Support for the X-band waveguide calibration kit for WR90 / WG16 / R100 (model X-WR90-3)

Low cost X-band waveguide calibration kit

Introduction to wavegude calibration

The Kirkby Microwave X-WR90-3 is a low-cost VNA calibration kit for wavegude WR90, also known as WG16 and R100. The kit includes the bare minimum of components necessary for waveguide calibration, which keeps costs down as much as possible. The kit includes a precision short, a quarter wave shim, and an excellent condition, but used HP load.

Anybody using a calibration kit for waveguide is likely to know the basic principles of VNA calibration for coaxial devices. This page assumes that is the case.

For coaxial devices it is normal to use a short, open and load for calibration. The purpose of using the short and open is to provide the VNA with two voltages which are as close to 180 degrees apart in phase. An open reflects a signal in phase and a short out of phase, so the phase difference is close to 180 degrees. The magnitudes of the reflection coefficient are both close to 1, or a return loss of 0 dB. For waveguide calibration it is not possible to use an open. Leaving the waveguide opens results in producing an antenna, with a return loss around 13 dB, at a phase which is not well defined. An open waveguide is therefore not suitable for VNA calibration. Instead it is usual to use two shorts, which differ in length by a quarter wave (90 degrees). Then the reflected signal is 180 degrees different from the incident signal. So the phase shifts are similar to what is achieved in coax. The use of two shorts is inherently narrow band, as a quarter wave at one frequency would be a half-wave at twice that frequency, which would result in the incident and reflected signals being in phase. It is the narrow band nature of the use of two shorts which makes it uncommon for coaxial calibration, although it is used for coax in some instances. However, the fact waveguide is normally used over a narrow range of frequencies means that the use of offset shorts is practical. The Kirkby Microwave X-WR90-3 uses 3 components for the calibration

The following process is required for a single-port calibration.
  1. Connect the VNA to a waveguide to coaxial transition
  2. Enter the definitions of the calibration kit into the VNA
  3. Select the calibration routine in the VNA, and when prompted use the following component, talking care to tighten the devices properly using 8-32 screws, nuts and washers (not supplied).
  4. Remove the calibration components and attach the DUT

For a two port calibration, the above should be repeated at the second port, then the two waveguide transitions joined for a zero length thru.

Either the length or the delay of the quarter wave shim needs to be entered into the calibration kit definition. Some VNAs need a length, and others a delay. The thickness of the shim, as measured on a very high quality micrometer, will be given, and so the delay can be computed, taking into account the velocity of propagation in air, which may be assumed to be 2.996953 x 108 m/s. This will which give a delay of approximately 32 ps.

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