MASSACHUSETTS I N ST I TU TE OF TEC’H .VOLOGY
R DI TION L OR TORY SERIES
B oa rd of E dit ors
LOU IS N. RID ENOU R, Editor- in-Chief
G EORG EB . C OLLINS , Deput y Ed i t or -i n -Ch i ef
B RITTON C HANC E, S . A. G OU DS MT, R. G . H EI I B , H U B E RT M. J AME S, J U LIAN K. KNIP P ,
J AME S L. LAWS ON, LE ON B . LINF ORD , CAROL G M ON TG O ME R Y, C . N E WTON , AL B E R T
M. S TONE , Louxs A. TU ENE R, G EORG E E . VALLE Y, J R., H E RB E RT H . WH EATON
1. R AD AR S YS TE M E N G I N E E i+ lxG -R id en ou r
2. R AD AR AI D S TO ~ AVI G ATI OX—H a ll
3. R AD AR B E AcoN s—R ob er t s
4. L oE Ax —P i er c , M cKew ie and Woodward
5. P U L S E G en er a t or s—6?a s oe and Lebacqz
6. MICR OWAVE MAG NETRONS—COllinS
7. K L YS TR ON S A?J D M I C R O WAVE Tm oD E s -H a m i lt on , Kn i pp an d Kuper
8. P R I N C I P L E S O F M I C R O WAVE C m m u r rs -M on t g om er y , Dtcke and Purcel l
9. MICROWAVE TRANSMISSION CIrwuITs—Raga n
10. WAVE G U I D E H AN D B O OK —M a r C U r Iit Z
11. TECH NIQUE OF ?YIICROWAVEME,4sumnmsw-Montgomery
12. M I C RO WAVE AN TE N NA TH E O RY AN D D E s ~ G i x-S ik er
13. P R OP AG ATI ON OF S H OR T R AD I O WAv E s—K ew
14. MI C ROWAVE ~ upLE xE rm+ -S mulzin a nd M ontgomer y
15. C RYS TAL R E C TI F IE R S — Torrey a nd Whitmer
16. MICI IOWAVE vfIxERs-P ound
17. C O MP O N E N TS H AN D B o oK —B l a c kb ur n
18. VAC U U M TU B E AMP LIF IE RS —~ ’U @ a nd Wa lma n
19. wA vE~oR~s—Chan ce H ughes M ac.NT ichol .Sayre a nd Wi l l i ams
20. E L E C TR ON I C TI M E M ea s u r em en t s —C h a n ce, H ul sizer .Wac.Yichol
and Williams
21. E L E C TR ON I C I N s’r R uh iE N m -G r e en w ood , Holdam and MacRae
22. C ATI I OD E R AY TU B E D I spL AYs —S o12er , Starr a nd Va lley
23. .~ lC RO WAVE RE cE 1vE Iw — Va n Voorhis
24. THRESH OLD SIG NALs—Law son and Uh l enbeck
25. THEORY OF SERvoiuEcIIANIsivs-J ames, Nichol s and Ph i l l i p s
26. R AD AR S C AN NE R S AN D R AD OXE S —C U d r J , Karel i tz and Turner
27. C OM P U TI NG K I E C HAN IS MS AN D LINnGm- oboda
28. I NDEx—Henney
1947
COP YR IGH T 9 4 7 BY T HE
J1cC R iW-HILL BOOK CoMP.iNY INC.
PRINTELI IX THE UNITED STATES OF AM ERI ..<
A u lr qhh rf?send
‘1’h is hook., or
pert: ihrrrqf, ?na~j no he reprod7wd
in 07 v Jm-TII wilhottl permission oj
the publishers.
 
CONTRI UTING UTHORS
W. M. C .4D Y
R. E . C LAP P
C ,. B . C OLLINS
W. W. H ANS E N
L. J . H AWORTH
M. M. H U B B ARD
P . C . J AC OB S
M. H . J OH NS ON
W. H . J ORD AN
J . V. LE B AC QZ
F . B . LIXC OLX
R. D . O’XE AL
C . F . J . OVE RH .WE
E . C . P OLLARD
C . V. ROB INS ON
A. J . F . S IE G E RT
R. L. S IIWH EIME R
D . C . S OP E R
G . F . TAP E
M. G . WH ITE
J . M. WOLF
Foreword
T
H E t remendous resea rch a nd development effor t t ha t w ent int o t he
development of ra da r a nd rela t ed t echniq ues during World Wa r I I
result ed not only in hundreds of ra da r set s for milit a ry (a nd some for
possible pea cet ime) use but a lso in a grea t body of informa tion a nd new
t echniques in t he elect ronics a n high-frequency fields. B eca use t his
ba sic ma ter ia l ma y be of grea t va lue t o science a nd engineer ing, it seemed
m st import a nt t o publish it a s soon a s secur it y permit t ed.
The Ra dia tion La bora to y of MIT, w hich opera ted under t he super-
vision of t he Na t iona l D efense Resea rch C ommit t ee, under t ook t he grea t
t a sk of pr epa r in g t hese volum es.
The w ork descr ibed herein , how ever , is
t he collect ive result of w ork done a t ma ny la bora t or ies, Army, Na vy,
universit y , a nd indust r ia l, bot h in t his count ry a nd in E ngla nd, C a na da ,
a n d ot h er D om in ion s.
The Ra dia t ion La bora t ory, on ce it s proposa ls w ere a pproved a nd
fina nces provided by t he Office of S cient ific Resea rch a nd D evelopment ,
chose Louis N. Ridenour a s E dit or-in-C hief t o lea d a nd direct t he ent ire
project . An edit or ia l st a ff w a s t hen select ed of t hose best qua lified for
t his t ype of t a sk. Fina lly t he a ut hors for t he va rious volumes or cha pt ers
or sect ions w ere chosen from a mong t hose exper t s w ho w er e int ima tely
fa milia r w it h t he va rious fields, ,a nd w ho w er e a ble a nd w illing t o w rit e
t he summa ries of t hem. This ent ire st a ff a greed t o rema in a t w ork a t
MIT for six mont hs or more a ft er t he w ork of t he Ra dia t ion La bora tory
w a s complet e. These volumes st a nd a s a monument t o t his group.
These volumes serve a s a memoria l t o t h unna med hundreds a nd
t housa nds of ot her scient ist s, engineers, a nd ot hers w ho a ct ua lly ca rr ied
on t he resea rch , development , a nd engineering w or k t he result s of w hich
a re herein descr ibed. There w er e so ma ny involved in t his w or k a nd t hey
w orked so closely t oget her even t hough oft en in w idely sepa ra ted la bora -
t or ies t ha t it is impossible t o na me or even t o know t hose w ho cont ribut ed
t o a pa r ticula r idea or developm en t.
Only cert a in ones w ho w rot e repor t s
or a rt icles ha ve even been ment ioned. B ut t o a ll t hose w ho cont r ibut ed
in a ny w a y t o t his grea t coopera t ive development ent erpr ise, bot h in t his
count ry a nd in E ngla nd, t hese volumes a re dedica ted.
L. A. D U B RI D G E .
Yii
Preface
T
H E ea rliest pla ns for t he Ra dia t ion La bora tory S eries, ma de in t he fa ll
of 1944, envisa ged only books con ce ned w it h t he ba sic microw a ve
a nd elect ronic t heory a nd t echniq ues t ha t ha d been so t horoughly devel-
oped during t he w art ime w ork on ra da r.
These pla ns w er e la id a side for
a t ime w hen it beca me clea r in t his count ry t ha t severa l mont hs of fight ing
rema ined in t he E uropea n w ar.
When w ork on t he S eries w as resumed in t he ea rly summer of 1945, t he
books pla nned, a s before, dea lt w it h ba sic ma tt ers a nd w it h t echniq ues.
E very effor t w a s ma de t o point out t he genera l a pplica bilit y of t he w ork
repor t ed a nd t o a void specia l empha sis on it s a pplica tion t o ra da r, since
ra da r it self w as t hought t o ha ve only a limit ed import a nce.
The end of t h e P acific w a r ma de it possible t o put more effor t on t he
job of prepa ring t he S eries t ha n ha d been a va ila ble ea rlier . The books
on t h eory a nd t echniq ues ha ving been pla nned a s comprehensively a s
a ppea red t o be w ort h w hile, t he w ork w a s ext ended by t he a ddit ion of
five books concern ed w it h ra da r a nd a llied syst ems.
Of t hose five books, t his is t he only on e t ha t dea ls w it h ra da r it self.
One book t a kes up t he use of ra da r in na viga tion, on e con cerns t he design
of ra da r sca nners a nd ra domes, one t rea ts t h design a nd const ruct ion of
bea con s, a n d on e descr ibes h yper bolic n a viga t ion a l sy st em s—in pa r ticu a r
Loran.
This book is in t ended t o serve a s a genera l t rea t ise a nd referen ce book
on t he design of ra da r syst ems. No a pology seems t o be needed for t he
fa ct t ha t it dea ls prima rily-t hough by no mea ns a lt oget her-w it h micro-
w ave pulse ra da r. Thousa nds of t imes a s much w ork ha s gone int o pulse
ra da r a s int o a ny ot her kind, a nd t e overw hehning ma jorit y of t his w ork
ha s been concerned w it h microw a ve pulse ra da r. The superiorit y of
microw a ves for a lmost a ll ra da r purposes is now clea r.
The first eight cha pt ers of t his book a re int ended t o provide a n int ro-
duct ion t o t he field of ra da r a nd a genera l a pproa ch t o t he problems of
syst em design. C ha pt ers 9 t hrough 14 t a ke up t he lea ding design con-
sidera t ions for t h e va rious import a nt component s t ha t ma ke up a ra da r
set . These cha p ers a re so t horough in t heir t rea t ment t ha t C ha p. 15,
w hich gives t w o fa ir ly det a iled exa mples of a ct ua l syst em design, ca n be
q uit e brief. C ha pt ers 16 a nd 17 t a ke up t w o new a nd import a nt a ncilla ry
ix
x PREFACE
t echniq ues t ha t a re not dea lt w it h fully elsew here in t he S eries: moving-
t a rget indica t ion a nd t he t ra nsmission of ra da r displa ys t o a r emot e
indica tor by ra dio mea ns.
F or fuller informa tion t ha n ca n be found in t his book on a ny det a iled
point of design, t he r ea der is refer r ed t o one of t he ot her books of t he
S er ies. In a sense, t his book specia lizes t o ra da r t he t echniq ues repot ied
more fully elsew here in t he S er ies.
Ra da r is a ver y simple subject , a nd no specia l ma thema tica l, physica l,
or engi eer ing ba ckground is needed t o r ea d a nd underst a nd t his book.
B eca use t he book cover s t he ent ire field of effor t of t he Ra dia t ion
La bora tory a nd t he ot her w a rt ime ra da r est a blishment s, it s cont r ibut ing
a ut hors a re mor e numerous t ha n t hose list ed for most ot her volumes of
t his ser ies. I a m especia lly gra t eful t o L. J . H a w or t h a nd t o E . M.
P urcell, w hose cont r ibut ions ha ve been mor e ext ensive t ha n t hose of
ot her a ut hors, nd w hose a dvice on edit oria l problems ha s oft en been
ext remely helpful. In a ddit ion t o t he a ut hors a lrea dy list ed, w hose
na mes a ppea r in t he book in connect ion w it h t he ma t eria l t hey ha ve
w rit t en , I w ish t o t ha nk t he follow ing men for t heir w or k in provid-
ing essent ia l ba ckground ma t eria l t ha t did not event ua ly find it s w a y
int o t he book: R. M. Alexa nder , A. H . B row n, J . F . C a rlson, M. A.
C ha ffee, L. M. H olling w or t h, E . L. H udspet h, R. C . S pencer , a nd I . G .
S w ope. C ha nging pla ns for t he book a lso reduced t he a know ledged
cont r ibut ion of E . C . P olla rd fa r below t he ver y considera ble q ua nt it y of
ma t er ia l h e pr epa r ed.
I ow e a n a pology t o a ll t he a ut hors for t he liber t y I ha ve oft en
t a ken in a lt er ing t heir or igina l t ext t o fit t he fina l fr a mew or k of t he
book a nd my ow n idea s of st y le.
B eca use most a ut hors left t he La bora -
t or y immedia tely on fi ishing t heir w r it ing, a nd much of t he edit oria l
w or k ha d t o be defer r ed unt il t he book w a s subst a nt ia lly complet e, it ha s
not a lw a ys been possible t o a djust w it h t he a ut hors t he a lt era t ions in t heir
ma nuscript s t ha t ha ve seemed desira ble t o me.
The genera l a cknow ledgment s I ow e a s E dit or-in-C hief of t he S er ies
a r e set for t h in t he S er ies I ndex. In connect ion w it h t he prepa ra t ion
of t his b ok, how ever , it is a plea sure t o t ha nk D r . B . ~ . B ow den, of t he
B rit ish Air C ommission, not only for his a ssist a nce a s a n a ut hor but a lso
for his genera l comment s on t he book a s w hole. I a m gra t eful t o
Lois C a pen for her w or k in follow ing t he prepa ra t ion of illust ra tions, a nd
t o P hyllis B r ow n for genera l secret a r ia l a ssist a nce.
LOUIS N. RIDENOUR.
C AM B R ID G E , M AS S .
Jum 1946
Contents
F OR E WO RD E I YL . A. D U B M D G E
vii
C H AP .1. INTROD U C TION . . . . . . . . . . . . . . . . . ..1
l.l Wha t Ra da r D oes.....
1
3
1.3 C om ponent sof a Ra da r S yst em . ..
6
1.4 The P erform a nce ofRa da r . 8
1.5 Ra da r S yst ems .,..., . . . . . . 12
1.6 The E a rly H ist ory of Ra da r . .,
13
1.7 Wa r t im e R a da r D evelopm en t i t h e U .S ..
15
C H AP .2. TH E RAD AR E QU ATI ON. 18
TH E RAD AR E QU ATI ON FOR F RE E -S P AC E P ROP AG ATI ON 18
2.1 The Mea ning of F ree-spa ce P ropa ga t ion 18
22 Ant enna G a in a nd Receiving C ross S ect ion.
19
2.3 S ca t t er ing C r oss S ect ion of t he Ta rget 21
2.4 The Ra da r E q ua t ion.
21
2.5 B ea ms of S pecia l S ha pes 22
2.6 The B ea con E q ua t ion. 27
TRE MI NI MU M D E TE C TAB LE S IG NAL. 28
2.7 N oise . . . . . . . . . . . . . 28
2.8 Receivers, I dea l a nd Rea l. 3
2 9 Receiver B a ndw idt h a nd P ulse E nergy 33
2.10 The S t a t ist ica l P roblem 35
2.11 E ffect of S t ora ge on Ra da r P er forma nce
41
MI C ROWAVE ROP AG ATI ON 47
2.12 P ropa ga t ion over a R eflect ing S ur f a r c 47
2.13 The Round E a rt h. 53
2.14 S uper refr a ct ion 55
2.15 At t enua t ion of Microw a ves in t he At mosphere 58
C H AP . 3. P ROP E RTI E S OF R AD AR TARG E TS .,. 63
S I MP LE TARG E TS .,.,.... ..63
3.1
32
33
3.4
3.5
3.6
3.7
C r oss S ect ion E xpressed in Terms of t he Field Qua nt it ies 63
Ra yleigh S ca t t er ing fr om a S ma ll S pher e. . 63
S ca t t er ing of a P la ne Wa ve by a S phere
64
Approxima tions for La rge ilfet a l Ta rget s
65
67
Ta rget S ha ping t o D iminish C ross S ect ion 68
U eof Absorbent Ma t er ia ls. . 69
xi
73
3.8 Ret urn from TWO I sot ropic Ta rget s . 73
39 Act ua l G omplex Ta rget s. 75
3.10 C ompound Ta rget s E xt ended t hrough S pa ce 81
311 E xt ended S ur fa ce Ta rget s 85
G Ft OWNDP AINTI NG B Y AI RB ORNE RAD A 88
3.12 S pecula r a nd D iffuse Reflect ion 89
3.13 S ea Ret ur n a nd G round Ret urn . 92
3.14 Mount a in Relief.
96
3.16 C it ies . . . . . . . . . .
3.17 N’a viga t ion . . . . . . . . . . . . . . . ...108
C H AP . 4. LIMITATI ONS OF P U LSE RAD AR 116
4 1 Ra nge, P ulse-repet it ion F req uency , a nd S peed of S ca n 116
4.2 B a ndw idt h, P ow er, a nd I nforma t ion-ra t e. 121
4.3 P uke R a da r a nd C -w Ra da r . 23
4.4 C lut t er . . . . . . . . . . . 124
C H AP . 5. C -W RAD AR S YS TE MS
127
127
5.3 E ffect of Ta rget .
130
5.4 C la ss of S yst ems C onsidered
131
55 U t ilit y of C w S yst ems. .
132
S P E C IF I C S YS TE MS . . . . . . . . ..132
5.6 S imple D oppler S ystem. 132
57 Ra nge-mea sur ing D oppler S ystem. 139
5.8 F -m Ra nge-mea suring S yst em.
143
59 Mult iple Ta rget F -m Ra nge .Mea surement 147
5.10 Alt erna t ive F -m Ra nging S ystem
149
150
157
C H AP . 6. TH E G ATH E RI NG AND P RE S E NTATION OF RAD AR D ATA
160
6.1 I nfluence of Opera t iona l Req uirement s. 160
TYP E S OF RAD AB I N~ I C ATOW. 161
6.2 D efinit ions . . . . . . . . 161
6.3 S umma ry of I ndica t or Types
163
6.4 One-dimensiona l D eflect ion-modula ted D ispla ys.
164
65 Represent a t ion of t he H or izont a l P la ne.
167
6,6 P la ne D ispla ys Involving E leva t ion
171
174
175
X l l l
RXAMP LE S OF TH E MAJ OR OP E RATI ONAL RE OU I E E ME NTS 175
69 E a r ly Aircra ft Wa rning Ra da r 175
6.10 P P I Ra da r for S ea rch , C ont rol, a nd P ilot a ge 182
6.11 H eight n ing I nvolving G roun d Reflect ion . 184
6.12 H eight -6nding w it h a Fr ee-spa ce B ea m. 187
6.13 H oming . . . . . . . . . . .196
6,14 P recision Tra cking of a S ingle Ta rget 203
6.15 P recision Tra cking dur ing Ra pid S ca n 210
C H A P . 7. TH E E lf P LO~ 31E NT OF RAD AR D AT.k
213
7.1 Th e S igna l a nd I t s LTse. .213
E XTE RNAL A1nS TORAD AE ~ -S E . 214
7.2 Aids t o I ndivid a l A’a viga tion .
214
7.3 Aids t o P lot t ing a nd C ont rol
218
7.4 The Rela y of Ra da r D ispla ys. 225
E XAM P L E S O F R AD AR O RG AN I ZATI O NS
226
7.5 Ra da r in t he R AF F ight er C omma nd 226
7.6 The U .S . Ta ct ica l Air C omma nds 229
7.7 C lose C ont rol w it h SC R-584 238
7.8 Tehera n . . . . . . . . . . 240
C H AP . 8. RAD AR B E AC Oh W.
RADAR-B EACON SYSTEMS.
8,1 Types of Ra da r-bea con S yst ems.
8.2 S yst ems P la ning
8.3 G enera l I dent ifica ti n S yst em—I FF.
8.4 Ra da r I nt er roga t ion vs. S pecia l I nt erroga t or s
8,5 I ndependence of I n t err oga t ion a nd R ply.
8.6 F req ue cy C onsidera tions
243
246
246
250
251
252
254
260
8.7 I n t err oga t ion C odes . . . . . .
263
265
8.10 U nsynchron ized Replies
268
C ..,. 9. AXTE NNAS , S C A~ ~ E R , AND S TAB I LI Z.\ TI OX.
271
9.1 The Ant enna E q ua t ion . 271
9,2 Round a nd C ut P a ra boloid Ant enn a s 272
93F a nB ea ms . . . . . . . . .274
94 Nonsca n ning Ant enna s. . 277
9.5 C on st ruct ion of Ra da r Ant enna s.
279
RAn ARS C ANNI NG P A~ E E NS ..,, 280
9.6 S imple S ca ns . . . . . . . 281
97C omplexS ea ns . . . . . . . . . . . . ...281
 
