A planar microstrip terahertz (THz) bandstop filter has been proposed with defected ground structure with high insertion loss (S21) in a stopband of ？25.8？dB at 1.436？THz. The parameters of the circuit model have been extracted from the EM simulation results. A dielectric substrate of Benzocyclobutene (BCB) is used to realize a compact bandstop filter using modified hexagonal dumbbell-shape defected ground structure (DB-DGS). In this paper, a defected ground structure topology is used in a λ/4, 50？？ microstrip line at THz frequency range for compactness. No article has been reported on the microstrip line at terahertz frequency regime using DGS topology. The proposed filter can be used for sensing and detection in biomedical instruments in DNA testing. All the simulations/cosimulations are carried out using a full-wave EM simulator CST V.9 Microwave Studio, HFSS V.10, and Agilent Design Suite (ADS). 1. Introduction THz frequency range spans from 0.1 to 10.0？THz in the electromagnetic spectrum. This THz regime of EM spectrum has been effectively exploited in a variety of applications such as medical spectroscopy, security, space, imaging and measurement of overlaid dielectric substrate [1, 2]. New trends emerging in the development of technology in optical communication systems motivate the requirement of terahertz transmission lines and components [3]. The THz technology is slowly unfolding to mature to meet needs and application specific requirements. Defected ground structures offer compact solutions for the design of microstrip antennas and other passive microwave/millimeter wave components [4]. A lot of literature is available based on defected ground structures for the size reduction of the microstrip filters [4–14]. However, at terahertz frequencies, no considerable work has been reported for the filter design using the defected ground structure (DGS). In this paper, a modified hexagonal dumbbell-shaped DGS/slot in the ground plane is employed in microstrip line at terahertz frequency regime to achieve bandstop characteristics. In this work, a Benzocyclobuten (BCB) substrate is used for the filter design [15]. 2. Properties of BCB Dielectric BCB is a promising organic material which is showing stable permittivity values and low loss over a broad frequency range [15]. The manufacturer [16] claims a dielectric constant , with a few percent variations between 10？GHz and 1.5？THz, and the loss tangent of the dielectric BCB is also varying between 0.0008 and 0.002 within the frequency interval from 1？MHz to 10？GHz. Additional data are also available
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