I've thought about this somewhat, you don't necessarily want to let the
neighborhood burglars open your gate automatically;

For situations where you want only "Authorized" vehicles to open the
gate, you could have a coil under the vehicle & inductively
(transformer) couple between the two - use some kind of Challenge/Reply
protocol (Could involve a rolling code or a KeeLoq onboard the vehicle,
rather obvious use for a KeeLoq.)

If you want authentication, you need to SET the frequencies used by
each, and use the coils as a transformer, not just detect an impedance
change.

Also, if you wind the coil carefully, you could use a concrete saw &
install the coil without digging the entire driveway up (though it IS
probably easier to wind the coil in a PVC pipe, bury the pipe, and
prevent any possible expansion/contraction from breaking your wires) -
The system I worked on which had such a coil used heavy wire (12 gauge
IIRC) for the buried loop, IIRC they just poured concrete directly over
the wires (Wires were taped together to the rebar stakes used to set the
coil form, IIRC.)  That was 6 years ago, and I wasn't involved in that
side of the project (I was doing the Windows comm/control code on that.)

  Mark

> From:   pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU] On
> Behalf Of H. Smit
> Sent:   Wednesday, July 21, 1999 2:00 PM
> To:     PICLIST@MITVMA.MIT.EDU
> Subject:        help wanted on detection loop
>
> hello,
>
> I'm working on a project to operate my driveway gate by using satellitedish
> motors. for the mechanical part it went ok, but for the electronical part i
> have some difficulties. I want to operate my gate by driving over an
> detection loop in my driveway, the kind they use by traffic lights. maybee
> one of you can give me a hint of how these detection loops are working.
>
> Please advise,
> Regards,
> Henk Smit.