A high frequency signal is inputted to a receiver from an antenna for receiving the high frequency signal. The receiver is provided with: an amplifier element for amplifying the inputted high frequency signal; n (n: natural number not less than 2) pieces of coil elements, which are serially connected between the antenna and an input side of the amplifier element and construct a low pass filter for the inputted high frequency signal; and an AGC circuit including an impedance variable device, which is connected at one end thereof to an intermediate connection point of two coil elements adjacently disposed in the n pieces of coil elements and is connected at the other end thereof to a ground, and which impedance between one end and the other end is varied in accordance with a level of the inputted high frequency signal, for controlling an input level of the inputted high frequency signal for the amplifier element by changing an attenuation characteristic thereof in accordance with the change in the impedance of the impedance variable device.

A transformer having a core of soft magnetic material around which a first winding and a second winding are wound and a leakage inductance based on the magnetic coupling between these windings. The effective leakage reactance is minimized by adding a third transformer winding serially connected to a capacitor, the latter of which is varied to reduce the leakage reactance over a broad frequency range.

An antenna input circuit for an electronic tuning type AM receiver for automobiles, which is connected to a capacitive antenna represented by an equivalent circuit composed of an antenna capacitance connected in series with an antenna electromotive force and a cable capacitance connected in parallel with them. A tuning transformer and a variable capacitance diode are connected in series with the capacitive antenna, and an additional capacitance is connected to the tuning transformer so that the current flowing in the additional capacitance also flows in one part of the tuning transformer, thereby providing an improved S/N ratio in the medium-frequency receiving band. Another capacitance is connected to the output terminal of the antenna input circuit, to reduce its output impedance. A capacitance for image interference ratio improving use is connected to the additional capacitance and the output impedance reducing capacitance.

An electronic tuning circuit for an AM radio receiver in which a signal of a specific frequency is selected from the input signal from an antenna is provided, this tuning circuit having a tuning transformer with a first winding and a second winding, the number of turns on the first winding being greater than the number of turns on said second winding, the input from the antenna being applied to the second winding of the tuning transformer, and further having two varactor diodes, each of which is connected in series with a DC-blocking capacitor, these series-connected varactor-capacitor combinations being connected in parallel with the first winding of the tuning transformer such that the polarities of the varactor diodes are mutually opposing.

A window glass antenna system for the reception of AM and FM radio broadcasts in an automobile includes an antenna grid applied to a window of a vehicle, that is electrically separate from a heater grid applied to the rear window. An antenna module connected to the vehicle window antenna grid is comprised of a low noise amplifier, matching, and overload protection circuits. The overload protection circuit is included between the antenna grid and the RF amplifier. When the received radio antenna signal is at the typical operating level of the amplifier, the overload protection circuit connects the output of the antenna grid to the RF amplifier. When the received signal is higher than the linear dynamic range of the amplifier the overload protection circuit provides linearity of the signal that is applied to the car radio. This process consists of two steps. In the first step, the overload protection circuit automatically decreases the signal applied to the amplifier so that the amplifier output signal does not change. In the second step, when the output antenna signal is very high, the overload protection circuit automatically provides bypassing of the amplifier and connects the antenna directly to the car radio without the amplifier stage.