xiv
CONTENTS
ME C H ANI C AL S CANNE F IS ..,... 282
9,8 The K inema t ics of Mecha nica lS ca nners 282
9.9 The Weight of Mecha nica l S ca nners. 283
910 R-fTra ns iesion Lines. 283
9,11 Da ta Transmission .
284
9.12 E xa mplesof Mecha nica l S ca nners. 284
ELE CTRICAL SCANNERS.
291
9.13 The AN/AP Q-7 (E a gle) S ca n ner 291
9.14 S ch w a rzschifd Ant enna .
295
9.15 S C I H eight Finder .. .298
9.16 Ot h er Types of E lect r ica l S ca nners. : 302
TH E S TAB I L I Z ATI O N P R O B L E M .
304
9.17 S t a biliza t ion of t he B ea m.
305
311
9.1 I nst a lla t ion of Airborn e S ca nners 312
9.2 I nst a lla t ion of S urfa ce-ba sed S ca n ners 313
9.2 Ra domes . . . . . . . . . 314
9.2 E lect rica l Tra nsmission. 316
9.24 S t ruct ura l D esign of Ra domes 316
9.2 E xa mples of Ra domes
317
GrrAF.10. TH E MAG NE TRON AND TH E I ’U LS E R 320
10.l C onst ruct ion . . . . . . .321
10.2 The Resona nt S yst em . . . . . .325
10.3 E lect ron Orbit s a nd t he S pa ce C ha rge 330
10.4 P er forma nceC ha rt s a nd Rieke D ia gra ms 336
10.5 Magnetron Cha ra cter is t icsAffect ing Over-al l Systems Design
340
10.6 Ma gnet ron C ha ra ct er ist icsAffect ing P ulser Design 352
TE E P uller . . . . . . . . . . . 355
10.7 P ulser C ircuit s .,..... . ..” . . . . . . . . . ...356
10.8 Loa d Req uirement s,. . ..362
109 The H a rd-t ube P ulser
367
10.11 Miscella neous C omponent s 383
C H AP . 11. R-F COMP ONE NTS , . . . . . . . . . . . . . . . . ...391
11.1 The R -f Tra nsmission P roblem 391
11.2 Coa xia l Lines . . . . . . .393
11.3 Wa veguid . . . . . .398
405
407
CONTENTS xv
MI C R OWAVE COMP ONE NTS OF TH E R E C E I VE R 411
11.6 The Mixer C ryst a l . . . . . . . . . . . . . . . . . . ...412
11.7 The Loca l Oscilla t or . . . . . . . . . . . . . . . . . ...414
11.8 The Mixer . . . . . . . . 416
11.9 Aut oma t ic F req uency C ont rol. 418
MOU NTING TH E R-F P ARTS .... . . . . .
419
11.10 Rea sons for a n R -f P a cka ge. 419
11.11 D esign C onsidera t ions for t he R-f H ea d 421
11.12 I llust ra t ive E xa mples of R-f H ea ds 425
C WP , 12. TH E RE C E I VI NG S YS TE M—RAD AR RE C E I VE RS 433
INTROD U C TI ON . . . . . . . . . . . . . . . . . . . . . . . . . . ..433
12.1 The Role of the Receiving S yst em. 433
12.2 A Typica l Receiving S yst em 435
TH E R E C E I VE R . . . . . . . . . . . . . . . . . . . . . . . . . .441
12,3 S pecia l P roblems in Ra da r Receivers. . 441
12,41 -f Amplifier D esign... . . . . . . . . . .. 442
12.5 S econd D et ect or . . . . . . . . . . . . . . .. 449
12.6 Video Amplifiers . . . . . . . . . . . . . 4W
12.7 Aut oma t ic F req uency C ont rol. 453
12.8 P rot ect ion a ga inst E xt ra neous Ra dia t ion. 457
TYP I C .AL RE C E I VE RS . . . . . . . . . . . . 460
12.9 A G enera l-purpose Receiver . 462
12.10 Light w eight Airbor ne Receiver 464
12.11 An E xt remely Wide-ba nd Receiver
470
C H AP . 13. TH E RE C E I VI NG S YS TE M—I ND I C ATORS . 475
TH E C ATirOnE -RAYU B E . . . . . 475
13.1 E lect r ica l P roper t iesof C a t h ode-ra y Tubes. 475
13.2 C a t h ode-ra y Tube S cr eens 479
13.3 Th e S elect ion of t h e C a t ode-r a y Tube 483
C Oor mlN Am ON WI TH TH E S C AN NE R
486
486
., .
490
B AS I C E L E C TR I CAL C I RC U I TS
.,. 492
13.6 Amplifiers . . . . . . . . . 492
13.7 The G enera t ion of Rect a ngula r Wa veforms.
496
13.8 The G enera t ion of Sha rp P ulses. 501
13.9 E lect ronic S w it ches . . . . . .. 503
13,10 S aw toot h G enera tors.
510
513
13.11 Angle I ndices . . . . . . . . . . . . . . . . . . . . . . .514
 
