an oseiiLator

of approximatu ly 300 megyeles/second (1 meter wave leagth) has been seleet. A very stable frequency ebaracteristie was achiefed through the use of a successfUl design by Mr. A. Peters*& of N. I. T.

(b) Selection of the Oseillator Tubes

Due to its ruggedness, whieh is particularly suitable for field tests, the R. C.*A. 9b5 (acorm tube) was used. The maoluum eatput at 500 megaoyelea/sec. is loe than 1/2 watts.

(a) Design at the Tank Circuit.

The esillator cireuit is of the tmed plate type as shan in Fig. 6. The overall tank capacity Is 45 iand,

aM the overall tnak indesteace, 6.21 wa. The design

details are illustrated in Fig. * a and b, and Fig. . For go0 agacycles/secad,

LC a 1/S

=

a2 x 10"01

Assbing a total capacity of 45 vafds, the eerresp. ending inductance is 6.25 emw.

tube, this means a length of X a L/ExlA(b/a) -6.25/(2z1.1) - 2.64 ne. Long or 1.1199 or 1.25' approximately.

This gives a revised figure for

L

of 6.3 eea. and a corresponding

C of 44.6 afds. Therefore, 46Safds

is sufficiently accurate.

For tube loadings

C'f + Cf + 2C 1 6 ufds

The effect of inductance of loads will approximately double this or 12 aufts. Stray effects such as capacity of comeeting leads, etc. will introduce further shunt capacity so that

it can be assumed a total capacity effect of 20 uufds. Reace, we have to design a tank capacity of about 21 uufds. Assuming the end plate introduces about 3 umfds, the concentric

condenser is then tc have about 22vuufds.

C a 51 /9xln(b/a) or ln(b/a)

=

- xlC/9C

For a length of 4 cem.,

ln(b/a)

=

20/198

=

0.101 or &/a

=

1.106 If a a 3', b - 5.518'

Ui

ay~

Wet

b

-

a(*/1#)#

-

S~alaa

UbM

1610

sezx0(b/a)/i

- (

o.Os

m - .

e...

er 1.s

or 3(9/18)' From the previous desiga, the oscillator frequemey without modulation system was experimentally fomnd to b

28 msga-cycles/se.e or about 1.0 meterk 4W wave length. The ezprimental determination was done by beating the

scillater wave with the 14th baraaic of a knoaa stable eseIllator built by Mr. Peterson I& General Radio Co.

The maxim= laput used was V a M volts

Iaput I ,. watts

The maximm output was estimated to be less than 1/2 att.

The v (roefreanes Uz, .

Two types of receiver were used throughout the ex-perisents. To design a receiver for this type of work, several requiremeats are esseantials

1. light weight

8. smal energy consuuption B'. high sensitivity

4. broad selectivity

At first a self-quenched super regenerative receiving device was built because of its extreme sensitivity and

broad tuning especially fcr ultra-high-frequeney receptions. The circuit diagram given in Fig. 9 shows the arrangeaent

of the elements.

volm*

to *p*rate a pair of earwheaes. The whole usit asure only Gt*$t 44 Oaw was ttea so tkait the le

la the radic frequency circuit were redeed to minimu legth Olw~mer pOSsiel.. By aing a half wave aluslams

tubei aatesa without retsWter re..ptioa was quite gloar p to thre* hawred yards with tha plate sauppl of the

t=mitter

0oeSte directly frme the Ij a. seatse.

Beymd

this ram* the baekgreomt

.ois.

became pre.dsaa. After sowe 3ang taste the athers fvumd this type of receiver *ot suitable for their purpose. A slight

*hafge

of resistor As shifts tming to sush an extest

that a resetting

of

the esa ser is assaqry. The self-queChing frqueoy detector is 4uite ustable. *tta

the q6enehing freqUency

stopped

ed the roeeiver became umapeepsive. The flant heating is very rtua2 too.

The hracteristio hissing Smase of sOPerregserative reediVer wiUl be h6ssing iftbe filwest voltage is

uderwated With freqeey meaiti ap to aromd three -ega-eyelos, the

sigal is

almost dut off.

