Tiny UHF Tracker Transmitter





Introduction: Tiny UHF Tracker Transmitter

This is a little circuit that could be used to track an object up to 400m.

It is essentially an SAW stabilized OOK modulated RF transmitter. The modulation is done with two low frequency ultra low power oscillators that activate the transmitter every two seconds for a short period.

With the setup shown here I got up to 400m range. Current consumption is about 180uA average so it'll work for a couple of days with the little button cell. Frequency 915MHz.

Step 1: Circuit

The first oscillator to the left activates the second to its right every 2 seconds or so. The second oscillates at about 800 to 900Hz. Its output signal modulates the RF transmitter which is essentially just a SAW based oscillator with some of the RF energy coupled to a whip antenna.

The adjustment of the RF oscillator can be tricky but works fine with the components shown here. The jumper resistor over the SAW element allows the frequency to be adjusted near the SAW fundamental frequency, then the jumper is removed and the circuit will oscillate at the SAW frequency.

The lower you go in frequency the easier this adjustment will be, so you could go for 433MHz for example too. The component to be changed would be the inductor then (about 22nH).

Use NPO caps for the RF area. The type of the inductor is not critical, I used ceramic.

The circuit would actually benefit from a buffer stage or a matched antenna output, but frankly I didn't fell like investing more time in it. :-) If you want to experiment, I added a pic with a matching circuit for 433MHz that worked pretty well, The inductor for the oscillator changes to about 22nH in that case.

(If you click on the image twice and then on "original DIY file" just below the low resolution picture it will open in hi-res.)

Step 2: Build

Building it requires a hotplate and solder paste or a soldering iron with a fine tip and steady hands.

Make your own PCB layout or download mine from here: Google drive link These are EAGLE files, Schematic and BOM are also included.

Upload the .brd file to your favourite cheap PCB manufacturer, I used Oshpark.com, will take two to three weeks and then:

1. Put solder paste on every pad a component will be placed on

2. Place all components

3. Heat the entire board on a hotplate and wait until the solder paste liquifies

4. Remove the board form the hotplate , let it cool down

5. Flip the board around and solder the battery holder on it

6. Solder the antenna wire into the hole

7. Important: Put some conformal coating or silicon etc on the component side. This will protect the circuit from contamination and humidity. The LF oscillators use pretty high resistance values, which means they are easily detuned if for example you put your finger on it.

Step 3: Range and Stability

The RF frequency is SAW stabilized so shouldn't drift. I did not test the circuit in extrem conditions, but it worked fine from room temp to minus 15C.

Range was about 400m line-of-sight ( does that make sense in this case? :-) )

You can play around with the antenna length and also try to increase the ground area adding some conductive material to the GND pin of the battery holder for example. The short green wire increased the range in my case.

Step 4: Receiver

The receiver is comprised of a YAGI antenna, an adjustable attenuator and a RTL-SDR receiver.

The RTL-SDR dongle is connected to a cellphone that runs a paid app called RF analyzer. It's not expensive.

If you mount the antenna on a car for example the dongle could be connected to a Windows PC though, and there is free software available for Windows.

The YAGI antenna design came form here: https://273k.net/gsm/designing-and-building-a-gsm-antenna/yagi/

There are many other designs on the net and you could also buy an antenna.

The RTL-SDR dongle comes from here: https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/

It's an incredible versatile and very useful gadget for the occasional RF Hobbyist, AND its price is unbeatable.

The attenuator is made of a shielded box with three DPDT switches and attenuates 10dB for each stage. Use small resistors and short connections. Its performance at these high frequencies I didn't feel like evaluating but it attenuates a good amount and that's all that counts. I didn't use any particular website for this part so you have to look this up for yourself. Search for How-Tos of RF attenuators with resistors.

Step 5: See It in Action



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    53 Discussions

    Hi, I would like to make this but I can't understand the section about tuning. What is it that needs tuning, are some of the components adjustable with a screwdriver? They look too small. Or maybe I need to change some of the components to different values in order to tune this? Can I use an oscilloscope? I've made microcontroller and surfacemount pcb stuff before but never rf so I have no idea. If you could tell me or direct me to another website which explains this that would be really appreciated. Thanks!!!

    7 replies

    .... sorry, I mean the section where you say

    "The adjustment of the RF oscillator can be tricky but works fine with the components shown here. The jumper resistor over the SAW element allows the frequency to be adjusted near the SAW fundamental frequency, then the jumper is removed and the circuit will oscillate at the SAW frequency".

    p.s. thank you for posting this it looks really great! :)

    hi again, if I didn't use NP0 capacitors will that make a big difference do you think?

    For RF circuits, everything that influences the oscillation frequency should be NPO.

    No need to tune anything if using component values from the schematic. If you change the SAW resonator you will need to reajust some values as I described.

    ah, ok, thanks!!! :) I will have a go and see how it goes, I plan to use it for tracking animals (for my post-grad. zoology course). If I can get it working I will then try to find a way to make the battery last longer. Thanks again!

    LOOK my friend also very cheap and READY to use with an app on your phone, you can see track movement, where the watch was in FOOTPRINTS on a map, you can set boundaries, and MUTCH MUTCH MORE.

    LINK. https://www.banggood.com/Anti-Lost-Smart-Watch-GPS...

    and look soon on my website how i even make it smaller to use it on smaller drones..www.wannaduino.com and all for FREE

    i help others i love OUR COMMUNITY and OPEN SOURCE

    WannaDuino www.wannaduino.com

    sorry to be rude, but to walk around with that GIANT ANTENNA setup, would not work.

    if you can make the receiver as small as the telephone oke, but this is nothing compared with the transmitter i already have and receivers for my drones, called RSSI work amazingly and i use also GPS in a watch style cost 14USD and works by an app, so in that case you work backwards in time instead of forward?

