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, UHF inductors 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.

circuit is built on a small single sided board.

Q1: 2SC3358 npn RF transistor

VR1: 10kohm potentiometer

Bat: CR2032 3V battery & socket

Antenna: 10 cm wire (approx)

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).