It's probably not worth designing your own RF circuits for something like this. Lots of people offer hybrid modules and chips for TX/RX. They've done all the hard work for you, and they're cheap too. Again, try (Hi, guys...how about a discount on my next order...) I think they're made by a company called Radiometrix, who might have a web-site of their own. There's a 400MHz band AM transmitter that has two pins (V+, Gnd) that you can run off a PIC output, and a hybrid carrier-detect AM receiver module selling for 2 pounds sterling any-quantity. They also do 27MHz stuff if you *must* use that band. and as Tjaart has just recently pointed out: >http://www.rfm.com/ sells some really nice one-chip-radios. Actually, if you're already putting ultrasonics on board each unit, why not use it for both directions? That will double your time delay, and improve your resolution. Alec =========================================================================== ________ / ______/ W5 Ltd. ______ ______ _ / /___ \ \ / \ / /____ \ 33 Sneath Avenue \ \ / \ / __ / / London NW11 9AJ \ \ / \ / / /_/ / United Kingdom \ \/ /\ \/ \_____/ \ / \ / Telephone +44 181 922 7778 \ / \ / Fax +44 976 650 110 \______/ \_______/ eMail mail@W5.co.uk Technology * Innovation * Design * Solutions