Study of radiation characteristics for reflector antennas using the planar near-field measurements of primary feed with area truncation level

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1 Special Cluster on Antennas and Propagation Technologies in Conjunction with Main Topics of ISAP016 Study of radiation characteristics for reflector antennas using the planar near-field measurements of primary feed with area truncation level Michio Takikawa a), Yoshio Inasawa, and Hiroaki Miyashita Mitsubishi Electric Corporation, Ofuna, Kamakura, Kanagawa , Japan a) Takikawa.Michio@ak.MitsubishiElectric.co.jp Abstract: We study an analysis method for radiation characteristics of reflector antennas using measurement values from planar near-field antenna measurement (NFAM) of a primary feed. In planar NFAM, area truncation error (ATE) is an important error factor which must be considered. However, the effects of ATE are not clear in this analysis method. In this paper, we clarify the influence of area truncation level (ATL) on radiation characteristics for this analysis method by simulation. We use an offset parabola antenna, and compare the results using this analysis method and the physical optics method (PO), taking the edge level (EL) of the reflector and ATL as parameters. Keywords: plane-wave expansion, near-field antenna measurements, reflector antennas, area truncation Classification: Antennas and Propagation References [1] D. M. Kerns, Plane-wave scattering-matrix theory of antennas and antennaantenna interactions: Formulation and applications, J. Res. Natl. Bur. Stand. Sect. B Math. Sci., vol. 80B, no. 1, DOI:10.608/jres.080B.003 [] G. E. Evans, Antenna Measurement Techniques, Artech House, [3] J. Bennett, A. Anderson, P. McInnes, and A. Whitaker, Microwave holographic metrology of large reflector antennas, IEEE Trans. Antennas Propag., vol. 4, no. 3, pp , DOI: /TAP [4] J. J. Lee, E. M. Ferren, D. P. Woollen, and K. M. Lee, Near-field probe used as a diagnostic tool to locate defective elements in an array antenna, IEEE Trans. Antennas Propag., vol. 36, no. 6, pp , June DOI: /8.119 [5] A. C. Newell, Error analysis techniques for planar near-field measurements, IEEE Trans. Antennas Propag., vol. 36, no. 6, pp , DOI: / [6] C. A. Bakanis, ed., Modern Antenna Handbook, pp , Wiley,

2 [7] P. Angeletti and M. Lisi, Multimode beamforming networks for space applications, IEEE Trans. Antennas Propag., vol. 56, no. 1, pp. 6 78, Feb [8] M. Takikawa, Y. Inasawa, and H. Miyashita, Radiation pattern analysis for reflector antennas using the near-field measurements of primary feed, Proc. 016 Int. Symp. Antenna propagat. (ISAP 016), Oct [9] S. Silver, ed., Microwave Antenna Theory and Design, McGraw-Hill, Introduction Plane-wave expansion (PWE), in which the radiated electric field is expressed by combining plane wave spectrum, is known as a method for finding the far-field from the near-field, and the near-field from the far-field, of the radiated electric field of antennas [1]. Planar NFAM is an antenna measurement method using PWE []. As subjects of analysis using field conversion, the method is applied not only to radiation characteristics, but also to radio holography [3], and fault diagnosis of phased array antennas [4]. Various types of measurement error are analyzed in planar NFAM [5]. ATE due to finiteness of the measurement range is known as one kind of measurement error in planar NFAM. To suppress ATE, it is generally necessary to keep ATL to 30 db or less, i.e., to measure up to a range where the radiated electric field is sufficiently small [6]. Recently, multi-beam antennas of the array-fed reflector antenna type, whose purpose is high-capacity communications, have garnered attention as onboard satellite antennas [7]. Due to the frequency band used, a large reflector is needed, and it is difficult to evaluate the entire antenna. In this case, there may be a need to forecast the characteristics of the reflector antenna from the results of planar NFAM of the array antenna that serves as the primary feed. The ability to evaluate while isolating the primary feed and reflector is also likely to be an advantage. Planar NFAM is used because the aforementioned array antenna often has a planar aperture, and comparatively high directionality. Therefore the equipment configuration is simple, and is thought to be suitable for PWE, where the fast Fourier transform can be used. We investigate the application of PWE not only to field conversion, but also to analysis of radiation characteristics via a reflector [8]. The effect of ATE in planar NFAM on measurement results has been reported. However, the effects of ATE are not clear in this analysis method. In this paper, we present an analysis method for radiation characteristics of reflector antennas using planar NFAM measurement values for the primary feed, and use a simulation to verify the influence due to ATL on reflector antenna radiation characteristics. Analysis method The near-field measurement coordinate system is shown in Fig. 1(a). The unit vectors for each axis are u x, u y, u z, respectively. The plane where z n ¼ Z 0 that is parallel to the x n y n -plane is taken to be the near-field measurement plane. k is the wavenumber vector, and k 0 is the free space wavenumber. When the vector r is 337