D IS P LAY S YNTH E S IS . . . . . . . . . . . . . . . . . . . . . . . . .524
13.13 The D esign of A-scopes. ,524
13.14 B -S cope D esign . . . . . . . . . . . . . . . . . . . . ..528
13.15 P la n-posit on Indica t or . 532
13.16 The “ Resolved Time B a se”
Met hod of P P I S ynt hesis . 534
13.17 Resolved-current P P I 538
13.18 The Met hod of P re-t ime-ba se Re olut ion . 544
13.19 The R a nge-height Indica t or . 545
S IG NAL D IS C RIMINATION, RE SOLU TION, AND CONTRAS T. 54s
1320 Resolut ion a nd Cont ra st 548
13.21 S pecia l Receiving Techniques for Air-t o-la nd Observa t ion. 550
CH AP . 14. P RIME P OWE R SU P P LIE S F OR RAD AR. 555
.41RC RAFT SYSTEMS
Wa ve S ha pe . . . . . . . . .
D ir ect -dr iven G en er a tor s.
Mot or-a lt erna tor S et s
Volt a ge R egu la t or s.
S peed Regula t ors . . . . . . .
D yna mot ors . . . . . . . . . .
Vibra tor P ow er Supplies
S umma ry of Recommenda t ions for Aircra ft Ra da r P ow er
555
557
561
563
571
579
581
582
G E OU ND AND S H IFB OARD S YeTE lS . :83
1410FixedLoca t ions. . . 583
14 11 La rge Syst ems Where No C ommercia l P ow er Is Ava ila ble, 584
1412 S ma ller Mobile U nit s
585
14.13 U lt ra port a ble U nit s 5s5
14.14 S hip Ra da r S yst ems. 586
C H AP . 15. E XAMP LE S OF RAD AR SYSTE M D E S IG N 588
1511 n t roduct ion . .,.,. . . 588
15 The Need for S yst em Test ing. 590
D E S IG N OF A H IG H-P ERF OE XANC E RAnAR FOB Am S U R VE I L L AN C E AN D
CONTROL, . . . . . . . . . . ..592
153 Init ia l P la nning a nd Object ives 592
15.4 The Ra nge E qua t ion. ,595
155 Choice of P ulse Le gt h .596
15.6 P ulse Recurrence F requency 598
15.7 Azimut h S ca n R a t e... . . . . . . .. 599
15.8 Choice of B ea m Sha pe. 600
159 C hoice of Wa velengt h . ,604
15.10 Component s D esign 606
15.11 hfodifica t ions a nd Addit ions,
609
CONTENTS
xvii
D E S IG N OF A L IG H TWE IG H T AI RB ORNE RAD AR F OR NAVI ~ ATI ON 611
15.12 D esign Object ives a nd Limit a t ions. 611
15.13 G enera l D esign of the AN/AP S -10. 614
15.14 D et a iled D esign of the AN/AP &10 616
C H AP . 16. MOVI NG -TARG E T I ND IC ATI ON 626
INTROD U C TION . . . . . . . . . . . . ...626
161 The Role of Moving-Ta rget Indica t ion.
626
162 B a sic P r inciples of MTI. : : : ~ : : : : 626
16.3 A P ra ct ica l MTI S yst em. . . 632
16.4 Alt erna tive Met hods for Obt a ining C oherence.
635
P E RF ORMANC E C R I TE RI A AND C H OI C E OF S YS TE M C ONS TANTS . 638
16.5 S t a bilit y Req uirement s. 638
16.6 I nt erna l C lut t er F luct ua t ions . 642
16.7 F luct ua t ions D ue t o S ca nning 644
16.8 Receiver C ha ra ct erist ics : 646
16.9 Ta rget Visibilit y . . . . . . . . .,...,..,.....649
16.10 C hoice of S yst em C onst a mt s. 653
MOVING -TAB G E T IND I C ATION ON A MOVI NG S YS TE M . 655
16.11 C ompensa t ion for Velocit y of S yst em . . 655
1612 The Noncoherent Met hod. . 656
16.13 B ea t ing D ue t o F init e P ulse P a cket 657
C OMP ONE NT D E S IG N . . . . . . . . . . . . . . . . . . . . . . . . .658
16.14 The Tra nsmit t er a nd I t s Modula t or . 658
16.15 The S t a ble Loca l Oscilla t or . 659
16.16 The C oherent Oscilla t or 662
16.17 The Receiver . . . . . . . . . . . . ...665
16.18 The S upersonic D ela y Line. 667
16.19 D ela y-line S igna l C ircuit s : . . . 672
16.20 D ela y-line Trigger C ircuit s 675
16.21 S pecia l Test Eq uipment 677
C H AP . 17. RAD AR RE LAY.... . . . . . . . . . .. 680
INTROD U C TION . . . . . . . . . . . . . . . . . . . . . . . . . ...680
17.1 The U sesof Ra da r Rela y. . .680
17.2 The Element s of Ra da r Rela y . 681
M IS TH O D S OF S C AN NE R D ATA TR AN S MI S S I ON
17.3 G enera l Met hods of S ca nner D a t a Tra nsmission.
17.4 Met hods of C omba t ing I n t er feren ce
17.5 The Met hod of I ncrement a l Angle.
17.6 The P ha se-shift Met hod
17.7 Met hods of Rela y ng S ine a nd C osine
17.8 P ulse Met hod for Rela ying S ine a nd C osine.
...
CONTENTS
TH E RAD IO-F RE QU ENC Y E QU IP ME NT . . . .
713
17.10 Ant enna s, F req uencies, a nd t he Ra dia t ion P a t h. 713
17.11 G enera l Tra nsmit t er a nd Receiver C onsidera t ions 717
17.12 A 300-Mc/sec Amplit ude-modula t ed E uipment . 719
17.13 A 100-Mc/sec F req uency-modula t ed E quipment . 721
17.14 Microw a ve S yst em for P oint -t o-point S ervice. 723
RA~ AR R LAY S YS TE MS . . . . . . . . . . . . ...726
17.15 A G round-t o-ground Rela y S yst em
726
17.16 Rela y S yst em for Airborne Ra da r 732
IND E X . . . . . . . . . . . . . . . . . . . . . .
INTRODUCTION
B Y LOU IS N. RI D E NOU R
101. Wha t Ra da r D oes. —Ra da r is a n a ddit ion t o ma n’s sensor
eq uipment w hich a ffords genui ely new fa cilit ies.
I t ena bles a cert a in
cla ss of object s t o be “seen”
—t ha t is, det ect ed a ndloca t ed—a t dist ances
fa r beyond t h ose a t w hich t hey could be dist inguished by t he una ided
eye. This ‘(seeing” is unimpa ired by night , fog, cloud, smoke, a nd most
ot her obst acles t o ordin a ry vision.
Ra da r fur t her p rmit s t he mea sure-
ment of t he ra nge of t he object s it “sees” (t his verb w ill herea ft er be used
w it hout a pologet ic quot a tion ma rks) w it h a con venience a nd precision
ent irely unknow n in t he pa st .
I t ca n a lso mea sure t he inst a nt a neous
speed of such a n object t ow a rd or a w a y from t he observing st a t ion in a
simple a nd na t ura l w ay.
The superior it y of ra da r t o ordina ry vision lies, t hen, in t he g ea ter
dist a nces a t w hich seeing is possible w it h ra da r, in t he a bilit y of ra da r t o
w ork rega rdles of light condit ion a nd of obscura tion of t he object being
seen, a nd in t he unpa ra lled ea se w it h w hich t a rget ra nge a nd it s ra t e of
cha nge ca n be mea sured. In cer t a in ot h er respect s ra da r is definit ely
infer ior t o t he eye. The det a iled definit ion of t he pict ure it offers is very
much poorer t ha n t ha t a fforded by t he eye.
E ven t he most a dva nced
ra da r eq uipment ca n on]y show t he ross out lines of a la rge object , such
a s a ship; t he eye ca n—if t ca n see t he ship a t a ll—pick out fine det a ils
su h a s t he ra ils on t he deck a nd t he number or cha ra ct er of t he fla gs a t
t he ma st hea d. B eca use of t his grossness of ra da r vision, t he object s
t ha t ca n usefully be seen by ra da r a re not a s numerous a s t he object s
t ha t ca na be dist inguished by t he eye.
Ra da r is a t it s best in dea ling w it h
isola ted t a rget s loca ted in a rela tively fea tureless ba ckground, such a s
a ircra ft in t he a ir , ships on t he open sea , isla nds a nd coa st lines, cit ies in
a pla in, a nd t h e like. Though modern high-definit ion ra da r does a fford
a fa ir ly det a iled present a t ion of such a complex t a rget a s a cit y view ed
from t he a ir (see, for exa mple, Fig. 335), t h ra da r pict ure of such a
t a rget is incompa ra bly poorer in det a il t ha n a vert ica l phot ogra ph t a ken
under fa vora ble condit ions w ould be.
On e furt her pr oper t y of ra da r is w ort h rem a rking: it s freedom from
difficult ies of perspect ive, B y suit a ble design of t he equipment , t he
pict ure obt a ined from a ra da r set ca n be pr esen t ed a s a t rue pla n view ,
1
HOW RADAR WORKS 3
t he ra da r pict ure w ould ha ve been una ffect ed w hile phot ogra phy or
ordina ry vision w ould ha ve been useless.
1.2. H ow Ra da r Works.—The coined w ord roda ris der ived from t he
descr ipt ive ph ra se
“ra dio det ect i n a nd ra nging. ” Ra da r w orks by
sending out ra dio w a ves from a t ra nsmit t er pow erful enough so t ha t
mea sura ble a mount s of ra dio energy w ill be reflect ed from t he object s t o
beseenby t hera da r t oa ra dio receiver usua lly loca ted, for convenience,
a t t he sa me sit e a s t he t ra nsmit ter .
The proper ties of t he r eceived ech oes
a re used t o form a pict ure or t o det ermine cer t a in proper t i s of t he object s
t ha t ca use t he echoe . Thera da r t ra nsmit t er ma y send out c-w signa ls,
or freq uency-modula t ed c-w signa ls, or signa ls modula ted in ot her w a ys.
Ma ny schemes ba sed on t ra nsmissions of va rious sor t s ha ve been proposed
a nd some of t hem ha ve been used. C ha pt er 5 of t his book t rea t s t he
genera l ra da r problem, in w hich a ny scheme of t ra nsmit t er modula tion
ma y be used, in a ver y funda ment a l a nd elega nt w a y .
D espit e t he grea t number of w a ys in w hich a ra da r syst em ca n in
principle be designed, one of t hese w a ys ha s been used t o such a n over-
w helming degree t ha t t he w hole of t his book, w it h t he except ion of C ha p.
5, is d vot ed t o it . When ra da r is ment ioned w it hout qua lifica t ion in
t his book, pulse ra da r w ill be mea nt .
N TOa p olog y f or t h is s pecia l iz a t ion
is needed. Thousa nds of t imes a s much effor t a s t ha t expended on a ll
ot her forms of ra da r put t oget her ha s gone int o t he rema rka bly sw ift
development of pulse ra da r since it s origin in t he yea rs just before World
Wa r II .
In pulse ra da r , t he t ra nsmit t er is modula t ed in such a w a y t ha t it
sends out very int e se, ver y br ief pulses of ra dio energy a t int erva ls t ha t
a re spa ced ra ther fa r a pa rt in t erms of t he dura tion of ea ch pulse. D uring
t he w a it ing t ime of t he t ra nsmit ter bet w een pulses, t he receiver is a ct ive.
E choes a re received from t he nea rest object s soon a ft er t he t ra nsmission
of t he pulse, from object s fa rt her a w a y a t a slight ly la t er t ime, a nd so on.
When sufficient t ime ha s ela psed t o a llow for t he r cept ion of echoes from
t he most dist a nt object s of int erest , t he t ra nsmit t er is key d a ga in t o
send a not her very shor t pulse, a nd t he cycle repea t s.
S ince t he ra dio
w a ves used in ra da r a re propa a t ed w it h t he speed of light , c, t he dela y
bet w een t he t ra smission of a pulse a nd t he recept ion of t he echo from
a n ob ect a t ra nge R w ill be
(1)
t he fa ct or 2 ent er ing beca use t he dist a nce t o t he t a rget ha s t o be t ra versed
t w ice, once out a nd once ba ck, F igure 1.2 show s schema t ica lly t he
pr inciple of pulse ra da r .
 
