A simple wireless microphone is perhaps the most basic form of a transmitter. Its purpose is to connect a microphone to an audio or PA system without the need for a hazardous intervening cable. The wireless microphone functions as a one-way radio link, transmitting signals to a nearby FM receiver. The received signals are then directed to the audio or PA system, replacing the traditional wired microphone. In professional-grade wireless microphones, a crystal-controlled receiver and transmitter are utilized, operating outside the FM broadcast band at frequencies specifically designated for this purpose. Frequencies around 170 MHz are commonly employed for wireless microphone systems. The following shows a typical circuitry of a wireless FM microphone.
The FM transmitter circuit comprises an audio electret microphone amplifier that channels audio signals into the bias network of a free-running oscillator circuit operating within the FM broadcast band. An electret microphone inputs audio signals into the audio amplifier stage (Q1). The biasing of the microphone is achieved through R1, which can be adjusted according to the microphone being used. In the absence of specific data from the manufacturer's datasheet, a recommended value of 4.7 K can be used as a general standard. Audio signals are coupled to the base of Q1 through C2, while R2, R3, and R4 provide biasing to maintain Q1 at approximately 4 volts and 0.5 milliamps. It is advisable to employ a low-noise, high-gain audio transistor, such as the 2N3565, although other similar transistors with high gain and low current should also suffice.
The amplified audio at the collector of Q1 is coupled to the base of oscillator Q2 through the RC network comprising C3 and R8. Q2 functions as a grounded base oscillator, with feedback provided by C8. R5 and R6 supply the initial bias for the oscillator transistor, while R7 offers emitter bias.
You can use a VHF transistor, such as 2N3563, 2N5179, or MPSH10, as the oscillator component. Any high-quality NPN transistor with a frequency rating of 500 MHz or higher should work, although the modulation characteristics may vary to some extent. Transistors with larger geometries or lower frequency ratings generally have higher capacitances, potentially leading to better modulation capabilities. However, it's important not to assume that a higher-frequency transistor will necessarily perform better.
The frequency of oscillation is determined by the tank circuit L1 C6, along with the stray circuit and transistor collector-to-base capacitance. The collector-to-base capacitance is influenced by the collector-base voltage, which is modulated by the applied audio from R5. This modulation directly affects the oscillator frequency. As the collector voltage also affects the oscillator power output, some amplitude modulation (AM) component will be present. However, in this application, the presence of AM does not cause significant issues and is considered a trade-off for simplicity.
L1 is usually tapped at around 10-30% of the total turns from the RF ground end (Vcc rail or C9 in this case). It's preferable to have the tap as close to the ground as possible while maintaining good signal output. Placing the tap closer to the collector increases the influence of the antenna on pulling the transmitter frequency. This effect is undesirable as it can make it challenging to set the transmitter on the desired frequency.
C6 is typically a trimmer capacitor made of polyethylene or Teflon, with a length of 5 or 7.5 mm. Its purpose is to adjust the oscillator frequency. The recommended value for C6 is typically around 3-5 picofarads per meter of the operating wavelength.
Given that the FM broadcast band operates at approximately a 3-meter wavelength (100 MHz), the capacitance value for C6 would be roughly 9-15 pf. A 10 pf capacitance would be suitable, considering the transistor and stray capacitances are around 2-5 pf. Alternatively, C6 can be fixed while L1 can be adjusted using a slug or by manipulating turns. However, this method might be inconvenient for certain applications, making the variable capacitor a more practical choice for frequency adjustment.
Typically, the antenna is constructed to be approximately one-tenth of the operating frequency's wavelength, which amounts to around 30 cm (approximately 1 foot) at 100 MHz. However, the actual length of the antenna depends on factors such as the application, mechanical limitations, required signal strength to comply with legal or FCC regulations, and the desired transmission range.
C7 serves as a DC-blocking capacitor and is generally chosen to have a similar value as the tank-tuning capacitor. It's important to note that these values are merely rough guidelines and provide initial values for the components. Optimal performance should be achieved by calculating or empirically determining the final values on functional models.
L1, the inductor used in the FM broadcast band, has an approximate value of 0.25 microhenry. It is commonly created by winding several turns of wire on a 1/4-inch coil form or can even be an air core coil. To minimize mechanical vibration and microphonic effects, it is important to mount the coil securely.
For a coil with a diameter of 1/4 inch and made from #22 wire, a 0.25-microhenry inductor would typically have around seven closely spaced turns. If a smaller coil is required to save space, using #28 wire with a diameter of 1/8 inch, one can achieve 13 turns. The tap can be connected directly to the coil by soldering or a small, twisted loop can be brought out at the appropriate point. Another option is to wind a few turns of wire around the ground end of the coil to act as a secondary winding, eliminating the need for a tap.
The power supply for this circuit can be a 9-volt transistor battery or six AA cells connected in series. It is recommended to include an on-off switch to control the DC power. When packaging the circuit, various suitable configurations can be used, but it is important to consider the requirement for short leads in the oscillator circuit, especially at VHF frequencies. As a cost-effective solution, the entire circuit can be housed in a case obtained from a discarded microphone found in a CB set. You can inquire at your local CB supplier or repair shop, as they may have a collection of non-functioning microphones that you could acquire easily.
Regarding the antenna, if you are operating within short ranges of up to 25 feet, it is possible that you may not require one. Adequate radiation from the circuit might occur, particularly if you encase the circuit in a plastic case. Alternatively, a short length of music wire can suffice. However, it's important to be cautious and prevent any accidental harm. To do so, ensure that you form a loop or attach a bead to the wire's end, as music wire can be sharp. Another option is to utilize a short, collapsible whip antenna designed for cell phone replacement purposes.
The process of tuning up this microphone is quite straightforward. Begin by powering up the circuit and tune C6 while listening on a blank or unused FM channel in your area. Adjust C6 until you detect a carrier signal in the receiver, indicated by a sudden reduction in receiver noise as you tune C6 to match the channel frequency. Speaking into the microphone should result in audible audio in the receiver. If not, verify your wiring connections and check the dimensions of L1. This may require some trial and error, especially if L1 has been altered from the dimensions provided in the text. If available, you can use a frequency counter coupled to L1 to determine the transmitter frequency. In the case of excessive audio gain, increase the value of R5 as necessary. Conversely, if the audio is insufficient, consider using a more sensitive electret microphone or incorporating a second audio preamplifier stage similar to the one used in this circuit.
