The circuit (1) has a two (16,17) antenna leads. An antenna amplifier (20) is also connected to a receiver input circuit (19) via an antenna d.c. supply line (18). An electronic controller (2) has two input clamp pairs and an output. The first clamp pair (3,4) has a high ohmic input impedance and is connected parallel to the antenna supply line. A reference voltage (8) is applied to the second clamp pair (6-4). The output signal (10) at the output (9) of the controller is derived from the difference between the instantaneous average antenna voltage (5) and the reference voltage.A controllable d.c. source (7) is connected across the antenna supply line. The HF noise current is small and independent of the direct current. The source control connection (12) is connected to the control circuit output. The instantaneous average of the antenna voltage equals the ref. voltage.