3 given indicating an arbitrary observation point q on this plane, the radiated electric field E q ðrþ is expressed as in eq. (1) by using the vector plane-wave spectrum bðk t Þ. ZZ bðk t Þ¼ ejz ðe q ðrþe jk Þdxdy ð1þ r ¼ þ Z 0 u z ¼ xu x þ yu y þ Z 0 u z k ¼ k t þ u z ¼ k x u x þ k y u y þ u z qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ k 0 k x k y bðk t Þ is expressed by using the transverse vector plane-wave spectrum Bðk t Þ [4]. Bðk t Þ is given by eq. (), using the radiated electric field E t ð; Z 0 Þ expressed with the orthogonal unit vectors e 1, e in the plane where near-field measurement was conducted. bð1; k t Þ;bð; k t Þ indicate the plane wave spectrum of the E == component (TM) and E? component (TE), respectively. bðk t Þ¼Bðk t Þ jk tj ¼ bð1; k t Þk 1 þ bð; k t Þk jk tj ¼ ejz 0 X ZZ e n E tn e jkt dxdy jk tj n¼1 k 1 ¼ k t jk t j ; k ¼ u z k 1 E t ¼ E q E z u z ¼ E t1 e 1 þ E t e e 1 ¼ sin p u x þ cos p u y ; e ¼ cos p u x þ sin p u y The incident magnetic field H q ðrþ on the reflector can be found as in eq. (3). H q ðrþ ¼ 1 X Z B n ðk t Þ 1 1! k fe n ðe n k t Þe z ge jkr dk ð3þ n¼1 The reflector antenna coordinate system is shown in Fig. 1(b). In the figure, ðx n ;y n ;z n Þ is the coordinate system of near-field measurement shown in Fig. 1(a). For the point r m on the reflector surface, R is taken to be the unit vector pointing from the origin O to the observation point p. If the distance R to the observation point is sufficient, the far field radiation pattern of the reflector antenna can be found with eq. (4) by using PO [9]. Here, H q in eq. (4) can be found by repeatedly applying eq. (3) to each point r m on the reflector surface. ðþ (a) Near-field Measurement Fig. 1. Coordinate system (b) Analysis of Reflector Antennas 338

4 E s ðrþ ¼ j! e jkr Z fn H q ½ðnH q ÞRŠRge jrmr ds R S ð4þ 3 Verification The configurations of an offset parabola antenna used for verification are: aperture diameter:. m, focal length:.05 m, offset angle: 37, and subtended semi-angle: 7. For the primary feed, three cases of an ideal HE11-mode horn are assumed, where EL of the reflector is ¼ 10 db, 15 db, and 0 db. We verify the effects on reflector antenna radiation characteristics when ATL is set to 10 db, 0 db, 30 db, and 40 db. Verification is conducted by comparing the results calculated with PO only, against the results calculated using this analysis method, by using simulation to find the planar electric field distribution obtained through planar NFAM of the primary feed. The analysis frequency is 10 GHz, and polarization is vertical-polarization. Fig. shows a comparison, for the horizontal cut plane of the offset parabola antenna, of the co-polarization pattern (Co-pol.) and cross-polarization pattern (a) EL = -10dB (b) EL = -15dB Fig.. (c) EL = -0dB Comparison of radiation pattern with ATL 339

5 (a) 3dB-Beam width (b) 1 st -Sidelobe Level (c) Cross-polarization Level Fig. 3. Comparison of near-axis s radiation characteristics with ATL (X-pol.) using this analysis method and PO. From Fig. it can be confirmed that, if for both Co-pol. and X-pol. ATL ¼ 0 db or less in the case when EL ¼ 10 db, or if ATL ¼ 30 db or less in the case when EL ¼ 15 db or 0 db, then there tends to be a good agreement of radiation patterns. Fig. 3 is a comparison of nearaxis s radiation characteristics for a 3 db-beamwidth, 1 st -sidelobe level, and peak value of X-pol. For the 3 db-beamwidth indicated in Fig. 3(a), if it is assumed that ATL ¼ 0 db or less, then there is a match with PO results regardless of EL For the 1 st -sidelobe level in Fig. 3(b), if EL ¼ 0 db, then the difference with PO is 3 db when ATL ¼ 0 db, and 0. db when ATL ¼ 30 db. For the peak value of X-pol. in Fig. 3(c), the difference with PO is 0. db or less if ATL ¼ 0 db or less, regardless of EL. Based on the above, a satisfactory evaluation is likely possible for near-axis s radiation characteristics of Co-pol. when EL is high, even if ATL ¼ 0 db. If the measurement range becomes smaller, this may also lead to reduction of measurement time. On the other hand, when EL is low and evaluation is done up to a low level of Co-pol. and X-pol., then a sufficient measurement range up to ATL ¼ 40 db or less will likely be necessary, which is lower than the typical ATL analyzed with planar NFAM. 4 Conclusion We investigated the analysis method for radiation characteristics of reflector antennas using measurement values from planar NFAM of the primary feed, and clarified the influence of ATL on radiation characteristics by simulation. Prediction of the characteristics of an entire antenna based on the results of planar NFAM of the primary feed can be regarded as an effective means of evaluation for the multibeam reflector antennas for installation in satellites which have garnered attention in recent years, because this method can also be used for analysis of reflector antenna manufacturing error and thermal deformation error. 340