# VHF-UHF RF Sniffer

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## Introduction: 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 ) https://en.wikipedia.org/wiki/Lecher_lines .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 ( https://en.wikipedia.org/wiki/Ground_plane ).

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

circuit is built on a small single sided board.

## Step 2: Circuit Diagram & Main Components

Q1: 2SC3358 npn RF transistor

VR1: 10kohm potentiometer

Bat: CR2032 3V battery & socket

Antenna: 10 cm wire (approx)

## Step 7: 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". https://en.wikipedia.org/wiki/Lecher_lines . 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).

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## Questions

Nice work. Any suggestions on how the circuit can be modified to give an indication of the transmission frequency. I'd like to know the frequency of various wireless thermostats from ~300 MHz to 2.4 GHz. My first thoughts with C1 being so comparatively small was a filter to the transistor base but this is not my area. Best wishes!

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

Hi! not possible with this simple circuit.

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

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

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

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.

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?

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!!

Hello!
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!

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!!

awesome great work mate :)

greetings!

Very simple and very usefull :)

Thank yous

Merci beaucoup!

BFR96 is a good replacement.

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

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

Vy 73

Frank

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.