978-1-5386-6599-2/19/$31.00 ©2019 IEEE Abstract—NPL has launched a comprehensive study on its primary waveguide TRL calibration ¾-wave Line standards, in order to establish traceability in S-parameter measurements at terahertz frequencies. The WM-380 band (500-750 GHz) has been chosen to demonstrate the approach that is being taken. High precision dimensional measurements have been carried out on these Line standards and the waveguide sections that are used as measuring instrument test ports. Electromagnetic modeling and simulations have been performed to quantify the impact of the dimensional errors on the electromagnetic performance of these devices. In this paper, some preliminary results are reported and discussed. Index Terms—Waveguides, measurements, TRL calibration, WM-380, measurement traceability, terahertz frequencies. I. INTRODUCTION There is an increasing demand for traceable electrical measurements at terahertz frequencies, driven by applications such as short-range communications, radar sensors, multi-pixel security imaging systems, medical diagnosis, remote sensing of the Earth, and astronomy. Waveguide is extensively utilised in these applications, mainly owing to its low loss characteristics. Waveguide components are typically characterized using a Vector Network Analyzer (VNA) for S-parameter measurements, and there exist different types of calibration techniques, including TRL (Thru-Reflect-Line) [1], LRL (Line- Reflect-Line), and SOLT (Short-Open-Load-Thru). Among them, TRL, a self-calibration technique, is considered very attractive for precision metrology applications, in that TRL does not require all standards to be known items except for the Thru connection (where the test port reference planes are joined together) [2]-[3]. Flush short-circuit is commonly utilised as the reflection standard for TRL calibration. To achieve the optimum performance, the Line standard usually has an electrical length of 90° (i.e. ¼ guided wavelength) near the mid-band frequency of the waveguide band. With rising frequencies, the size of waveguide decreases and the guided-wavelength reduces, resulting in ¼-wave Line with small length. For example, the ¼-wave Line for WM-380 band (500-750 GHz) has a length of 0.155 mm - calculated at the mid-band frequency of 625 GHz. Such Line standards can be easily deformed during use and therefore are not ideal. In [3], TRL calibration using two ¾-wave Line standards has been proposed. These lines are considerably longer and therefore are more robust. Two lines are employed so that their corresponding electrical lengths exclude 180° and 360°, where the TRL calibration does not work correctly. Each of these two lines is used to cover part of the full frequency range and both lines provide full coverage across the waveguide band, as described in detail in [3]. Here, the same technique was used to design the TRL lines for WM-380 band. The nominal designed lengths of these two lines are 0.431 mm and 0.568 mm, respectively. Fig. 1 (a) shows one of the Line standards. These Line and Reflect standards were manufactured by Swissto12 [4] using galvanic growth of materials based on a photolithography structured substrate. These devices were designed to incorporate flanges meeting the new IEEE specifications [5]. In practice, the fabricated Line standards have dimensional errors (e.g. rounded inner corners, imperfect waveguide internal sizes, etc). Such imperfections are negligible at lower frequencies, however their impact on electrical performance becomes significant when frequencies rise to terahertz [2]. (a) (b) Fig. 1. Photographs of the WM-380 devices. (a) One of the ¾-wave Line standards for TRL calibration; (b) one of the 2-inch long waveguide section (attached to the test port of VNA) being measured under optical microscope. Preliminary Study on WM-380 Waveguide TRL Calibration Line Standards at the UK’s National Physical Laboratory Xiaobang Shang, Nick Ridler, Wenjuan Sun, Peter Cooper, and Alan Wilson National Physical Laboratory, Teddington, TW11 0LW, UK WM-380 waveguide section WM-380 waveguide aperture with nominal dimensions: a=380µm, b=190µm