Investigation On Switchable Broadband Absorptive Frequency Selective Structures

Switchable Absorptive Frequency Selective Structure is evolved from traditional Absorptive Frequency Selective Structure(AFSS).By loading PIN diodes on the AFSS,the frequency response characteristics can be adjusted to adapt to the complex and changeable electromagnetic environment.The traditional reflective AFSS reflects electromagnetic waves in the band and absorbs electromagnetic waves outside the band.It is often used as antenna reflector.When the antenna is not working,it can reflect electromagnetic waves and easily reveal the orientation of radar system.The switchable reflective AFSS can be presented as microwave absorber,which can absorb electromagnetic wave and improve the stealth of radar system.Currently,it has been reported that single-polarization switchable bandpass AFSS can reflect cross-polarized electromagnetic waves.When it acts as radome of different polarized antennas,the normal operation of the antennas working in cross-polarization can be affected.Therefore,switchable bandpass AFSS of fully transmitting cross-polarized electromagnetic waves is of great significance.Based on the effective analysis methods,this thesis introduces many novel design methods.This thesis designs UWB microwave absorber,a new wideband switchable reflective AFSS based on monolithic structure and a wideband switchable bandpass AFSS of fully transmitting cross-polarized electromagnetic waves.The main contents of this thesis are as follows:1.A new type of ultra-wideband microwave absorber is designed by using a two-layer lossy cascade.The top layer design of UWB microwave absorber is based on a cross-finger resonator and lumped resistances.The middle layer design is based on a dipole and lumped resistances.The bottom layer is a metal surface.Design and simulate the structure of single-polarization and dual-polarization ultra-wideband microwave absorber.Construct and simulate its equivalent circuit model.The properties of microwave absorber are compared and discussed.2.Based on the ultra-wideband microwave absorber,a new wideband switchable reflective AFSS based on monolithic structure is designed by introducing a parallel LC resonator loaded with a PIN diode into the second lossy layer.When the PIN diode is turned on,the switchable reflective AFSS appears as a reflective AFSS,while achieving a low insertion loss of reflective band.When the PIN diode is turned off,the switchable reflective AFSS appears as a microwave absorber,while ensuring a large absorption bandwidth.By controlling the state of the PIN diode,the switching between the reflective AFSS and the microwave absorber is realized,while the larger absorption bandwidth is guaranteed at the same time.By using cross capacitor and zigzag line in the cell structure,the cell size is reduced and the angular stability of the structure is improved.3.Based on the weakness of single-polarization switchable bandpass AFSS reported so far that reflects cross-polarized electromagnetic waves,a new broadband switchable bandpass AFSS of fully transmitting cross-polarized electromagnetic waves is designed by replacing the third metal layer of switchable reflective AFSS with parallel LC resonator loaded with a PIN diode.When the PIN diode is switched on,the parallel LC structure of three layers is designed to resonate at the same frequency point.Therefore,the switchable bandpass AFSS can be switched to be bandpass AFSS.When the PIN diode is cut off,the third layer of FSS is presented as a metal reflecting surface and the two lossy layers absorb electromagnetic waves of different frequency bands.Thus,the switchable bandpass AFSS is presented as microwave absorber.The switchable bandpass AFSS has great application potential in multi-polarization and multi-antenna systems.