Improved preselector filters for RF receivers

A pre-selector tuneable RF filter includes a first circuit 1 which is a tuneable resonant circuit and a second circuit 2 which is a tuneable resonant circuit and a reactance coupling between the first and second circuits. The reactance coupling comprises a voltage controlled device, such as a varactor 55, providing a reactance which varies with applied voltage. The filter also includes means 21 for applying to the voltage controlled device a voltage which is varied whereby the coupling factor of the reactance coupling is maintained to be substantially constant as the resonant frequency of the first and second circuits is varied. Undesirable changes in bandwidth and selectivity are avoided. Also described is a wideband receiver including one or more of the filters.