The optimum design choice for the MST radar antenna was considered. The following factors are required: directivity and gain; beam width and its symmetry; sidelobe levels, near and wide angle; impedance matching; feeder network losses; polarization diversity; steerability; cost effectiveness; and maintainability. The directivity and related beam forming aspects of various antenna elements and directivity aspects when such elements are formed into an array are discussed. Array performance for important variables, in particular, the spacing of the elements is considered

Koshy, V. K.

English

NASA Technical Reports Server

456 
7.1A DIRECTIVITY AND SPACING FOR THE ANTENNA ELEMENTS 
V. K. Koshy 
Bharat Elec t ronics  Ltd 
Bharatnagar P. 0. - 201008 
Ghaziabad (U.P. 1, India 
While deciding on the optimum design choice fo r  the MST radar  antenna, the  
following f a c t o r s  a r e  required t o  be considered: 
beam width and i t s  symmetry; sidelobe levels - near and wide angle; impedance 
matching; feeder network l o s s e s ;  po la r i sa t ion  d ive r s i ty ;  s t e e r a b i l i t y ;  cost-  
e f fec t iveness ;  and main ta inabi l i ty .  
d i r e c t i v i t y  and ga in ;  
The scope ot t h i s  no te  w i l l  be r e s t r i c t e d  t o  the d i r e c t i v i t y  and r e l a t e d  
beam-forming aspec ts  of var ious  antenna elements and a l s o  the d i r e c t i v i t y  
aspec ts  when such elements a r e  formed i n t o  an array. 
considered i n  regard t o  important va r i ab le s ,  i n  p a r t i c u l a r ,  the spacing of the  
elements. 
ANTENNA CONFIGURATIONS 
Array performance w i l l  be 
Al t e rna t ive  conf igura t ions  poss ib le  fo r  MST radar  antenna are the 
following: coaxial  c o l l i n e a r  a r ray ;  d i s c r e t e  d ipole  a r r ay ;  Yagi a r ray ;  d i sh  
antenna; and short  backf i re  a r ray .  
COAXIAL COLLINEAR ARRAY 
A c o l l i n e a r  antenna (BALSLEY and ECKLUND, 1972) i s  constructed of a series 
of ha l f  wavelengths of coaxia l  cab le  t h a t  have been connected together by 
e l e c t r i c a l l y  interchanging the  inner and outer  conductors a t  each junction. 
half-wave d ipole  has a length higher than 0.5X to  compensate fo r  the propaga- 
t i o n  ve loc i ty  of the cable  (0.671 fo r  RG-8 cable).  
c o l l i n e a r  antenna could be any even number and typ ica l  cases a r e  26-element, 16- 
element e t c .  
A 
The number of d ipoles  i n  a 
The c o l l i n e a r  antenna (BALSLEY and ECKLUND, 19721, by v i r t u e  of its 
cons t ruc t ion  or l a rge  numbers of r ad ia to r s ,  inherent ly  has high d i r e c t i v i t y  i n  
the  plane or the antenna. A 26-element co l l i nea r  antenna has shown a 
t h e o r e t i c a l  beam width of 5.6" a t  49.8 MHz. The d i r e c t i v i t y  i n  the plane per- 
pendicular t o  the c o l l i n e a r  antenna i s  i so t rop ic ,  as appl icable  i n  the case of 
any dipole.  
The r a d i a t i o n  pa t t e rn  of a c o l l i n e a r  antenna of n halfwave d ipoles  fed  a t  
the center  summetrically (BALSLEY and ECKLUND, 1972) i s  expressed a s  
th Where 0 i s  the angle from the  broadside ax i s ,  AK the amplitude of K 
from the cen te r  and J, = ( 7 ) d  s ine ,  d being d is tance  between elements. 
Sunset (GIOIm et al., 19811, Poker F l a t  r ada r s  (BALSLEY et  al., 1981) of M X N 
elements produces a d i r e c t i v e  pa t t e rn  on the broadside of the  array.  
pa t t e rn  i s  obtained f r o n  a product of the element pa t t e rn  and the  a r r ay  space 
f ac to r .  
element 2s 
The c o l l i n e a r  antenna when b u i l t  i n t o  an a r r ay ,  a s  done i n  the  case of 
The a r r ay  
The a r r ay  space f a c t o r  (ALLMAN and BOWBILL, 1976) i s  given by 
https://ntrs.nasa.gov/search.jsp?R=19840019086 2020-03-22T09:47:06+00:00Z
457 
s i n  (WX/ 2) sin(N$ /2 
s xy = s x s y M 1 sin(Jlx/2) 1 i l s i n ( l ~ 2 )  1 
The spacing elements i n  the plane of the co l l i nea r  antenna does not  provide 
I n  the plane perpendicular t o  the antenna elements, much scope for  adjustment. 
the spacing should be decided from consideration of the following f ac to r s :  
(1) 
leading t o  l a rge  impedance var ia t ions.  
of elements and hence the d i r e c t i v i t y ,  would be reduced. 