+ )
FI G .1.2.—Theprincipleof pulsera da r.
(a ) P u l se ha s ju st b een em it t e df rom r a d a r
aa t . (b)P ulaereachesta rget . (. ) Scat teredenergy,eturnsfromt a rget; t ra nsZnit tedpul`e
carrieOn, (d) E chopulserea chesra da r.
 
5
t heclue t ot heea se t it h t ich ra nge ca n remea sured y ra da r . Ra nge
mea surement is reduced t o a mea surement of t ime, a nd t ime ca n be
mea sured perha ps m ore a ccura tely t ha n a ny ot her ba sic physica l qua n-
t it y. B eca use t he velocit y of light is high, t he int erva ls of t ime t ha t
must be mea sured in ra da r a re short .
Num rica lly , t he ra nge corre-
sponding t o a given dela y t ime is 164 yd for ea ch microsecond ela psing
bet w een t he t ra nsmission of t he pulse a nd t he recept ion of t he echo. I f
it is desired t o mea sure ra nge t o a precision of 5 yd, w hich is necessa ry in
some a pplica t ions of ra da r , t ime int erva ls must be mea sured w it h a
precision bet t er t ha n & psec. Modern elect ronic t iming a nd displa y
t echniques ha ve been developed t o such a point t ha t t his ca n rea dily be
done.
One of t he simplest w ays in w hich ra da r echo signa ls ca n be displa yed
is show n in Fig. 1.3. The bea m of a ca t hode-ra y t ube is ca used t o begin
a sw eep from left t o r ight a cross t he fa ce of t he t ube a t t he inst a nt a pulse
is sent from t he t ra nsmit ter .
The bea m is sw ept t o t he right a t a uniform
ra te by mea ns of a sa w t oot h w a veform a pplied t o t he horizont a l def ect ion
pla t es of t he C RT. The out put signa ls of t he ra da r receiver a re a pplied
t o t he vert ica l deflect ion pla tes.
To ensure t ha t t he w ea kest signa ls
t ha t a re a t a ll det ect a ble a re not missed, t he over-a ll ga in of t he receiver
is high enough so t ha t t herma l noise origina t ing in t he receiver (S ec. 2.7)
is percept ible on t he displa y. The t w o signa ls t ha t r ise significa nt ly
 
6
INTRODUCTION
[SEC.1.3
pulse lea king int o t he receiver , a nd on t he right , t he echo signa l from a
ra da r t a rget . Th t a rget in t he pa rt icula r ca se of F ig. 1“3 is t he ea rt h ’s
moon.
The mea surement of ra nge by mea ns of ra da r is t hus a st ra ight forw a rd
pr oblem of t im e m ea sur em ent .
I t is a lso desira ble t o be a ble t o mea sure
t he direct ion in w hich a t a rget lies a s view ed from a ra da r st a t ion. In
principle, t his ca n be done on t he ba sis of t r ia ngula t ion, using ra nge
informa t ion on t he sa me t a rget from t w o or more sepa ra te ra da r loca t ions.
Alt hough t his met hod p rmit s of grea t a ccura cy a nd ha s occa siona lly
been used for specia l purposes, it is fa r more desira ble from t he st a nd-
point of simplicit y a nd flexibilit y t o mea sure direct ion, a s w ell a s ra nge,
from a single ra da r st a t ion. Mea surement of t a rget bea ring w a s ma de
possible by t he development of ra dio t echniq ues on w a velengt hs short
enough t o permit t he use of highly direct iona l a nt enna s, so t ha t a more
or less sha rp bea m of ra dia t ion could be produced by a n a nt enna of
rea sona ble ph ysica l size.
When t e pulses a re sent out in such a bea m, echo s w ill be received
only from t a rget s t ha t lie in t he direct ion t he bea m is point ing.
I f t h
a nt enna , a nd hence t he ra da r bea m, is sw ept or sca nned a round t he
horizon, t he st rongest echo w ill be received from ea ch t a rget w hen
t he bea m is point ing direct ly t ow ard t he t a rget , w ea ker echoes w hen t he
bea m is point ed a lit t le t o one side or t he ot her of t he t a rget , a nd no echo
a t a ll w hen it is point ing in ot her direct ions.
Thus, t he bea ring of a
t a rget ca n be det ermined by not ing t he bea ring of t he ra da r a nt enna
w hen t ha t t a rget gives t he st rongest echo signa l.
This ca n be done in a
va riet y of w ays, a nd more precise a nd convenient mea ns for det ermining
t a rget bea ring by mea ns of ra da r ha ve been developed (C ha p. 6), but t he
m et hod described h er e illust ra t es t he ba sic principle.
I t is onvenient t o a rra nge t he ra da r displa y so t ha t, inst ea d of show -
ing t a rget ra nge only , a s in Fig. 1.3, it show s t he ra nge a nd a ngula r
disposit ion of a ll t a rget s a t a ll a zimut hs. The pla n-posit ion indica tor,
or P P I , is t he most common a nd convenient displa y of t his t ype. F igure
1.1 is a phot og a ph of a P P I-scope.
The direct ion of ea ch echo signa l
from t he cent er of t he P P I show s it s direct ion from t he ra da r; it s dist a nce
from t he cent er is proport iona l t o t a rget ra nge. Ma ny ot her forms of
indica tion a re convenient for specia l purposes; t he va rious t ypes of indi-
ca t or a re ca t a loged in C ha p. 6.
1.3. C omponent s of a Ra da r S yst em.— ra da r set ca n be considered
a s sepa ra ble, for t he purposes of design a nd descript ion, int o severa l ma jor
component s concerned w it h different funct ions.
F igure 1.4 is a block
dia gra m of a simple ra da r set broken up int o t he component s ordina rily
dist inguished from one a not her .
 
7
fir ing of t he modula tor . This sends a hi h-pow er , high-volt a ge pulse t o
t h e m a gnet ron, w hich is t he t ype of t ra nsm it t ing t ube a lmost universa lly
used in m oder n ra da r . F or t he br ief dura t ion of t he modula t or pulse,
w hich ma y t ypica lly be 1 ~ sec, t h e m a gnet r on oscilla t es a t t he ra dio
freq uency for w hich it is designed, usua lly some t housa nds of m ega cycles
per second. Th e r -f pulse t hus pr oduced t ra vels dow n t he r-f t ra nsmis-
sion line show n by double lines in F ig. 1.4, a nd pa sses t hrough t h e t w o
sw it ches designa t ed a s TR a nd ATR. Th ese a r e ga s-discha r ge devices
of a ver y specia l sor t . The ga s discha r ge is st a r t ed by t h e high-pow er
Rotatingantenna
—1
4
FIG.1.4.—Blockdiagramof a simpleradar.
r -f pulse fr om t h e t ra nsm it t er , a nd ma int a ined for t he dura t ion of t ha t
pulse; during t his t im e t he TR ( or t r a nsm it receive) sw it ch connect s
t h e t ra nsm it t er r -f line t o t he a nt enna , a nd disconnect s t he mixer a nd t h e
r est of t h e ra da r receiver show n below t he TR sw it ch . The ATR (for
a nt i-TR) sw it ch , w hen fired, simply permit s t he r-f pulse from t he t ra ns-
m it t er t o pa ss t hr ough it w it h negligible loss. B et w een pulses, w hen
t h ese ga s-discha rge sw it ches a r e in a n unfir ed st a t e, t h e TR sw it ch
conn ect s t he rn her t o t he a nt enna , a nd t he ATR disconnect s t h e m a gne-
t ron t o pr even t loss of a ny pa rt of t he feeble r eceived signa l.
Aft er pa ssing t hr ough t hese t w o sw it ches, t h e t ra nsmit t er pulse
 
INTRODUCTION [SEC.1.4
a nt enna is designed in such a w a y t ha t t he bea m sha pe it produces is
suit a ble for t he req uirem ent s t he ra da r set must m eet . I t is m ount ed on
a sca nner w hich is a rra nged t o sw eep t he bea m t hrough spa ce in t he
ma nner desired; simple a zimut h rot a t ion is indica ted in F ig. 1“4.
Aft er t he t ra nsmission of t he pulse, t he discha rges in t he TR a nd
ATRst it ches cea se a ndt hesyst em isrea dy t o receive echoes. E choes
a re picked up by t he a nt enna a nd sent dow n t h e r-f line t o t he mixer .
The m ixer is a nonlinea r device w hich , in a ddit ion t o receiving t he signa ls
from t he a nt enna , is supplied c-w pow er from a loca l oscilla tor opera t in g
a t a freq uen cy only a few t ens of m ega cycles per second a w a y from t h e
m a gnet ron freq uency. The difference freq uency t ha t result s from mixing
t hese t w o signa ls cont a ins t he sa me i t elligence a s did t he origina l r-f
ech oes, but it is a t a sufficient ly low freq uency (t ypica lly , 30 Me/see) t o
be a mplilied by m ore or less co vent iona l t echniq ues in t he int ermedia t e-
freq uen cy a mplifier show n. Out put signa ls from t h e i-f a mplifier a re
dem odula ted by a et ect or , a nd t he result ing unipola r signa l a re fur t h er
a mplified by a video-freq uency a mplifier simila r t o t h ose fa milia r in
t elev is ion t ech n iq u e.
Th e out put signa ls of t he video a mplifier a re pa ssed t o t h e indica tor ,
w hich displa ys t hem , let us sa y for definit eness, in pla n-posit ion form .
In order t o do t his, it must receive a t iming pulse from t he modula t or , t o
indica t e t he inst a nt a t w hich ea ch of t he uniform ra n ge sw eeps out from
t he cen t er of t he P P I t ube should begin. I t must a lso r eceive from t h e
sca nner informa t ion on t h e direct ion in w hich t h e a nt enna is point ing,
in order t ha t t h e ra nge sw eep be execut ed in t he proper direct ion from t he
cen t er of t h e t ube. C onnect ions for a ccomplishing t his a re indica t ed in
t he F ig. 1.4.
In C ha ps. 9 t o 14, inclusive, t he det a iled design of ea ch f t he com -
p nen t s show n i F ig. 1.4 is t rea ted.
In a ddit ion, considera t ion is given
t o t he problem of supplying pr ima ry pow er in a form suit a ble for use w it h
a ra da r set ; t his is especia lly difficult a nd im port a nt in t he ca se of a irborne
r a d a r .
1.4. The P er form a nce of Ra da r .-In discussing t he per form a nce of
ra da r, on e usua lly refers t o it s a nge pw forrnunce-t h a t is, t he ma ximum
dist a nce a t w hich som e t a rget of in t erest w ill ret urn a sufficient ly st ron g
signa l t o be det ect ed. The fa ct ors t ha t det erm ine ra nge per form a nce a re
num erous a nd t h ey int era ct in a ra t h er com plica t ed w a y. C ha pt er 2 is
devot ed t o a discussion of t hem , a nd C ha p. 3 dea ls w it h t he import a nt
m a tt er of t h e proper t ies of ra da r t a rget s.
The usua l inverse-sq ua re la w w hich governs t he int ensit y of ra dia t ion
from a point source a ct s t o det erm ine t h e ra nge depen dence of t he fra ct ion
of t he t ot a l t ra nsm it t ed ener gy t ha t fa lls on a t a rget . S o fa r a s t he ech o
 
