Active window antenna for motor vehicles

Claim 1

An active window antenna for motor vehicles comprising an antenna conductor arranged in or on a windshield and a pre-amplifier circuit located nearby, said antenna and pre-amplifier circuit comprising: a double-gate MOS field-effect transistor, said antenna conductor being connected to a first gate of said transistor and amplification of said transistor being controlled by a control voltage applied to a second gate of said transistor, said transistor providing a high input resistance in the AM range and a comparatively low input resistance in the FM range; and a control circuit which controls the amplification of said pre-amplifier circuit as a function of the output voltage of said pre-amplifier circuit, said control circuit comprising: a junction field-effect transistor, coupled to an output of said double-gate transistor, a storage device having a discharge time constant of approximately one second or more, said storage device being coupled to said junction field-effect transistor to form a DC voltage which is proportional to the output voltage of said pre-amplifier circuit, and a Zener diode coupled between said second gate and said storage device such that the voltage at said second gate is controlled by that stored by said storage device only when said Zener diode operates in the breakdown region.