A study of intermodulation interference due to non-linearities in metallic structures

Intermodulation products are spurious frequency components generated when two or more signals mix in devices with non-linear characteristics. In multifrequency communications environments such as communal antenna sites, intermodulation products generated by active devices (transmitters and receivers) and passive components (metallic contacts and conductors) can cause serious radio interference. The interference caused by passive intermodulation products is a well known phenomenon and is sometimes called the 'rusty-bolt' effect. This study is directed towards the characterisation, detection and location of external passive intermodulation interference sources at land mobile radio sites used by the UK Emergency Services. The thesis describes the development of computer-controlled passive intermodulation product measurement facilities. The laboratory and field measurement work investigates the parameter related to the generation of intermodulation products. The measurements concentrate on the lower order products and results are given for combined input powers of up to 63 W at 150 MHz. The types of test conditions and samples used are similar to a typical radio site environment. A review of previous investigations related to passive intermodulation is presented. The various non-linear mechanisms at metallic contacts are described and the most dominant one is identified. Some analytical techniques for predicting intermodulation signals are considered and the theoretical results based on a power series model are compared with measurement results. The basic principles of detecting and locating non-linear metallic contacts are explained. Experimental studies of audio and radio frequency detection techniques are conducted and strategies for detecting and locating external passive intermodulation interference sources are outlined.