    You can buy it here,


    GPS type: GPS tracker
    Screen size: 0.96" LED
    Communication Module: GSM / GPRS
    GPRS:Class 12 TCP IPbuild in GSm MODULE
    GPS Senstivity: -159 dbm
    GPS Positioning Accuracy: 10m (2D RM)
    Gsm Positioning Accuracy: 50-200mm
    Speed Accuracy: 0.1m/s
    Maximum Altitude: 18000m
    Stand-by time: 100 hours
    Battery Capacity: 400mA

    instr watch.png
    4 replies

    the RSSI is already in EVERY transmitter these days, it is a home beakon that beeps and give you live data on screan if you are close or the wrong way,

    BUT your receiver needs to have it too, but mots of them have it with our hobby becauzzz of the HIGH cost of our drones

    You are comparing apples to oranges. Yes it's backwards in time you are right, the point is to use a single miniature button cell and a PCB size of what I showed though. If anybody is willing to post an instructable for free how to built an GPS based transmitter that size I would be happy to see it. :-)

    Your link doesn't lead to an actual product.

    Hello can you send me more info about price, to email
    Full complekt system

    Hey, i'm totally out of my element but this might be just the thing I need for not loosing my Disk golf driver when I have a wild throw. I actually been thinking about thing just like this to help me but I was thinking earlyer about those UHF tracker they make for finding lost keys... but found those have been a bit too expensive and nobody could tell me if they actually work on a disk. I was looking for something that is really light, and has good distance to its transmission since a disk throw can travel some 400 meters or so.. I do like to know if you can maybe answer some of my questions to help me better understand if this is a good solution to my issue.

    The disks i'm using range in weight from as little as 170 grams to a little over 200 grams.. I could add a little bit more weight. (a few grams) but obviously the more I add the worse it will for me to be able to throw it. just how much does this weigh as a unit? Do you see this being something that can be light if required?

    The reason I need this is I will probably loose my disk in heavy brush...The brush can get really thick... and I had it where I walked over a disk not to notice it... so how close would I have to get to be if heavy brush reduces the transmission distance? I normally have a good idea about where it went but brush can really obscure a disk so what I'm hoping is I can just point this and find in what general direction I need to walk to to find the disk. Can I use more battery power to boost the signal? would a bigger Antenna on the transmitter help? also would having the antenna only on one side of the disk be a issue?

    I plan on hot gluing the UHF tracker to the center of the disk and then spiraling the antenna round that so that the antenna does not unbalance the disk. Can the antenna be made longer, and be bend in a spiral shape?

    Finally, could I make the receive bigger to catch the signal better, I see myself just making a new disk bag out of receiver frame and holding that in front of me to find where the disk landed.

    I really appreciate if you found the time to answer these questions and maybe do another demonstration of this units capabilities and limitations to better understand how it might be applied to use.

    4 replies

    The unit as shown weighs 1.26g including battery. A bigger battery and some modification to the circuit could increase transmitting power yes. The disk cannot be conductive, that would severely degrade antenna performance, they are made out of plastic right?

    Through thick brush you are good for at least 100m with this setup I`d guess, didn`t try it out though.

    When getting near the transmitter you might have problems determining an exact direction and your object is relatively small so that would be something to experiment with.

    Since you already know where the disk landed (more or less) maybe an audible signal would be better, with a little buzzer that makes a beep every 1 or 2 seconds, but what could be the range of that? Maybe about 10 to 15m? Just a thought.

    Even at 2-3 grams is not going to burden the throw of the disk at all so I could actually afford to even put a luxury item like a power switch into the design to save battery power. (the battery switch would be needed to be set so it turn on if the centrifugal force of the throw could jar it though..))

    Disk are non conductive plastic. https://www.innovadiscs.com/home/disc-golf-faq/pla... the more expensive they are, the more durable and easier to grasp onto they tend to be.. I am just a little worried would I need to put in a second antenna for both sides of the disk to send data out. Could I maybe just use a metal foil sticker of some kind as the antenna? that would also be a good way adhear the device to the disk.

    Its good to know the size is maybe a issue... I already am using vary brightly colored disks, but because the course are not maintained vary well the grass is more then long enough and thick enough that the disks disappear into them. If all this can do is point me in the right direction, it already will help me out soo much.

    I am willing to try a auto-able noise thing too.. the only issue with that being It have to be reasonable well defined. I can off as much as 50 meter from my throw since I have to sometimes blindly throw over a hill towards a target. Being able to wand the air and say, "its this way!" seems like a much better option over trying to listen for a noise.

    If you watch this video... you can see just how bad a disk golf park can get... when you screw up your thrown you go into thick garbage and can't find anything in that kind of stuff. https://www.youtube.com/watch?v=H1QVLQriAlM

    Wow that's some distance. For the antenna other conductive material could be used too, sure, like copper tape for example. You only need one antenna. The receiver antenna is pretty big, I dont know if you want to walk around with this on a regular basis though. The only advantage is its low tech, small and inexpensive

    Actually the receiver could be built into my disk golf bag. I normally carry around at least 6 other disks, a water bottle,small plastic rain poncho for if I should get caught out in the rain, and a towel to wipe off the disk should it hit the mud...((This is vary minimum most people carry anyway as far as disk golf is concerned..)) so I need something to carry it all in anyway. building the receive to fit around the bag would not be a huge issue. The bonus would be that now I also have a dedicated spot for a phone to sit. :-)