{"id":1927,"date":"2021-09-09T12:09:30","date_gmt":"2021-09-09T16:09:30","guid":{"rendered":"http:\/\/blog.pasternack.com\/?p=1927"},"modified":"2021-09-16T12:12:59","modified_gmt":"2021-09-16T16:12:59","slug":"all-about-waveguide-shorts-and-shims","status":"publish","type":"post","link":"\/blog\/waveguides\/all-about-waveguide-shorts-and-shims\/","title":{"rendered":"All About Waveguide Shorts and Shims"},"content":{"rendered":"

\"\"For many RF test applications in the higher frequencies, such as millimeter-waves, or for high power use cases, rectangular waveguides are the interconnect solution of choice. In a similar wave to short-open-load-through (SOLT) calibration standards used for vector network analyzer (VNA) calibration with coaxial interconnect, VNA using waveguide interconnect requires waveguide calibration standards.<\/p>\n

Even with coaxial interface VNA, there are many cases where waveguide may be used after the VNA coaxial ports. For instance, a frequency converter system may be used to increase the frequency range of operation of a VNA to hundreds of gigahertz, which typically results in the use of waveguide interconnect for the higher frequency portion of the test apparatus. In order to calibrate to the device-under-test (DUT) ports, waveguide calibration standards are then needed.<\/p>\n

Coaxial calibration standards for SOLT calibration ideally included a zero-ohm short, an infinite ohm open, a termination load perfectly matched to the characteristic impedance of the coaxial line with zero reflection, and a perfect through matched to the coaxial line characteristic impedance. With a waveguide, however, there is no way to build a perfect waveguide open. For instance, an open waveguide will simply radiate as a waveguide antenna with a substantial return loss that invalidates the use of an open waveguide as an open calibration standard. Instead, a shim, or spacer that is matched to a quarter wavelength of the desired test frequency backed by a solid metal waveguide plate (waveguide short) is used. Given that the shift is exactly a quarter wavelength at a desired frequency, the reflected wave from the shim-short pair will be offset in phase of 180 degrees at the opening of the waveguide. In this way, a waveguide shim-short may be used as a waveguide open for the purpose of VNA calibration, and other measurement tool calibrations. For a waveguide calibration kit, all that is needed is a quarter wave shim, a short, and a good waveguide termination.<\/p>\n

It is important to note that for wideband applications, the phase delay of a shim-short for a waveguide will not generate a perfect 180 phase shift across the entire waveguide frequency band. There will be some error for frequencies that have a wavelength different from the \u00bc wave shim. This error can typically be calculated and corrected within the VNA software, however, so it should not pose a significant increase in measurement uncertainty.<\/p>\n

It is also possible to manufacture an indented short that is exactly a \u00bc wavelength deep for a given application, though is generally more widely applicable to use a flush short and a shim, with various thickness shims, for optimum calibration for a given use case. This is because some testing requirements may be for different areas of the waveguide operating frequency range and would result in better accuracy using a quarter wave shim centered at the frequency area of interest.<\/p>\n

Learn more about Pasternack\u2019s extensive offering of Waveguide Shorts an Shims, as well as other waveguide products:<\/p>\n