Abstract: A changeover device provides for contactless interchange of power between an r.f. source such as a transmitter Tx.sub.1 and two loads, such as an aerial 2 and a test load 3. The device may be used in a changeover system for exchanging main and standby transmitters in the event of failure of the main transmitter between the aerial and a test load, or for connecting transmitters in parallel in such a way as to leave them well matched in the event of failure of one while permitting half power operation from the surviving transmitter.

Abstract: Directional couplers 7.sup.1 -7.sup.20 are used to couple power from r.f. power amplifier modules 5.sup.1 -5.sup.20 to a transmission line 6 or other type of resonator, from which it is withdrawn by directional coupler 8. The power withdrawn from the continuous transmission line, which is an integral number of wavelengths at the operating frequency of the modules in length, is less than the power circulating around the transmission line 6. The balance load may be removed to enable the output load to be combined with the output of another r.f. power amplifier operating a different frequency so that e.g. sound and vision signals can be combined at an aerial and, in the latter case, means may be provided for improving the resonance of the main loop 6.

Abstract: The circuit is capable of selectively adding r.f. inputs at A, B dependent on the bias voltage applied to varactor diodes D1, D2. In one bias condition of the diodes and hence one value of capacitance, the reactance between A and B and between the output and point D is equal to the reactance of transmission lines 10, 11 formed by co-axial cables, and quadrature inputs at A and B are combined at output O/p. At another value of acpacitance of the varactor diodes, corresponding to another bias voltage, the capacitance of the diodes and the series capacitances C1, C2 resonate with the inductances L1, L2 and input A is effectively isolated from input B. The circuit can be used to produce a power amplified composite video r.f. signal, input A being connected to a power amplifier for the video signal only and input B being connected to a pulse amplifier for the sync pulses only. This permits more efficient use of the amplifiers compared to a single amplifier for amplifying the composite video signal.