Test & Measurement


Using waveguide shorts and shims for RF testing

27 October 2021 Test & Measurement

For many RF test applications in the higher frequencies, such as millimetre-waves, or for high-power use cases, rectangular waveguides are the interconnect solution of choice. In a similar vein to short-open-load-through (SOLT) calibration standards used for vector network analyser (VNA) calibration with coaxial interconnect, VNA using waveguide interconnect requires waveguide calibration standards.

Even with coaxial interface VNA, there are many cases where waveguides 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 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 by 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 degree phase shift across the entire waveguide frequency band. There will be some error for frequencies that have a wavelength different from the quarter-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 quarter-wavelength deep for a given application, though it is generally more appropriate to use a flush short and a shim, with shims of various thickness, 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 centred at the frequency area of interest.

For more information contact Andrew Hutton, RF Design, +27 21 555 8400, andrew@rfdesign.co.za, www.rfdesign.co.za


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