Introduction: VHF-UHF RF Sniffer

Picture of VHF-UHF RF Sniffer

This is a multi-chapter instructable. I will be describing the making of a short/medium range RF remote-control using the UHF 433Mhz frequency. It´s impossible to setup & adjust a RF transmit-receive link if you are not sure the transmitter is working properly.At 433MHz, your multimeter or even a regular oscilloscope are totally useless.

In this chapter (1) I will show you this ultra-simple RF sniffer circuit with which you can visualize with an LED:

(1) if the transmitter is oscillating.

(2) its relative power output by the brightness of LED.

(3) check the frequency of the oscillator with a simple school ruler by measuring the distance between nodes (as did Ernst Lecher 120 years ago ) .At 433MHz, distance between nodes (0-crossing points) is 323mm on copper wire (half wavelength).

UHF comprises frequencies between 300Mhz and 3Ghz. At these frequencies the physical layout of the components is crucial; the same circuit may work or not depending on how it´s built. A single millimeter of wire or component lead is an inductor and affects the circuit. You cannot use a solderless breadboard because it´s plagued with parasitic capacitances and inductances which at UHF frequencies (and VHF) behave as actual components. To avoid crosstalk between traces & ground loops UHF circuit must share a common ground plane (eg:,double sided PCB) to which all ground points are connected ( ).

On the good side, UHFinductors are of low values and are usually air-core and made with few turns of wire or even printed on the PCB. The same applies for antennas: at 433Mhz a 1/4 wave antenna is only 17cm long. Capacitor values are also very low. All these properties translate into a very small & economical circuit.

Step 1: Overview

Picture of Overview

circuit is built on a small single sided board.

Step 2: Circuit Diagram & Main Components

Picture of Circuit Diagram & Main Components

Q1: 2SC3358 npn RF transistor

VR1: 10kohm potentiometer

Bat: CR2032 3V battery & socket

Antenna: 10 cm wire (approx)

Step 3: 2SC3358 Is a High Gain - High Frequency Transistor (up to 7 GHz)

Picture of 2SC3358 Is a High Gain - High Frequency Transistor (up to 7 GHz)

Step 4: 10pF Cap Blocks Lower Frequencies

Picture of 10pF Cap Blocks Lower Frequencies

Step 5: Adjust VR1 for Minimum LED Brightness

Picture of Adjust VR1 for Minimum LED Brightness

Step 6: Sniffer Will Detect RF Radiation From Many Sources.

Picture of Sniffer Will Detect RF Radiation From Many Sources.

Step 7: Use Sniffer to Find Nodes on a UHF Standing Wave

Picture of Use Sniffer to Find Nodes on a UHF Standing Wave

Ernst Lecher(1856-1926 ) measured the distance between adjacent nodes of a standing wave to calculate the frequency with his "Lecher Wires". . In a similar way we can measure the frequency of our oscillator by finding the nodes sliding the sniffer antenna over a wire connected to the oscillator out. LED will be dark over the nodes. The distance between 2 nodes is equal to 1/2 wavelength. Frequency is the propagation speed divided by wavelength. For copper wire speed is around 280,000 Km/second. We could also measure the 60 HzAC line frequency with this method, but nodes would be 2350Km apart ! . (wavelength: 4700Km).

Step 8: See It in Action, Watch the Video. Thank You!


ShredderP (author)2017-04-06

Hi there, tank you for sharing your wonderful designs, how can I expand range of detection about 5 to 10 meters or more?

simpletronic (author)ShredderP2017-07-22

Hi! not possible with this simple circuit.

eltoro5 (author)2017-01-11

in a technical high school in the electronics technology trade hoe long might it take to make with all parts at hand? Just wondering for time restraint purposes for projects

simpletronic (author)eltoro52017-01-15

The translation is not very clear. Please repost your question. Thank you!

JeoleX made it! (author)2016-02-14

Dear Author,

Firstly thank you for sharing your knowledges with us. I am an impatient guy, so even I don't have 2SC3358 transistor, I made this circuit with 2N3933 (750 mhz). All parts are soldered as you said. I observed that all the time led is on. Then searched why it is like that and realised that my transistor (2N3933) can also detect low frequencies about min 60 mhz. Is it why led is always on ? I ordered 2SC3358 and will try with this one. But first I have to clean my concerns :)

Best regards

simpletronic (author)JeoleX2016-02-15


as far as I can discern from your image, the wiring looks OK. Now, when the potentiometer wiper is fully down the transistor should cut off (and led turn off) .

