Friday, July 4, 2014

A Transmitter Circuits

1 Valve 3.5MH CW Transmitters

1 Valve CW Transmitter

1.5 Volt Tracking Transmitter

1.5 Volt Tracking Transmitter 2

10W HF Linear Amplifier

150mW FM Transmitter

1W CW Transmitter

2 Transistor FM Transmitters

2 Transistor FM Transmitters:

2 Valve 40m CW Transmitter

2 Valve CW Transmitter

20M, 4W QRP Transmitter

250mW HF CW Transmitter

27MHz AM/CW Transmitter

2N2222 40 Meter CW / DSB Tranceiver

3 Watt FM Transmitter

30 Meter QRP Transmitter for Morse Code

3W HF QRP Linear Amplifier

4 Transistor Tracking Transmitter

4 Transistor Transmitter

433MHz Transmitter using SAW Resonator

5 Watt HF CW Transmitter

500mW HF Linear Amplifier

56K RF Modem

5W PLL Transmitter

7Mhz AM/CW Amateur Radio Transmitter

7MHz QRP Transmitter

7MHz SSB Transceiver:  Circuit digram and brief description of 7MHz SSB Transceiver for Hams. The circuit is designed around two numbers of MC1496. It can push around 80 Watts with IRF840 in the final. You can down load HTML version or the printer friendly word document.

80 Meter DSB Transmitter

807 and 1625 Valves:  data on vacuum tubes 807 and 1625 used in ham radio transmitters. Describes various pin voltages and different operation modes.

AM DSB Transmitter for Hams:  circuit diagram of simple double side band suppressed carrier (DSBSC) transmitter for hams. Circuit uses crystal oscillator, crystal can be switched for multi band operation. .

AM oscillator for Wireless Microphones

AM Transmitter

Antennas for Ham Transmitters:  Describes how to construct various type of antenna for Ham Radio Transmitters.

AT Volt Repeater Controllers

Basic FM Radio Transmitters

Basic RF Oscillator #1

Basic RF Transmitter for PIR Sensors

Battery operated FM rebroadcast transmitter :  Gives you 10 to 20 meters range and runs for months on a single penlight cell.

Ceramic Filter BFO:  Receive SSB and CW transmissions on your BC receiver. Simple BFO is build around 455 KHz Ceramic Filter.

Crystal Controlled FM Transmitter

Current Transmitter With Linear Voltage Transfer Rejects Ground Noise:  08/07/00 Electronic Design - Ideas for Design / Many systems use current signals to control remote instruments. The advantage of this method is the ability to operate with two remotely connected power supplies even if their grounds are not the same. In these cases, it's necessary for the output. . .

Design of Brookdale AT Volt Repeater System Exciter:  uses a pair of Hamtronics model TA4512-watt narrow-band FM voice transmitters to develop video and audio carriers on439.250 MHz and443.750 MHz

Easy 2 Meter Transmitter:  This project is a simple transmitter using only one crystal and will cover 145.00 to 146.00 MHz. The crystal is a 44.9333 MHz crystal for 145.500 receive, as used in the Trio (Kenwood) 2200, PYE, Motorolla, Tait equipment, to name but four. The frequency of the crystal is not critical as almost any other xtal for the 2-meter band will function

Experimental Data Transmitter for Fiber optics

Fibroptic transmitter

FM Band Monaural Transmitter

FM Beacon Transmitter (88 108 MHz):  This circuit will transmit a continuous audio tone on the FM broadcast band (88-108 MHz) which could used for remote control or security purposes. Circuit draws about30 mA from a 6-9 volt battery and can be received to about100 yards.

FM Broadcast Audio Transmitter :  Monophonic FM band transmitter for home use.

FM Bug

FM Radio Bug

FM Radio Telephone Transmitter

FM Radio Transmitter

FM Radio Transmitter #1

FM Radio Transmitters With OpAmp

FM Transmitter

FM transmitter

Four Channel Wireless Transmitter & Receiver:

Four Transistor Tracking Transmitter

Frequency Agile 80m CW QRP Transmitter

High Power FM Bug

Infra / Radio Remote Control Transmitter / Receiver

Infrared Transmitter and Receiver Schematic Diagrams

Infrared Transmitter Circuit:

Infrared Transmitter for Audio:  (Amplitude Modulated IR)

Laser Diode Transmitter

Laser Transmitter Schematics

Light Sensing RF Transmitter

Li'l 7 AM Transmitter Schematic

Long Range FM Transmitter

Low Power FM Transmitter

Micro Power AM Broadcast Transmitters:  In this circuit, a 74HC14 hex Schmitt trigger inverter is used as a square wave oscillator to drive a small signal transistor in a Class C amplifier configuration. The oscillator frequency can be either fixed by a crystal or made adjustable VFO with a capacitor/resistor combination.

