An illustration of leakage “forwarding” by surface waves

The picture illustrates the potential safety problem associated with the formation of surface (Zennek) waves at flat load surfaces in a tunnel oven. It was obtained by microwave modelling using the QW3D software.

A load is located inside the tunnel to the left, and extends to the right out of the tunnel. A human finger is located further out from the tunnel opening and points at the load.

The microwave power density in the load and finger is shown in a colour scale with magenta = high and dark blue = < 10 % of the high value.

The surface wave “sticking” to the load provides energy to cause a quite high power deposition in the finger. Note that a microwave leakage instrument used in the standardised fashion (minimum 5 cm away from the top of the load) might not sense the surface wave at all.

The new standardised method of leakage measurement in situations like this is by using the modified probe with the rod in accessible locations at the tunnel end.