High sensitivity, low power FM transmitter

The radio frequency (RF) oscillator section of this FM transmitter is connected to a high-sensitivity, wide-bandwidth audio amplifier and a capacitive microphone. The microphone has a field effect transistor (FET) inside to modulate the base of the RF oscillator, as shown in the figure below.

Transistor Q1 forms a relatively stable RF oscillator, whose frequency is determined by coil L1 and tuning capacitor C4. The desired operating frequency is determined by setting C4, and the tuned circuit is designed to produce a maximum frequency of 110MHz in the standard FM broadcast band. Capacitor C2 provides the necessary feedback voltage through resistor R3 in the emitter circuit of Q1 to maintain the oscillation condition. Resistors R1 and R2 provide the emitter junction bias voltage required for correct operation, while capacitor C1 bypasses any RF signal applied to the base to ground. Capacitor C3 provides an RF path for the energy storage circuit formed by L1 and C4, while blocking the supply voltage applied to the collector of Q1.

The audio section uses a highly sensitive capacitive microphone (M1) with built-in field effect transistors and will clearly pick up low level sounds in the speech spectrum. The speech voltage generated by M1 across R9 is coupled to the base of audio amplifier transistor Q2 via capacitor C6. You will notice that the DC bias for the oscillator transistor Q1 is provided by resistors R1 and R2. Now, a signal voltage generated across R6 is coupled to the base of Q1 via non-polarized capacitor C5 via resistor R4. The gain of Q2 is controlled by the ratio of R7 and R6. The DC operating point is set to allow the maximum range of the collector to vary by the amplified signal. The amplified speech signal now FM and AM modulates the oscillator circuit by a small amount by moving the DC operating point of the base section. Resistor R10 decouples the oscillator and audio circuits to prevent feedback and other undesirable effects.

After proper assembly, clear sound should be received when the receiver is properly tuned to the transmitter frequency. Note that a capacitor can be connected to the base of transistor Q1 to reduce sensitivity. The circuit made with the components listed works best at the high end of the FM band, a clear point without interference from FM broadcast stations. However, satisfactory performance is obtained in the restricted frequency range above 110MHz. Since this is the aviation communication band, it must be used with caution.
It should not be used near airports (component parameters are shown in the table below).

1. Assembly steps
Assemble the circuit as shown in the figure below.
(1) Cut a piece of perforated board with length × width × thickness = 2in × 1in × 0.1in (1in = 2.54cm).
(2) Wrap 8 turns of 16# busbar tightly side by side on an 8# screwdriver to make coil L1. This will make an 8-turn coil with an inner diameter of approximately 0.135in. Insert it into the appropriate hole and solder it as shown.
(3) Insert the trimmer capacitor C4 into the hole shown in the figure. You can put this component on either side of the PC board. This is your choice and you can determine it based on the final assembly diagram you need. Here, 3 holes are prepared for this component, two of which are electrically the same point. This design is very necessary because some trimmer capacitors may have 3 pins. Make sure to connect the common pins to the same electrical point and connect the extra pins to other points.
(4) If you are using perforated board to make it, it is recommended to insert the components from the lower left corner. Pay attention to the polarity of capacitors with polarity symbols and all semiconductor components. Install the pins of the components as shown in the figure, solder them, and cut off the excess wires. Try to use a certain number of pins or 24# busbar sections as connections. The dotted lines on the assembly diagram indicate the routing on the bottom surface of the mounting plate.
(5) Connect the external pins of the battery clamp (CL1).
(6) Check the quality of the routing and solder joints repeatedly to avoid jumpers, short circuits, and being too close to other components. If jumpers are necessary, put insulating sleeves on the pins to avoid any possible short circuits.
(7) Tune an FM receiver to a station with a strong signal at the high end of the frequency band (108MHz or higher). Turn up the volume and place the receiver 25 to 50 feet away from the circuit.
(8) Connect a milliammeter in series with the battery pins. This can be achieved by removing one of the snap buttons on the battery clamp and connecting the meter to the free contacts, as shown in Figure 3. The meter should read 5 to 10mA. Starting from one end of C3, use a short piece of bare wire to contact one turn of the coil at a time. Note that the current indicated on the meter will drop or change as you move forward turn by turn away from C3.
(9) Tracking with the battery connected, use an insulated tuning rod to slowly turn C4 until a station about 108 MHz received by the radio disappears or is switched out in the audio feedback. Because the tuning is very sensitive, it will be difficult to find the signal the first time. Also note that several points in the adjustment may be wrong and the signal will be weak and unstable. The correct signal should be strong, stable and clear; and this can be verified by testing the transmission distance.
(10) Once the expected setting for C4 is found, the frequency value should be marked.
(11) After operation has been verified, it is best to enclose the board in a plastic box with a battery to facilitate tuning and operation.

2. Precautions
One of the things that requires attention when using this type of device is the correctness of the tuning. The adjustable capacitor C4 is very sensitive and only needs to be moved very slightly to change the frequency, so always use a tuning rod. If you are not familiar with the device, it is very easy to tune to a wrong signal. This phenomenon is easy to happen when the device is close to the monitoring receiver. As mentioned earlier, if it is a modulated signal, the wrong signal will be weak, distorted and unstable (it is often mistaken for the main signal and attributed to poor performance of the device), while the main signal will be strong, stable and undistorted. Several tuning experiments should be made before using the device for the desired application.
Also, whenever possible, the device should be used in the 108-109 MHz band, which is on the edge of aviation communications and above the FM broadcast band. When the desired point is found, make a final tune at the receiver to obtain a clear signal. In most areas, these "high" frequencies of the FM broadcast band are clear and allow uninterrupted use, while at the lower end of the band, frequencies can be swamped by strong broadcast station signals if they vary slightly from a clear frequency. If you are near an airport or aircraft flight path, do not use frequencies above 108 MHz.
Most FM radios can be easily detuned slightly, moving the 108 MHz mark on the dial to 109 MHz. This is accomplished by carefully adjusting the "oscillator", the trimmer capacitor located above the main tuning capacitor. The antenna peaking tab should now be adjusted to obtain the maximum signal at the high end of the frequencies.
A high quality receiver with an analog slide-rule type tuning dial will be required to obtain the best performance. Digitally tuned receivers do not perform well for this task.