All About Waveguide Shorts and Shims
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.
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.
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.
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 ¼ 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.
It is also possible to manufacture an indented short that is exactly a ¼ 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.
Learn more about Pasternack’s extensive offering of Waveguide Shorts an Shims, as well as other waveguide products:
Courtesy of Pasternack