SIDELOBE LEVEL REDUCTION BY LWROVEMENT OF STRUT SHAPE

Naoto Hatsunaka Shinichi Betsudan Takashi Katagi

Kokusai Denshin Denwa M t s u b i s h i E l e c t r i c co- Mitsubishi Electric Co. Co., LTD (KDD) Corn. Eqpt. Works Kamakura Works Tokyo, Japan Amagasaki, Japan Kamakura. Japan

1. Introduct ion

From the v ieupoin t o f e f fec t ive u t i l i za t ion of bo th the geos ta t ionary o r b i t and the f requency spectrum in the satel l i te cornmicat ion system, i t

charac te r i s t ics in o rder to min imize the in te r fe rence f roml to the ad jacent is s t rongly required of the ear th s ta t ion antenna to have low sidelobe

s a t e l l i t e and t h e t e r r e s t r i a l microwave l inks . The reference diagram of the s idelobe for a large antenna recommended by CCIR i s drawn as follows:

g a i n r e l a t i v e t o an isotropic antenna and 0 is a n o f f s e t a n g l e from the main lobe axis in degrees .

In the case of axisymmetrical Cassegrain antenna operated in the

d i f f r a c t i o n by t h e s t r u t exceeds that shown i n t h e above reference diagram circular polar izat ion, the level of the cross-polar s idelobe due t o t h e

By introduction of a nev type of strut , the corresponding sidelobe c+n be in the designated angular region on account of the strut arrangement,

method based on GTD f o r improving the strut shape is presented together suppressed below t h a t shown in the reference diagram. Herein. the design

with the measured values of an In te l sa t s tandard "A" ear th s ta t ion an tenna havfng the improved s t r u t . The e f fec t o f the improved s t r u t i s c l a r i f i e d by comparing i t with that of a convent ional s t rut .

2. Analysis of the Sidelobe due to D i f f r ac t ion by S t r u t

G=32-2510@. dB(1°505480) : &-lo, dB(8>48°). where G is the s idelobe

It i s knom tha t t he d i f f r ac t ion due t o t h e s t r u t c a u s e s t h e s c a t t e r - ing cone , resu l t ing in a re la t ive ly h igh s ide lobe wi th respec t to tha t s h o m i n t h e C C I R reference diagram.l) Figure 1 shows the measured and predicted s idelobe direct ions of t h e s c a t t e r i n g cone i n a conventional tr ipod case.

the reference diagram, it is u s e f u l t o d i r e c t t h e d i f f r a c t e d wave i n t o t h e near-axis region vhile not concentrating i t into the designated direct ion.

The d i f f r a c t e d wave Ed due t o an edge is expressed as fo1lovs:z) Ed EiD J"' e-Jks

In order to reduce the corresponding scat ter ing cone level re la t ive to

S(p+s) - (1)

where Ef is the i nc iden t f i e ld i n t ens i ty , k is wave number,s is the dis-

curvature of the diffracted wave-front and D is the edge d i f f rac t ion t a m e from edge to the observing point , p is the p r inc ip le rad ius of the

c o e f f i c i e n t .

ents corresponding to co-polar and cross-polar components are expressed as fo l lovs : TI TI

In the case of c i r cu la r po la r i za t ion , t he edge d i f f r ac t ion coe f f i c i -

e-jb s i n 1 D = ( n s inge cos+- ; for co-plar

.-s$ s i n 1 - (2)

D = ( n s h e . cos d+d' ) ; cross-polar n

vhere d. 4' and 0. a r e t h e angles shown in F igure 2 .

CH1672-5/81/0000-000496.$00.75 0 1981 IEEE.

496

L*,the r e s u l t i n g d i f f r a c t e d wave i s expressed by the following equations. When a plane wave i s i n c i d e n t t o t h e s t r a i g h t s t r u t h a v i n g l e n g t h o f

E t = E i (?)( D)(-) 7 sinkY e-jks

Y = sin0,m s i n (Qm-B,m) kY --- ( 3 )

2 where 0,m and 0m are the re la t ive angle o f the inc ident wave and tha t o f the observ ing d i rec t ion to t h e s t r u t d i r e c t i o n , r e s p e c t i v e l y . The ca lcu la ted d i f f r ac t ion l eve l due t o va r ious s t ru t shapes a r e shown in F igure 3.

changing the ver t ica l angle (2%) of t h e s t r u t c r o s s s e c t i o n and by at tach- The d i f f r a c t e d wave can be d i rec ted in to the des i red d i rec t ion by

ing a t h i n f i n a t t h e v e r t e x o f t he s t ru t c ros s - sec t ion .

