How accurate does the boundary have to be, and how fast do you have to
respond?  GPS units are accurate to a few meters, and are getting smaller
and cheaper by the day.  With one of the OEM GPS units and a PIC, you could
have a perimeter map in memory, and do all the control stuff with no
physical boundary markers at all.

> -----Original Message-----
> From: Jinx [mailto:joecolquitt@CLEAR.NET.NZ]
> Sent: Tuesday, February 20, 2001 6:12 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]:Directional transmitter
>
>
> > Your best bet is to use phase information. It is very hard to
> > confine an EM emission to a pure hemisphere (unless you're
> > up in the UHF or microwave bands). The best you can hope
> > for is a cardioide pattern.
> >
> > If you told us more about the application, rather than your pre-
> > conceived ideas of what solution to use (no offense), we could
> > probably make more meaningful suggestions.
>
> No offense taken, just being cautious.
>
> Imagine it this way. You've got a large area like a sports field in
> which you can move this object. But if you try to take this object
> past the perimeter of the field it is disabled. This area may be
> unfenced (meaning the object could be taken out at any point),
> or it may be fenced (meaning the object would have to be taken
> out through an exit). To make it work again it has to be brought
> back inside the perimeter, which is why the need for two signals
>
> The intention was to mount solar-powered battery transmitters
> on posts (as the venue isn't really a sports field, but it is outside,
> that's OK). There are many identical objects moving around, so
> the effect of the signals must localised to just the immediate
> infringer.
>
> I thought radio would be the best choice, mainly because of the
> power supply. IR and ultrasound I think may use too much power,
> and after sunset or on dull days that may be a problem. Lasers
> were thought of too but there's too much chance of the beams
> being blocked. The distances involved are too great for RFID,
> and inductor loops would mean too much setting up, and there's
> a lot of vehicular traffic around which may upset them. So radio
> seems to be the only option, the only problem being setting up
> distinct fields either side of a boundary
>
> --
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>
>

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