I have an application where I want a vehicle to detect whether it is
entering or exiting a particular area in a building. The accessways are
typically about 5m across and there have been problems using infra-red,
reflective tape and RFID technologies.

I would like to try 2 Stips across the accessway that are spaced about
1m apart and act as antennas for a 915MHz transmission.

One strip would transmit the first zone ID and the other the second
zone. The Receiver on the vehicle would use the RSSI and Zone ID to
determine which way it was travelling.

In theory I think this may work. I am considering a prototype using
leakt coax as the strip but the only type I can source here in Australia
so far is 28mm diameter (cause a bump in the vehicle by the time this is
placed into a conduit to protect it).

Would it be feasible to use simple metal strips as antennas that are
tuned to length to be exact multiple of 1/2 wave ? I would then expect
that there would be a peak signal every 1/2 wave along the length of the
metal strip.

The receiver antenna on the vehicle will only be around 500mm above the
ground so the transmit power can be low and high loss in the strip can
be tolerated. The lower the power the better as it will minimize
interaction between the strips.


Thanks for your comments

Regards

David