Proceedings of 2010 IEEE International Conference on Ultra-Wideband (ICUWB2010)

UWB Cross-Polarization Discrimination with Differentially Fed, Mirrored Antenna Elements Grzegorz Adamiuk, Student Member, IEEE, Lukasz Zwirello, Student Member, IEEE, Lars Reichardt, Student Member, IEEE, and Thomas Zwick, Senior Member, IEEE

Abstract—The paper describes a principle of broadband discrimination of cross-polarization in linearly polarized radiators. The cross-polarization suppression is achieved by the application of two differentially fed mirrored antenna elements. It is shown, that in such configuration the cross-polarized components radiated from both elements are out-of-phase and annihilate each other, whereas the co-polarization components are in-phase and interfere constructively in the far field of the radiator. For the verification of the idea the prototype of the single antenna with strong cross-polarized components is designed. Next an array comprising differentially fed, mirrored antenna elements is presented. The measurements show a significant reduction of the undesired cross polarization radiated from the antenna and simultaneous enhancement of co-polarization. The idea presented in this paper can be successfully applied in the design of UWB dual-polarized antennas.

the radiators, the E-field vectors are mirrored as well. This causes different influence of the signal interference, radiated from both elements, on the co-and cross-polarization. In order to suppress the cross-polarized components and enhance the co-polarization, the orientation of the second e-field vector must be inverted, which is realized by a differential feeding of the antennas. Such configuration is presented in Fig. 1(b). The advantage of such configuration is, in ideal case, a complete annihilation of the y-components of the electric field (x-pol) in the whole xy-plane. The z-components (co-pol) remain inphase and are radiated constructively.

Index Terms—Antennas, Arrays, UWB, polarization diversity.

I. I NTRODUCTION An application of polarization diversity in the UWB technology can significantly enhance the system performance. The advantages are e.g. higher data rates in communication systems or more detailed information about the target in radar/imaging [1]. It was shown by the authors in [2] and [3] that a differential feeding introduces an outstanding solution for the realization of UWB, dual-orthogonal-linearly polarized antennas. The presented radiators maintain such advantages like high polarization purity, same phase center of radiation for both polarizations, similar radiation characteristics in Eand H-planes. This papers focuses on the explanation of the functionality of the antennas basing on geometrical distribution of the field radiated from single antenna elements. II. A RRAY C ONFIGURATION Assumed is an arbitrary UWB antenna in Fig. 1(a) which radiates in positive x-direction. The desired polarization is linear and oriented parallel to the z-axis. The antenna possesses a non-ideal polarization marked in the figure with the continuous line, which is distributed into co- and crosspolarized components. A trivial solution of mechanical turn of the antenna cannot be applied, since the polarization may turn with the frequency and such solution is not robust against an elliptical polarization. For that purpose a second mirrored antenna is applied. Through the symmetrical arrangement of The Authors are with the Institut fuer Hochfrequenztechnik und Elektronik (IHE), Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131, Karlsruhe, Germany, email:[email protected]

978-1-4244-5306-1/10/$26.00 ©2010 IEEE

(a) Single antenna

(b) Array configuration mirrored elements

with

Fig. 1. Arbitrary antennas (grey figures) and schematical distributions of radiated electric field vectors (continuous line) into co-polarized (dashed line) and cross-polarized (dotted line) components

In the pulse-based UWB (IR-UWB) Systems a stable phase center of radiation over frequency is of big interest. The variation of it causes temporal distortion of the pulse, which results e.g. in higher inter-symbol-interference in communication systems or reduction of the resolution in radar [4]. The here presented array configuration allows for the reduction of variation area of the phase center. It is firstly assumed that the phase center of radiation in an arbitrary antenna varies with the frequency. This is schematically introduced in Fig. 2(a). Through the symmetrical arrangement of the second radiator, its phase center varies also symmetrically to the one of the first element. The resulting phase center of radiation of an antenna array is placed in the middle between the contributing phase centers. Hence it is clear from the Fig. 2(b) that the resulting phase center of the configuration is compensated along the

z-axis. Thus the further advantage of the configuration is revealed - partial compensation of the distorting properties of the single radiator. A mirrored arrangement of antennas for cross-polarization suppression has been already mentioned in some literature [6], [5]. The works focused however on narrowband systems and did not reveal the advantage of the principle for UWB frequency range - frequency independent behavior and stabilization of the phase center.

(a) Single antenna

(b) Array configuration with mirrored elements Fig. 3.