If it doesn't, possible causes could be: 1)faulty transistor 2)wrong wiring of transistor pins. 3)faulty potentiometer. Nevertheless, 2N3933 has an fT (gain=1) of 750MHz and won't catch GSM, wifi, etc. 2SC3358 has an fT of 7000MHz. There are many possible substitutes for it also. Greetings!

JeoleX (author)simpletronic2016-02-15

Thank you for your consideration. I can control led via potentiometer. When I fully turn off the potentiometer, led off. I can adjust brightness of the led with potentiometer. Only problem is led always on :) Probably The led is always on due to transistor. My transistor can detect low frequencies such as radio range (88-108 mhz), because of that it is always on. So I will change transistor with higher than 2 ghz and we will see :)

Thanks again.

simpletronic (author)JeoleX2016-02-15

You must calibrate pot as described in step 5, that is reduce the brightness of led until it is barely visible. (with no radiation source close to antenna). Now, placing antenna close to radiation source ( eg, fluorescent lamp ) you will see an increase of LED brightness. ---!>!---

JeoleX (author)simpletronic2016-02-15

I got it, but it was not calibrated. Because there are always frequencies that my transistor detect. So I think first I have to change my transistor (higher than 2 ghz). And calibrate it far away from radiaton sources (for example mobile phone, monitor etc). then approach antenna to radiation source. I hope I will see led is on. So I have also idea about using arduino for measure frequencies from radiaton source.

ShajidI1 (author)2016-01-12

Can i use 2n222 or 2n3904 Transistor?
If i use 2n2222or 2n3904 transistor so, what will be?

simpletronic (author)ShajidI12016-01-12

those transistors have a fT of 300MHz. (gain=1 at 300MHz) . No good for UHF ( wifi - GSM, etc ) . 2SC3358 i used has a fT of 7000MHz.

Betalbots (author)2015-12-12

I really like your three Instructables on UHF - they are very interesting, and I'm exited to try building them! I am thinking that I would like to try to design a PCB version, rather than building on top of a copper clad board, but from what you said about the issues associated with high frequency, I'm trying to decide if it would work to build on a PCB. What advice would you give me?

simpletronic (author)Betalbots2015-12-13

Hi!! PCB is fine, but it should be double-sided using one for ground-plane. It is difficult to predict the final behaviour of the PCB circuit (crosstalk between components ground loops / layout) and make mods once built. L1 inductor can be readily made with PCB trace and must not have ground plane under it. In the other hand, using "manhattan" construction is fast, sturdy and allows for post-tweaks. Greetings!!

Cicko (author)2015-10-23

I've read all of your instructables and I've seen that you have excellent knowledge of electronics (analog too, which is hard to find these days) and one can learn stuff just by reading your instructions. That's the way an instructable should be made! Job well done!

I will definitely learn something by building some of your projects.

Looking forward to more of the good stuff from you!

simpletronic (author)Cicko2015-10-27

Thank you very much Cicko ! I´m really glad you liked my posts. Right now I´m working on a UHF remote control receiver which will be my next instructable. Greetings!!

Bart Morse (author)2015-10-11

awesome great work mate :)

simpletronic (author)Bart Morse2015-10-14


Mic100 (author)2015-09-16

Very simple and very usefull :)

Thank yous

simpletronic (author)Mic1002015-09-17

Merci beaucoup!

simpletronic (author)2015-09-15

BFR96 is a good replacement.

Else, Any NPN with fT > 4GHz) will work.

frank40 (author)2015-09-15

Hello, nice construction. Just about equivalents of this transistor, who is in Europe just a little exotic, hi....

Vy 73


HenrikC (author)2015-09-06

Can i make a sniffer that can detect RFI in the 150Khz - 2Mhz ? and which component should i replace. I am working as a Tele tecnitcian and wee often have to find defective 12V transformers in households. This could be a very useful gadget to have in the toolbox :) Regards Henrik.

simpletronic (author)HenrikC2015-09-07

Just replace C1 for a larger value. You may also replace the 2SC3358 transistor with a cheaper one like BC548 or 2N2222. Greetings!

HenrikC (author)simpletronic2015-09-08

Great ! Thanks for the reply, i will get back with the results.

rkrishnan7 (author)2015-09-06

This is an outstanding 'ible! Thank You!

I wish I had built something a few years ago when my newly installed garage door would inexplicably open and close on it's own and I wanted to sniff around for random RF as a possible cause (of course it was not - the probability of catching the rolling code is low).

simpletronic (author)rkrishnan72015-09-07

Thanks for commenting, greetings!!

