FM Transmitter Mod 7

A portable FM transmitter that can be used to replace a FM microphone. Notes:This simple transmitter operates from a 9V battery as shown above. I personally built this for a purpose. This is how it happened:- "members of my singing group find it difficult to handle a FM mic in Church. The choir sings in acapella form and the congregation expect us to perform well. The old FM mic system was not a perfect solution, so I built this kit and a mixer for the receivers. using three separate transmitter circuits and a mixer on the receiever, the choir voices were perfect and cleanly reproduced."From the notes I made experimentally, C3 is vital to the circuit and without it the circuit may become unstable. C4 is in parallel with C5 and presents a moderate load impedance. Finally all transistors are NPN. The circuit works well and has proved reliable.L1 Details:In the circuit L1 has 6 turns and has a diameter of 5.5mm and has a length of 4.5mm. Using the above values, the inductance of L1 can be calculated. A diameter of 5.5mm = 0.22 inches, the radius is half this value or 0.11 inch, the length is 4.5mm and number of turns, n = 6. This gives L1 a value of: 0,156 uH

FM transmitter Mod 6

The power output of many transmitter circuits are very low because no power amplifier stages are incorporated. The transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometres.The circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signal power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2.For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using an aluminium sheet. Coil winding details are given below:L1 - 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.L2 - 2 turns of 24 SWG wire near top end of L1.(Note: No core (i.e. air core) is used for the above coils)L3 - 7 turns of 24 SWG wire close wound with 4mm diameter air core.L4 - 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.This transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a license is illegal in India.
WARNING: Transmitting on the UK Commercial FM band is also illegal in the UK, please see the general disclaimer. This circuit is shown for educational purposes only.

FM Transmitter Mod 5

This circuit provides an FM modulated signal with an output power of around 500mW. The input Mic preamp is built around a couple of 2N3904 transistors, audio gain limited by the 5k preset. The oscillator is a colpitts stage, frequency of oscillation governed by the tank circuit made from two 5pF capacitors and the inductor. Frequency is around 100Mhz with values shown. Audio modulation is fed into the tank circuit via the 5p capacitor, the 10k resistor and 1N4002 controlling the amount of modulation. The oscillator output is fed into the 3.9uH inductor which will have a high impedance at RF frequencies. The output stage operates as a class D amplifier , no direct bias is applied but the RF signal developed across the 3.9uH inductor is sufficient to drive this stage. The emitter resistor and 1k base resistor prevent instability and thermal runaway in this stage.

FM Transmitter Mod 4

A small FM voice transmitter for Band 2 VHF. This small transmitter uses a hartley type oscillator. Normally the capacitor in the tank circuit would connect at the base of the transistor, but at VHF the base emitter capacitance of the transistor acts as a short circuit, so in effect, it still is. The coil is four turns of 18swg wire wound around a quarter inch former. The aerial tap is about one and a half turns from the supply end. Audio sensitivity is very good when used with an ECM type microphone insert.

FM Transmitter Mod 3

Take care with transmitter circuits. It is illegal in most countries to operate radio transmitters without a license. Although only low power this circuit may be tuned to operate over the range 87-108MHz with a range of 20 or 30 metres.

I have used a pair of BC548 transistors in this circuit. Although not strictly RF transistors, they still give good results. It is a two terminal ECM, but ordinary dynamic mic inserts can also be used, simply omit the front 10k resistor. The coil L1 was again from Maplin, part no. UF68Y and consists of 7 turns on a quarter inch plastic former with a tuning slug. The tuning slug is adjusted to tune the transmitter. Actual range on my prototype tuned from 70MHz to around 120MHz. The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work. Although RF circuits are best constructed on a PCB, you can get away with veroboard, keep all leads short, and break tracks at appropriate points.One final point, don't hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations. I have had some first hand experience of this problem.

FM Transmitter Mod 2

An low power FM Transmitter using an op-amp as the audio preamp and a single transistor as the RF amplifier.

L1 is 0.112uH (this tunes to the middle of the FM band, 98 MHz, with VC1 at its centre value of 33pF).L1 is 5 turns of 22 swg enamelled copper wire close-wound on a 5mm (3/16") diameter former. Alternatively, you can have a fixed 33pF cap instead of VC1 and have L1 as an adjustable molded coil (eg UF64U from Maplin). VC1 will give you a tuning range of 85 - 125 MHz, and a possible choice is the Philips type polypropylene film trimmer (Maplin code WL72P).Two sets of oscillator bias resistors are given, the ones in the brackets give about 20% more RF power.Mike is our favourite Omnidirectional sub-mini electret (Maplin code FS43W). Ant is a (lambda / 4) whip monopole (eg 76 cms of 22 swg copper wire).Q1 is configured as a Clapp oscillator. Frequency modulation results from the audio voltage changing the transistor's base-emitter capacitance.

R1=4K7, R4 = 150K, R7=3K9 (2K7), R2 = 4K7, R5 = 220R, R8 120R (82R), R3 = 4K7, R6 = 4K7. All resistors except R8 are at least 0.25W rated. R8 is at least 0.5W rated(the 0.6W metal film M-series from Maplin can be used for R1-R8). C1 = 1n, C4 = 22uF, C7 = 10n, C10 = 1n, C2 = 4u7, C5 = 1n, C8 = 1n, C3 = 1n, C6 = 10n, C9 = 33pF, VC1 = 5-60pF, IC1 = LM358, Q1 = ZTX108.

FM Transmitter Mod 1

Here is the schematic, PC board pattern, and parts placement for a low powered FM transmitter. The range of the transmitter when running at 9V is about 300 feet. Running it from 12V increases the range to about 400 feet. This transmitter should not be used as a room or telephone bug.

L1 and L2 are 5 turns of 28 AWG enamel coated magnet wire wound with a inside diameter of about 4mm. The inside of a ballpoint pen works well (the plastic tube that holds the ink). Remove the form after winding then install the coil on the circuit board, being careful not to bend it. C5 is used for tuning. This transmitter operates on the normal broadcast frequencies (88-108MHz). Q1 and Q2 can also be 2N3904 or something similar. You can use 1/4 W resistors mounted vertically instead of 1/8 W resistors. You may want to bypass the battery with a .01uf capacitor. An antenna may not be required for operation.

Parts: C1 = 0.001uf Disc Capacitor, C2 = 5.6pf Disc Capacitor, C3,C4 = 10uf Electrolytic Capacitor, C5 = 3-18pf Adjustable Cap, R1 = 270 Ohm 1/8W Resistor (270 Ohm 1/4W Resistor), R2,R5,R6 = 4.7k 1/8W Resistor (4.7K 1/4W Resistor), R3 = 10k 1/8W Resistor (10K 1/4W Resistor), R4 = 100k 1/8W Resistor (100K 1/4W Resistor), Q1, Q2 = 2N2222A NPN Transistor (2N3904, NTE123A), L1, L2 = 5 Turn Air Core Coil, MIC = 1 Electret Microphone MISC = 9V Battery Snap, PC Board, Wire For Antenna