City Research Online

Structural intermodulation interference in mobile radio systems

Ho, P.S.W. (1991). Structural intermodulation interference in mobile radio systems. (Unpublished Doctoral thesis, City, University of London)

Abstract

Intermodulation interference at communal base stations, with co-sited transmitting and receiving aerials, represents a major obstacle to the improvement in the utilisation of limited radio frequency spectrum available to mobile radios. An investigation has been made of the phenomenon of Structural Intermodulation Interference : its sources, generation mechanisms, and the influences of environmental factors on the interference level. Studies have also been made of the methods to reduce its effect.

It was found that corrosion products exhibit non-linear current-voltage characteristics; and they are able to generate intermodulation interference. Furthermore, it was shown that in the case of steel lattice antenna structures, metal- semiconductor rectifying contact is the underlying mechanism responsible for the observed non-linear current-voltage relationship. Experimental results also supported that corroded metallic junction is a first order factor which determines the severity of intermodulation interference effect; surface corrosion products contribute relatively little to the intermodulation interference level.

A lumped circuit model of a non-linear junction was developed, which consists of a non-linear junction resistance in parallel with a linear junction capacitance. The resultant model was used to predict the third order intermodulation level of a test sample, and the prediction was in reasonable agreement with the measured result.

The effects of weather conditions on intermodulation level were also studied in depth. It was found that wind loading on the antenna tower structure can cause large and rapid changes in the intermodulation interference level. The interference level drops significantly when it is raining. Laboratory experiments shown that the high permittivity of water is responsible for the observed large drop in the interference level when it rains, rather than its conductivity. The effect of changes in ambient temperature do not produce large changes in the interference level in short space of time.

Lastly, methods to reduce and suppress intermodulation interference were investigated. The principle is to provide an alternative linear path for the induced radio frequency current. Various techniques were tried, and metal plating was found to be the most promising. Preventive methods, including high corrosion resistant coating, were also studied and are of particular interest in the construction of new antenna towers.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments: School of Science & Technology > Engineering > Electrical & Electronic Engineering
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