CEEM'2006/Dalian K-4

Some advancements in antenna near/far field calculation

for EMC prediction

Ben-Qing Gao Jang-Dong Geng Wei-Min Li

Department of Electronic Engineering, Beijing Institute of Technology, Beijing 100081, China

Abstract: Several novel approaches are 0 The additivity of radiation field ofsummarized, which are from our EMC discrete antenna elements;project practices for recent years. These 0 An array antenna do have discreteinclude: (1) A universal near/far field antenna elements and an aperturealgorithm,; (2) A deductive method for antenna can be divided into discretenear/far field estimate; (3) Elementtechnique combined with GTD approaches ealements on its aperture in terms of theto predict near/far field. Some examples areshown to support these approaches. They 0 The near/far field criterion is that themay be widely used in the antenna analysis observed points are in the far field ofand EMC prediction. each element, but may be in the near

field of the whole antenna.

I. Introduction p

*In some platform, there are often some | r,serious electromagnetic interference /7/ \/between electronic sets through the antennas.Of course, these belong to the case of the 3 / _near or far field produced from the antennas.Therefore, it is more important to estimate / / e2

the near/far field of an especial antenna in a If:Ayshort time. In order to accommodate therequirement, some efforts for us have been Apmade to find new approaches from our EMC ,/'project practices for recent years. In this /paper these approaches will be represented r '03 .

one by one in next sections. For each -- i-------approaches it includes: basic principle and 01 01,11consideration, main mathematics modes and ,2some examples. el

II. A universal near/far field algorithm Fig. 1 Configuration of rectangular aperture withA universal algorithm for an antenna both coordinate system

in near and far field calculation is presented. Without loss any generality we onlyThe scheme is relied on the principles: consider the case of planar aperture antenna.

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Fig. 1 shows the configuration of a published one, apparently, the near fieldrectangular aperture, where the aperture calculated here is in good agreement withplane is located on the x-y plane and it is the theory in figure (b).divided into N.? * ?.elements. According to _______________the equivalent principle, the equivalent_current is at x, y and z direction, ie, el, e2 10and e3 direction respectively. Next, the U

Cartesian coordinate and sphericalo 5-coordinate for each element will be set up,

and el, e2, e3 are polar axis for the equivalent ocurrent at different axis direction 1.0 1.5 2.0 2.5 3.0 3.5 4.0

respectively. In this case, the far fiend for Distance from OEG Aperture (in)each axis element has only component in (a) Near field calculatedpolar angle 0,i (i=1, 2, 3). The near/far field 15- OirtEho aeod tOEG)for planar antenna is obtained in the C10iF el Strer0tlo

-~~~~ ~Theory

following steps:TheAOpoMtd(1) Find the far field from each element. MIo(2) For each element its far fiend polar

5coordinate components are transformedinto (e 1, e2, e3) components, and (x, y, z)components. U ernOA 4

