Some RF Characteristics of Bonding Systems

The measurement of bonding impedance has been typically limited to dc or to relatively low (<20 MHz) frequencies. However, the impedance characteristics of the bonding system should not be simply ignored at higher frequencies. To do so may be neglecting a serious source of electromagnetic interference. This paper presents techniques for the measurement and interpretation of bonding impedance characteristics through the VHF region. A measuring device based on the insertion loss technique is described. Calibration curves of typical models of this device show its usefulness for measurements up to 400 MHz. The application of this device to the measurement of bonding systems in field installations is also explored. From data obtained with the insertion loss device, an equivalent circuit of a typical bonding system is developed which incorporates the contributions of the equipment case as well as those contributions of the bonding straps to the overall impedance characteristics. It is shown that typical bonding systems exhibit regions of parallel resonance which are often at relatively low frequencies. Within these regions, the impedance of the bonding path is often very high. Because of this high impedance, the equipment case can act as a very effective antenna. Evidence is presented to show that the case voltages induced by a radiated field in the resonant frequency region may increase as much as 25 dB for a bonded system over a system that is not bonded.