Wireless FM Transmitter

Keywords: FM transmitter, wireless microphone, single transistor,

The photo shows a wireless FM transmitter, pocket radio and yellow disk for size comparisons. Speak into the transmitter and others hear you on any FM radio. The transmitter can be built in an afternoon 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.

Motivation and Audience

Not to give you false expectations, 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.

One answer is that much can be learned and this tutorial is is appendixed with the underlying mathematics to calculate parameters like (1) tranmitter 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.

This tutorial's audience is thus electronics enthusiasts who:

• don't have much FM or radio-frequency (RF) experience
• want to build an FM transmitter
• want to learn how such transmitters work
• want to learn how components are selected e.g. calculating the number of turns when building an air coil, antenna length etc.

Again this tutorial emphasizes that the transmitter's performance is modest, but is learned in its construction. The tutorial breakdown is as follows:

• Parts List and Sources
• Construction/Schematic
• Theory
• Operation
• Where To Go From Here
• Author Information

Parts List and Sources

An effort was made to find a single source supplier of all parts. Jameco has almost every part cited in the tables. Details construction your air core inductor using a McDonald's soda straw will be described in the next section.

Construction

Schematic

fmTx031402a.pdf is the Acrobat file of the same schematic. You will need Adobe's free Acrobat reader to view it.

The schematic and constructing the circuit are relatively straight-forward. Some highlights and clarifications towards circuit construction are given next.

Electret Microphone

Batteries

2N2222A Transistor

The 2N2222A also comes in a black plastic casing (TO-92 style) which you can use if you want. The T0-18 is preferred because the can has a small tab that typically represents the emitter pin.

Make sure you correctly identify the 2N2222A's pinout and correctly wire the base, collector and emitter in the schematic. Often, circuit malfunctions because the pins were mis-wired.

Variable Capacitor

Inductor

Some on-line and printed articles describe winding the wire around a pencil. Unfortunately, pencils come in different diameters and hence a McDonald's soda straw was used; the yellow-red-white striped straw, found in every McDonalds in the world, is the same size. The straw's radius is exactly 0.1325 inches (diameter = 0.2650 inches) and 1/4 inches was snipped off the straw.

Next, a straight piece wire was wound around this 1/4 inch snippet six times and then soldered on the prototyping board. The end result is an inductor (also known as an air core coil) with an 0.1325 inch radius. If you wish, you can apply some womens' clear fingernail polish to permanently keep the wire on the straw snippet.

Antenna

Theory: How does the FM Transmitter Work?

The component values in the circuit are derived to better understand how this FM transmitter will work. The underlying math is rather simple and can be found in most undergraduate university physics textbooks.

Inductance of an Air Core Coil

For your McDonald's soda straw inductor, r = 0.1325 inches, x = 0.25 inches and n = 6 turns and results in L = 0.171 microHenry or 0.000000171 Henry.

The specific frequency f generated is now determined by the capacitance C and inductance L measured in Farads and Henry respectively:

Resonant Frequency of a Parallel LC Circuit

Given your variable capacitor ranges from 4 to 34 pF, your tank circuit will resonant between 66 and 192 MHz, well within the FM radio range. To compute these values for different values of C, n, r and x a simple Excel spreadsheet, called calcFreq.xls was created. Simply enter the values and the inductance and frequency are automatically calculated.

Antenna Length

Fixed Capacitors

A capacitor can be thought of as a frequency-dependent resistor (called reactance). Speech consists of different frequencies and the capacitor C1 impedes them. The net effect is that C1 modulates the current going into the transistor. Using a large value for C1 reinforces bass (low frequencies) while smaller values boost treble (high frequencies).

The C3 capacitor across the 2N2222A transistor serves to keep the tank circuit vibrating. In theory, as long as there is a supply voltage across the parallel inductor and variable capacitor, it should vibrate at the resonant frequency indefinetely. In reality however, the frequency decays due to heating losses. C3 is used to prevent decay and the 2N2222A spec sheet suggests a capacitance between 4 to 10 pF.

Resistor for Electret Mic

Resistors and the 2N2222A

The 2N2222A's maximum rated power is Pmax = 0.5 W. This power ultimately affects the distance you can transmit. Overpowering the transistor will heat and destroy it. To avoid this, one can calculate that the FM transmitter outputs approximately 124 mW and is well below the rated maximum. The mathematical details are given in rfMath.pdf.

The power is intimately related to the transmission range. At 124 mW and 30% radiation efficiencies, the maximum distance between your FM transmitter and a battery-powered radio will range betweem 35 to 112 feet. The calculations are given in rfDistance.pdf.

Operation

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 FM 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 wireless microphone!

• A hand-made coil that's 0.1 uH is difficult to test. Inductance meters to measure at the micro Henry range are expensive.
• Capacitance meters measuring picoFarads are also expensive.
• A 0.111 uH air core coil was purchased from Coilcraft, an inductor manufacturer.
• Spectrum analyzers costs thousands of dollars. A viable alternative is an $130 USD Elenco F-2800 frequency counter. This is a nice unit to acquire if you plan on building FM transmitters more seriously.

For the most part, the frequency counter displayed approximated 200 MHz even with the Coilcraft inductor! Thus most probably the variable capacitor is not truly giving a 4-to-34 pF range. Since the transmission frequency stayed closed to 200 MHz, calcFreq.xls reveal that variable capacitor actually stays close 4 pF rather than going up to the rated 34 pF. This should be expected since tolerances in capacitance are rarely precise. The net effect is that without picoFarad resolution capacitance you'll be transmitting at a 216 MHz harmonic yielding reduced range and susceptible to noise.

Final Words

Like the author, readers might be excited about the prospects of building FM transmitters. Many circuit designs and schematics exist on-line and in print but don't often provide much analysis. This tutorial attempts to fill this gap, especially for first-time FM transmitter builders. The analysis allows one to learn what roles and their values play in the circuit. Such analysis provides a reader a stepping point towards improving or customizing the circuit.

Illustrating the math and real-world operation is the tutorial's value. Some material towards learning more might be acquired from the references below. Happy building!

References

• Braga, N., Pirate Radio and Video Newnes Butterworth-Heinemann Publishing, 2001. ISBN: 0-7506-7331-1. An excellently written book that provides a dozen or so transmitter designs but gives little mathematical analysis
• The ARRL Handbook for Radio Amateurs, 79th Edition, The Americal Radio Relay League, 2002. ISBN: 0872591891 This tome covers practically every aspect of radio communications including the mathematics.
• alt.radio.pirate - a very helpful newsgroup
• Harry's Homebrew Homepage - the dozens of circuits are well documented for do-it-yourself builders

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