(3) Total field for P point in space are:

~~~ (1)E(i, ) (b)Near field in [1]

e mn Fig2. Open-ended waveguide electric field with the

It is easy to obtain field in ( r, 0, 0i5 distance

component, by coordinate transformation. It2)EdFrAnea

is necessary to point out: (1) above field 2)onsd-Faliern-ireAntennahacalculation is suit for both near and far Cosdralnred-ie rayttregion and without any approximation, (2) consists of N half wave dipoles located

process for implement is very simple, (3) the along y -axis with the spacing Ay, and theaccuracy calculation is enough for EMC axis of the dipoles are parallel to z axis,prediction. Some examples are as follows: shown in Fig.3 (a). This end-fire linear array

1) Near field of open-ended waveguide can be extended along the x axis with the

The OEG has an aperture of spacing of Ax to form an end-fire planarax b = 53.34 x 26.67cm,. and the working array.frequency f = 450MHzU with the

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* Hansen-Woodyard condition (H.W.C) that* * 0

y used for the maximum gain, are used here.* * * The results obtained are shown in Table 1.

* From the results, we can conclude: (1) In

the cases of the focus, the end-fire circulararray has almost same gain for different

(b) end-fire circulararray azimuth angle (0i5) in both near and far field;Fig.3 End-fire array but the end-fire linear array has different

An end-fire circular array shown in gain with the angle (p), where maximumFig.3(b). For the gain calculation of these gain happens at si = 0°, corresponding to aend-fire arrays, assume that parameters are: side-fire direction. (2) For the linear array,)frequency f =1.5GHz ; total element the gains vary with the phase controls in

both near and far field, where maximumnumber N=13; element spacing gi per we sn hAXc = Ay = 0.4A A -wavelength); array

gi per we sn h

A Ay0 ( w e g;Hansen-Woodyard condition. (3) Comparedmaximum size LI (linear array) = 4.82. the circular array to the linear array, in theL2 (circular array) = 1.62 . The exciting case of similar element number and element

amplitudes for all dipoles are uniform. spacing, the advantages of the circular arrayare that the maximum size is smaller and thegain is uniform in the azimuth angle; the

traveling wave (exciting) condition (T.W.C.) linear array is able to obtain the maximumthat happened along linear array (along gain in the specified direction.radius direction for the circular array),

Table 1 the gain of end-fire antennas (Note: o = goo in all cases)

Type parameters G (far field, rp = 4000m ), dB G (near field, rp = 5m ), dB

Focus T.W.C. H.W.C. Focus T.W.C H.W.C

Linear f = 1.5GHz 11.64 13.54 15.66 11.67 13.57 15.70

end-fire N=13 ( =b900) ( =b900) ( =b900) ( =b900) ( =b900) ( =b900)array Ay =O.4 13.61 13.10

Lm =4.8/1 (0= 0°0) (0= 00°

11.98

(ob = 450

Circular f = 1.5GHz 11.07 11.06

end-fire N =13 (Sb=90) ( Sb=90)array XcA y =O.41. 11.26 11.26

|Lm1 =1.6 ( =450) __l__| (sb=45 i |___l

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III. A deductive method for antenna enough and the sample step is small enough,near field prediction the whole details of the line source can be

Some electronic equipments are often acquired. In this way two line source along x

located in a platform, such as a ship, aircraft and y axis direction can be obtainedetc. In the case EMI prediction for near respectively. Next, "multiplying method of

fields are usually meted. For a specific the current" on the plane is used to obtain

antenna it is difficult to know its original current distribution at equal-spacinggridding points, as shown in Fig.4design data. Therefore, a deductive method

from far field parameter to near fieldcalculation may be interesting and useful. i

The deductive steps are as follows:(1) The beginning is from the far field x(o Y) = IY I(X Y) = / x I |parameters of the antenna, such as Beam 1width, Gain, SLL. As we all known, formost high gain antennas, the main-beam I(x,O) I Xwidth is required to be narrow and SLL to be Fig.4 Mutiplying thelow. So it is necessary to find the optimal current valuepoint between the main-beam width and theSLL. Dolph-Chebyshev line array method (3) near-field calculation method:and the Taylor line source method are two Using above algorithm presented in sectionmain synthesis methods to analyze this 2, we can obtain the near/far field of aproblem. Here, the Taylor line source specific antenna.

Two examples are in the following:method (using the function of the 1)Acruaprboidntn:Chebyshev multinomial to offer preceding Th A fieldparametersar

The far field parameters are shown inn sub-beam and attenuating following far Table.2 and its aperture diameter is 2.4m.sub-beam) is taken to deduce the current The results using the deductive method aredistribution on the antenna aperture by using also showed in Table.2. The far fieldthe far field parameters. parameters calculated are in good agreement(2) According to the Taylor line sourcemethAordian with the Tassumption thatte with the given values. To illuminate the gainSLL and the coefficient in have been varies with distance R, the results have beenknown, the current distribution is obtained, calculated and the curve is shown in Fig.5which is located on the main axis of anantenna aperture. Its general form is as Table.2 Reference parameters of an circularfollows: aperture antenna

