I have had a few inquiries about a low power FM transmitter, and this article should satisfy those who might want to build one.  It is designed to use an input from another sound source (such as a guitar or microphone), and transmits on the commercial FM band - it is actually quite powerful, so make sure that you select an unused position on the dial!

The FM band is 88 to 108MHz, but is getting fairly crowded nearly everywhere, but you should be able to find a blank spot on the dial somewhere.

NOTE: A few people have had trouble with this circuit.  The biggest problem is not knowing if it is even oscillating, since the frequency is outside the range of most simple oscilloscopes.  See Project 74 for a simple RF probe that will (or should) tell you that you have a useful signal at the antenna.  If so, then you know it oscillates, and just have to find out at what frequency.  This may require the use of an RF frequency counter if you just cannot locate the FM band.

The circuit of the transmitter is shown in Figure 1, and as you can see it is quite simple.  The first stage is the oscillator, and is tuned with the variable capacitor.  Select an unused frequency, and carefully adjust C3 until the background noise stops (you have to disable the FM receiver's mute circuit to hear this).

Because the trimmer cap is very sensitive, make the final frequency adjustment on the receiver.  When assembling the circuit, make sure the rotor of C3 is connected to the +9V supply.  This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft.  You can use a small piece of non copper-clad circuit board to make a screwdriver - this will not alter the frequency.

Note:  A reader has suggested that the frequency stability is improved considerably by adding a capacitor from the base of Q1 to ground.  This ensures that the transistor operates in true common base at RF.  A value of 1nF (ceramic) as shown is suitable, and will also limit the HF response to 15 kHz - this is a benefit for a simple circuit like this.

How It Works
Q1 is the oscillator, and is a conventional Colpitts design.  L1 and C3 (in parallel with C2) tunes the circuit to the desired frequency, and the output (from the emitter of Q1) is fed to the buffer and amplifier Q2.  This isolates the antenna from the oscillator giving much better frequency stability, as well as providing considerable extra gain. L2 and C6 form a tuned collector load, and C7 helps to further isolate the circuit from the antenna, as well as preventing any possibility of short circuits should the antenna contact the grounded metal case that would normally be used for the complete transmitter.

The audio signal applied to the base of Q1 causes the frequency to change, as the transistor's collector current is modulated by the audio.  This provides the frequency modulation (FM) that can be received on any standard FM band receiver.  The audio input must be kept to a maximum of about 100mV, although this will vary somewhat from one unit to the next.

With the value shown for C1, this limits the lower frequency response to about 50Hz (based only on R1, which is somewhat pessimistic) - if you need to go lower than this, then use a 1uF cap instead, which will allow a response down to at least 15Hz.  C1 may be polyester or mylar, or a 1uF electrolytic may be used, either bipolar or polarised.  If polarised, the positive terminal must connect to the 10k resistor.

Inductors
The inductors are nominally 10 turns (actually 9.5) of 1mm diameter enamelled copper wire.  They are close wound on a 3mm diameter former, which is removed after the coils are wound.  Carefully scrape away the enamel where the coil ends will go through the board - all the enamel must be removed to ensure good contact.  Figure 2 shows a detail drawing of a coil.  The coils should be mounted about 2mm above the board.

For those still stuck in the dark ages with imperial measurements (grin), 1mm is about 0.04" (0.0394") or 5/127 inch (chuckle) - you will have to work out what gauge that is, depending on which wire gauge system you use (there are several).  You can see the benefits of metric already, can't you?  To work out the other measurements, 1" = 25.4mm

The nominal (and very approximate) inductance for the coils is about 130nH. This is calculated according to the formula ...

L = N² * r² / (228r + 254l)

... where L = inductance in microhenries (uH), N = number of turns, r = average coil radius, and l = coil length.  All dimensions are in millimetres.

It is normal with FM transmission that "pre-emphasis" is used, and there is a corresponding amount of de-emphasis at the receiver.  There are two standards (of course) - most of the world uses a 50us time constant, and the US uses 75us.  These time constants represent a frequency of 3183Hz and 2122Hz respectively.  This is the 3dB point of a simple filter that boosts the high frequencies on transmission and cuts the same highs again on reception, restoring the frequency response to normal, and reducing noise.

The simple transmitter above does not have this built in, so it can be added to the microphone preamp or line stage buffer circuit.  These are both shown in Figure 3, and are of much higher quality than the standard offerings in most other designs.

Rather than a simple single transistor amp, using a TL071 opamp gives much better distortion figures, and a more predictable output impedance to the transmitter.  If you want to use a dynamic microphone, leave out R1 (5.6k) since this is only needed to power an electret mic insert.  The gain control (for either circuit) can be an internal preset, or a normal pot to allow adjustment to the maximum level without distortion with different signal sources.  The 100nF bypass capacitors must be ceramic types, because of the frequency.

The mic preamp has a maximum gain of 22, giving a microphone sensitivity of around 5mV.  The line preamp has a gain of unity, so maximum input sensitivity is 100mV.

Select the appropriate capacitor value for pre-emphasis as shown in Figure 3 depending on where you live.  The pre-emphasis is not especially accurate, but will be quite good enough for the sorts of uses that a low power FM transmitter will be put to.  Needless to say, this does not include "bugging" of rooms, as this is illegal almost everywhere.

I would advise that the preamp be in its own small sub-enclosure to prevent RF from entering the opamp input.  This does not need to be anything fancy, and you could even just wrap some insulation around the preamp then just wrap the entire preamp unit in aluminium foil.  Remember to make a good earth connection to the foil, or the shielding will serve no purpose.