page_title: Frequency selective surface
section_title: Bandpass FSS filter equivalent circuit and transmission response
hierarchical_section_title: Frequency selective surface / Equivalent circuits – introduction / Reflection and transmission properties of bandpass and bandstop FSS and equivalent circuits – introduction / Bandpass FSS filter equivalent circuit and transmission response
caption_attribution_description: English: Bandpass Transmission Line Filter
context_page_description: A frequency-selective surface is any thin, repetitive surface designed to reflect, transmit or absorb electromagnetic fields based on the frequency of the field. In this sense, an FSS is a type of optical filter or metal-mesh optical filters in which the filtering is accomplished by virtue of the regular, periodic pattern on the surface of the FSS. Though not explicitly mentioned in the name, FSS's also have properties which vary with incidence angle and polarization as well - these are unavoidable consequences of the way in which FSS's are constructed. Frequency-selective surfaces have been most commonly used in the radio frequency region of the electromagnetic spectrum and find use in applications as diverse as the aforementioned microwave oven, antenna radomes and modern metamaterials. Sometimes frequency selective surfaces are referred to simply as periodic surfaces and are a 2-dimensional analog of the new periodic volumes known as photonic crystals.
  Many factors are involved in understanding the operation and application of frequency selective surfaces.
context_section_description: The equivalent circuit for a mesh-type bandpass FSS is shown in Fg. 2.4.2-1.  The admittance of the parallel connection of inductor and capacitor is (Desoer, Kuh [1984]):
  and this admittance is zero (open-circuit condition) when
  When the parallel combination of inductor and capacitor produces an open circuit, all energy is transmitted.
  In the same way, the magnitude of the transmission coefficient of the bandpass filter is:
  Below resonance, the admittance of the inductor, 1/jωL is greater than the admittance of the capacitor jωC, therefore the mesh-type bandpass FSS is inductive below resonance.
caption: Fig. 2.4.2-1. Equivalent circuit for mesh-type bandpass FSS
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