9
t ion, so t ha t t he inverse-sq ua re la w must be a pplied a ga in t o det ermine
t he ra nge dependence of t he a mount of echo en ergy rea ching t he receiver .
In conseq uence, t he echo ener gy received from a t a rget va r ies w it h t he
inverse fourt h pow er of t he ra nge fr m t he ra da r set t o t he t a rget , ot her
fa ct or s bein g con st a nt .
To be det ect a ble, a signa l must ha ve a cer t a in minimum pow er ; let
us ca ll t h e minimum et ect a ble signa l t lt i~ .
Then t h e ma ximum ra n ge
of a ra da r set on a t a rget of a given t ype w ill be det erm ined by Smi. ,
a ccor ding t o t he expression
S .n = g,
wh er e K is a const a nt a nd Pt is t he pow er in t he t ra nsmit t ed pulse, t o
()
m.
E qua tion (2) displa ys t he difficult y of increa sing t he ra nge per form a nce
of a ra da r set by ra ising it s pulse pow er .
A 16-fold increa se in pow er is
req u red t o double t he ra nge.
10,CQO
10?m
Date
100
80
D a t e
H ow ever formida ble t his requir ement a ppea rs, one of t he most
rema r ka ble fa ct s of t he w a rt ime yea rs of developm ent of ra da r is t ha t
pra ct ica ble pulse pow ers in t he microw a ve freq uency ra nge (a bout 1000
Me/see a nd a bove) ha ve increa sed by a fa ct or of hundreds in a r ela t ively
sh ort t ime. This st upendous a dva nce result ed from t he invent ion a nd
ra pid improvement of t he mult ica vit y ma gnet ron, w hich is descr ibed in
C ha p. 10. Figure 1.5 show s t he hist ory of ma gnet ron developm ent , w it h
respect t o pulse pow er a nd effi iency, a t t he t hr ee most import ant micro-
w a ve ba nds exploit ed during t he w ar .
The cur ves a re ra t her a rbit ra rily
 
[SEC.14
st ep in out put pow er w a s due t o a n im provement in t he m a gnet ron it self.
The increa se a t 10-cm w avelengt h in t he ea rly pa rt of 1941 w a s brought
a bout by t he development of modula tors of higher pow er .
I t is import a nt t o rea lize t ha t t he curves of Fig. 1.5 lie a bove one
a not her in t he or der of increa sing w a velengt h not beca use development
w a s begun ea r lier a t 10 cm t ha n a t 3 cm, a nd ea r lier a t 3 cm t ha n a t 1 cm,
but beca use m a gnet rons of t he t ype used in ra da r a re subject t o inherent
limit a t ions on ma ximum pow er w hich a re m ore sever e t he shor t er t he
w a velengt h . The sa me is t rue of t he r-f t ra nsmission lines used a t
m icrow a ve freq uencies. The horizont a l da shed lines show n in Fig. 15a
show t he ma ximum pow er t ha t ca n be ha ndled in t he st a nda rd sizes o
‘‘ w a veguide”
used for r-f t ra nsmission a t t he t hr ee ba nds.
simila rly spect a cula r decrea se in t he minimum det ect a ble signa l,
due t o t he im p ovement of microw a ve ra da r receiver s, ha s m a rked t he
w a r yea rs.
In t he w a vele gt h ba nds a bove a bout 10 m, na tura l “st a tic”
a nd ma n-ma de int er ference set a ra t her high noise level a bove w hich
signa ls must b det ect ed, so t ha t t here is lit t le necessit y for pursuing t he
best possible r eceiver per for ma n ce.
This is not t rue a t microw ve fre-
Na tura l a nd ma n-ma de int er ference ca n be neglect ed a t t hese
freq ueficies in compa r ison w it h t he una voida ble inherent noise of t he
r eceiver . This ha s put a premium on t he developm ent of t he m ost
sensit ive receiver s possi le; a t t he end of 1945 m icrow a ve receiver s w er e
w it hin a fa ct or of 10 of t heoret ica lly per fect per form a nce. I m provem ent
by t his fa ct or of 10 w ould increa se t he ra nge of a ra da r set only by t he
fa ct or 1.8; a nd fur t her receiver improvem nt ca n t oda y be w on only by
t he m ost pa inst a king a nd difficult a t t ent ion t o det a ils of design.
Why Microw a ve s?-The rea der w ll ha ve observed t ha t w hen ra da r is
discussed in w ha t ha s gone before, microw ave ra da r is a ssumed. This is
t rue of t he ba la nce of t his book a s w ell. S o fa r a s t he a ut hors of t his
book a re concerned, t he w or d m.da r implies not only pulse ra da r , a s ha s
a lrea dy been rem a rked, but m icrow a ve pulse ra da r.
Though it is t rue
t ha t t he effor t s of t he Ra dia tion La bora tory w er e devot ed exclusively t o
m icrow a ve pulse ra da r , t his a t t it ude is not ent irely pa rochia lism. The
fa ct is t ha t for nea r ly ver y purpose served by ra da r , m icrow a ve ra da r
is prefera ble. There a re a few a pplica tions in w hich longer-w a ve ra da r
is eq ua lly good, a nd a ver y few w here long w a ves a re defin it ely prefera ble,
but for t he overw helming ma jorit y of ra da r a pplica t ions microw a ve ra da r
is demonst ra bly fa r more desira ble t ha n ra da r opera t ing a t longer
wavelengths.
The superior it y of micr w a ve ra da r a r ises la rgely beca use of t he
desira bilit y of focusing ra da r energy int o sha rp bea ms, so t ha t t he direc-
t ion a s w ell a s t e ra nge of t a get s ca n be det ermined. In conform it y
 
THE PERFORMANCE OF RADAR 11
t he bea m pa ssing t hrough a n a per t ure of given size depends on t he ra tio
of t he dia met er of t he a per t ure t o t he w a velengt h of t he ra dia t ion in t he
bea m, t he sha rpness of t e bea m produced by a ra da r a nt enna (w hich
ca n be t hought of a s a sor t of a per t ure for t he ra dio ener gy) depends on
t he ra t io of t he a nt enna dimensions o t he w a velengt h used. F or a n
a nt enna of given size, t he brea dt h of t he bea m produced is propor t iona l
t o t he w a velengt h. These st a t ement s a re ma de precise in S ec. 9.1.
P a rt icula r ly in t he ca se of a irborne ra da r , w here a la rge a nt enna
ca nnot be t olera ted for a erodyna mic rea sons, it is impor t a nt t o produce
sha rp ra da r bea ms w it h a n a nt enna st ruct ure of modest size.
This
dema nds t he use of microw a ves. Roughly spea king, microw a ves a re
ra dio w aves w hose w avelengt h is less t ha 30 cm.
Ra da r definit ion, it s a bilit y t o discr imina t e bet w een t a rget s close
t oget her in spa ce, improves a s he bea mw idt h is na rrow ed. Ta rget s a t
t he sa me ra nge ca n be dist inguished by ra da r a s being sepa ra t e if t hey
a re sepa ra t ed in a zimut h by a n a ngle la rger t ha n one bea mw idt h; t hus
t he q ua lit y of t he pict ure a fforded by ra da r improves a s t he bea mw idt h
is reduced. F or a n a nt enna of given size, t he bea mw idt h ca n be decrea sed
only by low er ing t he w a velengt h.
The finit e velocit y of light set s a limit t o t he desira ble bea mw idt h if
a region of finit e size is t o be sca nned a t a given speed by a ra da r set .
C ha pt er 4 considers t his a nd ot her limit a t ions of pulse ra da r in some
detail.
The Propagation of Micr ow a ves. —F ur ther limit a tions on t he per for m-
a nce of ra da r a rise from t he propa ga tion proper t ies of ra dio w aves in t he
microw a ve region of t he elect roma gnet ic spect rum.
Like light , micro-
w a ves a re propa ga t ed in st ra ight lines.
U nlike r a dio w a ves a t fr eq uencies
low er t ha n a bout 30 Me/see, microw a ves a re not reflect ed from t he
ionosphere. This mea ns t ha t t he ma ximum ra nge of a ra da r set w hose
per forma nce is not ot herw ise limit ed w ill be set by t he opt ica l hor izon
w hich occurs beca use t he ea r t h is round. This is in fa ct t he limit a t ion
on t he per forma nce of t he best ra da r S et s developed dur ing t he w a r .
U nder cer t a in condit ions, bending of t he microw a ve bea m a round t he
ea r t h is produced by met eorologica l condit ions (S ec. 2.14). This ca n
increa se t he ra nge of a ra da r set beyond t he opt ica l horizon, but such
phenomena a r e rela t ively r a re a nd essent ia lly unpredict a ble.
A low er limit on t he w a velengt hs w hich ca n be used for pra ct ica l
ra da r syst ems is fixed by t he onset of a tmospher ic a bsorpt ion of micro-
w a ve en er gy .
B elow a w a velengt h of a bout 1.9 cm, serious a bsorpt ion
occurs in moist a tmosphere, beca use of a molecula r t ra nsit ion in w at er
va por w hich ca n be excit ed by t he ra dia t ion (S ec. 2.15). F or t his rea son,
2 cm is a bout t he shor t est w a velengt h a t w hich ra da r syst ems of good
 
12
INTRODUCTION
[SEC.1.5
w here high a bsorpt ion ca n be t olera ted or is even w elcom e, short er w a ve-
lengt hs ca n be used, but 2 cm is a good pra ct ica l limit . The w a r t ime
development of ra da r component s a nd syst ems a t 1.25 cm a nt eda ted t he
discovery of t he st rong w a t er-va por a bsorpt ion a t t his w a velengt h. A
w a velengt h of 1.25 cm is, for t uit ously , very nea rly t he most unf or t t ina te
choice t ha t could ha ve been ma de in t he developm ent of a new short -
w a velengt h ba nd.
105. Ra da r S yst ems.—The uses ma de of ra da r w er e so va rious un der
w a rt im e condit ions t ha t ma ny different syst ems w ere developed t o fill
different needs. Th ese syst ems usua lly differed mor e in rega rd t o bea m
sha pe, sca nning mea ns, a nd mode of indica t ion t ha n in r ega rd t o a ny
ot her proper t ies. C ha pt er gives a brief conspect us of t h e principa l
va riet es of ra da r, w it h especia l empha sis on t hose t ypes t ha t prom ise t o
ha ve a n import a nt pea cet ime use.
Tw o exa mples of t he det a iled design
of ra da r syst ems a re given in C ha p. 15, a ft er t he component s of ra da r
syst ems h ave been discussed.
C on sider a ble use ha s been ma de of ra da r bea cons. These ,a re devices
w hich, on receivin g a pulse or a ser ies of pr operly coded pulses from a
ra da r set , w ill sen d ba ck in reply a pulse or a ser ies of coded pulses. A
gr ea t increa se in t he flexibilit y a nd convenien ce of t he use of ra da r under
cert a in condit ions ca n be obt a ined by t h e use of such bea cons.
A brief
a ccount of t heir pr oper t ies a nd uses, t hough not of t heir design, w ill be
foun d in C ha p. 8.
Tow ar d t he en d of t h e w ar , t w o m a jor ’ development s occurr ed w hich
promised t o ext en d grea t ly t he a pplica bilit y of pulse ra da r under unfa vor-
a ble condit ions. Mea ns w er e developed for r eproducin g ra da r indica t ions
a t a point dist a nt from t h e set t ha t ga ther ed t he origina l da ta ; t he int elli-
gence necessa ry w as t ra nsmit t ed from t h e ra da r t o t he dist a nt indica tor
by r a dio mea ns. This ra da r rela y, a s it ha s come t o be ca lled, is descr ibed
in some det a il in C ha p. 17.
C ha pt er 16 dea ls w it h a not her i port a nt developm ent —na mely, t he
modifica t ion of pulse-ra da r eq uipment so t ha t it w ill displa y only t a rget s
t ha t a re in m ot ion r ela tive t o t h e ra da r . S uch moving-t a rget indica tion
is pot ent ia lly of grea t impor t a nce in freeing ra da r fr om t he limit a t ions of
sit e. At t he pr esent , a ra da r sit e must be chosen w it h ca reful a t t ent ion
t o t h e surrounding t erra in ; hills or buildings w it hin t he line of sight ca n
ret urn st r on g “perma nent echoes” w hich ma sk t a rget signa ls over a la rge
pa r t of t he desira ble covera ge of t he set . In mount a inous t ema t i, t his
problem is ver y ser ious. An a rra n gement t ha t gives signa ls only from
t a rget s t ha t a re movin g a ppea rs t o be t h e best solut ion t o t he perma nent -
ech o pr ob lem .
A fa ct t ha t ha s been t oo lit t le recognized w hen ra da r syst ems a re
 
