Research On Polarization Characteristics Of Heterogeneous Array And Its Applications

The high integration and performance improvement of modern radio communication equipment brings new challenges to the signal interception in high frequency(HF)communications.For a long time,the signal intercepting system and direction finding system in HF communications are independent of each other.Due to different functional requirements on the interception antenna array and the direction finding antenna array,it is difficult for the two different systems to share a same antenna array,and achieving a reconciliation of intercepting and direction finding in one system.The reconciliation requires high integration of the intercepting system and direction finding system,enabling intercept-and-detect and same source data between the two systems,which can greatly improve the reconnoitre efficiency.However,there are some difficulties when using the interception antenna with high directivity(such as the log-periodic antenna)as a direction finding antenna.In order to achieve a wide signal coverage area,the directional antenna elements in an antenna array must point to different directions and have different polarizations as well.In this case,the antenna array is called a heterogeneous array.In some other applications such as direction finding,the antenna elements of an antenna array in traditional DF systems are required to have same orientation.The antenna array in this case is often referred to as a homogeneous array.Therefore,here comes a technical contradiction: is it possible for a heterogeneous array in which each antenna element has different orientation to be used as a direction finding array.In order to realize a high performance system with the function of both intercepting and direction finding,the first problem to be solved is the application of the heterogeneous array in such a system.In this paper,the characteristics of the heterogeneous array and its practical applications in direction finding systems are fully described.Then,a high performance heterogeneous direction finding array is developed for system applications and is validated experimentally,laying a physical basis for future research of intercepting-and-direction finding systems.The main contributions of this paper are summarized as follows: 1.Based on the horizontally polarized dipole,the spatial polarization characteristics of the horizontal electric dipole are studied.In addition,the mechanism of polarization sensitivity in a heterogeneous array composed of horizontal electric dipoles is also revealed through mathematical derivations.It is found that 1)the spatial polarization characteristics of the horizontal electric dipole vary with the spatial observation angles;2)the horizontal electric dipole does not exhibit the linear polarization state at all observation angles.The state of linear polarization can be only observed at the maximum radiation angle.However,the polarization of this dipole antenna turns into elliptical state when the observation is off the maximum radiation angle.2.The general formulas of the heterogeneous direction-finding array placed in free-space and on earth surface are derived in detail.To make it adoptable in the analysis of the polarization characteristics of the heterogeneous array,a modified MUSIC algorithm on direction finding,Pol-MUSIC,is proposed,through which the direction finding is not needed in the polarization space and the direction of signals with unknown polarization parameters can be obtained by spatial search.The simulated results show that the proposed algorithm exhibits good robustness and accuracy.It is also proved that the spatial correlation in the heterogeneous array is weak and therefore the array is able to discriminate two signals from almost the same angle.In addition,it is demonstrated that the heterogeneous array shows better angular resolution than the homogeneous array.This is because that the heterogeneous array has different array responses to signals with different polarizations.The direction finding errors for the physical ground surface with non-uniform parameters are also evaluated.3.A heterogeneous array composed of 24 HF horizontal log-periodic dipole antennas(LPDA)with uniform distribution is designed for the application of direction finding.A novel method of designing a log-periodic antenna is presented.Different from the Carrel's method,the proposed method manipulates the moving speed of the active radiation region by continuously changing the scale factort and spacing factor s of each pair of dipole in a LPDA,effectively extending the distance between the phase center and the feeding point of the antenna under the condition of fixed antenna length.Thus,the effective radius of a circular heterogeneous array formed by the LPDAs is increased correspondingly,which is a very important benefit for the direction finding antenna.Besides,by introducing inductance loading the lateral length of the LPDA are reduced,resulting a reasonable baseline distance to achieve a more compact inward circular heterogeneous array.Each LPDA element in the array is oriented to different orientation to achieve an effective coverage of 0-360°.The experimental results show that the VSWR of a LPDA element is less than 2.0 over the frequency range of 2MHz-30 MHz and the maximum gain is13 d Bi.The amplitude and phase pattern are in good agreement with the theoretical calculation.4.Based on the circular heterogeneous LPDA array,a phase center measurement method for LPDA is proposed.The accurate phase center position is obtained by the least square method with the phase differences between LPDA elements.The phase differences are extracted by utilizing a multi-channel receiver.This method can be used in short-wave antenna out-door measurement.The measured phase center of the LPDA agrees well with the calculated ones,which effectively validates the accuracy of the proposed measurement method.5.The performances of the existing methods of beam synthesis based on phase weighting are theoretically analyzed.It is mathematically proved that these methods are only applicable to the homogenous array formed by the elements having the same polarization.However,for the horizontal LPDA circular array,the existing phase weighting methods exhibits a limited benefit.They can only work well when the horizontal polarization components dominate the incident waves.In view of this,a joint phase weighting method based on the polarization and spatial domain is proposed.The simulated results show that the joint weighting method can enhance the performance of the beam synthesizer.6.The simulated and experimental results demonstrate that the traditional MUSIC algorithm results in instability of the observation direction and large errors at low frequencies for the circular heterogeneous LPDA array.This is mainly because of the polarization effect existed in the circular heterogeneous array.At low frequencies,the spatial polarization varies dramatically with the observation angles due to strong couplings within the heterogeneous array.By applying the modified Pol-MUSIC algorithm,the performance of direction finding can be greatly improved.This suggests that the circular heterogeneous LPDA array which is an interception antenna array can be also used in the direction finding systems,making it possible for realizing a high performance system with the function of both intercepting and direction finding.