This long-range FM radio transmitter has an additional RF power amplifier stage, after the oscillator stage. The extra RF power amplifier used to strenght up the power output to become 200-250 milliwatts. With a great matching multi-element Yagi antenna or 50-ohm ground?plane antenna, this transmitter can give great good signal strength up to a distance of about 2 kilometres.

The circuit designed using transistor T1 (BF494) as 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 signals” power four to five times. Thus, 200-250 milliwatts of power is produced at the collector of transistor T2.

Here the coil winding specifications:
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 3mm diameter air core.
L4 ? 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)

Potentiometer VR1 is utilized to set the centre frequency whereas potentiometer VR2 is used for power control. For humfree 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 centre frequency near 100 MHz.

For better comes about, build 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.

*L1 = 0.1 Uh, 6 to 8 turns of 22 ga. hookup wire close wound around a 1/4 inch diameter non-conductive core such as a pencil

Circuit Notes:
Typically the default for the capacitors model is ceramic, preferably the npo 1% type or equivalent. However, generally almost nothing critical right here. Work with any capacitor you’ve laying arround, but DO NOT use any electrolytic or tantalum capacitors. Only in case you intend to apply this circuit outside the home you may need to use capacitores which have more temperature stability.

To find the signal on your receiver, make sure that there is a signal coming into the microphone, otherwise the circuit won’t work. I use an old mechanical alarm clock (you know, with those two large bells on it). I put this clock by the microphone which picks up the loud tick-tock. I’m sure you get the idea… You can play your phone music and place near the microphone, or you can just lightly tap the microphone while searching for the location of the signal on your receiver.

Here the simple and low cost FM transmitter circuit. The frequency range of this FM transmitter should be about 89MHz ? 109MHz. Output power is about 9mW at 9V.

Components List:

Resistors:
R1, R6        : 12K
R2, R4        : 100K
R3            : 22K
R5            : 1K
R7            : 100R

Capacitors:
C1            : 10uF/25V
C2            : 22n
C3            : 10n
C4            : 3n3
C5            : 2p7
C6            : 22p 
C7            : 100uF/16V
Trimmer       : 0-30pF tuning capacitor

Mic           : ECM-60P

Transistors:
Q1            : BC338
Q2            : BC548B

The battery supply rails have been well tied together with respect to radio frequencies (C1, C2 and C7.) The tracks are also thicker. This makes the circuit a single “solid” block eliminating RF currents in different parts of the circuit. This also means the battery no longer has RF on it which makes the whole unit a lot more frequency stable. A battery is perfect choice.

Download the complete explanation of this FM Transmitter (include circuit diagram, component list, the circuit’s work) in PDF format:
[wpdm_file id=42]

Source: kitrus.com

Main advantage of this circuit is that power supply is a 1.5Volts cell (any size) which makes it possible to fix PCB and the battery into very tight places. Transmitter even runs with standard NiCd rechargeable cells, for example a 750mAh AA size battery runs it about 500 hours (while it drags 1.4mA at 1.24V) which equals to 20 days. This way circuit especially valuable in amateur spy operations

Transistor is not a critical part of the circuit, but selecting a high frequency / low noise one contributes the sound quality and range of the transmitter. PN2222A, 2N2222A, BFxxx series, BC109B, C, and even well known BC238 runs perfect. Key to a well functioning, low consumption circuit is to use a high hFE / low Ceb (internal junction capacity) transistor.

Not all of the condenser microphones are the same in electrical characteristics, so after operating the circuit, use a 10K variable resistance instead of the 5.6K, which supplies current to the internal amplifier of microphone, and adjust it to an optimum point where sound is best in amplitude and quality. Then note the value of the variable resistor and replace it with a fixed one.

The critical part is the inductance L which should be handmade. Get an enamelled copper wire of 0.5mm (AWG24) and round two loose loops having a diameter of 4-5mm. Wire size may vary as well. Rest of the work is much dependent on your level of knowledge and experience on inductances: Have an FM radio near the circuit and set frequency where is no reception. Apply power to the circuit and put a iron rod into the inductance loops to chance it”s value. When you find the right point, adjust inductance”s looseness and, if required, number of turns. Once it”s OK, you may use trimmer capacitor to make further frequency adjustments. You may get help of a experienced person on this point. Do not forget to fix inductance by pouring some glue onto it against external forces. If the reception on the radio lost in a few meters range, than it”s probably caused by a wrong coil adjustment and you are in fact listening to a harmonic of the transmitter instead of the centre frequency. Place radio far away from the circuit and re-adjust. An oscilloscope would make it easier, if you know how to use it in this case.

Every part should fit on the following PCB easily. Pay attention to the transistor”s leads which should be connected right. Also try to connect trimmer capacitor”s moving part to the + side, which may help unwanted frequency shift while adjusting. PCB drawing should be printed at 300DPI, here is a TIFF file already set.

Mini FM Transmitter PCB Layout:

Technical data:
Supply voltage : 1.1 - 3 Volts
Power consumption : 1.8 mA at 1.5 Volts
Range : 30 meters max. at 1.5 Volts

The coil L1 was consist 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 this 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.

Refer to the above diagram, the input source is an electret condenser microphone (you may try to? use other input sources) and signal gain from the input can be adjusted by using the potentiometer P1. You can easily prepare the PCB by using any PCB editor and apply it to the board by using the ironing method. The coils and the antenna can be designed as a part of the PCB made up of the copper area.

Components List:

P1 act as condenser microphone volume level. For FM, coil will be small. Use thin gauge enamel magnet wire. the diameter of coil will be a couple mm: use ink tube from pen to form, and try 8-12 turns. Small inductance coils make for much guess work.

Good luck.