ANTENNA GAIN EVALUATION BASED ON WEIGHTING NEAR-FIELD MEASUREMENTS

Transcription

1 Forum for Electromagnetic Researc Metods and Application Tecnologies (FERMAT) ANTENNA GAIN EVALUATION BASED ON WEIGHTING NEAR-FIELD MEASUREMENTS Liliana Ancidin (1,), Razvan D. Tamas (1,), Adrian Androne (), and George Caruntu (1) (1) Department of Electronics and Telecommunications, Constanta Maritime University, C.P., O.P., 978 Constanta, Romania () Doctoral Scool of Electronics and Telecommunications, Politenica University of Bucarest, Bd. Iuliu Maniu 1-3, Sect. 6, 6171, Bucarest, Romania Abstract Antenna gain is generally extracted from near-field measurements wen te size of te measuring site (anecoic camber or open area test site) does not meet te far-field constraints tat include bot distance and probe size limitation. Near-field measurements are usually performed by scanning te field on a closed surface around te antenna wit a small size probe. In tis paper, we sow tat te distance averaging metod tat we ave previously proposed for gain evaluation in a multipat environment can also be employed for measurements in te near-field zone. We introduce an alternative tecnique to extract te gain by applying weigting functions on te near-field data wen te size of te probe cannot be neglected compared to te wavelengt. We validated our metod by comparing te results to te gain measured at distances in te far-field region. Keywords Antenna gain; near-field measurements; weigting functions; averaging metod *Tis use of tis work is restricted solely for academic purposes. Te autor of tis work owns te copyrigt and no reproduction in any form is permitted witout written permission by te autor.* 1

2 Liliana Ancidin was born in Dragoiesti, Romania, on September 9, Se is a P.D. student wit te Doctoral Scool of Electronics and Telecommunications at Politenica University of Bucarest, Romania. Se is currently an assistant professor wit te Electronics and Telecommunications Department at Constanta Maritime University. Her field of interest includes antenna teory, more specifically near-field caracterization tecniques. Liliana Ancidin is a Graduate Student Member of te IEEE. Razvan D. Tamas was born in Navodari, Romania, on May 1, 197. He received te P.D. degree jointly from Politenica University of Bucarest, Romania, and from Institut Polytecnique de Grenoble, France, and te Habilitation degree (Habilitation à Diriger des Recerces) from Institut Polytecnique de Grenoble, France. He is currently a Professor, ead of te Electronics and Telecommunications Department at Constanta Maritime University, Romania, and a P.D. supervisor wit te Doctoral Scool of Electronics and Telecommunications at Politenica University of Bucarest, Romania. He was formerly a Senior Researcer wit te Navy Scientific Researc Center of te Military Equipment and Tecnology Researc Agency, Romania. He also worked wit te Institute of Microelectronics, Electromagnetism and Potonics of Grenoble, France on te R.N.R.T. AUBADE project and e was an Invited Professor wit Université de Rennes 1 and Université Josep Fourier, France. His field of interest includes antenna teory, numerical metods for electromagnetism, time-domain caracterization, electromagnetic compatibility, and naval communication systems. Dr. Razvan D. Tamas is a Senior Member of te IEEE. He founded te IEEE AP-S Capter in Romania and e currently serves as a Cair.

3 Adrian Androne is a P.D. student wit te Doctoral Scool of Electronics and Telecommunications at Politenica University of Bucarest, Romania. His field of interest includes antenna teory, more specifically frequency-selective surfaces. George Caruntu was born in Constanta, Romania, on July 6, He received te P.D. degree from Politenica University of Bucarest, Romania. He is currently a Professor wit te Electronics and Telecommunications Department at Constanta Maritime University, Romania. His field of interest includes transducers and measurement tecniques. Dr. George Caruntu is a Member of te IEEE. 3

