Waveguide calibration of a vector network analyzer (VNA)

rectangular waveguide like WR90

Introduction to rectangular waveguide

Rectangular waveguide is a very low loss transmission medium, that is mainly used above a few GHz, since the size of waveguide is very large at low frequencies. The internal width needs to be at least half a wavelength. So at 1 GHz, where the wavelength is approximately 300 mm, the internal width of the waveguide needs to be more than 150 mm. As such, except for a few specialist applications, waveguide is not used below a few GHz. WR2300 waveguide, which has an internal width of 584.2 mm (24") and height of 292.1 mm (11.5") is used between 320 MHz and 450 MHz, but it is rarely used. For more information about waveguide, see the Microwave101.com website or the everythingRF website

Why a short, open load and thru (SOLT) calibration is not possible for waveguide

For coaxial connectors, short, open and load calibration standards are normally used. The ideal properties of these are:

The short reflects all the incident power, so |S11|=1 (0 dB return loss)
The open reflects all the incident power, so |S11|=1 (0 dB return loss)
The load absorbs all power put to it, so S11=0 (an infinite return loss in dB)
The phase difference between the short and the open calibration standards are approximately 180° at all frequencies.

It is possible to make very good waveguide loads, and a waveguide short is easy to make too. The left hand object in the photograph below is used to create a short across the waveguide.

WR90 rectangular waveguide short and shim.

Unfortunately, it is impossible to make any waveguide device that acts like an open in a coaxial calibration kit. Leaving the waveguide open, will cause the waveguide to radiate, making an antenna, with a return loss of about 13 dB, although it changed across the frequency range. Some simulations performed with HFSS on WR90 waveguide can be found in a paper Rectangular waveguide radiator miniaturization using electromagnetic infinity-shaped metamaterial resonator. Although at the waveguide cutoff frequency all power is reflected, through the frequency range of normal use, the return loss is around 13 dB. Since most power is radiated, an open-ended waveguide is not suitable as a VNA calibration standard.

The get around this problem, an offset short is used in waveguide calibration. A spacer which is a quarter wave (λ/4) is added to the waveguide, and then the short attached. The electromagnetic radiation reflects off the short, but since it has twice passed though the (λ/4) the phase has been delayed by 180°, which meets the requirements specified earlier, that the difference in phase between ttwo calibration standards should be close to 180°. Due to the fact that the wavelength in waveguide is not a simple function of frequency, in practice the phase difference between the two reflection standards will vary significantly across the waveguide band. However, this variation can be calculated by the VNA, and is not sufficient to cause problems.

The steps to perform a waveguide calibration are depend on whether a one port or two port calibration is required.

One port waveguide calibration of a vector network analyzer using a short, offset short and load.

Purchase a suitable calibration kit. Kirkby Microwave normally keep the X-WR90-3f WR90 waveguide kit in stock, but others can be built to order
Set up the VNA to use the waveguide kit. In particular the VNA needs to know the delay of the spacer (taking into account the relative permittivity of air), and the TE10 cutoff frequency of the waveguide. When kits are purchased from Kirkby Microwave, we can provide all the information required, and for many VNAs, can provide a file readable by the VNA with the correct data in it.
Configure the VNA to perform a one port calibration.
Put a flush short across the waveguide when the VNA asks for it.
Put the spacer on the waveguide, then the flush short, when the VNA asks for an offset short. (The spacer plus short is called an offset short)
Attach the waveguide load when the VNA requires this.

That completes a 1-port waveguide calibration on a VNA

Two port calibration of a vector network analyzer using a short, offset short load and thru.

The steps are similar to above, but with some differences.

Configure the VNA to perform a two port calibration.
Put a flush short across the waveguide which is connected to port 1 of the VNA.
Put an offset short across the waveguide which is connected to port 1 of the VNA.
Put a waveguide load across the waveguide which is connected to port 1 of the VNA.
Put a flush short across the waveguide which is connected to port 2 of the VNA.
Put an offset short across the waveguide which is connected to port 2 of the VNA.
Put a waveguide load across the waveguide which is connected to port 2 of the VNA.
Join the two pieces of waveguide to perform the thru calibration.

There are a number of variations of this method, such as using a load, then a load with a spacer, to make an offset load, or TRL calibration of waveguide.

Kirkby Microwave Ltd is registered in England and Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT.