Mode-stirring technology in reverberation chambers
Reverberation chambers require a statistically uniform field in which to conduct research. Their typical rectangular shape and material properties do not, however, create such a field naturally. Mechanical stirrers are needed to create uniform field through a process of called ‘mode-stirring.’ Comtest supplies a number of different stirrer configurations depending on customer requirements and chamber size. Call our engineers directly at +31 71 541 75 31 for more information.
Creating a statistically uniform field
Every reverberation chamber (RC) is basically an RF shielded enclosure with conducting walls which typically have a rectangular shape. When RF energy is injected inside such a closed cavity, the total field distribution could be then represented by the superposition of all the resonant modes propagating and significantly contributing to the uniform field inside the chamber. The field distribution found in RC clearly exhibit a strong nonuniform behavior, which is the opposite of what reverbs are supposed to deliver, a statistically uniform field (working volume).
To attain a statistically uniform field in a reverberation chamber, the process of mode-stirring is therefore essential and required to take place inside the cavity. An effective solution to generate an ideally statistically uniform field is to integrate a mode-stirring mechanism, also known as stirrers, tuners or mixers. While mode-stirrers are moving, it acts as a complex scatterer and produces a remarkable change in the contributing resonant modes by altering the boundary conditions of the chamber.
To obtain the best possible results for various reverb chamber designs, Comtest Engineering incorporates different types of mechanical stirrers depending on customer requirements.
The ‘Z-fold’ rotating mode stirrer is well-developed and a widely recognized approach to provide the necessary perturbation of total field distribution inside the cavity. These classical rotating paddle systems are commonly employed in Comtest Engineering RCs of moderate and small sizes.
Oscillating Stirrer (Quasi-Wall)
For RCs characterized by a lowest usable frequency (LUF) of 80 MHz ~ 200 MHz, the oscillating wall stirrer is typically used. The stirrer operates as an interior moving scatterer analogous to the traditional rotating stirrers. This innovative technique and patented design is developed by our in-house Engineering and R&D department.
The OWS consists of four irregular (non-flat) horizontal panels. The upper and lower panel are mechanically attached by hinges to the outer chamber shielding. Using a linear motor, the OWS can effectively stir the contributing resonant modes by performing expanding-contracting oscillating movements.
The foremost advantage of the OWS over rotating stirrers is that it occupies the volume confined within RC much more efficiently and notably increases the working-to-total volume ratio.