Jon_Fuge (author)2015-09-07

Nice article and nice simple circuit; I wish this was shown at school in physics. Just a small note about units; kilo should be "k" as the upper case K is Kelvin (temperature). 2350km etc.

simpletronic (author)Jon_Fuge2015-09-07

Thanks for commenting!

Luno1 (author)2015-09-06

Great way to introduce someone to RF! Simple design and easy to show results. Thanks!

simpletronic (author)Luno12015-09-07

thanks for commenting!

dresch (author)2015-09-06

Very cool. I wonder if you could do a shorter (coiled?) antenna and stack a couple of circuits along a rotating arm. Much like a propeller clock. Then you might be able to visualize fields in two dimensions 'instantly'. Also, use a few different color LEDs, each to represent different frequency bands (with associated BP filters). Many thanks for a nice sniffer tool! Bill

simpletronic (author)dresch2015-09-07

Creative imagination! greetings!

emerson.john (author)2015-09-06

This is a clever little design, and it looks like a project I can build entirely from the junk box except for the battery. I will add a power switch.

Thanks for bringing it all together.

Thanks for your comment , Greetings!!

manuka made it! (author)2015-09-06

Bravo-this makes a wonderful Lecher wire demonstrator! It could suit antenna design,radiation pattern checking or transmitter tuneups too That low noise 2SC3358 may be hard to find however- for lower freq UHF work & strong signals almost any regular NPN may do?

You really need to qualify it's RF detectability however & mention ranges & transmitter power responses -many UHF devices use quite weak transmitters! Wireless doorbells & backyard weather stations etc may be only a few milliWatts and you'd need you sniffer right beside them for detection. Hacking a dirt cheap 433 MHz ASK data module as a general ISM band sniffer may appeal instead, and audio can also be heard. Detection ranges with such modules can be 10s to 100s of meters. See =>

simpletronic (author)manuka2015-09-07

Thanks for your comment! Yes, this will only detect RF sources which are relatively close. I will use it to verify & tune a UHF transmitter (next post). The circuit you posted is of course very sensitive as it is a complete regenerative receiver, however it will only detect 433Mhz RF. Greetings!

WilliamP1 (author)2015-09-06

Took a look at your circuit, and it is basically a broadband receiver.

Which is "tuned" by adjusting the length of the antenna. A simple LC filter between the antenna and the base of the transistor would help get rid of out of band signals.

A variable C or L would be even better.

simpletronic (author)WilliamP12015-09-07

Sure, and a tunable parallel LC would make it a wavemeter . The main purpose of this circuit is to verify & tune a UHF oscillator which will be the subject of my next post. Greetings!

corbs (author)2015-09-06

I wonder whether it could be used to find my remote car key. I think I lost it somewhere in the house. Don't know the frequency.

What circuit board do we use?

WilliamP1 (author)corbs2015-09-06

Nope. The key fob would have to be transmitting :-)

corbs (author)WilliamP12015-09-06

In fact the key fob does transmit. A signal is sent out from the fob all the time and, when picked up by my Toyota, turns on some lights and enables the door to unlock when I pull a door handle. Also, what circuit board?

simpletronic (author)corbs2015-09-07

This circuit will detect RF from sources which are relatively close to it. For the circuit board I used the Manhattan-Style construction on a single sided board.

smrp1984 (author)corbs2015-09-07

The board is a plain unetched single sided board, and as far as finding your key fob you simply need to carry your car around the house intil the doors unlock and the lights come on... then you know the key fob is near.

oldboffin (author)corbs2015-09-06

I think you will find this is a RFID device, the car is transmitting all the time, and when you bring the keyfob near to the car ,it uses the rf to power generate a response.

WilliamP1 (author)corbs2015-09-06

Most key fobs only transmit when the button is pressed to conserve battery power. What makes you think yours is always transmitting.

Samiran (author)2015-09-05

Can be used as bug detector (Snitch in the GANG).


simpletronic (author)Samiran2015-09-05

Greetings !!

Electron_flow (author)2015-09-03

Cool project! One small-ish thing I'd respectfully ask though...the GIF you're using for the main image about dropped me to the floor. For epileptics fast-strobing lights and colors is no good - it can easily trigger seizures in many of us. I love your idea (very clever), but I'll probably (unfortunately) have to choose to avoid your instructables just in case of more intensely strobing images. But thanks for sharing your knowledge and ideas!

I am really sorry and I apologize. I was not aware of the potential risk of injury from a strobing GIF.I have now changed it to a still image. If I include gifs in future posts, they will be safe. Thanks for your comment !! --|>|--

About This Instructable




Bio: Music: my profession for over 40 years... Electronics: my beloved hobby always.
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