Introduction: UHF Remote Control Transmitter
Nowadays, you can buy ready-to-use RF remote control transmitter-receiver links for your projects. They are reasonably cheap and can be easily found in the marketplace. Now, for us, electronics lovers and hobbyists, making them yourself and from scratch is much cheaper and full of magic. However, working at these frequencies (UHF) is very challenging and can easily lead to failure and surrender (it happened to me!). There are many basic rules to follow when building UHF circuits (and RF in general) . Connections between components should be as short as possible. The saying is: "If you can see it it´s too long". To reduce crosstalk between components and ground loops the circuit should share a common ground plane https://en.wikipedia.org/wiki/Ground_plane . Your digital multimeter or even a regular oscilloscope are totally useless at UHF frequencies. On my previous instructable I showed you a very simple circuit with which you will be able to verify and tune this UHF transmitter.
That simple 1 transistor circuit will be "your eyes" to verify, setup and tune this project.
Step 1: Finished Oscillator Circuit Board
Step 2: Oscillator Circuit Diagram
This is basically a common-base COLPITTS OSCILLATOR circuit.
Step 3: Circuit Details
L1 and C3 form the TANK and determine the frequency of the oscillator. C4 and C5 are for feedback but also have some influence on the frequency.
Step 4: L1 & Tank
The "TANK" is a parallel LC circuit that resonates at the desired frequency. L1 is made of 1.5mm copper wire. Large diameter wire is used to minimize the "SKIN EFFECT"https://en.wikipedia.org/wiki/Skin_effect
an keep resistance of L1 low at high frequencies . Shorting bar makes L1 a variable inductor, allowing the coarse tuning of oscillator between 200MHz and 900MHz. C3, a ceramic trimmer cap is for fine tuning. Set it at mid-position before adjusting L1.
L1 also radiates electromagnetic waves and is the antenna of the transmitter. This type of coil/antenna is ideal for implementing on a printed circuit board. The trace can be wide with enough area to minimize skin-effect. There must not be a ground plane under the coil trace on the other layer of the PCB.
Step 5: Tank & Shorting Bar
Step 6: Understanding the Position of Trimmer Cap (c3)
to adjust the trimmer use a non-metallic tool.
Step 7: Construction
circuit is built on an unetched copper-clad board. Small square pieces ( 4x4 mm) of board material are glued on the main board and components are soldered to these. This is called "Manhattan style construction" and is the best prototyping technique for RF circuits. All ground connections are made to the main board. You cannot use a solderless breadboard as it is plagued with parasitic capacitances and inductances which at UHF behave as actual circuit components.
Step 8: Building Steps
Transistor 2SC3358 is a delicate component. Avoid static & excessive heat when soldering.
Step 9: Test & Tune
To test and tune the oscillator, place a single-turn secondary coil next to L1. Connect a 2m wire loop to its ends and extend it on a tabletop. Sliding the RF sniffer over one of the wires you will detect the nodes (0 crossing points) as dark led spots. The distance between 2 adjacent nodes is 1/2 wavelength of oscillator frequency. This method was used by Ernst Lecher in 1880 to measure UHF frequencies with his "Lecher Wires" (or lines).
Step 10: Measuring the Frequency
The distance between 2 adjacent nodes is equal to 1/2 wavelength.
propagation on copper wire is 280,000Km/second
wavelength= propagation speed / frequency
If you are tuning to 433MHz , wavelength=280,000 / 433 or 646mm (323mm between nodes)
If you are tuning to 300MHz , wavelength=280,000 / 300 or 933mm (466mm between nodes)
use the same formula for any other frequency.
Step 11: NOTICE !
Step 12: For More Details, Watch Video. Thank You!
On next instructables I will be describing MODULATION and RECEIVER circuits.