(2) 
l i m i t s  the angle of s t e e r a b i l i t y .  
A c lose spacing, say 0.5h or less, would r e s u l t  i n  strong mutual coupling 
Also, the array area fo r  a given number 
A large spacing, say 1.0h or higher causes generation of grat ing lobes and 
A spacing between 0.5h to  1 . O X  i s  considered acceptable i n  p rac t i ca l  
s i t ua t ions  and 0.6h t o  0.8h may be deemed as optimum (KRAUS, 1950). 
Further, spacing of the dipole  from the ground influences the d i r e c t i v i t y  
and impedance. 
r ea l i s ed  f o r  spacings between O . l h  and 0 . 2 h ,  although the bandwidth w i l l  be 
narrow and the input impedance w i l l  be sens i t i ve  t o  ground va r i a t ions ,  fo r  such 
spacings. 
bandwidth, tolerance t o  ground undulations and minimum re f l ec to r  losses.  When 
wide-angle s t e e r a b i l i t y  i s  required, a spacing of 3Al8 can ensure b e t t e r  gain 
performance a t  wide angles (OLINER and MALECH, 1966). 
For a half-wave dipole,  highest  d i r e c t i v i t y  (KRAUS, 1950) i s  
A spacing of XI4 i s  considered optimum from considerations of good 
Direct ivi ty  of an array of l i nea r  dipoles i s  a l so  affected t o  some extent 
by mutual coupling. 
backed by a r e f l e c t o r ,  the e f f ec t  of mutual coupling i s  negl igible  and the gain 
va r i e s  only by 0.1 dB from the value computed from the physical area of the 
array (DEVANE and DION, 1962). 
HOwwer, when the array i s  very large and dipoles a re  
The d i r e c t i v i t y  of the array a l s o  depends on the amplitude i l lumination of 
the elements. While a uniform il lumination gives maximum d i r e c t i v i t y  of unity 
but poor sidelobe l eve l  ( t heo re t i ca l ly  13.2 dB), a tapered i l luminat ion (KOSHY 
et a l . ,  1983) can improve the sidelobe l eve l  by 6 to  8 dB, a t  the expense of re- 
duction i n  d i r e c t i v i t y  of about 1 t o  1.5 dB. 
DISCRETE DIPOLE ARRAY 
Dipoles, half-wave or full-wave, fabr icated out of aluminum tubes form the 
basic element. 
and moderately d i r ec t ive  i n  the plane of the dipole. The 3-dB beam width for  
half-wave and full-wave dipoles are 78' and 47", respectively.  
The beam i s  i so t rop ic  i n  the plane perpendicular t o  the dipole  
An array of dipoles i s  constructed by arranging them co l l inea r  i n  columns 
and with several  rows o t  such columns. 
dependent on the number of elements i n  both planes, element d i r e c t i v i t y ,  spacing 
etc .  , the same way as  i n  the case of the coaxial  col l inear  antenna explained 
e a r l i e r .  
BOWHILL and MAYES (19791, and MAYES and TANNER (1981) as par t  of t h e i r  work on 
the Urbana MST radar. 
Direct ivi ty  of an array of dipoles i s  
Excellent design d e t a i l s  are provided by ALLMAN and BOWHILL (1976), 
Both the coaxial  col l inear  and d i sc re t e  dipole  arrays require two 
independent orthogonal arrays t o  produce polar izat ion i n  E-W and N-S direct ions.  
I n  respect of s teer ing,  while co l l i nea r  coaxial  a r r ay  i s  capable of s teer ing 
only i n  the plane perpendicular t o  the dipoles,  the d i s c r e t e  dipole array can be 
made t o  s t ee r  in  orthogonal planes by providing su i t ab le  progressive phase 
s h i f t s  i n  both planes (MAYES and TANNER, 1981). 
458 
YAGI A R U Y  
Yagi antenna element i s  compact i n  hardware and provides a high d i r e c t i v i t y  
i n  both p r inc ipa l  planes. 
(CZECHOWSKY and MAmR, 1980) and MUR (FUKAO e t  al., 1980) gives a gain of 8.7 
dB . 
A four-element Yagi antenna, as used i n  the SOUSY 
When formed i n t o  an array,  the Yagi type requires a lesser number of 
elements i n  comparison t o  the dipole  type, f o r  r ea l i s ing  a given d i r ec t iv i ty .  
The a r r ay  d i r e c t i v i t y  depends on the number of elements and i s  computed by the 
standard pat tern mult ipl icat ion method. 
element pat tern and discrete-element aper ture  avai lable  t o  i l lumination taper ,  
can ensure low sidelobes (FUKAO et al .  , 1980). 