Prototypes for the verification of the concept

(a) Single antenna

(b) Array configuration mirrored elements

HArray (f, θ, ψ) =

with

Fig. 2. Schematical distribution of phase centers of radiation fo arbitrary antennas (grey figures) and frequencies

 

III. M ATHEMATICAL DESCRIPTION

−H 1,ψ (f, −θ, ψ)uψ 

OF RADIATION

In the following the antenna placed in the positive z-values is indexed antenna ”1” and the one in the negative - antenna ”-1”. The feeding voltages of both radiators are differential to each other and are described by: 1 1 U 1 (f ) = √ and U −1 (f ) = √ ej(π) . 2 2

H 1,θ (f, −θ, ψ)uθ

(1)



H 1,θ (f, θ, ψ)uθ H 1,ψ (f, θ, ψ)uψ



 · √1 ej 12 β0 d(f ) cos(θ) + 2

(3)



 · √1 e−j 21 β0 d(f ) cos(θ) 2

Remarkable is the same leading sign in front of the transfer functions at the θ-components and different at the ψ-terms. This implies different impact of the array configuration on the both polarizations - constructive for θ-components and destructive for ψ-components (regarding the direction (θ, ψ) = (90◦ , 0◦ )). In the following the prototypes are introduced and the principle is verified by the measurements.

The complex transfer function of the array HArray comprising differentially fed, mirrored radiators yields

HArray (f, θ, ψ) = H−1 (f, θ, ψ) · U −1 (f )e−jβ0 (−1) +H1 (θ, ψ) · U 1 (f )e−jβ0 (1)

d(f ) 2

d(f ) 2

cos(θ)

cos(θ)

(2) , where H−1,1 are complex transfer functions of the respective antennas, d(f ) is the frequency dependent distance between the phase centers of the antennas and β0 is a free space propagation constant [7]. The distribution of the transfer functions into their polarization components (θ- co-pol, ψ- x-pol) and including the property of symmetry in the spherical coordinate system, the Eq. 2 yields

Fig. 4. Orientation of the array configuration with mirrored elements in the coordinate systems.

IV. V ERIFICATION For the verification of the principle a planar elliptical antenna (also known in the literature as diamond-shaped antenna or volcano-smoke antenna) optimized for the FCC frequency range form 3.1 GHz to 10.6 GHz is used [8]. The antenna radiates in co-polarization with two beams symmetrical to the antenna surface. In the H-plane of the antenna strong crosspolarization components are present. This plane is used for the verification of x-polarization suppression capability of the proposed arrangement. The model of the designed prototype with feeding network is presented in Fig. 3(a). The schematics of an array consisting of two elliptical antennas with feeding network is shown in Fig. 3(b) and the orientation of it in the coordinate system is depicted in Fig. 4. The E-Plane of the antenna is defined for the angle (θ, ψ = 90◦ ) and the H-plane for (θ = 90◦ , ψ)). As already mentioned, the antenna maintains strong x-polarization components in the H-plane. For that reason an symmetrical arrangement w.r.t. the ”y”-axis is performed in order to suppress the undesired components in the respective plane. Due to the lack of symmetry of the antenna along z-axis, the phase center of radiation changes its position along the respective z-axis. Due to the symmetry of the antenna along y-axis, the phase center of radiation does not change its position in the respective direction. In such case the varying phase centers of the single antennas are compensated by the array arrangement and the resulting phase center of radiation is in the origin of the coordinate system. The feeding network of the array must perform a 3 dB signal division in order to feed the single elements. For that purpose a T-junction is used (see Fig. 3(b)). The microstrip lines are formed to twin lines and the differential feeding is obtained by a respective soldering of the outputs of the feeding network to the respective parts of the elliptical antennas. In the literature such configuration has been presented recently [9], [10]. The authors however did not considered the cross-polarization properties of the arrangement and advantages regarding the stabilization of the phase center. Hence the advantage of the configuration in UWB polarization-diversity systems has not been sufficiently revealed. In Fig. 5(a) and Fig. 5(b) the simulated and measured crosspolarized gain in the H-plane of the single antenna from Fig. 3(a) are presented, respectively. The antenna radiates strong cross-polarized components in the plane, which makes the radiator unusefull in polarization diversity systems. The creation of the lobes is observed over very wide frequency range and the measurement results comply very well with the simulated ones. The simulated and measured results of the array in the same plane are presented in Fig. 5(c) and Fig. 5(d), respectively. A clear suppression of cross-polarization by the presented array is observed. The creation of four lobes is avoided over large frequency range. In the high frequency range some strong cross-polarized components are present. The simulation shows that the radiation is caused mainly by the polarization rotation during the scattering of the radiated signal at the feeding network. This is also the reason for the occurrence of the

components mainly in the angle range −90◦ > ψ > 90◦ (halfplane containing feeding network). The measured co-polarized gain for the single antenna and antenna array in the H-plane are shown respectively in Fig. 6(a) and Fig. 6(b). It is observed that the co-polarization, in opposite to the x-pol, is radiated constructively with higher gain w.r.t. the single element. At the same time the radiation pattern (apart form the amplitude) remains nearly unaffected.

