Introduction: 433 MHz UHF Lost Model Radio Beacon

About: Retired educator/writer

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 =>

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 => & a Dorji based 433 MHz "sniffer" receiver => ), the Instructable is presently still Keymark module based.

Update -Nov. 2012 : A superior Dorji based tape measure antenna Instructable is now available =>

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!

Step 1:

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!

Step 2: Circuitry

A popular & cheap (~ US$3) PICAXE-08M can modulate the 433 transmitter with a wide range of encoded beeps, tones, morse ID, tunes or even Sequentional Multi-Tone (SMT) Hellschreiber! Refer the makers web site (UK firm Rev.Ed) at => , with skinflint programming details => PICAXE can further allow microAmp level "snoozing" between locator transmissions, potentially thus offering greatly enhanced beacon battery life.

Step 3:

Assembly onto solderless breadboard is initially recommended for evalution - all the UHF circuitry is on the TXC1 module, with just simple signals & voltages elsewhere. Circuit layout is not critical,so even "rats nest" layouts to fit available space may do,but a dedicated PCB is being considered. This may assist those who want a compact and easily assembled device.

The trial shown here explored attaching the module directly to the antenna (a "Slim JIM") - no real benefits arose however.

Step 4:

Although many may cringe with this dated hobbiest technique, assembly can be made onto a 10 hole x 12 hole "stamp of Veroboard. A PICAXE programming socket (made from a cut down DIP8 IC socket) is even provided for easy program tweaking. Note -the 3V lithium cell shown here is cheap,light and convenient, but has only limited service life of course. With low transmission duty cycles (averaging perhaps a current drain of under a mA) a few days of beacon service may be expected.

Step 5:

Here's the matching Veroboard "stamp" layout- note that the mid board conductive strips MUST be broken or short circuits WILL occur! The classic way to do this is with a drill bit spun between your fingers. Ensure all copper scraps are removed. The PICAXE-08M has it's I/O channels referred to as "pins" - pin 4 here is feeding the TXC1 module Vcc supply voltage, while pin 2 delivers the tones and pin 1 has a "spare" LED to perhaps indicate beacon action pre lift off. This LED could be swapped for a pulsed ultrabright version to assist in after dark location.

Pre programmed PICAXE control of these channels allows tremendous versatility. The beacon could even hibernate after lift off, only turning on perhaps after 30 minutes (when the model may be assumed lost).

Step 6:

The controlling PICAXE naturally will need to be programmed, although commands such as HIGH, LOW, SLEEP and GOTO are easy enough to master! The high level PICAXE Editor (download free from ) runs on even an old W98 clunker PC, and is very user friendly. The program is passed into the PICAXE via a simple 3 wire serial lead- the DIY one used here is made from some ribbon cable & 3 header pins. PCs without a D9 serial port will need a USB-serial adapter of course..

Step 7:

For simple use a ¼ wavelength vertical antenna can be made from a scrap of wire. A length of about 170 mm should do it- this is just under an adult handspan. The antenna works best when elevated and in the clear, but even a flexible wire may do OK for nearby (~50-100 metres) locations.

Do not expect brilliant performance unless the TX antenna and receiver have a good propagation path between each other! Ranges in cluttered terrain may well be quite small- although even then invaluable when trying to find a nearby downed plane/rocket/balloon in tall grass etc.

Step 8:

Superior performance results however when the antenna is elevated. For pre arranged hidden transmitter "fox hunting" a "Slim JIM" type can be made onto a wooden strip or peice of bamboo etc. Slim JIM radiation is near horizontal so this type may not suit vertical hunts so well!

Step 9:

Weather protection can be made by encasing the entire beacon and antenna inside a plastic pipe offcut. Battery life with cheap AA cells may be months if low duty cycles are programmed. Note - 4 x fresh AA cells = 6 Volts of course, which is above the rated supply of both the PICAXE & 433 TX module. Using a dummy cell (a nail inside a piece of bamboo) reduces this to 4½ V, but still allows 4 older cells or NiCd/NiMH types to be inserted.

Step 10:

A slim JIM antenna, along with the attached beacon transmitter & battery pack, can be hauled up a tree with a light cord. It may be even possible to solar power the setup of course, but naturally good sunlight would be needed, which is not often available under a forest canopy!

Step 11: Companion "sniffer" Receiver

A UHF scanner or ham radio ($$-$$$) suits reception of the locator, but for tight budgets & modest needs consider a simple "sniffer" receiver. Aside from use here, these may be very handy for assorted 433 MHz device checking as well- wireless doorbells,back yard weather stations, energy meters etc. Output from the companion ASK Keymark RXB1 receiver can readily drive a piezo sounder when simply NPN boosted, and an undocumented RSSI tap may even be handy. See details =>

Step 12:

A simple 170mm whip can again be used on the receiver, but a "cotanga" Yagi antenna made from coat hanger wire will usefully boost ranges and allow better beaming. See Yagi constructional details =>

Step 13: YouTube

Check the YouTube field testing, c/w research assistant "Jack" => For receiving clarity a morse "HI" ( = .... / .. ) was PICAXE generated, as the hillside test site experiences considerable 433 MHz "hash" from Wellington city -some 10km away cross harbour! The 170mm whip transmitter was not detectable at 500m LOS (line of sight) with the RX whip, but easily heard with the ~ 6dB gain Yagi.

Update- October 2012: Use of the 25mW Dorji ASK transmitter in the beacon gave far superior performance, with the TX beacon signal then easily heard LOS to several km- even without a directional antenna ! The Dorji TX module is hence recommended instead- it's also cheaper!

UPDATE #2 - Sept. 2014: Aust/NZ electronics outlet now stock a pin for pin compatible PT4302 RF engined SpiritOn module as their ZW3102. RSSI (received signal strength) on this module has turned out as a breeze to organise- refer pictures! Signal clarity and module supply versatility (down to 2.4V) are appealing, and sensitivity is quite amazing. A Darlington pair driven LED allows visual signal confirmation. Reception was found good thru' light vegetation & wooden buildings to 100m, and LOS ( line of sight) signals were still "armchair copy" at 500m !