13
informa t ion a fforded by ra da r is usua lly a t lea st a s import a nt a s is t he
ra da r it self. A good orga niza t ion ca n ma ke excellent use even of infer ior
ra da r informa t ion, a s w a s proved by t he success of t he B rit ish H ome
Cha in of ra da r st a t ions, t he first la rge-sca le ra da r inst a lla t ion t o be ma de.
An ina dequa t e orga niza t ion l set -up ca n do poor job, even t hough
provided w it h splendid ra da r from t he t echnica l st a ndpoint . The ma ny
problems t ha t ent er int o t he crea t ion of a n a deq ua te orga niza tion for t he
use of ra da r da t a ha ve not received t he st udy t ha t t hey shoul . D espit e
t his fa ct , Cha p. 7 a t t empt s t o provide a n int roduct ion t o t his sort of
pla nning, a nd t o ra ise some of t he import a nt problems, even t hough t hey
ma y not yet be sa t isfa ct orily solved.
106. The E a rly H ist ory of Ra da r.—Though t he complet e hist ory of
t he origins a nd t he grow th of modern ra da r is a long a nd complica ted one, 1
it w ill be of some int erest t o sket ch here it s ma in lines, w it h especia l
r ef er en ce t o Allied d ev elopm en t s.
S uccessful pulse ra da r syst ems w ere developed independent ly in
America , E ngla nd, Fra nce, a nd G erma n during t he la tt er 1930’s. B a ck
of t heir development la y ha lf a cent ury of ra dio development for commu-
nica tion purposes, a nd a ha ndful of ea rly suggest ions t ha t, since ra dio
w a ves a re know n t o be reflect ed by object s w hose size is of t he order of a
w a velengt h, t hey might be used t o det ect object s in fog or da rkness.
The fa ct t ha t ra dio w a ves ha ve opt ica l propert ies ident ica l w it h t hose
a ssocia ted w it h ordina ry visible light w as est a blished by H einrich H ert z
in 1886, in t he fa mous series of experiment s in w hich he first discovered
ra dio w a ves. H ert z show ed a mong ot her t hings, t ha t ra dio w a ves w ere
reflect ed from solid object s. In 1904 a G erma n engineer , H ulsmeyer ,
w a s gra nt ed a pa t ent in severa l count r ies on a proposed w a y of using
t his proper t y in a n obst a cle de ect or a nd na viga t iona l a id for ships. In
J une 1922, Ma rconi st rongly urged t he use of short w a ves for ra dio
detection.
first used in 1925 by B r eit a nd Tuve, of t he C a rnegie Inst it ut ion of Wa shing-
t on, for mea suring t he height of t he ionosphere. 2 Aft er t he successful
experiment s of B reit a nd Tuve, t he ra dio-pulse echo t echniq ue beca me t he
est ablished met hod for ionospheric invest iga t ion in a ll count ries.
The
st ep fr m t his t echniq ue t o t he not ion of using it for t he det ect ion of a ir-
cra ft a nd ships is, in ret rospect , not such a grea t one; a nd va rious indi-
vidua ls t ook it independent ly a nd a lmost simult a neously in America ,
1F or t h e fu lles t t r ea t m en t of r a d a r h is t or y a v a ila b le, t h e r ea d e r is referredto the
officia l h is t or y of D iv . 14, N D RC ,
“Ra da r” by H . E . G uerla c, t o be published by
L it t le, B r ow n , & C o., B os t on .
2M. A. Tuve and G , B reit , “Terrestr ia l Magnet isma nd AtmosphericElectr ici ty , ”
 
14
INTRODUCTION
[SEC.1.6
E ngla nd, F ra nce, a nd G erm a ny, a bout t en yea rs a ft er t he or igina l w or k
of B reit a nd Tuve.
The resea rch a gencies of t he Am erica n Arm y a nd Na vy ha ve a long a nd
complica ted hist ory of ea rly exper im ent , t ot a l fa ilure, a nd q ua lified suc-
cess in t he field of ra dio det ect ion . The int erest ed rea der w ill find t his
dea lt w it h a t lengt h in D r . G uer la c’s hist ory . 1 H ere it w ill be sufficient
t o repor t t he ea rliest full successes.
In ea rly 1939, a ra da r set designed
a nd built a t t he Na va l Resea rch La bora t ory w a s given exha ust ive
t est s a t sea during ba t t le ma neuvers, inst a lled on t he U.S.S. New York.
The first cont ra ct - for t he com mercia l m a nufa ct ure of ra da r eq uipment
w a s let a s a result of t hese t est s, for t he co st ruct ion of six set s, designa t ed
a s C XAM (S ec. 6.9), duplica t ing t ha t used in t h e t r ia ls. In Novem ber
1938, a ra da r posit ion-finding eq uipment in t ended for t he cont rol of
a nt ia ircra ft guns a nd sea rchlight s, designed a nd built by t he S igna l C orps
La bora t ories of t he Arm y, w a s given ext ensive t est s by t he C oa st Art illery
B oa rd, repr sent ing t he using a rm.
This set a lso w ent in t o q ua nt it y
ma nufa ct ure, a s t he S C R-268 (S ec. 6.14). An Arm y long-ra nge a ircra ft -
det ect ion set w hose developm ent ha d been req uest ed ea rlier y t he Air
C orps w a s dem onst ra t ed t o t he S ecret a ry of J t ’a r by t he S igna l C orps
La bora t ories in Novem ber 1939. A cont ra ct for t he product ion of t hk
eq uipment , t he S C R-270 (a nd S C R-271; see S ec. 6.9) w a s let in August
1940.
B rit ish ra da r w a s developed a t a bout t he sa me t ime but it s a pplica t ion
pr oceeded a t a somew ha t fa st er pa ce under t he immedia t e t hrea t t o
E ngla nd a nd w it h considera bly gr ea ter rea lism during t he ea rly yea rs of
t he w a r. D uring t he w int er of 1934-1935, t he Air Minist ry set up a C om -
m it t ee for t he S cient ific S urvey of Air D efense.
Am on g t he suggest ions
it r eceived w a s a ca refully w or ked out pla n for t he det ect ion of a ircra ft
by a pulse m et hod, submit t ed by a S cot t ish physicist , now S ir Rober t
Wa tson-Wa tt , t hen a t t h e hea d of t he Ra dio D epa rt m ent of t he Na t iona l
P h ysica l L a bor a t or y.
The first exper iment a l ra da r syst em of t he t ype suggest ed by Wa tson-
Wa t t w a s set up in t he la t e spring of 1935 on a sma ll isla nd off t he ea st
coa st of E ngla nd. D evelopm ent w or k during t h e summer led t o t he
blocking-out of t he ma in fea t ure of t he B rit ish H om e C ha in of ea r ly-
w a rning st a t ions (S ec. 6“9) by fa ll.
Work bega n in 1936 t ow a r d set t ing
up five st a t ions, a bout 25 miles a pa r t , t o pr ot ect t h e Tha mes est ua ry .
B y Ma rch 1938, a ll t hese st a t ions—t he nucleus of t he fina l C ha in-w ere
com plet e a nd in opera t ion under t he ch a rge of RAF personnel.
B rit ish ra da r developm ent eff r t w a s t hen brought t o bea r on a irborne
ra da r eq uipm ent . Tw o t ypes w er e envisa ged: a set for t he det ec ion of
sur fa ce vessels by pa trol a ircra ft (ca lled AS V, for a ir t o sur fa ce vessel),
1op. ant.
15
a nd a n eq uipment for ena bling night fight ers t o home on enemy a ircra ft
( a lled AI , for a ircra ft in t ercept ion). Work w a s concent ra ted on AS V
first , a nd a n exper iment a l eq uipment w a s successfully demonst ra ted
dur ing fleet ma neuv rs in S ept ember 1938. E xperiment a l AI eq uipment
w a s w orking by J u ne, 1939, a nd it w a s demonst ra ted t o t he chief of RAF
F ight er C omma nd in August of t ha t yea r . The Air Minist ry a sked t ha t
30 such syst ems be inst a lled in a ircra ft in t he next 30 da ys. B efore t he
end of S ept ember a ll t hese syst ems ha d been inst a lled, four ha ving been
rea dy on t he da y w a r br oke out .
E mpha sis on a irborne ra da r underline t he point t ha t, if sha rp ra da r
bea ms w er e ever t o be produced by a nt enna s sma ll enough t o ca rry in a n
~ P la ne, w a velengt hs shor t er t ha n t he 1+ m used in ea r ly B rit ish a ir-
borne eq uipment w ould ha ve t o be employed. This led t o t he effor t
t ha t t he B rit ish put int o developing a genera tor of mic ow a ves w hich
could give pulse pow er a deq ua te for ra da r use.
B y ea rly 1940, a B rit ish
version of t he mult ica vit y ma gnet ron ha d been developed t o t he point
w here it w a s a n ent irely pra ct ica ble source of pulsed microw a ve energy ,
a nd t he hist ory of modern ra da r ha d begun.
1.7. Wa rt ime Ra da r D evelopment in t he U nit ed S t a t es. —B efor e t he
end of 1940, t he w or k on ra da r of America n a nd B rit ish la bora t ories ha d
been combined a s a result of a n a gr eement bet w een t he t w o government s for
excha nge of t echnica l informa t ion of a milit a ry na t ure. A B rit ish Technica l
Mission a rr ived in Wa shingt on in S ept ember 1940 a nd mut ua l disclosures
w er e ma de of B rit ish a nd America n a ccomplishment s in ra da r up t o t ha t
t ime. Members of t he B rit ish mission visit ed t he N’a va l Resea rch
La bora t ory , t he Army S igna l C orps La bora t ories a t F or t Monmout h ,
a nd t he Aircra ft Ra dio La bora tory a t Wright F ield, a s w ell a s ma nufa c-
t uring est a blishment s enga ged in ra da r w ork. They demonst ra ted t heir
version of t he ca vit y ma gnet ron a nd furnished design informa tion t ha t
ena bled U . S . ma nufa ct urers t o duplica te it prompt ly .
In discussions w it h t he Microw a ve C ommit t ee of t he Na t iona l
D efense Resea rch C ommit t ee, w hich ha d been set up a few mont hs before,
members of t he B rit ish Mission pr oposed t w o specific pr oject s w hich
t hey suggest ed t ha t t he U nit ed S t a tes under t a ke: a microw a ve a ircra ft -
in t ercept ion eq uipment , a nd a microw a ve posit ion finder for a nt ia ircra ft
l ire control.
To implement t heir decision t o follow t hese suggest ions, t he Micro-
w a ve C ommit t ee of t he ND RC decided t o set up a development la bora -
t or y st a ffed pr ima rily by physicist s fr om a number of universit ies.
They w ere encoura ged in t his st ep by t he success t ha t t he B rit ish ha d
a lrea dy exper ienced w it h civ lia n w a r t ime ra da r development a gencies
st a ffed w it h physicist s ha ving no specia l ra dio exper ience but good
 