3 . Design of the Improved S t r u t

in Figure 4 was se l ec t ed . The respective edges denoted as Edges 1. 2 and 3 a r e so des igned t o be l a id z ig -zag a s t o d i r ec t t he d i f f r ac t ed wave i n t o the vide angular region. Dividing each edge into several sect ions, the t o t a l d i f f r a c t e d wave E d i s expressed as follows:

A s a r e s u l t o f p rev ious d i scuss ions , the c ross sec t ion of s t ru t shown

Ed = Ed ,jk &-(%;-fT) --- ( 4 ) m m-

where vectors Rm, s and show the cen ter of each sect ion, the observing d i r e c t i o n and the i nc iden ta l wave d i r ec t ion , r e spec t ive ly , The ca lcu la ted maximum s ide lobe leve l due to the des igned s t ru t , whose shape is shown i n Figure 5, is presented in F igure 6 , where the s ide lobe l eve l i s normalized t o t h a t shown i n t h e C C I R reference diagram.

cross-polar wide-angle radiation patterns are shown i n Figure 8, where t h e i s equipped wi th the des igned s t ru t as shown in F igure 7, and the measured

comparison i s a l s o made with those obtained when using a convent ional ly shaped s t r u t . The improvement of t he s ide lobe l eve l i s obvious on seeing i t i n not only i t s p r inc ip l e p l ane bu t a l so o f f - ax i s p l ane .

4 . Conclusion

The s tandard "A" e a r t h s t a t i o n a n t e n n a , i n s t a l l e d i n Yamaguchi, Japan,

of the axisymmetrical Cassegrain antenna can meet t h a t shown i n t h e C C I R r e fe rence d i ag ram in a l l d i r ec t ions fo r bo th co- and cross-polar senses in t h e 6682 band.

By adopting a uniquely shaped s t rut , the resul t ing wide-angle s idelobe

Denshin Denwa Co. LTD, for his constant encouragement. Thanks a r e a l s o due to the engineers of Kokusai Denshin Denwa Co. LTD and the engineers o f Mi t sub i sh i E lec t r i c Co. LTD. fo r t he i r va luab le d i scuss ions and e f f o r t s in manufacturing.

The authors wish to extend their thanks to Dr.T.Satoh of Kokusai

Reference (1) R.W.Kreute1, "Wide-Angle Sidelobe Envelopes of a Cassegrain Antenna",

(2) R.G.Kouyoumjian and P.H.Pathak, "A Uniform Geometrical Theory of Comsat Technical Review, Vo1.6. No.1, Spring, 1976.

D i f f r a c t i o n f o r an Edge i n a Perfectly Conducting Surface", Proc. IEEE, Vo1.62, No.11, Nov. 1974.

497

A, Angle Figure t . Scattering Cone

Figure. 2 Diffraction Wave

Fqre. 4 Strut Cross Section

Elevation Angle (Degree)

F i i r e . 6 Calculated Maximum Sidelobe Level due to Improved Strut (Tripod)

498

f & iirelmive onale 10 the direction

Figure. 3 Calculated Diffraction Level

-‘ L c ”- c

B - s 2

AA’ BE’

Fqre. 5 Designed Strut Construction

Figure. 7 Slandord"A* Earth Slation Anlenna Equipped Wifh Improved Strut. IK D D, YAMAGUCHI , Japan 1

_---_ CClR Reference Diagram

El = 0'

Azimulh Angle (Degree)

( a ) I n case of Improved Tripod

-_-_ CClR Reference Diagrom

€ 1 = 0"

20 30 40 Azimuth Angle (Degree)

5 0 60 70 80 90

( b ) I n case of convenfonal Tripod

Figure. a Effecl of the Improved Strut (Cross-polar, 6. GHz)

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