Parameter BeamFrequency SLL Gain

i(s)= L1±+2ZF(nl,A,fn)COSr(2ffllns) . j (2) s87 width

Given -28dB ... 33dB3000MHz

By sampling the continued Taylor linesource, we can acquire an array with N 2900MH4z -27.37dB Pdiscrete elements. Evidently, if N is large no* 0 dB

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blockage and scatting effects of the objectsfar each element, i.e., ray path of eachelement can be traced and the ray coordinatesystem can be established to find the field ofeach element respectively.0 The total field from all elements can beobtained by the superposition of the field

24 gdue to each element.Xm Z ZXInorder to describe how to use the GTD

Fig.5 Distance from the antenna approach, let us consider the effect fromcentre to observation point (in) vertical stabilizer in the aircraft shown in

2) An plane aperture antenna Fig.6The plane aperture has dimension of 9.1x

4.75 m, and its far field parameters and theVertical stabilizer-

calculated values are shown in Table.3Table.3 Reference parameters of an plane

Diffraction point

aperture antenna ____

BeamParameters Frequency SLL Gain Diffraction point

width

1215Given -25dB 3- 38.9dB

1400MHzSource point

Approx

Calculation 1250MHz -22.3dB 38.96dB Fig.6 Diffraction on the edge of the vertical stabilizer30

The total field E(p) at the point P nearbyIV. Element method combined with GTD the aircraft can be expressed asWhen the prediction of the near/far field E(p)) El (p) + £ (p) + Ed (p) + E (P) (3)

of an antenna is preformed in a complicatedplatform environment, some blockage and Where E' (p) is direct field (due to thescattering effects from nearby objects should antenna ) Er(p) is the reflection fieldbe considered. To solve this problem, an (from a blockage, such as the fuselage ), andElement method combined with GTD El (p) ,E2i (p) are the diffraction field(fromapproach has been studied. The main points different part of a blockage, such as thefor the combined approach are: vertical stabilizer). The direct field can be* Antenna aperture can be divided into obtained by the radiation field due to themany elements (In term of sampling element in the antenna aperture (same as theprinciple). The near field region of the approach in section 2).whole antenna may be in the far field region According to the theory of UTD theof any element. diffraction field is:* Any nearby object positioned in near Ed=D E'(0) * 4*field region of antenna can be considered tobe located in far field region ofany element. where D is the matrix of the dyadicTherefore, GTD can be used to find the diffraction coefficient, 0 is the diffraction

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point.A special example to demonstrate the

approach as follows: the configuration ofnear field analysis of an airborne antenna

array is shown in Fig.7. the antenna array isan ellipse planar array with 24 element invertical direction and 130 elements inhorizontal direction. 2 4' flt)t1I2140r 6( 2

(b)Fig.8 the pattern of antenna array on an airplane

Lit regionn

Antenna array REFERENCE[1] Motohisa Kanda and R. David Orv, Near-field

Shadow region gain of a horn and open-ended waveguide:comparison between theory and experiment,IEEE Trans. Antennas Propagat., Vol.AP-35,PP.33-40, Jan. 1987

[2] Y.T. Lo and S.W. Lo, Antenna Handbook:

Fig.7 Geometry of a airplane with a vertical stabilizer Theory, Applications, and Design, Van

To verity our approach , two far field Nostrand Reinhold Company, Part B, 8, PP.

azimuth patterns for above array have been 817,11988[3] Gao Ben-qing,Wu Jian,Ma Lin, etc. "Aobtained and compared, in which one Generalized Algorithm For Antennapattern is from the classical far field analysis Near-field Computation in EMCand another is from our approach. The Prediction" Proceedings, Asia-Pacificconclusion is that one result is in good Conference on Environmentalagreement with another one. Electromagnetic CEEM'2003 Nov.4-7,2003

The elevation patterns of the antenna Hangzhou, china -

array with effect from the vertical stabilizer [4] Ma Lin, Gao Ben-qing A deductive methodfor antenna near-field computation in EMCare shown in Fig.8 (a). For comparison theprediction. Proceedings, 2004 3'th

pattern for same array in free space iS shownprdcin Poedng, 20 3't

patternFor.8 same ara nfe pc ssonInternational Conference on Computationalin Fig.8 (b) Electromagnetic and Its Applications

ICCEA2004 Nov.1-4, 2004 Beijing, China[5] Jiang-dong Geng Wei-ming Li Zheng-hui

Xue Ben-qing Gao. Near-field Analysis ofAirborne Antenna Array by the UTD Method.Proceedings, IEEE 2005 InternationalSymposium on Microwave, Antenna,

40 ~~~~~~~~~~~Propagation And EMC Technologies for;0 t 9 9 ~~~~~~~~Wireless Communications Aug. 8-12,2005,

_____________________________[6] Kouyoumjian R. G. "A Uniform Geometrical{)X)4 g xllljga l t)S Theory of Diffraction for Edge in Perfectly

El.i<tt.t.lxAXg.X)fX1il.'s.!}Conducting Surface," Proc. IEEE, 1976, vol.(a) 62, Nov. 1974.

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