ee87kjh@cc.brunel.ac.uk (H J Harris) (02/22/91)
Here in the UK there seems to be an enourmous market for car burglar alarms which utilise a tiny radio-contrlled keyswitch operating on 418 MHz. It doesnt take a genius to realise that at 418 MHz, something which measures one inch by two inches isnt using a half-wave dipole as an antenna. Examination of the PCB reveals only a thin strip of track connected directly between the terminals of a variable capacitor and presumably part of the oscillator tuned circuit. This piece of track being (roughly !) as shown; ________________ | /----------\ |<---PCB (approx 1" x 1.5") | \----||----/ | | ^ | variable cap.(& rest of tuned circuit/osclillator/coding chip) Does anyone out there have any idea how these transmitters work, as I am currently working on a similar project which needs a *very* compact, low-power, short-range (100ft max) transmitter operating at 173 MHz. (can't these antennae be used to receive as well, as the car alarms always utilise a 1/4 wave monopole - a piece of wire, as the receiving antenna ?) If I cant use this method, does anyone have any other methods I might try within a limited, enclosed, portable environment ? Many thanks, Kevin Harris.