Here's a simple 433.92 MHz short range radio beacon that may be handy for locating downed R/C planes, lost ballons, model rockets - or perhaps hidden transmitter "fox hunts"! 433.92 MHz is internationally a free band slot, and although only low power transmitters (10-25 mW) are legal, UHF signal punch thru' light vegetation etc is superior to 2.4 GHz microwaves.
It's based around a cheap (~US$5-10) low power (a few mW) Keymark/Spirit-On TXC1 data transmitter, fooled into transmitting audio tones generated by a cheap PICAXE-08M microcontroller. Ranges are up to a km line of sight, dropping to 100-200 metres thru' light vegetation and wooden buildings etc. Compared with flashing lights & beepers, the beauty of a wireless locator relates to all hours convenience & simple direction finding even thru' light vegetation etc. A cheap "sniffer" receiver ( based around a companion RXB1 Keymark receiver module) is shown at => http://www.picaxe.orconhosting.net.nz/433rx.htm
NOTE- keep this simple locator beacon approach in perspective! Do not expect it to locate the likes of your $$$$ FPV (First Person View) model plane downed miles from anywhere in deep woods. For serious locator use, more powerful transmitters & GPS encoding should be considered. These of course can be costly, have higher battery drain and may be tedious to configure .
UPDATE - early 2012: Chinese firm Dorji have recently released some cheap (< US$10 a matched TX/RX pair) ASK 433 MHz data modules that significantly improve this beacon! Their transmitter is particularly appealing, as it's power is boosted to a (legal) 25 mW, giving ranges some 4-5 times greater than the KeyMark/SpiritOn equivalent ! A sensitive UHF scanner can still detect it thru' light vegetation etc to ~500 metres, and LOS (line of sight) several km ! The Dorji receiver, which usefully can also be persuaded as a band monitor, is somewhat more sensitive than the Keymark as well.
Since these Dorji ASK modules have shown themselves clearly superior, more versatile, smaller and far cheaper they're now recommended instead of the Keymark! As circuitry & layout will need to be slightly changed however (see => www.picaxe.orconhosting.net.nz/dorjiask.htm & a Dorji based 433 MHz "sniffer" receiver => www.picaxe.orconhosting.net.nz/dorji-tonerx.jpg ), the Instructable is presently still Keymark module based.
Update -Nov. 2012 : A superior Dorji based tape measure antenna Instructable is now available => www.instructables.com/id/433-MHz-tape-measure-antenna-suits-UHF-transmitte/
Update -Sept 2014: An improved PT4302 engined SpiritOn RXN3-B module is now stocked as Jaycar's ZW3102 receiver. Performance has been found very pleasing, & the versatile supply voltage ( 2.4V-5.5V) is especially welcome. Refer RSSI wiring details under comments at this Instructable end.RECOMMENDED!
Classic Keymark ASK data modules are only modest performers, but they're cheap (~ US$5-10), very popular, widely available,reliable and easy to use. More sophisticated powerful and sensitive 433 MHz transceiver modules are now being marketed by the likes of Appcon & HopeRF,but these new offerings can be a real pain to configure!