Frequency Selective Surface Miniaturization Technology And Its Application In Antenna Design

Frequency selective surface is an artificial surface formed by arranging the sub-wavelength unit structure aperiodically or periodically on a two-dimensional planed/curved surface.It shows different filtering characteristics for the frequency,polarization state and incident angle of incident wave.Therefore,it is widely used in many fields,such as antenna technology,stealth technology,microwave and optoelectronic devices.This dissertation mainly focuses on the miniaturization design method of frequency selective surfaces and the application of these frequency selective surfaces in ultra-wide band antenna gain enhancement,compact array antenna decoupling and broadband radar cross section reduction for microstrip antennas.The main research contents and contributions of this dissertation are as follows:1.Based on the design method of single-/double-side printed unit with branch structures,two different miniaturized two-dimensional band-stop frequency selective surfaces are proposed for ultra-wideband antenna gain enhancement.The first structure adopts the branch-loading method,and the designed unit size is 8.25 mm×8.25 mm,which is about 1/12 of the wavelength corresponding to the lowest operating frequency.The second structure uses a single-layer doublesided printing of similar metal units to achieve a miniaturized frequency selective surface design.The unit size is 6.25 mm×6.25 mm,which is about 1/16 of the wavelength corresponding to the lowest operating frequency.The dispersion diagram is used to evaluate the overall performances of the structures.The two designed structures have good angle and polarization independence from 0 to 60°,and the relative operating bandwidths are 122%and 138%,respectively.The working principle of the structure is explained through the equivalent circuit model and lumped parameter calculation.The designed antenna and frequency selective surface were processed and tested.Without affecting the bandwidth of the antenna,the frequency selective surface reflector increased the gain of the original monopole antenna from 2-6 dBi to 8-9 dBi and 7.5-8.5 dBi,respectively.The in-band gain fluctuation is reduced from the original 4 dB to 1 dB.The designed antenna and frequency selective surface are processed and measured,and the measured results are in good agreement with the simulation results.2.A design method of 2.5D frequency selective surface(denoted as 2.5D FSS)based on metallized vias is proposed,based on which a miniaturized 2.5D FSS is designed for ultrawideband antenna gain enhancement.The equivalent inductance and capacitance of the metallized vias are used to change the parameter distribution of the structure and reduce the resonant frequency of the unit.The size of the designed unit structure is only 4.2 mm × 4.2 mm,less than 1/20 of the wavelength corresponding to the lowest working frequency.Compared with the above two-dimensional planed structures,namely FSS1 and FSS2,the unit size is reduced by about 50%and 33%respectively.At the same time,the designed 2.5D FSS structure has excellent polarization stability and angular stability.Under TE polarization and TM polarization,the incident angle is within the range of 0°-80°,the resonant frequency changes are only 0.008 f0 and 0.015 f0.Using this 2.5D FSS as the reflector of ultra-wideband printed monopole antennaz,one can effectively make use of its band-stop filtering characteristics and the linearly-varying phase frequency characteristics to improve the gain from 4.3 dBi to 8.0 dBi and its flatness from 3.0 dB to 1.0 dB,reduce the backward radiation in the whole working frequency band and enhance the directivity of the antenna.3.In order to solve the problem of compact array coupling,two different types of frequency selective surface decoupling structures are designed according to the principle of field cancellation,which are the patch type decoupling structure and the slot type decoupling structure.The field distribution generated by the patch-type decoupling structure cancels the field distribution between the original antennas,blocking the propagation of the field between the antennas,and the isolation between the antennas is increased by 36 dB when the unit spacing is less than 0.04 λ0(λ0 is the wavelength corresponding to the center frequency of the antenna).By guiding the coupling field between the antennas to the decoupling structure and generating opposite phase between indirect and direct coupling fields,the slot-type decoupling structure improves the port isolation by 22 dB.4.Based on the scattering field of the antenna structure mode term,the antenna performances are compared and analyzed under two different conditions:using the frequency selective surface alone as the ground plane or combine it with the traditional metal ground plane to form a hybrid ground structure.Then,by combining different element structures with high pass characteristics and band stop characteristics,a band stop frequency selective surface design method with steep edge drop characteristics is proposed to build a hybrid ground structure for microstrip antenna.The results show that this design can promote the relative bandwidth of 10 dB out-of-band radar cross section reduction up to 51.1%and 58.0%,respectively,for the two orthogonal polarization directions,while maintaining the antenna port performance and radiation performance.In addition,considering the application value of frequency selective surface absorber on in-band radar cross section reduction for the antenna,a single-layer dual-frequency absorber is designed and the two resonant frequencies are independently adjustable.Through independent unit branch adjustment,the adjustable range of low-frequency band reaches 9.6%and the absorptivity in this band is better than 96%;the adjustable range of high-frequency band is up to 35%and its absorptivity is also better than 93%.Meanwhile,the presented absorber has desired polarization and angle stability.