Single chip(MAX 2606) based FM transmitter circuit

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Here’s a single chip FM transmitter circuit using Maxim semiconductors IC MAX2606. The MAX2606 is a compact, high-performance intermediate frequency VCO specially designed for wireless communication circuits. They have monolithic construction with low-noise and a low-power operation in a compact 6-pin SOT23 packing .Th1s low-noise IC feature an on-chip varicap diode and feedback capacitances that avoid the need for external tuning components, making the MAX2606 perfect for portable systems. Only an external inductor is needed to set the oscillation frequency.In addition to this, an
integrated differential output buffer is also there for driving a mixer or prescaler.The MAX2606 can be operated from a single +2.8 V to +5.4V supply and consumes very less current .The chip can be operated from 45MHz to 650MHz .

In the circuit the nominal frequency is set to 100 Mhz by inductor L1, (390nH) . The left and right channel audio signals from your source are added by R3 and R4, and attenuated by the POT R2. R2 can be used as a volume control .POT R1 can be used to select a channel of transmission between 88Mhz and 108Mhz.Use 80 cm long wire as the antenna .

Notes

• Assemble the circuit on a good quality PCB or common board.
• Use a battery for powering the circuit.It will reduce noise.
• An FM antenna from a old radio is a better option than the wire antenna.

The output is between 88MHz and 108MHz, and allows transmitting audio signals to FM radios for remote listening. A schematic is provided with all component values to complete the project. Output power is ~ -21dBm, and the IC operates on 3V.

IC1 is a voltage-controlled oscillator with integrated varactor. Its nominal frequency of oscillation is set by inductor L1, and a 390nH value places that frequency at 100MHz. Potentiometer R1 then lets you select a channel by tuning over the FM band of 88MHz to 108MHz. Output power is about -21dBm into 50 (most countries accept emissions below 10dBm in the FM band).

The audio unit's left and right audio signals are summed by R3 and R4, and attenuated by the (optional) potentiometer R2. R2's wiper signal serves as a volume control by modulating the RF frequency. Signals above 60mV introduce distortion, so the pot attenuates down from that level.

In the absence of a standard FM radio antenna, 75cm (30 inches) of wire will suffice as a transmitting antenna. For best reception, it should be mounted parallel with the receiving antenna. The IC operates on a single supply voltage in the range 3V to 5V, but one must regulate the applied voltage to minimize frequency drift and noise.

Long Range FM Transmitter Circuit

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This circuit is a circuit diagram fm transmitter. This circuit is somewhat different from the previous fm transmitter circuit. Transmitter circuit described here has the additional RF power amplifier stage, after the oscillator stage, to increase the power output of 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element yagi antenna, this transmitter can provide a good enough signal strength to a distance of about 2 kilometers. The circuit built around transistor T1 (BF494) is the 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 power amplifier. This increases the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power produced at the collector of transistor T2. For better results, assemble the circuit on a good quality glass epoxy board and house the transmitter in the case of aluminum. Shield the oscillator stage using aluminum sheets. Transistor T2 must be mounted on the 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.

Coil winding details are given below:
L1 – 4 changes of 20 SWG wire close wound over 8mm diameter plastic former.
L2 – 2 changes of 24 SWG wire near top end of L1.
(Note: There is no core (ie air core) is used to coil on top)
L3 – 7 changed from 24 SWG wire close wound with 4mm diameter air core.
L4 – 7 changed from 24 SWG wire-wound on ferrite beads (choking)
Potentiometer VR1 is used to change the fundamental frequency whereas potentiometer VR2 is used as power control.