The Yagi array,  owing t o  the d i r ec t ive  
The e f f ec t  of spacing of elements on d i r e c t i v i t y  of a Yagi array i s  
governed by s i m i l a r  considerations as i n  dipole  arrays. 
where mutual coupling would be s ign i f i can t ,  the element f ac to r  generally 
sharpens so as t o  improve the d i r e c t i v i t y  i n  low-elevation angles (FUKAO e t  a l . ,  
1980). 
DISH ANTENNA 
A t  c lose distances,  
The d i sh  antenna i s -a  s ingle  element antenna consisting of a large 
parabolic ref  l ec to r  and a prime-focus or cassegranian feed system. 
The d i r e c t i v i t y  of the d i sh  antenna depends p r h a r i l y  on the aperture  area 
of the r e f l ec to r  (SILVER, 1949) and the gain i s  expressed by 
where R i s  the antenna efficiency which i s  the product of a number of eff ic iency 
f a c t o r s  considering losses  due t o  aperture i l lumination, spi l lover ,  blockage, 
surface e r ro r ,  VSWR, cross  polarization, etc. By proper control  of the feed 
i l lumination, very low sidelobe l eve l s  can be achieved i n  d i sh  antennas with 
moderate loss of gain. 
The only 'spacing' involved i n  dlsh antennas i s  t h a t  of the feed which i s  
required t o  be a t  a unique posit ion,  namely, the focus of the r e f l ec to r  €or 
optimum d i r e c t i v i t y .  Movement of the feed i n  the foca l  plane perpendicular t o  
,the axis of the dish gives rise t o  s teer ing of the beam. 
SHORT BACKFIRE ANTmNA 
The short  backfire antenna (JBRENSPECK, 1965; EHRENSPECK and STROM, 1977) 
i s  a compact high-gain element, consisting of a s lo t t ed  dipole  feed, a r e f l ec to r  
d i sc  of diameter 0.5h and a c i r c u l a r  p l a n a r  r e f l ec to r  with a r i m  of width 0.5X. 
For a planar reflecqor diameter of 2X, t h i s  antenna gives a gain of 15.1 dB. 
I n  an array,  the high element gain of the SBF antenna gives the advantage 
of high array d i r e c t i v i t y  for.  a s m a l l  number of elements. 
array of 2 c i r c u l a r  SBF antennas can realise a gain of 39 dB (PRYSICAL RESEARCB 
A 16 x 16 uniform 
LABORATORY, ,1981) 
Nevertheless, the inter-element spacing of 2h  required i n  such an array 
would produce grat ing lobes a t  30" and 90" from the array a x i s  and hence would 
r e s u l t  i n  high sidelobe l e v e l s  a t  these angles. Good sidelobe levels ,  be t t e r  
than 20 dB, can be achieved by reducing the element spacing t o  l h  (EHRENSPECK 
. and STROM, 1977). Consequent t o  th i s ,  the gain of the 16 x 16 element array 
459 
would, however, reduce t o  around 34 dB. 
CON CLUS ION 
I n  the foregoing sections,  a brief r ecap i tu l a t ion  i s  given of the 
d i r e c t i v i t y  and r e l a t ed  f ea tu res  of the various elements avai lable  f o r  r ea l i z ing  
a n  optimum antenna fo r  MST radar.  The ac tua l  choice of a pa r t i cu la r  element and 
array configuration should, however, be based not only on d i r e c t i v i t y  but a l s o  
on a number of other considerations a s  mentioned i n  the f i r s t  paragraph of t h i s  
paper. 
REFERENCES 
Allman, M. E. and S. A. Bowhill (19761, Feed system design f o r  the Urbana 
incoherent s ca t t e r  radar antenna, Aeron. Rep. No. 71, Aeron. Lab., Dep. 
Elec. Eng., Univ. Ill., Urbana-Champaign. 
Balsley, B. B. and W. L. Ecklund (19721, A portable coaxial col l inear  antenna, 
I E E  Trans. on Antennas and Prop.. AP-20, 513-516. 
Balsley, B. B., W. L. Ecklund, D. A. Carter, P. E. Johnston and A. C. Riddle 
Current s t a tus  and capab i l i t i e s ,  (19811, The Poker F l a t  MST radar system: 
20th Conference on Radar Meteorology, Nov. 30 - Dec. 3. 
Bowhill, S. A. and P. E. Mayes (19791, Proposal for  engineering and cost  study 
of a phased array antenna fo r  Urbana coherent s ca t t e r  radar,  Aeron. Lab., 
Dep. Elec. Eng., Univ. Ill., Urbana-Champaign. 
Czechowsky, P. and K. Mayer (19801, Antenna system for  SOUSY VHF radars,  Max- 
Planck I n s t i t u t  f u r  Aeronomie Report MPAE - T-00-80-19. 
