Calibration kits are an essential tool in ensuring vector network analyzer (VNA) measurements are accurate and consistent. Without calibration before testing, the algorithms used in modern VNAs would not be able to sufficiently account for the variations in interconnect and components inline to the device-under-test (DUT), and these variations would result in extensive uncertainty. Most VNAs have coaxial test ports. Hence, most VNA calibration kits are based on coaxial interconnect. This arrangement serves well for DUTs that have coaxial interconnect also but may not be ideal for DUTs with waveguide interconnect. This is why there are also Precision Waveguide Calibration.
There are some distinct differences between coaxial VNA calibration kits and waveguide VNA calibration kits. Part of the differences stems from the reality that the standard method of VNA calibration, short/open/load/through (SOLT), isn’t accessible to waveguide calibration. The short in a SOLT kit requires a very good complete reflection of incident power with S11 = 1. The same goes for an open, where a short and open ideally have 0 dB return loss. The short and the open must have a 180-degree phase difference at all frequencies for this to work properly. The load is designed to absorb all of the signal energy with zero reflection, hence infinite return loss.
It is possible to make precision loads and shorts with a waveguide. However, making a waveguide open with 0 dB return loss has yet to be achieved due to the intrinsic operation of a waveguide open as a radiator. To overcome this challenge, an offset spacer is used to offset the short from the end of the waveguide. These spacers are typically ¼ wavelength, ⅛ wavelength, and ⅜ wavelength. Hence, with a ¼ wavelength spacer, the reflected signal from the short can be phase shifter by about 180 degrees, which functionally acts as an open for the waveguide calibration. This is called a short-short-load-thru (SSLT) calibration.
Since this isn’t a perfect approach and waveguides are not ideal over all frequencies, especially considering manufacturing and assembly tolerances of the short/spacer combination, then there is some variation over frequency that is different from the ideal 180 degrees. This can often be accounted for though by VNA software algorithms and a precision calibration can be made in this way. However, the VNA will need to have a precise figure for the delay of the spacers used. A one port calibration with a short, then spacer/short combination, and lastly a load step can be used to the 1-port calibration of a VNA. A 2-port VNA calibration looks much like a SOLT calibration with a coaxial VNA calibration kit with the exception that the short/spacer combination replaces the open.
Thru-reflect-line (TRL) calibrations can also be performed with waveguide VNA calibration kits when that feature is needed. It is also often advantageous to have waveguide straights when performing laboratory testing on waveguide interconnect based DUTs. It is important that these straights are also precision waveguide components to minimize the impact of manufacturing tolerances, the same way precision adapters and cable assemblies are used for coaxial VNA calibrations.