VHF Video Transmitter 60-200 MHz

Here's a simple video transmitter for VHF TV channel will accept baseband video input, hence it can be driven by most CCD cameras and VCR video outputs. It ouputs roughly 80mW and when used with a 40cm telescopic antenna over 100 metres range is possible.
The transistor of the video transmitter can be a BC108, BC546, BC337 or a 2N2222. L1 is wound on a 10 mm air former. Use 6 turns 24 SWG for frequency 60-80 MHz, 4 turns for 150-180 MHz, and 2 turns for 180-200 MHz

You can use this with a monochrome or color video signal. To transmit sound just build the wide band FM transmitter and tune it to the audio channel.

preamplifier 88-108Mhz

This VHF amplifier working on Band 2 Radio Spectrum tuning approximately 88 - 108 Mhz

The Preamplifier circuit uses two 2N3819 FET's in cascade configuration. The lower FET operates in common source mode, while the upper FET, operates in common gate, realizing full high frequency gain. The bottom FET is tunable allowing a peak for a particular station.

Coil details follow:

L1 4 turns of 18swg air spaced with a 1cm diameter, the tap is one turn up from earth end...

L2 4 turns of 18swg air spaced with 1 cm diameter. The coupling coil is 1 turn interwound from the supply end. Enamel coated wire must be used. More info ...

FM Wireless Mike

FM Wireless Mike can transmit voice signals to any FM Radio receiver 100 meters away. The circuit is basically a frequency modulated transmitter working at 100 MHz. The frequency of the transmitter can be varied slightly by changing the trimmer C5. You can use ordinary condenser mike in this circuit. The transistors can be replaced by any low power transistors like BC148 or BF494.

The coil L1 is air core 6 turn 24 SWG. Third turn is tapped and connected to telescopic antenna. You can replace telescopic antenna with a small piece of wire.

Miniature MW Transmitter

Here is a very simple, inexpensive and interesting project which provides lot of fun to a home experimenter or hobbyist. This simple transmitter can transmit speeches or songs within a short range.

The circuit uses only one transistor. The entire circuit can be easily assembled on a prototyping printed circuit board. After assembling all the components properly put the whole assembly in a plastic enclosure provided with a telescopic antenna. Now keep your MW radio and the transmitter on a table about one meter away from each other. Switch on the radio receiver and turn to a clear spot where no broadcasting station is present. Now switch on the transmitter and turn the gang condenser. At some position loud hissing sound will be heard from your receiver. Stop the gang condenser at this position. Speak some thing to the speaker which serves as the microphone. Now turn the radio receiver to get clear and loud sound.

The transmitter have a range of 200 meters. You can increase the range by using an external antenna and sensitive receiver at receiving end.

AM DSB Transmitter for HAMS

This circuit of AM transmitter is designed to transmit AM(amplitude modulated) DSB (double side Band) signals. A modulated AM signal consists of a carrier and two symmetrically spaced side bands. The two side bands have the same amplitude and the carry same information. In fact, the carrier itself conveys or carries no information. In a 100% modulated AM signal 2/3rd of the power is wasted in the carrier and only 1/6th of the power in each side band

In this transmitter we remove the carrier and transmits only the two side bands. The effective output of the circuit is three times that of an equivalent AM transmitter.

Opamp IC741 is added here as a microphone amplifier to amplify the audio signals from the condenser mike. The output of the Opamp is fed to the double balanced modulator build around In4148 diodes. The modulation level can be adjusted with the help of preset VR1.

The carrier using cristal wired around BC548 transistor T2. The carrier is further amplified bt transistor T1, which also acts as a buffer between modulator. The working frequencey of the transmitter can be changed by using the cristals of the diffreent frequencies. For multi-frequency operation, selection of different cristals can be made using a selector switch. The level of the carrier coupled to the DEM(double Balanced Modulator) cna be adjusted with the help of preset VR2.

The output of the DBM contains only the product (of audio and carrier) frequencise. The DBM supress both the input signals anf produce the double side band supressed carrier(DSBSC) at its output. However, since the diodes used in the balanced modulator are not fully matched , the output of the DBM does contain some residual carrier. This is known as carrier leckage. By adjusting the 100-ohm preset(VR2) and the trimmer (c7) you can nullify the carrier leckage.