[SEC.1°7
Microw a ve C ommit t ee persua ded t he Ma ssa chuset t s I nst it ut e of Tech-
nology t o a ccept t he responsibilit y of a dminist er ing t he new la bora tory ,
The Ra dia t ion La bora t ory , a s it w a s na med, opened it s doors ea r ly in
November 1940. The direct or of t he la bora t ory t hroughout it s 62
mont hs of life w a s D r . L. A. D uB ridge.
The Army a nd Na vy development la bora t or ies w er e gla d t o depend
on t he new Ra dia t ion La bora tory for a n invest iga tion of t he usefulness
for ra da r of t he new microw a ve region of t he ra dio spect rum. They w er e
fully occupied w it h t he urgent engineer ing, t ra ining, a nd nst a lla t ion
problems involved in get t ing ra da r eq uipment t ha t ha d a lrea dy been
developed out int o a ct ua l milit a ry a nd na va l service. At t he end of
1940, t he use of m icr ow a ves f or r a da r pur poses seemed highly specula t ive,
a nd t he S ervice la bora t ories q uit e proper ly felt it t heir dut y t o concent ra t e
on ra da r t echniq ues t ha t ha d a lrea dy been w orked out successfully .
D ur ing 1941, w hile t he Na vy w a s inst a lling long-w a ve sea rch ra da r
a nd med um-w a velengt h fire-cont rol ra da r on ships of t he fleet , a nd t he
Army w as sending out S igna l Aircra ft Wa rning B a t t a lions eq uipped w it h
t he S C R-270 a nd a nt ia ircra ft ba t t er ies w it h t he S C R-268, n t a single
it em of ra da r eq uipment ba sed on t he new microw a ve t echniq ues w a s
delivered for opera t iona l use. H ow ever , development w ork a t t he
Ra dia tion La bora tory ha d broa dened fa r beyond t he t wo specific project s
suggest ed by t he B rit ish Technica l Mission, a nd microw a ve eq uipment
w a s show ing grea t promise for ma ny w art ime uses.
A few impor t a nt da t es w ill indica t e t he w a y in w hich t his develop-
ment w a s proceeding. On J a n. 4, 1941, t he Ra dia t ion La bora t ory ’s f rst
microw a ve ra da r echoes w ere obt a ined. A successful flight t est of a
w orking “brea dboa rd” model of a n a irborne ra da r int ended for AI use
w a s ma de on Ma rch 10, in a B -18A furnished by t he Army Air C orps.
In t his fight it w a s found t ha t t he eq uipment w a s ext remely effect ive in
sea rching for ships a nd sur fa ced subma rines a t sea .
In t he la t e spring of 1941, a n exper iment a l microw a ve sea -sea rch
ra da r eq uipped w it h a P P I w a s inst a lled on t he old dest r oyer U.S.S.
Semmes. On J une 30, t he Na vy let t he first product ion cont ra ct for
microw a ve ra da r eq uipment ba sed on t he w or k of t he Ra dia tion La bora -
t ory . This w a s for a product ion version of t he set t ha t ha d been demon-
st r a t ed o t he S emmes.
At t he end of a y, a prot ot ype of t he microw a ve a nt ia ircra ft posi ion
finder developed a t t he Ra dia t ion La bora t ory w a s in opera t ion . I t
a ccomplished t he t hen-a st onishing fea t of t ra cking a t a rget pla ne in
a zimut h a nd eleva t ion w holly a ut oma t ica lly .
These a nd ot her ea rly successes led t o a n increa sing S ervice in t erest in
microw a ve ra da r , w hich ha d seemed so specula t ive a vent ure in 1940.
 
17
by t he fa ct t ha t t he personnel of t he Ra dia t ion La bora t ory, w hich ha d
been a bout 40 a t t he beginning of 1941, rose t o nea rly 4000 by mid-1945.
S imila rly , t he Ra da r S ect ion of t he Na va l Resea rch La bora tory increa sed
it s personnel t o 600. The Ra dio P osit ion Finding sect ion of t he S igna l
C orps La bo a t ories gr ew int o t he E va ns S igna l La bora t ory , w it h a pea k
personnel of m or e t ha n 3000. A simila r grow t h t ook pla ce a t t he Air-
cra ft Ra dio La bora tory a t Wright Field.
A t rem endous a mount of w or k w a s ca rr ied out dur ing t he w a r by t he
resea rch a nd engineering st a ffs of ma ny indust r ia l concerns, bot h la rge
a nd sma ll. In some ca ses, t hese firms, w orking eit her independent y
or on developm ent or product ion cont a ct s w it h t he a rm ed forces or w it h
ND RC , engineer ed cer t a in t ypes of ra da r set s a ll t he w a y t hrough from
t he ba sic idea t o t he finished product .
To a la rger ext ent , t he cont ribu-
t ion of indust ry w a s t o t a ke t he pr ot ot y pe eq uipment produced in
govern m ent la bora t or ies a nd m a ke t he design suit a ble for q ua nt it y
ma nuf a ct ure a nd for ser vice use t inder comba t condit ions.
The a rt
a dva nced so ra pidly in t h e ea r ly yea rs t ha t ma nufa ct urers w er e oft en
ca lled pon t o m a ke m a jor cha nges during t h e course of product ion in
or der t o t a ke a cc unt of n ew lessons from bot h t he la bora t or ies a nd t he
battlefields.
The grow th of t he ra da r indust ry , w hich sca rcely exist ed before 1940,
is indica t ed by t he fa ct t ha t by t he end of J une 1945, a pproxima t ely
$2,700,000,000 w ort h of ra da r eq uipm ent ha d been delivered t o t he Arm y
a nd t he Na vy. At t h e end of t he w a r , ra da r eq uipment w a s being
produced a t a ra te of m or e t ha n $100,000,000 w ort h per mont h .
The enormous w a r t ime invest m ent of m oney , skill, a nd product ive
fa cilit ies in ra da r pa id t he Allies ha ndsome dividends w it h t he fleet , in t he
a ir , a nd on t he ba t t lefield. 1 The uses of ra da r in a pea ceful w or ld w er e
jusb beginning t o be w orked out in 1946. S o e of t hese a re dea lt w it h
in Vol. 2 of t his ser ies. B ut t he t echnica l a chievem ent r epr esent ed by
t h e w a r t im e developm ent of ra da r seems ver y nea r ly unpa ra lleled. In
t erm s of t he t im e int ervening bet w een ‘t he recept ion of t he first ra da r
signa ls a nd t he la rge-sca le use of ra da r in t he w a r, it is a s if, seven yea rs
a ft er t he first fa lt er ing flight of t he Wright brot her s a t K it t y H a w k, t he
a irpla ne ha d been developed int o a pow erful w ea pon of w hich t housa nds
w er e in c nst a nt use.
1Th e s t or y of r a da r ’s oper a t ion a lu se i n t h e w a r is t old , in a w a y t h a t is som ew h a t
blu rr ed a b ou t t h e ed ges by t h e cen sor sh ipobt z in in g ju st befor e t h e en d of t h e w a r
w it h J a pa n , in a pa m ph let en tit led “R a da r : A R epor t ,on S cien cea t Wa r ,” r elea s ed
by t he J oint B oa rd on S cien t if ic I n for m a t i on P olicy on Au g . 15, 1945. I t is ob t a i n-
a ble fr om t h e S uper in t en den tof D ocum en t s, U .S . G over nm en t P r in tin g Office,
\Vashington,D.C.
B Y E . M. P U RC E LL
The opera t ion of a ra da r set depends o t he det ect ion of a w ea k
signa l ret urned from a dist a nt refle t ing object .
The a ct ors w hich
cont rol t he st rengt h of t he signa l so received a re clea rly of first import a nce
in det ermining t he ma ximum ra nge of det ect ion of a given t a rget by a
specified ra da r set .
In S ees. 2.1 t o 26 w e sha ll formula t e a nd exa mine
t h ba sic rela tion bet w een t hese qua nt it ies, w hich is commonly know n
a s t he “ra da r equa t ion. ”
S pecifica lly , w e w ant t o derive a n expression
for t he pea k ra dio-freq uency signa l pow er S , a va ila ble a t t he t ermina ls
of t he ra da r a nt enna , w hich w ill involve mea sura ble propert ies of t he
t ra nsmit t ing a nd receiving a nt enna syst em, t he t ra nsmission pa t h
t hrough spa ce, a nd t he t a rg t it self.
Now t his rela ion w ill not suffice
t o fix t he ma ximum ra nge of det e t ion unless t he minimum pow er
req uired for det ect ion , S k, is know n.
This import a nt qua nt it y S ~ t i w e
prefer t o discuss sepa ra tely , beginning in S ec. 2.7 below . I t w ill be found
t o depend on ma ny ot h er fa ct ors, not a ll rea dily a ccessible t o mea sure-
ment , ra nging from t herma l noise in a resist or t o t he int egra t ing propert y
of t he eye of t he ra da r observer .
Thus w e choose t o divide t he problem
int o t w o pa rt s, by a fict it ious bounda ry, a s it w ere, bet w een t he ra da r
a nt enna a nd t h rest of t he set .
The rela tions w hich w e sha ll develop
in S ees. 2.1 t o 2.6 a re w holly geometrical ones in t he sense t ha t t he fa ct ors
upon w hich t he received pow er S depends a re a ll lengt hs, a pa rt from
t ra n sm it t ed pow er P, t o w hich, of course, S is a lw ays proport iona l.
TH E R AD AR E Q U ATI ON F OR F R E E -S P AC E P R OP AG ATI ON
2.1. The Mea ning of F ree-spa ce P ropa ga t ion.-F ort una tely ,
t he
t he
qua si-opt ica l na t ure of microw a ve propa ga t ion permit s us t o concent ra t e
our a t t ent ion a t t he out set on a ver simple ca se, w hich w e sha ll ca ll
“ fr ee-s pa ce pr opa g a t i on .”
Th e circum st a nces im plied w ould be rea lized
exa ct ly if ra da r set a nd t a rget w ere isola ted in unbounded empt y spa ce.
They a re rea lized w ell enough for pra ct ica purposes if t he follow ing
con dit ion s a r e fu lfilled :
1, No la rge obst a cles in t ervene bet w een a nt enna a nd t a rget , a long
a n opt ica l line of sight .
18
19
2. No a lt erna t e t ra nsmission pa t h, via a reflect ing sur fa ce, ca n be
fo low ed by a subst a nt ia l fra ct ion of t he ra dia ted energy.
3. The int ervening a tmosphere is omogeneous w it h r espect t o index
of refra ct ion, a t t he freq uency used.
4. The int ervening a tmosphere is t ra nspa rent , i.e., does not a bsorb
energy from t he w a ve, a t t he freq uency used.
C ondit ion 1 rest r ict s our a t t ent ion t o t a rget s w it hin t he hor izon.
C ondit ion 2 ba rs, for t he present , considera t ion of ra da r sea rch a t low
a ngles over w a t er , a lt hough la ter w e sha ll include t his ca se by a suit a ble
modifica t ion of t he ra da r eq ua t ion.
Microw a ve ra da r over la nd a ppea rs
t o be rela t ively free, even a t low a ngles, from t he reflect ion effect s w hich
a re so pronounced a t longer w avelengt hs. In a ny ca se, if t he direct ivit y
of t he a nt enna pa t t ern is such t ha t very lit t le energy st r ikes t he reflect ing
sur fa ce, C ondit ion 2 is fulfilled. Any implica tions of C ondit ions 3 a nd 4
w hich a re not self-evident w ill be cla rified in t he la st pa rt of t his cha pt er ,
w her e ot her t ypes of propa ga t ion w ill be discussed.
I f, now , t hese condi-
t ions of free-spa ce propa ga t ion a pply, t he result is very simple: The
t ra nsmit t ed w a ve, a t a ny considera ble dist a nce from t he a nt enna , 1 h a s
spherica l w a vefront s—limit ed in ext ent ,. of course, by t he ra dia t ion
pa t t ern of t he a nt enna —w hich sprea d so t ha t t he int ensit y of t he dis-
t urba nce fa lls off w it h t he inverse squa re of t he dist a nce.
2.2. Ant enna G a in a nd Receiving C ross S ect ion. -I f t he t ra nsmit t ing
a nt enna w ere t o ra dia t e energy isot ropica lly t ha t i , uniformly in a ll
direct ions—t he pow er flow t hrough unit a rea a t a dist a nce R from t he
a nt enna could be found by dividing P, t he t ot a l ra dia t ed pow er , by
4TRZ. A directive a nt enna , how ever , w ill concent ra t e t he energy in
cer t a in direct ions. The pow er flow observed a t some dist a nt point w ill
differ by some fa ct or G from t ha t w hich w ould be produced by a n a nt enna
ra dia t ing isot ropica lly t he sa me t ot a l pow er .
This fa ct or G is ca lled t he
“ga in” of t he a nt enna in t he direct ion in q uest ion.
B y our definit ion,
t he ga in of t he hypot het ica l isot ropic ra dia tor is 1 in ever y direct io . F or
a ny ot her a nt en a G w ill be gr ea t er t ha n 1 in some direct ions a nd less
t ha n 1 in ot hers. I t is clea r t ha t G could not be grea t er t ha n 1 in ever y
direct ion, a nd in fa ct t he a vera ge of G t a ken over t he w hole sphere must
be just 1.
U sua lly w e a re int erest ed in a nt enna s for w hich G ha s a ver y pro-
nounced ma ximum in one direct ion, t ha t is t o sa y, a nt enna s w hich
1The limita t ion implied is to dis ta ncesgreat erthan al l/X,w here d is t h e w id th of
t h e a n t en na a p er t ur ea n d x t h e w a v elen gt h . At dist a n cesR less tha n this (less than
366ft , for exa mple, for x = 3 cm, d = 6 ft ), t h e in t en sit y d oes n ot fa ll off a s 1/R J .
Althoughthis reg ion has been unt il now of no interes t for rada r applica t ions ,one can
a nt icipatethe development of short-ra nge,very-high-resolut ionra dar for which thc
nearzone,so defined,w ill be.of prima ry import a nce.
 