This a

Wah

1s

not bres eseUg for the preaset purpos. th Mdeal respsse wea 4 he a -aost eharacteristie ae). sestiity withia ba4d Width of thirty to forty OW.a

cyees. "a.

to the streag bekg4una noise of a swper. regULerati receivers a higher input signal is nessary

fig. 10.

This simple

1551 receiving device

w4s found very

- satisfactory

ex-cept that it was

re :not sufficiently

sensitive. _{A high}

r r, .gain amplifier was

ncessary after the de tec tor. The sonetimes picked up stray field interf*.rerice if not rell shielded. Rowever, this disadvantage can be cffset by increasing the

signal stregth ofthe trans4oer to reauce the necessary aiplification in the receiver.

The antenna weas tuned to a half wave length, &nd the

antenna coupled inductance coil is only an extension of

the antenna rod about oae inch long. Becausc of the

ultra-high-frequency carrier used, C&, C& and Cs are only strip brass connection _leads bypassed to grouna through aica.

several omees and *coupies a space of three sauare inhe*. Shield wires were employed in the input leads to the

am-plifier se as to minimize the stray field pick up. The frequency discrimination within a maximum band width of 9 megamycles, was mnoticeable.

Th freassans Mters (reforence 26.)

The frequency meter used throvghout the experiments was based an a circuit f irst mployed by Dr. F. V. hut with slight modifications. Fig. 11.

This frequency meter has the advantage over similar devices of other types in that the frequency inAcatias are iadepeadent of the asplitude and wave form of the

impressed signal. It increases the ease of calibration by the diretc reaalng scale ca a standard alerosUmeter. It is capable of respanding to rapid variations in the

frequency of the applied signal. Assuming that thyrtres R

is aon-conducting at the time, and that thyratroa A carries

its normal plate current, the condenser C5 associated with tube A, an ci-adenser C each becomes charg ed to a voltage Er equal to the drop across A while the other CK is us

affected. When an input signal is applied to the transformaer Ta , as soon as the grid of tube D. becomes sufficiently positive, a& are discharge in B will be initiated and the

potential of the eathode of B. will be raised abruptly

,a sbarge insttamsealy, th eatheod of tube A will semom

tarily be carrIed positive with respeet to its aade b a vltage equal to -y lek s the tub drop eOes A# Simes

the grM of habe A is at Whe sam ti"e negative with re apewt to the *athe , the wr I& A will be eza1 islhbO4.

2*a

sImilar fashtes, ainea the voltages erosea Cu

*gsmet

*bane instaItanses7Y, beth aued et the ouulem41od will be raised maestaril to a positive VOLtage a ad a eurreat psi.. will isbhage through the sieoraaMnter

My Usitg differst values of Cu ad C, the ster aa In".& *ate fil seale readings for different ranges of frqueasiesw

mm " he .armt 1Mpulses disharging throughb.sh Ster 49Opemd wpm "1s voltage asaanes beeoiag Oharget4 It is essetial that the plate voltage sheaUd be kept 640-statie. Itweer, it was ft that a deviaties of twe or three welts 4o" not affect the "scuraq t is &eWies. The filaset supply may beobtaae4 from a M volt AC.

trasformer or & volts D.C. without any appreciable

4iffer-Figs. 12, 16 and 14 are typical characteristic ourwes of this direet reading tfretuey mster for different range at full seale iniestions. At 2000 c.p.. it was f4und that the devise faled t* work. This was due to he fast that at suih high frequeacis the tubes can not 4eienis eoapletely at seh eyele.

M7I 44g

gi

7!li i5i1ii i~~~ Liggg t

gigigti

gi7a0gie + Kt77- --

Er,1I

ILI :

-f-

i.nj.

4

tT 7-t it .. ... ; . . M+ -Lt - ; g -4t17--11

77.1 A

7i

L t: . . . .. . . .. .. 7 :A. v jr IL 77 .