To receive DSb signals you need a Beat Frequency Oscillator(BFO) to reinsert the missing carrier. If you dodn't have a BFO, or want to transmit only AM signal, adjust preset VR2 to leak some carrier so thar you can receive the signals on any ordinary radio receiver. In AM mode 100% modulation can be attained by adjusting presets VR1 and VR2.
The DSBSC signal available at the output of the balanced modulator is amplified by two stages of RF libnear amplifiers. Transisor 2n2222A(t3) is used as an RF amplifier., which provides enough signal amplification to drive the final power amplifier around transistor SL100B. The output of the final power amplifier is connected to the antienna.

All coils are to be wound on ferrite balun cores(same ast used in TV balun transformer of size 1.4cm x 0.6 cm) using 24 SWG enameled copper wire. proper heat-sink should be provided for SL100B transistor used as final power amplifier.

Range of the order of a few kilometers can be easily achieved by proper choice of the site, type of antenna (such as a resonant half-wave dipole of lenght 10 meters for 7.08 Mhz frequency) and proper matching of transmitter to the antanna. Use good quality shielded wire of short length to connect the crystals.

60W Linear amplifier

The 60 Watt linear amplifier is simple all solid state circuit using power mosfet IRF840. The IRF series of power transistors are available in various voltage and power ratings. A single IRF840 can handle maximum power output of 125 watts. Since these transistors are used in inverters and smps they are easily available for around Rs: 20/-.

The IRF linear amplifier can be connected to the out put of popular VWN-QRP to get an output of 60 Watts. The circuit draws 700 ma at 60 Volt Vcc. Good heat sink is a must for the power transistor.

Alignment of the circuit is very easy. Connect a dummy load to the out put of the circuit. You can use some small bulb like 24V 6Watts as the dummy load. I have even used 230V 60Watts bulb as dummy load with my IRF840 power amplifier working at 120Volts. Adjust the 10K preset to get around 100 ma Drain current. I used gate voltage of 0.8V with my linear amplifier. A heigh gate voltage can make the power transistor get distroyed by self oscillation. So gate voltage must be below 2V and fixing at 1V will be safe.

Bifalar transformaer T1 is wound with 8 turns 26SWG on 1.4 x 1 balun core.

The coil on the drain of IRF is 3 turns 20 SWG wound on 4 number of T13.9 torroids (two torroids are stacked to form a balun core). The RFC at the Vcc line is 20 Turns 20 SWG wound on T20 torroid.

Ta7378P FM wireless microphone circuit

TA7378P using FM radio integrated block the production of FM wireless microphone, external components small, simple and easy system, work stability, particularly suitable for the production of radio enthusiasts. Figure FM wireless microphone for the specific circuit, ICl (TA7378P) includes RF amplifiers, mixers, to enlarge the buffer, the local oscillator circuit and bias, the regulator circuit.

The circuit Qiaomiaodejiang combination of internal circuits, use of the circuit-and AFC (automatic frequency control) the use of a variable capacitor oscillator circuit to generate FM (FM) waves.

In order to reduce the impact of external, in the oscillation between the mixer and a buffer amplifier has. Finally, RF amplifiers zoom through launch FM radio antenna.

15 W transmitter power amplifier 88-108MHZ

The power amplifier can be reactive 1-2 W ,88-108MHZ power FM transmitter As for the expansion of 10-15 W, a single C Larger and multi-level low pass filter components, has a high conversion efficiency and strong Yi-wave suppression.

Circuit see attached map shows, using high-power launch of C1972, its parameters are as follows: 175 MHZ, 4A, 25W, power gain ≥ 8.5 db, as shown by parameters, circuit work center frequency of about 98 MHZ, the importation of about 2 W of RF power , The rated output up to 15 W.