[S E C .22
r a dia t e a w ell-d efin ed beam. This ma ximum va lue of G w e sha ll denot e
by G O. The na rrow , concent ra t ed bea ms w hich a re cha ra ct er ist ic of
microw a ve ra da r require, for t heir forma tion, a nt enna s la rge compa red
t o a w a velengt h . In nea rly very ca se t he ra dia t ing syst em a mount s t o
a n a per t ure of la rge a rea over w hich a substa nt ia lly pla ne w a ve is excit ed.
F or such a syst em, a funda ment a l rela t ion connect s t he ma ximum ga in
G ,, t he a rea of t he a per t ure A, a nd t he w a velengt h :
(1)
The dimensionless fa ct or .f is equa l t o 1 if t he excit a t ion is uniform in
pha se a nd int ensit y over t he w hole a per t ure; in a ct ua l a nt enna s f is oft en
a s la rge a s 0.6 or 0.7 a nd is ra rely less tha n 0.5. An a ntenna formed by a
pa ra boloida l mirror 100 cm in dia met er , for a w a velengt h of 10 cm,
w ould ha ve a ga in of 986 a ccording t o E q . (1) w it h ~ = 1, a nd in pra ct ice
might be designed t o a t t a in G O = 640.
The connect ion bet w een ga in a nd bea mw idt h is ea sily seen. U sing
a n a pert ure of dimensions d in bot h direct ions, a bea m ma y be f rmed
w hose a ngula r w idt h, 1 det ermined by diffra ct ion, is a bout X/d radians .
The ra dia ted pow er is t hen ma inly concent ra ted in a solid a ngle of X2/dZ.
An isot ropic ra dia or w ould sprea d t he sa me pow er over a solid a ngle
of 4T.
Th erefore, w e expect t he ga in t o be a pproxima t ely 4rd2/X’, which
is consist ent w it E q . (1), s nce t he a rea of t he a pert ure is a bout d’. For
a more rigorous discussion of t hese q uest ions t he rea der is referred t o
Vol. 12, Cha p. 5.
A complement a ry proper t y of a n a nt enna w hich is of import a nce
equa l t o t ha t of t he ga in is t he e.fective receiving cross section. This
qua nt it y ha s t he dimensions of a n a rea , a nd w hen mult iplied by t he pow er
densit y (pow er per unit a rea ) of a n incident pla ne w ave yields t he t ot a l
signa l pow er a va ila ble a t t he t ermina ls of he a nt enna .
Th e ef fect i ve
r eceiv in g cr os s s ect ion A, is rela ted t o t he ga in a s follow s:
A . G @2.
4rr
(2)
Not e t ha t G , not G o, ha s been w rit t en in E q . (2), t he a pplica bility of
w hich is not rest r ict ed t o t he direct ion of ma ximum ga in or t o bea ms of
a ny specia l sha pe. Once t he ga in of t he a nt enna in a pa rt icula r direct ion
is specified, it s effect ive receiving cross sect ion for pla ne w a ves incident
jrom t ha t direct ion is fixed. E qua t ion (2) ca n be ba sed rigorously on t he
Reciprocit y Theorem (see Vol. 12, Cha p. 1). C ompa ring E qs. (2) and
(1) w e observe tha t , f t he fa ct or j is unit y, t he effec ive receiving cross
1Wh er ever a pr ecise defin it ion of bea m w idt h is in t en ded, w e sh a ll m ea n t h e
 
21
sect ion o a n a nt enna in t he principa l direct ion is precisely t he a rea of
t he a pert ure; in ot her w ords, a ll t he energy incident on t he a per t ure is
a bsorbed. Quit e genera lly A, w ill depend on t he a rea of t he a nt enna
a pert ure a nd not on~ , w herea s G Ow ill depend on A/h2.
2.3. S ca t t er ing C ross S ect ion of t he Ta r et .—We ha ve t o consider
h ow t he t a rget it self ent ers t he ra da r problem. E vident ly w e need some
mea sure of t he a mount of pow er reflect ed by t he t a rget . F or t his
purpose w e define t he sca t t er ing cross sect ion of t he t a rget u a s follow s:
u (dimensions of a n a rea ) is t o be 4T t imes t he ra t io of t he pow er per unit
solid a ngle sca tt ered ba ck t ow a rd t he t ra nsmit t er , t o t he pow er densit y
(pow er per unit a rea ) in t h e w a ve incident on t he t a rget . In ot h er
w ords, if a t t h e t a rget t he pow er incident on a n a ea u pla ced norma l
t o t he bea m w ere t o be sca t t ered uniformly in a ll direct ions, t he int ensit y
of t he signa l received ba ck a t t he ra da r set w ould be just w ha t it is in t he
ca se of t he a ct ua l t a rget . In some respect s “ra da r cross sect ion” is a
more a ppropria te na me for u in so fa r a s it indica tes t ha t w e a re con cerned
only w it h t he pow er sca t t ered direct ly ba ck t ow a rd t he t ra nsmit t er.
I t is essent ia l t o rea lize t ha t t he cross sect ion of a given t a rget w ill
depend not only upon t he w a velengt h but upon t he a ngle from w hich t he
t a rget is view ed by t h e ra da r. The fluct ua t ion of u w it h “t a rget a spect ,”
a s it is ca lled, is due t o t h e int er ference of reflect ed w a ves from va rious
pa rt s of t he t a rget (see C ha p. 3).
only for cert a in specia l ca ses ca n u be
ca lcula ted rigorously ; for most t a rget s u ha s t o be inferred from t he ra da r
da t a t hem selves.
U sua lly t his ca nnot be don e in a ny uniform w a y
beca use of t h e fluct ua tion referred t o, a nd it ma y be w ell t o a ssert a t t his
point t ha t in t elligent use of t h e formula s w hich w e sha ll der ive, in a ll of
w hich u a ppea rs, req uires a n a pprecia t ion of t hese limit a t ions.
2s4. The Ra da r E qua tion. -Wit h t he pert inent qua nt it ies defined it is
now a simple ma tt er t o formula te t he ra da r eq ua tion .
I f S is t he signa l
pow er received, P t he t ra nsmit t ed pow er , G t he ga in of t he a nt enna , A th
w a velengt h , a t he ra da r cross sect ion of t he t a rget , a nd R t he dist a nce
t o t he a rget or ra nge, t his rela tion must hold:
‘ ‘(s)(&)(z)
(3a)
The qua nt it y in t he first pa rent hesis is t he pow er densit y in t he incident
w a ve a t t he t a rget . The first t w o t erms in pa rent heses t oget h er give t he
pow er densit y in t he ret urning w a ve a t t he ra da r a nt enna , a nd t he la st
fa ct or w ill be rec gnized a s t he receiving cross sect ion of t he ra da r
a nt enna , from E q. (2). Rea rra nging t erms, for compa ct ness,
S= P=.
THE RADAR EQUATION [SEC.25
Aga in w e ca ll a t t ent ion t o t he fa ct t ha t E q . (3b), like E q . (2), cont a ins G
ra t her t ha n G o, a nd is not rest r ict ed t o a ny pa rt icula r direct ion or t o
bea ms of a ny specia l sha pe. The sole rest r ict ion w hich ha s not yet been
ma de explicit is t ha t G should not va ry significa nt ly over t he a ngle w hich
t he t a rget subt ends a t t he ra da r a nt enna .
U sua lly w e sha ll be int erest ed in t he signa l t ha t is ret urned w hen t he
t a rget lies somew here a long t he ma ximum of t he ra da r bea m, a nd w e
should t hen repla ce G by G ,. I t is inst ruct ive t o proceed t hen t o elimi-
na t e G Oby mea ns of E q . (l), obt a ining
S = P(TA ‘j2
(4a)
Not e t ha t AZnow a ppea rs in t he denomina tor , w hile t he numera tor con-
t a ins t he sq ua re of t he a rea of t he a nt enna a pert ure. A furt her ma nipu-
la tion of E q . (4a ) is of int erest .
S uppose t he minimum pow er req uired
for sa t isfa ct or y det ect ion , Smin, is know n; w e ma y solve E q . (4a) for t he
ma ximum ra nge of d t ect ion, R~..:
4 PuA ‘jz
(4b)
At t his point it ma y be w ell t o get a n idea of t he order of ma gnit ude
of t he q ua nt it ies involved by insert ing numbers not unusua l in w art ime
pulse-ra da r pra ct ice. I f w e choose X = 0.10 ft ( = 3.0 m), P = 10’
w a t t s, A = 10 ft 2, j = 0.6, u = 100 ft z (t ypica l for sma ll a irc a ft ),
R~.. =
We observe t ha t a 16-fold increa se in t ra nsmit t ed pow er is req uired t o
double t he ma ximum ra nge; on t he ot er ha nd, it w ould a ppea r t ha t R~..
could be doubled by doubling t he linea r dimensions of t he a nt enna . B ut
t he la t t er st ep w ould a t t he sa me t ime reduce t he bea mw idt h by a fa ct or
of 2 a nd a s w e sha ll see in S ec. 2“11 t ha t t his indirect ly a ffect s S t i.. A
cha nge in w a velengt h is even more difficult t o discuss a s it ent a ils cha nges
in S ~ i., P, a nd possibly u a s w ell.
2.5. B ea ms of S pecia l S ha pes.—In severa l a pplica tions of ra da r, use
is ma de of a n a nt enna designed t o prea d t he ra dia t ed energy ou over a
considera ble ra nge in a ngle in one pla ne.
The object usua lly is t o
increa se t he a ngula r region covered a t one t ime. An exa mple of such a
ra dia tion pa tt ern is t he simple
“fa n bea m” sket ched in Fig. 2.1. I t is
ea sy t o produce such a bea m by mea ns of a n a nt enna w hose effect ive
a pert ure is w ide in t he direct ion in w hich t he bea m is t o be na rrow a nd
n rrow in t he direct ion i