Because of the high impedance grid circuit of the

thyratrons, it is necessary to work this device from the

plate cirenit of an amplifier. An amplitude variation from

6 to 100 volts input did nit affect the acouracy of this

instrument.

Ocoaionally the frequency meter broke into oscilla-tions. It may be due to one of the several causes. First and most probably that the triasformer say be resonaat with

the plate circuit of the amplifier. A distinct hissing sound could be heard within the transformer. Second, it might be the radio frequency induced in the circuit. The requency odulatie

The frequency modulation system consists of a otor

runaing _{at cens tant speed to vary periodioally the capacity}

of a condenser which is connected electrically in parallel

to the tank circuit of the oscillator. The system used

is descrited in fig. 15 and 17.

From Equation "

/3

(E2/a)h

where

(3 - heat frequency in cycles/sec.

i a altitude in meters

a velocity of light x 08 8 m./see. and m- F/(T/2) a rate of frequency variation where F ft, - fa - maximum of frequency variation

and T/2 a time in sec. during which the

beat frequ~ency capabiL of bel~itU a"Ie b~y Qrii~ry, audiio. f requeney transf critrs an~a th . (,-Irsct re ii fre!.. ut .cy

x.Aers wouald ;.e from k0 to 5OCC cyc~cs/s*c. %.. e, th~e value of (S$/h or /as8o10 therefore a-= (8-IOC)AE a .. ,...(8)

Fes

U15. 4b, whon'the aotor sakes one revoluties, the frequency variation will take two etplete cycles, i.e. from the minim. frequency to the maxim and back to the mininm twice.

senwe,

1/rp.a. of motor i ST or r.p.. of ator

=

1/2T therefore, r.p.m. of motor a 60/2T * &O/T

Lot

£

a 1/2 a nodulatioa frequoy then, N a ropo ./.

Synchronous ators of both 1800 r.p.m. and 360t r.p.a. had been found available in the laboratory.

For r.po. a 1800

T a 30/100 a 1/60 &*a.

Since by equatica (#) a M (4 to S*)a

and also a a F/(/)

therefore AF/(T/ ) 0 (4 to bO)a

and A ? a (4 -50)(/12O)a.

Rene* A F M (4--O)SZ5z10/1t0 Cyelea/seo.

a (10.125) aegaycles/seo. For a mean frequency of the oscillator to, of approd-aately 00 Sesa-eyeles/seo. It corresponds to a frequency variation of .3 to 40%.

#A consideratica of the space available, the smallast pceta.ge of frequency nodulation of a.i% was chosem.

-7 . ... q 40 17 .77 - ZL---1:7 7-p _1,04 :v q 001 7 77 --. V, 71 7-. 7'-:, Ac F7 bi 7 q

1. A/h a S, or / = ft t.. A f 10 Segacycles

S. motor speed a 1800 r.p.m., ot 5600 r.p.a. The design of the variable condenser will requiret

1. A linear frequency variatica characteristic. 2. A maxinam frequency variation of 10 megacycles. In fig. 17, it can be seen that tte total capacity, gB, C1 + C a Ca + _C1

,

and the totAl Inauetance,

S-

6.Ueen.

sine,

r i 1/WU a 1/204L(C0 +C1) Ihemfore, the mximn freqtueny,

t 1/24%c whlon Cx a 0. From physics we know,

Cx a A/4d am..

The area of the *uadeaser A varies when one of the plates is rotating.

Let us limit the discussion from t

=

0 to t

=

T/2. Then, A a (1/2)D/4 0 cn,

where a at

and of(S /60 bad./sec. Substituting the value of .,

A - O/8 (Se/S)[l- a kt. wheare k -(DI*/ (*/***S.-...0

C

-A/4"4 *a ~r0w~~k44~

t

Let

"Sy

or Gx-ayof cc*

wha USfe IY)AY i

ftr a smLl perematage yp saY less

than 10%w

By binomial Owpasioas fb(lm Y)

pance*

the

frequency

Variation

is

linear whon

a

wall preintag

.kkamo of capacitY is vzod4.

at t a oi tb

amd at % a 1/2 , f~

tf&

fb (I-~ )

&? r 3xI*G

Oyeles/8660

1 /60 seet.