To maintain 88 ~ 108 MHZ with any frequency output reached rating, according to the level before the center frequency of some components to make suitable adjustments. May, when necessary, to reduce low-ball-series, to increase power output. The expansion of the power signals from three low-pass filter Yi-filtered high element of the transmitting antenna feed.

Components choice: In addition to electrolytic capacitor, the other tiles with high-frequency capacitors, C11, C12, C14 use high-frequency characteristics of a good, stable performance of adjustable capacitors, inductors Choke RFC1, RFC2 finished with inductors, must pay attention to the current RFC2 Carrying capacity, should use the coarse diameter Cores with the inductors.
L1-L6 available ø0.8mm the high-intensity enameled wire system, a diameter of about 5 MM, a few laps in the plans to "T" for the units indicated. Q1 ordinary Q9 socket, and supporting the use of plugs. Q2 used for 50 Ω RF output connectors, and then of resistance is smaller, more conducive to impedance matching.
Larger effective power more common for the launch of the C1972, of course, especially if you sufficient money to buy blocks C2538 contour of the gain, power will be even greater.

Debug circuit, be sure to pay attention, the power circuit, we must connect false load (I use 30 1 W, 1500 Ω high-precision metal film resistors made parallel), and there must be enough in the cooling devices, normal working hours Power Of not less than 2.5 A, the antenna impedance strictly equivalent to 50 Ω, can not be used Duanbang drawbars antenna, or a strong current of RF feedback circuit will create their own interference, most of RF energy to space and can not be convergence in the consumption of power, to overheating Damage must be launched for 50 Ω coax, tabled Reply to launch outdoor antenna.

Circuit the normal work of the key lies in whether the circuit debugging, the whole process had to very carefully.

Debugging, enter only the smaller the incentive power supply voltage drop to 9 V, using high-frequency voltage (can not use ordinary multimeter) monitoring false load at both ends of high-frequency voltage value, regulating C12, C14, L3, L4, L5, L6 So that the voltage range of 15-20 V around, and then adjust C11, L1 voltage to the largest.

And then gradually raise the voltage, each raising a voltage repeatedly adjusted C12, C14 and C11, L1 so that the maximum output voltage, noted that the input voltage and RF power simultaneously increasing incentives to ensure the accuracy of the results of debugging. Reach rating, 13.8 V supply voltage of about 2 A current work around, 50 Ω-load resistance at both ends voltage ≥ 40 V, RF power output of 15 W.

With the RF power amplifier with 50 Ω-wide umbrella to the vertical launch antenna (gain of about 2 dB), to ordinary FM radio test fired from the coverage of not less than 15 KM

VHF UHF TV modulator

Simple TV Modulator that working on VHF UHF Band, the oscillator generates frequency is modulated with the video signal and the modulated carrier wave thus generated is fed into the TV set's aerial input via a cable. Then all that remains to do is tune the VHF UHF TV set to the correct frequency.

The harmonics generator converts the oscillator signal into a sort of frequency spectrum containing all the multiples of 27 MHz up to about 1800 MHz. The TV modulator's output signal is made up of a large number of little peaks, each of which is a complete transmitter signal. At least one of these will always be in band I (VHF channels 2. . . 4), one in band III (VHF channels S. . .12) and many of them will be in bands IV and V (UHF channels 21.. .69).

TV Transmitter Band I and III

This TV transmitter working on VHF Band I and III, using negative sound modulation and PAL video modulation. This is suitable for countries using TV systems B and G, like Australia and Indonesia.

This circuit has not been tested at UHF frequencies. The modulated sound signal contains 5.5 -6MHz by tuning C5. Sound modulation is FM and is compatible with UK TV Transmitter System I sound. The transmitter however is working at VHF frequencies between 54 and 216MHz (band I and Band III) and therefore compatible only with countries using Pal System B and Pal System G.