4 I. INTRODUCTION Gain measurements are usually performed by placing a calibrated probe as far from te antenna under test, as far-field constraints would be fulfilled. Compliance wit tat limitation migt not be possible wen large antennas are measured. Most of te measuring metods [1], [], [3] use small probes placed on a surface surrounding te antenna under test, and near-field to far-field transforms are applied on te measured data. We ave previously presented [4], [5], [6] a metod for antenna gain evaluation in a multipat environment, based on averaging data measured at different distances between te probe antenna and te antenna under test. Weigting on measured data was solely used to compensate te effects of te propagation (i.e., delay and attenuation), as it would be in te far-field region. In tis paper, we sow tat distance averaging can successfully be applied on a data set entirely acquired in te near-field zone wit a probe of a size comparable to te wavelengt. An alternative tecnique for processing near-field data, based on weigting functions, is proposed. We firstly set up a far-field limit for transmission between two antennas of finite size. Ten we define te weigting functions to be applied on te measured, near-field data in order to remove te effect of te pase deviation between different pairs of source points and field points. Te metod was validated by comparing weigted data measured in te near-field region to data measured on a set of distances mostly in te far-field region. 4

5 II. NEAR-FIELD TRANSMISSION BETWEEN TWO FINITE SIZE ANTENNAS Probe antenna: transmitting antenna Z 1 Z Antenna under test: receiving antenna Let us consider te transmission between two linear dipoles of a total lengt 1 and, respectively (Fig. 1). Gt I1(z ) O -1 θ R d E1,θ (z) z O - I(z) Gr Fig. 1 Near-field transmission between two linear dipoles II.1 Revision of te far-field limit for transmission between two antennas of finite sizes Te maximal pase deviation between two waves incident on te receiving antenna occurs wen te field point is located on its top i.e., z =, and te source points are located one at te same eigt as te field point and te oter one at te bottom end of te transmitting antenna, respectively; tat is, z 1 = and z = 1. By assuming tat d 3( 1 + ) te pat lengt difference between te two incident waves can be expressed as: ( 1 + ) Rmax d. d As for a single antenna te maximal pase deviation can be set at π/8 [7] as a reasonable error margin. It comes out tat te far-field range for te configuration given in Fig. 1 is ( + ) 8 1 d λ 5

6 We define weigting functions tat near-field measurements sould be multiplied by in order to assess te far-field. For a given direction one can define on bot receiving and transmitting antennas constant, equivalent current distributions along te main axis of polarization. Te weigting functions can be derived by expressing te normalized voltage at te output of te receiving antenna from te mutual impedance [8], as follows V, norm d = d exp( jk II. Definition of weigting functions for near-field data d) V exp θ [ jk ( R d) ] 1 exp[ jk ( R d) ] 1 3 I1( z') I ( z)sin dz dz' d I1( z') I ( z) - = V R 1 = lim d exp( jkd) V I1( z') I ( z)dz dz',, norm d -1 R dz dz', V F( f, d) = V, norm, norm = d exp R [ jk ( R d) ] 3. dz dz' 6

7 II.3 Definition of te average transfer function and AUT gain extraction An average transfer function can be calculated from te scattering parameters measured at different distances between antennas: S 1 d N n 1 = S1, n F( f, dn) N n d were d n is a set of N distances, d is te reference distance (usually set at 1 m), and S 1,n are te corresponding transfer functions. Te AUT gain can ten be extracted as G r = 1 G t 4πd λ R S 1 ( 1 11 ) Ra 1 S S wit R te normalizing impedance and R a te real part of te input impedance of te antenna under test. 7

8 III. EXPERIMENTAL RESULTS We consider a typical setup consisting of an antenna under test, a probe antenna, and a vector network analyzer. Measurements were performed in a reflection free environment in order to only investigate te impact of te near-field zone. As an antenna under test, we took a monopole on a square ground plane. Te monopole was 8.4 cm ig and te side of te ground was 1 cm long (Fig. a). Te antenna under test resonates around 8 MHz and.4 GHz. A calibrated, biconical dipole was employed as a probe antenna (Fig. b). Te total eigt of te biconical dipole was 1 =14 cm. By taking into account a full size image for te monopole under test te far-field region at 1GHz starts at 68 cm between antennas. a) b) Fig. Experimental setup: antenna under test (a), and probe antenna (b) 8