Devane, M. E. and A. R. Dion (1962), The E l  Campo so la r  radar antenna, MIT 
Lincoln Lab., Tech. Rep. No. 276. 
Ehrenspeck, H. W. (19651, The short  backfire antenna, Proc. IEEE,  53, 
1138-1140. 
Ehrenspeck, H. W. and J. A. Strom (19771, Short backfire antenna - a highly 
e f f i c i e n t  array element, Microwave Journal. 20, 47-49. 
Fukao, S., S. Kato, T. ASO, M. Sasada and T. Makihira (19801, Middle and upper 
atmosphere radar (MUR) under design i n  Japan, Radio Sci.,  15, 225-231. 
Green, J. L., J. M. Warnoch, W. L. Clark, F. J. Eggert and K. J. Ruth (19811, 
Modification to the sunset radar t o  provide antenna beam steer ing,  20th 
Conference on Radar Meteorology, Nov. 30 - Dec. 3. 
Koshy, V. K., A. K. Majumdar and K. K. Gupta (19831, Plon-uniform exc i t a t ion  of 
a coaxial  col l inear  array ( t o  be published). 
Kraus, J. D. (19501, Antennas, Mc-Graw H i l l ,  N e w  York. 
Mayes, P. E. and D. R. Tanner (19811, Preliminary d r a f t  on design s tudies  o f  
phased a r r ay  fo r  Urbana coherent s ca t t e r  radar, Aeron. Lab., Dep. Elec. Eng., 
Univ. 111. , Urbana-Champaign. 
Oliner ,  A. A. and R. G. Malech (19661, Microwave Scanning Antennas, (ed. R. C. 
Hansen), Academic Press, New Pork. 
460 
Physical Research Laboratory (19811, A proposal for  atmospheric s tudies  using 
high power VRF radar,  Physical Research Laboratory, Ahedabad, India. 
Si lver ,  S. (19491, Microwave Antenna - Theory and Design, Mc-Graw H i l l ,  New 
York. 
APPENDIX 
COAXIAL COLLINEAR ANTENNA ARRAY 
ELEMENT PATTERN 
2 r  where J, = - d sine 
ARRAY FACTOR 
MAXIMUM GAIN OR DIRECTIVITY OF ARRAY 
Where the re  is no r ad ia t ion  below the ground plane. 
DISCRETE DIPOLE ANTENNA ARRAY 
ELEMENT PATTERN 
ARRAY FACTOR 
MAXIMUM GAIN OR DIRECTIVITY OF ARRAY 
2 
4* IET(O,O) Sx (O,O)( 
IE.p(e,Q) sxy(eyQ)12 s i n  e d e d Q 2n a12 
G =  
I, j o  
where the re  is no r ad ia t ion  below the ground plane. 
461 
YAGI ARRAY 
ELEMENT PATTERN 
sin(kL/ 2) (cos0-K) 
E(e) = (kL/2) (cosfJ-K) 
k~ - 0,468 + L/h 
k - L/h K = -  
Where L = l ength  of t he  Ya i E ement 
cos k s = cos ks  - l /2eksf fh ,a j  
1 cos ka cos k h ( 9 )  - kh cos kh (- ka 
X 
s i n  kh - kh cos kh f(h,a) = 
a f o r  s >> a,  h >> a,  s >> - 2 
ARRAY FACTOR 
MAXIMUM GAIN OR DIRECTIVITY OF ARRAY 
Where t h e r e  i s  no r a d i a t i o n  below t h e  ground plane 
DISH ANTENNA 
FIELD PATTERN 
E plane: 
H plane: 
462 
5 = f r e e  space impedance. 
l r  8 '  = 2 tan  - - 2f 
2 2  4f + r 
4 f  P =  
Gain = IC[?] 
Where 'k' is ef f ic iency .  
SHORT BACKFIRE ANTENNA ARRAY 
ELEMENT PATTERN 
E-plane 
Eo = IE(A,9) sin$ 
H-plane 
E = IH(A,e') cos$ 
I ~ ( A , B )  = ( 1  + case) 
4 
LX-2 2 2  c o s  2A c o s ( 2 1 ~ s i n i 3 ~ ) d z  
1- (4sineJ(A2 - z > I  
I H ( A , 9 ' )  = ( 1  + sine')  {t cos 2 cos(2nz cos0')dz 
where 8 '  = 8 - 90' 
A = Dr? is t h e  electrical ape r tu re  rad ius  > physical ape r tu re  rad ius  2 
ARRAY FACTOR 
MAXIMUM GAIN OR DIRECTIVITY OF ARRAY 
Where t h e r e  i s  no r a d i a t i o n  below the  ground plane. 


Directivity and spacing for the antenna elements

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