(a) Single antenna

(b) Array configuration with mirrored elements Fig. 6. Measured gain GH−P lane,Co−P ol (f, θ = 90◦ , ψ) in the copolarization for the H-Plane over frequency f

V. C ONCLUSIONS In this paper an explanation of the principle for the broadband suppression of the cross-polarization in the linearlypolarized, mirrored antennas has been described. The successful discrimination of cross-polarization and simultaneous enhancement of the co-polarization are verified by the measurements. The concept is also able, due to the reduction of the variation area of the phase center, to compensate the distorting properties of the radiators. By an addition of a second pair of radiators rotated by 90◦ a dual-polarized radiator with the same phase center of radiation for both polarizations is

Fig. 5.

(a) Single antenna, simulation

(b) Single antenna, measurement

(c) Antenna array, simulation

(d) Antenna array, measurement

Simulated and measured gain GH−P lane,X−P ol (f, θ = 90◦ , ψ) in the cross-polarization for the H-Plane over frequency f

feasible. Hence the principle introduces a solution for the realization of planar, dual-polarized UWB radiators, which can be successfully applied in e.g. UWB-Radar or UWB-MIMO. ACKNOWLEDGMENT The authors would like to thank to the German Research Fundation (DFG) for the financial support. R EFERENCES [1] R. Zetik, J. Sachs, R. Thomae, ”UWB Short Range Radar Sensing” IEEE Instrumention and Measurement Magzine, vol. 10, pp. 39 - 45, April 2007 [2] G. Adamiuk, W. Wiesbeck, T. Zwick, ”Differential feeding as a concept for the realization of broadband dual-polarized antennas with very high polarization purity”, Antennas and Propagation Society International Symposium, 2009. APSURSI ’09. IEEE, 2009, pp. 1 -4 [3] G. Adamiuk, S. Beer, W. Wiesbeck, T. Zwick, ”Dual-Orthogonal Polarized Antenna for UWB-IR Technology”, Antennas and Wireless Propagation Letters, IEEE, 2009, vol. 8, pp. 981-984 [4] W. Soergel, W. Wiesbeck, ”Influence of the Antennas on the Ultra Wideband Transmission”, EURASIP Journal on Applied Signal Processing, special issue UWB - State of the Art, pp. 296 305, March 2005 [5] P. Hall, ”Dual polarisation antenna arrays with sequentially rotated feeding Microwaves”, Antennas and Propagation, IEE Proceedings, 1992, vol. 139, pp. 465 -471

[6] J. Granholm, K. Woelders,”Dual polarization stacked microstrip patch antenna array with very low cross-polarization”, IEEE Transactions on Antennas and Propagation, 2001, vol. 49, pp. 1393 -1402 [7] K. Woelder, J. Granholm, ”Cross-polarization and sidelobe suppression in dual linear polarization antenna arrays”, IEEE Transactions on Antennas and Propagation, 1997, vol. 45, pp. 1727 -1740 [8] J.-Y. Tham,B. L. Ooi, M.S. Leong, ”Diamond-shaped broadband slot antenna”, IEEE International Workshop on Antenna Technology: Small Antennas and Novel Metamaterials, IWAT, 2005, pp. 431 - 434 [9] J. Powell, A. Chandrakasan, ” Differential and single ended elliptical antennas for 3.1-10.6 GHz ultra wideband communication”, Antennas and Propagation Society International Symposium, 2004. IEEE, 2004, vol. 3, pp. 2935 - 2938 [10] N. Telzhensky, Y. Leviatan, ”Planar Differential Elliptical UWB Antenna Optimization”, IEEE Transactions on Antennas and Propagation, 2006, vol. 54, pp. 3400 -3406 [11] A. Narbudowicz, G. Adamiuk, W. Zieniutycz, ”Clover array — polarisation diversity solution for ultra wideband systems”, Progress In Electromagnetics Research Letters, 2009, vol. 10, pp. 163-170