9 We performed two sets of measurements: one for distances between antennas ranging from 1 to 14 cm, and te oter one for distances comprised between 15 and 6 cm. Te distances in te first set are all in te farfield zone for frequencies up to 1.6 GHz; te second measuring range is completely below te far-field limit at 1 GHz and above. Te weigting functions for bot sets are sown in Fig. 3 and Fig. 4, respectively. -.3 F(f,d) [db], d=1 to 14 cm 3 F(f,d) [db], d=15 to 6 cm Frequency [GHz] Fig. 3 Weigting functions for d=1 to 14 cm Frequency [GHz] Fig. 4 Weigting functions for d=15 to 6 cm For distances in te first set (1 to 14 cm) and for a frequency range of.85 to GHz te magnitude deviation between any two weigting functions does not exceed.1db. Conversely, te weigting functions for te second set spread over 3 db for different distances. 9

10 In Fig. 5 we sow te measured, normalized transfer factor for te set of 5 distances ranging from 1 to 14 cm. Te reference distance tat was set at 1 m. Fig. 6 sows te normalized, transfer factor measured for 1 equally spaced distances between 15 and 6 cm. (d/d ) S 1 [db] (d/d ) S 1 [db] Average over weigted data Average over weigted data Frequency [GHz] Fig.5 Normalized, transfer factors for d=1 to 14 cm Frequency [GHz] Fig.6 Normalized, transfer factors for d=15 to 6 cm 1

11 Te gain figures resulting from te far-field set and from te near-field set are compared in Fig. 7. An unweigted average on te near-field data is also given, so as to observe te impact of te weigting on te accuracy. Table I gives te accuracy improvement defined as a difference between te relative error witout weigting and te relative error wit weigting; te far-field gain was set as a reference for computing bot error figures. GAUT [dbi] Measured, far-field Measured, near-field w/o weigting Measured, near-field wit weigting Frequency [GHz] Fig. 7 Gain of te antenna under test Freq. [GHz] Gain, far-field [dbi] Gain, near-field [dbi] w/o weigting w. weigting Accuracy improvement [%] Table I. Accuracy improvement on gain evaluation It comes out tat weigting on te near-field data results in improving te accuracy of gain measurements by.5 db for frequencies between 1 and GHz. 11

12 CONCLUSION Our metod can also be used for radiation pattern measurements wit an appropriate two-axis rotator for te antenna under test. Te distance averaging can be employed as a valid approac for gain measurements witin a distance range entirely below te far-field limit. By applying te weigting functions tat we introduced in tis paper on measured, near-field data one can accurately assess te gain of an antenna, provided tat te probe size is comparable to te wavelengt. Our study was focused on antennas operating at around 1 GHz. Te gain could be evaluated for frequencies of up to GHz, based on measurements performed witin a distance range of 15 to 6 cm, instead of placing te probe at above 1 cm as te fulfillment of te far-field constraints would require. Furtermore, if one would need to caracterize antennas operating at around 1 MHz by applying our tecnique te measurements migt be performed witin a distance range of 1.5 m to 6 m. 1

13 REFERENCES [1] A. D. Yagjw, An Overview of Near-Field Antenna Measurements, IEEE Transaction on Antenna and Propagation, vol. 34, pp.3-45, January [] R. C. Jonson, H. A. Ecker, J. S. Hollis, Determination of Far-Field Antenna Patterns From Near-Field Measurements, Proceedings of te IEEE, vol. 61, pp , December [3] R. C. Wittmann; B. K. Alpert; M. H. Francis, Near-field antenna measurements using nonideal measurement locations, IEEE Transaction on Antenna and Propagation, vol. 46, pp.716-7, May [4] R. D. Tamas, D. Deacu, G. Caruntu, T. Petrescu, An Indoor Measuring Tecnique for Antenna Gain, IEEE International Worksop on Antenna Tecnology, pp. 19-, Marc 13. [5] R. D. Tamas, D. Deacu, G. Vasile, C. Ioana: A metod for antenna gain measurements in nonanecoic sites, Microwave and Optical Tecnology Letters, vol. 56, pp , July 14. [6] R. D. Tamas, New Advances in Gain Measurements in Non-Anecoic Sites: Application to Narrow Band Monopoles, IEEE International Worksop on Antenna Tecnology, pp. 6-63, Marc 16. [7] C. A. Balanis, Antenna Teory Analysis and Design, 3rd edition, Wiley, New York, 5, pp , [8] J. Ricmond, N. Geary, Mutual impedance between coplanar-skew dipoles, IEEE Transaction on Antenna and Propagation, vol. 18, pp , May