Signal transmission system

The transmission system has an electrically conductive probe 2 connected to the output of a generator 3, 5, 6, 7 which generates a radio frequency signal at a frequency (eg. 1 MHz) at which the probe 2 is substantially non-radiative, and the probe only transmits the RF signal when it is proximate sensor means 8, 12 connected to an RF receiver 10. The system may be used to disable an alarm and operate locks of a vehicle. An electronic key fob 1 houses the probe 2 and the generator, which in response to closure of a power switch 4 causes an RF carrier to be modulated by a code generator 3. The switch 4 is closed by pressing the probe 2 against the metal bodywork 8 of the vehicle, the modulated RF signal being transmitted into the metal bodywork 8 by direct electrical contact or capacitive coupling. A detector 11 senses the signal between bodywork 8 and a static line 12 formed by an electrical conductor extending from within the vehicle into the surrounding atmosphere. "Anti-grabbing" protection, to avoid the security system being overcome by scanning/analysing equipment, includes a timer 5 which limits the duration of transmission from probe 2 to a period of about four code words, and an anti-scan circuit 17 which disables a decoder 18 in the receiver for a certain period if a received code word is longer than an expected code word.