88-108 MHz Preamplifier

This VHF amplifier working on Band 2 Radio Spectrum tuning approximately 88 - 108 Mhz

The Preamplifier circuit uses two 2N3819 FET's in cascade configuration. The lower FET operates in common source mode, while the upper FET, operates in common gate, realizing full high frequency gain. The bottom FET is tunable allowing a peak for a particular station.

Coil details follow:
L1 4 turns of 18swg air spaced with a 1cm diameter, the tap is one turn up from earth end...
L2 4 turns of 18swg air spaced with 1 cm diameter. The coupling coil is 1 turn interwound from the supply end. Enamel coated wire must be used.

Making Simple TV transmitter

(1) Soundless version, You are familiar with the most simplest FM transmitter" that I designed (left). Let's try to transform it into a TV transmitter. Just change the input from the audio to the video (video camera or VCR) and check the signal at your television set: in Europe, for instance, choose the channel 2 - 4 and turn the trimmer cap of the transmitter. You will find some images or might watch a clear image. This suggests that it will be not so difficult to build a TV transmitter.

(2) Advanced Version (AV) , When you succeed in this version to work, you may try the advanced version (pdf). ( http://www.translocal.jp/microtv/20070704tvtx_rtctk01.pdf )

In this version, stability and quality are improved. This can have even the audio too. However, in order to complete this, you have to use a proper frequency counter.

FM Transmitter With Transistor 2SC1815

Will have to introduce a system of simple and small FM transmitter. Not only does it meet the requirements of the launch distance, but also at the same time using microphone and voice input signal line, background voice. Modulation circuit surveillance has also joined the table first, so that they can better control and the proper use of transmitters, the following circuit diagram, read briefly, whether it's just like a radio station equipment as many functions. Yes, this small transmitter can help you easily set up an amateur FM radio stations! 500 M in the coverage around. Echocardiography the bar, act immediately!

Principle small FM transmitter circuit components and options:

Signal to the microphone after intake, the C1 coupling into the BG1 and external circuit voltage of the single negative feedback amplifier, the voltage signal amplification weak enough to enlarge the extent of U1A and the line to enter the input signal with a mixed U1B. Mixed signal modulation way to the R17 from BG2 FET and the surrounding circuit composed of common than LC finishes the test, the test than the LC oscillator are characterized by a general three-capacitance oscillator simple, but also with high efficiency and high stability. Waveform, and modulation bandwidth, as this amateur production of FM transmitters appears to be particularly important.

Finally BG3 cast by the post-amplifier to enlarge the field by launching antenna fired by general can listen to FM radio broadcast. Another component of the signal through the amplifier to enlarge U1C after the wave rectifier D2 SR DC drive microamps table, Near and surveillance signal modulation rate. Use the system for general should not exceed 85 per cent suitable. GM and other integrated circuits using LM324 four operational amplifier and form a single power reverse input, both of the input voltage is set at half the supply voltage. Table surveillance for the first u-200, BG1.BG3 choose 2SC1815. BG2 three DO2 FET, D2 varactor diode using a S2267. Antenna require the use of 1 / 4 wavelength, such as the use of the best pull rod antenna not less than one metre. Otherwise effect will be less than overstaying launch. Final say about, please do not interfere with normal radio!

FM transmitter With uPC1651

The circuit has been produced by Japan's NEC as a major upc1651 IC devices, the high gain circuit, work stability, thereby ensuring the microphone performance. Use of FM transmitter. Used it 40 - 50 cm soft drag line for antenna, the effective range of more than 30 launch M. L carefully spacing adjustment and fine-tuning capacitor, will launch frequency coverage for 88 - 108 Mhz. Map L enameled wire with diameter of 0.51 mm in diameter, 4 mm cylindrical Tuitai from around 5 laps

Macsot MR-700 High Quality wireless microphone

This is collected by the Macsot MR-700 wireless microphone system launch of the circuit for maintenance reference. High Quality Audio on receiver output.

