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From: ee87kjh@cc.brunel.ac.uk (H J Harris)
Newsgroups: sci.electronics
Subject: keyless burglar alarms
Message-ID: <2009@Terra.cc.brunel.ac.uk>
Date: 22 Feb 91 12:53:38 GMT
Organization: Brunel University, Uxbridge, UK
Lines: 30

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.


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