RF Wireless FM Transmitter circuit and explanation

RF Wireless FM Transmitter

RF Wireless FM

Here’s a rf wireless FM transmitter can be built with simple, affordable and widely available parts. Construction is fun and much can be learned although performance is modest; for example, your voice gets difficult to hear at distances greater than 25 feet.

This FM transmitter is far from perfect offering only modest performance. First, tuning the transmitter can be frustrating. Even slight turns in the variable capacitor can result in large frequency changes. Second, transmitter tuning often resulted in a harmonic frequency. Instead of the intended 108 MHz for example, capacitor tuning yielded a 216 MHz transmitter frequency. In addition to hearing your voice one could slightly hear radio station broadcasts.

If performance is modest, why would I build this transmitter?

One answer is that much can be learned and this tutorial is is appendixed with the underlying mathematics to calculate parameters like (1) transmitter frequency, power output and range (2) antenna length and (3) required coil winding. Often on the web, one just finds a schematic. By adding the analysis (with high school level math), one can conceive improvements on transmitter performance.

RF Wireless Circuit

Construction
A combination of wire wrapping and soldering was used to construct the FM transmitter. Jameco’s prototyping card provides enough room for (non-critical) part placement. You should try to keep all parts close together and keep wire leads short.

RF Wireless-Inductor

RF Wireless-Bottom

RF Wireless FM Transmitter Operation
First, use a battery-powered pocket radio as a receiver. AC powered boom-boxes and home stereos (110 or 220 V) are not recommended; battery-powered radios are much better at receiving transmissions than AC-powered units.

1. Tune your radio to dead air, i.e. frequencies within the FM radio band that are silent or only have some hiss. Frequencies near 108 MHz are typically dead air. The Radio-Locator web page lists local radio stations in your area. This can help you identify dead air frequencies.
2. Turn on your RF wirelessFM transmitter, extend its antenna and keep the transmitter approximately 2 feet away from your FM radio. Speak into the mic while slowly adjusting the variable cap. Use your fingernail or non-metallic screwdriver until you hear yourself over the radio. This process is frustratingly tedious, requiring careful capacitor tuning. You are tuned once you hear howling (also known as a hot mic) which indicates transmitter-receiver feedback.
3. Increase the transmitter-to-radio distance. Congratulations – you have a rf wireless microphone!

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