C a C ₀ + C1, Con 46 to

Cam

a =to.

approxitately.

gem* we am determine x fre equaties (11).

lrb (11),

(T?/Cg) * 44y/f

-

4(4/fb)

-

4(/a

(40/S)%

tbrefore, S.Lla x 60 x 50 x 10* m "

xo10*"

k/d

and,

k/4 a(16 )a z9 1 x

=Ox

WIG* x"9.

(Do/8

U/60)x

-- *

trefors for 600 rey

D.

(]D*/d) - 198 e9.

Aesga 4 - 1/6 Inch a 0.16 cm from space considr.

ation,

thesn

or, D a 6.5 ca. a 1*1 inches.

From a space censiaeratica, the diameter of the

plat =as ehosen as 2 inches. This gives a slightly smaUerr4 of less than 10 megmaeyC/see.

I ftg. 22, frem experiental result, it is fasnd to be 9 OMgacyOls/me. which eChets with theiretic& predictlem very closely.

Ia order to have the frequeney variation linear, It Is meatinod that the valve of y must be saler than 10.

max* y at

mux./Co a x *)

From

(g),

thwtot*,

rmAK*

4/O- / 0

This

to =*Ulle %"a 2406 l~wefcrs 60 em Oipeet a

U1ar~

twequW~

vawltia. 2bis isch

d

Q*t~

I* ag

d4.1~ ad

85. law a"YoottLOI variIGor

O

the

twqulwao "

bo

Inau

Oh xPerimsUa

rsults la ft**

*Uaa

ukd "is Pwobb

a". to

the oomticj

betLyou tf

rt~a aft . &Ohk inviah "t

From wet e:.ae S? a eyllmdrloil parabolic e .tv

to sbo~a In 16' I&* ws

"444404

11

(4f)

%ftao(

uiire

&ftlv, of rt.1

Sw.r~l,

Ober*

wave " muth ofUma Ih "'La Su

5.4

, * I~th *f

the Pu*abr4ic

refloew

$Iae In our tosI~a Umi amaUet MlIA otwecL

ail*~

(f

to "0iwabae, the lar&eSt &V4aLeI

f sboa&*

loia

asSets

vr* *boa& 2he

piabillc

ous~a is umme

pI~tte4 SOOuL4in

to the M~thad .eatce

In

Im &

4calI

drawing ato etm

tvea1 J.itA is knowu.

te

to"I 3*mth

is

.s

A a

a* flu e. a A4v* Ion th at L41 a,*

The matoau* for both& io.

rec-sor lve

*or, o si&-u4 so ttOi i.1n4th .ro trAl wit"&

tt

£±ai

Of (.L/

1

jA±

(1u)A

or bb

tU

ay7

dG~ft sup wo CAU tuneLi t,4 wtewoad tL ary aby

atesirable oree)~e

'rbs original.

theory L4LpaM

ta*

%&Us

wow'

ara Lhti.1

tt

%"% U"

s

b"" of~

a

X*U

ift ibs)*

&"a""

can airpLmw

abevo vqwAm w4A from

taw

reatt ~

twqwwy

of the

41re.t

*mt*C!twUA

a4Lo mw,&

mw th

post

the

ph&a*. "Opaqet ww~v wwoa *&a

Umu

Uoetad

fllowlft

to

b.Ul*

Um. &"&e too

*a*a of

lbs vktbi-?*ao obtaIa. frs~a

tU.

tIWA~

Uto on. UI"Aam

ehi..

Aftaer aweral

&"if

Isatc

Ot Mo

aeem"ss

-"0

the Most ptedkhsL~t Otas. tSMt .t%u"I

stsa aw

Obeiut* OaUt~

ea

#~qa~

*L

O thwwaaattr te a& m*61Avttam desu%

~a'~

easbelo Is.e

116t

4

havS

at &&&twuksss

I* ftawiags w~4 ehat for rctata

ANSAamai

16 14 !Pattw #IC v VAw to ASO awq