The Design Of A Monopulse Antenna And The Feeding Network

As for the requirement of tracking directional accuracy in radar is increasing now,the monopulse technology as a kind of precise direction tracking technology,has been applied widely.According to the theory of circular array antenna,a dual-circularly polarized monopulse receiving antenna with the broadband frequency range of 0.9GHz-6GHz had been designed.We also designed the broadband passive feeding network.Firstly,a monopulse antenna array is designed based on uniform circular array in this thesis.The uniform circular array has an excellent performance in realizing the wide-band sum mode difference mode and dual-circular polarization.The antenna unit used a variant structure of the vivaldi mode with ultra-wideband performance.And the antenna unit has a good standing-wave ratio at 700MHz-8GHz.For forming circular polarization,the number of elements of the circular array must be more than 6.The more elements,the better axial ratio of the circular polarization.Taking the complexity of the network into account,we chose an 8-cell array.The simulation results in HFSS proved the design project was credible.The measured VSWR of each unit of the fabricated antenna array less than 2 within the range of 0.8GHz-8GHz over the relative frequency of 170%.Meanwhile,the sum-mode radiation pattern are deformed when the frequency greater than 8 GHz.In the simulation of the antenna array,the peak gain ranges from 7.7db to 11 db.The zero depth of the difference pattern is less than-30 db.The HPBWs of 1GHz,2GHz,4GHz and 6GHz are 84°,82°,68° and 32° respectively.The HPBWs of the entire range of 0.8GHz-8GHz is maintained above 30 degrees.Next,the thesis designs the wideband sum-difference beam forming network for the antenna array.Because of the bandwidth limitationthe of the boradband passive devices,the entire network is divided into two sub-networks of 0.9GHz-2.35 GHz and 2.35GHz-6GHz by using the duplexer.In the network,four kinds of equal amplitude signals with phase difference of ±45° and ±90°are formed.The network consists of 90-degree phase-shifter and 3d B hybrid.The 90-degree phase-shifter uses a weakly parallel coupling line and loading a quarter wavelength short circuit at the end of the coupling line.This structure ensures the precision of broadband phase shift while miniaturizing the device.The 3d B bridge is composed of two cascaded-8.34 db multi-node coupler with cross coupled stripline structure.Because the devices need to be cascaded,their reflection coefficient should satisfy S11?-20 db,the phase error of the phase shifter should less 4 degree,the amplitude error of the coupler is less than 0.3db at low frequency and less than 0.2d B at high frequency.In the entire network,the output amplitude error of each port is within 2d B at low frequency and 1d B at high frequency.The phase error is within ±10°.Compared with other feeding networks,this passive feeding network achieves 2.5:1 bandwidth.Finally,all the ports of the two feeding networks at different frequency are cascaded through the duplexer,then they are sequentially connected with the antenna unit to form the monopulse antenna system.It is measured that the reflection coefficients of each port are below-10 d B,forming a relatively stable sum-mode and difference-mode pattern.However,the radiation pattern of the antenna shows a certain deviation in the direction of maximum gain relative to the antenna axis in the sum-mode pattern.In the difference-mode pattern,it presents the zero offset and the zero value uplift.