Estimation and Reduction of Weather Radar Safety Distances

The siting of a weather radar always requires a safety analysis, which determines the minimum distances necessary to prevent RF hazards. This analysis has considerable influence on the selection of important parameters of the infrastructure of the radar site, like the height of the radar tower. It may even exclude a preselected site i.e if occupied buildings are too close.

Because of the implications of the safety analysis it is important to have a simple method which provides reliable estimations of the field strengths in the vicinity of the radar antenna. The estimations must have a sufficient safety margin, so that measurements of the field strengths which are performed during the acceptance testing of the radar do not question the selection of the site or the height of the antenna tower.

Such a method is presented in this paper. It is based on simple yet accurate approximations of the field strengths in different regions around the radar antenna. In particular the problem of the far-field equation which gives unrealistic high amplitudes close to the antenna is addressed in a practical manner without falling back on numerical simulations.

Most RF-related safety standards allow a long-term averaging of the radiated field strenghts, typically over 6 minutes. This fact can be advantageous in order to reduce the safety distances. Because all weather radars are permanently scanning in azimuth (and elevation), each building in the vicinity of the radar is only illuminated for a very short period per scan. Thus, if the radar features a failsafe inhibition of the transmitter in case of a stop of the antenna movement, the long-term averaging may be applied when estimating the safety distances. However, the transmitter inhibition must comply to certain design standards for safety equipment. A simple sensing of the antenna movement by monitoring the antenna position data provided to the signal processor and a software-controlled interruption of the transmitter trigger does not satisfy the requirements of modern safety standards.

This paper describes a technique for monitoring the antenna movement and inhibition of the radar transmitter which is compliant to the EU Machinery Directive, the relevant product safety regulation of the European Union. The technique allows the application of the long-term averaging of the radiated field strengths for the calculation of safety distances.