Active components:

• Microphone or Audio Preamplifier with IC MC4458
• Dynamic Audio processed with one NE571 From Philips
• Frequency Oscillator generated by one crystal 20.05 MHz and Varactor diode 2CB11A. No drift frequency is guaranted.
• 9V DC power supply or battery.

FM Stereo Transmitter With MC145151

This fm stereo transmitter use pll chip MC145151 from Motorola and stereo encoder chips BA1404 with vco built in. MC145151 IC used is to overcome the single-frequency instability in the BA1404. Modulation of the emission control BB910 frequency. B571C kept in control procedures, the procedures required to prepare another.

Parts List and Schematic

Component Values of the fm stereo transmitter you can find out the schematic. Please download schematic for enlarged image

Amplifier section of this FM Stereo transmitter uses a single 2sc2053 transistor with power out about 250 mWatt.

300mW FM Transmitter 2SC2538

The FM transmitter using a varactor diode way radio, plus a Class C amplifier, RF output power of up to 300 mW more open to more than one kilometer distance communications. Select the components of this fm transmitter: Q1 with ≥ 100 mA, Ft ≥ 300 MHz, β ≥ 100 tubes available 3 DG82, 3DG122, 3DG130, 2G711; Q2 with 2SC2538, 2SC1970, etc..

It must be noted in the fm transmitter assembly that RF Baffles circle L8, L9, L10; C13, C14, C17, C18, C19, and so can not be omitted, otherwise it would cause unnecessary self-excited oscillation. L1, L5 need to tap extraction, data such as icons, not otherwise due to impedance matching, the output power of less than maximum. Such as a battery-powered, Q3 (Darlington tube), C15, R6 can be omitted.

Debugging steps :

1. In the output termination of testing circuit to regulate the C7, C9, C12 to the largest multimeter readings,

2. Remove test circuit connected to one meter in length pole antenna to pull to fine-tune C12 simple reading of the largest field will be completed debugging.

500mW FM PLL Transmitter 88-108MHz

FM transmitters have always been fascinating and one can find thousand examples of them on the internet. Sadly most of them are full of error and miss leading information. Most of them also have low stability and frequency drift, many coils and components which are difficult to find. The output power is often set to several watts with just a transistor or two…..can't fool me. So therefore I decided to construct a simple transmitter with great performances.

Some contruction achievements I wanted were:

# Simple construction
# Commonly components
# High quality and stability
# Low number of coils
# High output power

The frequency of this transmitter can easy be changed with software and space/compress an air coil, simple don't you say? The basic hart of this transmitter is a colpitts oscillator. The oscillator is a VCO (voltage controlled oscillator) which is regulated by a PLL circuit and PIC micro controller. Don't get upset now…it is not that difficult after all. Let's check the schematic and I will explain the function.

Hardware and schematic

Click on the pic to see the large schematic. The main oscillator is based around the transistor T1. This oscillator is called Colpitts oscillator and it is voltage controlled to achieve FM (frequency modulation) and PLL control. T1 should be a HF transistor to work well, but in this case I have used a cheap and common BC817 transistor. The oscillator needs a LC tank to oscillate properly. In this case the LC tank consist of L1 with C1, C2, C3, and the varicap BB139. The coil is parallel with C1 and C2 which are in serial . The same with the varicap and C3. You can think that L is parallel with [ (C1//C2) + (Varicap//C3)] The value of C3 will set the VCO range. The large value of C3 the wider will the VCO range be. Since the capacitance of the varicap is dependent of the voltage over it, the capacitance will change with changed voltage. When the voltage change, so will the oscillating frequency. In this way you achieve a VCO function.

PLL and Microcontroller
The oscillator is made to work as "Voltage Controlled Oscillator" VCO.

To control the frequency a synthesizer circuit LMX 2306 has been added. The PLL circuit has a pickup coil (L2) connected to pin 6. This coil should be put close to the L1 coil for picking up some of the oscillating energy. The PLL in the LMX2306 will then use this frequency to regulate the VCO and lock it to desired frequency. The regulating system also need an external reference crystal. In this case I use 12.8 MHz.

At pin 2 of MX2306 you will find a PLL filter to form the Vout which is the regulating voltage of the VCO. The PLL try to regulate the Vout so the oscillator keeps the frequency locked to desired frequency. The desired frequency is programmed into the PIC EEPROM and is clocked into the synthesizer (LMX2306) at power up. I will below explain how to program the EEPROM for different frequencies. At pin14 of the synthesizer you have a control output. At this output you will find the reference frequency for testing. (I must warn you because the signal is not symetrical in shape. The positive pulse are only a few microsecond so you will have difficult to see it at oscilloscope.) I solved it by connecting it to a 74HC4020 (14-stage Binary Counter) to pin 10 Clock input. At Q0 (pin 9) you will have a symmetrical square wave with half frequency since the circuit is a counter. At Q1 pin 7 it will be divided by 4, see datasheets for more info.

LF input
The audio you wish to transmit should be connected to the Audio input (left side of schematic).

The signal will affect the varicap and thereby Frequency Modulate FM the RF carrier. A potentiometer P1 has been added to set the modulation depth (Wide FM or Narrow FM). You may have to play a bit with the value of P1 because it tends to modulate to much. You may have to add a 500k - 1M potentiometer instead. You test and find out yourself.

Buffer stage

Here you find another HF transistor and it is working in class C. The resistor R1 and the resistor Re2 set the DC current. In this case I found that 9.1k will give good output power and so the same with 150. If you wish to increase the power Re2 should be lower. You can add another 150 ohm resistor parallel.

In the table below I show you the output power with different voltages and values of resistor Re2. I advice you not to run this transmitter with to high output power. The transistor I use is a small one and tends to get hot. I advice you to run the unit from 0 - to 200mW. At 500mW the transistor will be in pain...*smiling* At the output you will find a T network. This "filter" will match the antenna impedance to the transmitter output stage. You have two variable capacitors 60pF to tune the transmitter for best performances.

The antenna I used I a 1/4 wave whip antenna (wire) about 75cm long. This type of antenna is smaller but not so good performance as a dipole. With a dipole you will be able to transmitter much longer distance.

How long can I transmit?
That is a very difficult question because the environment affect the transmitting distance very much. In a city environment with concrete buildings the transmitter will send maybe 200m.

I an open filed it will transmit 2000m. I did a filed test and with 70mW output power into the "bad" whip antenna placed indoors I could transmit 200-300m out into a park with no problem.

Output power

Table below show you the power measurements I have done. The Re2 is 150 ohm and in some test I connect a 50 ohm parallel. The output power in measured into a dummy load of 50 ohm.

Testing

The first thing you should test is that the oscillator is working. I disconnected the Vout from pin 2 of the PLL LMX2306. I then connected Vout to ground and check the oscillator. The oscillator should now oscillate at the lowest frequency. With my Wireless frequency counter I found that the oscillator was working at 100 MHz. I streatched the coil L1 a bit until it oscillated at 105 MHz. I then connected Vout to +5V and now the oscillator was oscillating at 108MHz. Great!, just as I wanted. By changing the Vout from 0 to +5V I could change the oscillating frequency from 105 to 108 MHz. I then reconnected the Vtune to the PLL.

Download PIC16F870 programs (INHX8M format)
The zip file contains several hex files made for different frequencies (88 to 108) MHz.

fm_500.zip( http://hem.passagen.se/communication/pic/fm_500.zip ) PLL software to FM transmitter (the hex files are zipped!).

Final word

This project is explaining how you can build a FM transmitter with great performances. I advice you not to use it because it is not legal. You can only use it with a dummy load, not with an antenna. If you choose to use an antenna I hope you will use it with good manner.

More Information --->( http://hem.passagen.se/communication/fm_500.html )