Micro Spy With FETs

Micro Spy With TTL

Micro Spy With USW

MicroPower FM Broadcasting Circuits

Miniature FM Transmitter #2

Miniature FM Transmitter #3

Miniature FM Transmitter #4

Miniature FM Transmitters #4

Miniature MW Transmitter:  circuit diagram of simple medium wave transmitter using BF494B. This simple transmitter have a range of 200 meters. .

MINIATURE TRANSMITTER:   What can I say about this circuit except brilliant I have actually built this one and was very impressed, I built it using leaded components maybe one day try a bit of smd make it even smaller, problems needs a big Ariel to transmit over any great distance.

One Valve 3.5MH CW Transmitter

Op Amp Based FM Transmitter

Phasing SSB Exciter

QRP HF Transmitter

QRP Keyer:  very simple keyer circuit using only one transistor.

QRP SSB Transmitter

Quality FM Transmitter

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Sensitive FM Transmitter

Shortwave Radio Transmitter

Shortwave Transmitter:

Simple FM Microphone

Simple FM Transmitter #1

Simple Low FER Transmitter

Simple RF Transmitter

Simple T Volt Transmitter #1

Simple T Volt Transmitter #2

Simplest RF Transmitter

Small circuit forms programmable 4 to 20 mtransmitter:  04/17/03  EDN-Design Ideas / One of the key challenges in the design of 4 to 20-mA current transmitters is the voltage-to-current conversion stage. Conventional transmitters use multiple op amps and transistors to perform the conversion function. These approaches have been around for a long time, but they are usually inflexible, have poor power efficiency, and have limited current compliance...

Small FM Transmitter #2

Small Radio Transmitter

Spark gap Transmitter

Surveillance Transmitter Detector:  This circuit can be used to "sweep" an area or room and will indicate if a surveillance device is operative. The problem in making a suitable a detector is to get its sensitivity just right, Too much sensitivity and it will respond to radio broadcasts, too little, and nothing will be heard.

T Volt Transmitter:  allows you to send video to any television in the house, Poptronix kit circuit

Telephone Transmitter

Three Watt FM Transmitters

Tracking Transmitter #1

Tracking Transmitter #2

Transmitter senses triple relative humidity figures:  09/26/2002  EDN - Design Ideas / The circuit in Figure 1 is a triple, relative-humidity sensor and radio transmitter. Sensors 1 and 2 form two gated oscillators with natural frequencies of 10 and 5 kHz, respectively, at relative humidity of 50%. The gated oscillators use variable resistances R2 and R3, respectively. Together, these two oscillators generate FSK-modulated outputs at output of IC1B, Pin 6..

Transmitter using LM317:

Two Transistor FM Transmitters

Two Valve 40m CW Transmitter

VHF / UHF T Volt Modulator:   Elektor January1985

VHF Audio Video Transmitter:  This circuit is a TV transmitter on VHF band.

VHF beacon transmitter

VHF FM Transmitter

VHF Transmitter

Video / Audio Wireless Transmitter:  circuit diagram and project description

Video to RF Modulator:  This circuit is a RF modulator which can be used for modeling of video signal.

Video/Audio Wireless Transmitter

Wire Tracer

Wireless IR headphone Transmitter

Wireless Microphone #1

Wireless Microphone #2

Wireless Microphone #3

Wireless Microphone Transmitter

WLW 500KW Transmitter Schematic

XTAL Locked tone Transmitter

detailsUSB FM Transmitter Circuit for PC and Laptop



Here's a small FM transmitter ciruit for your desktop or laptop to enjoy the movie and music from a distance. This FM transmitter, which is powered by USB, recovers output on your computer or your MP3 player to the relay on the tape FM (frequency 108 MHz). For Assemblying this FM transmitter kit, an electronics hobbyist will have built in about 30 minutes.

FM Transmitter Construction
It is not necessary to drill the transmitter PCB. All components will be soldered to the plate with their legs folded.

The two transistors and the LEDs are polarized:
The transistor has a flat side, the LED a foot longer than the other is the anode (A), the other is the cathode (K). The audio cable (minijack) must be transformed from a stereo cable into a cable.
Mono Sound:
Soldering together the white and red cables, leaving aside the yellow cable (mass). The frequency setting will be turning the variable capacitor gently with a screwdriver or thin cardboard but rigid.

FM Transmitter Parts List
* 1 Ohm resistor 510 (green - brown - brown)
* 100 resistor 1 kOhm (brown - black - yellow)
* 1 MOhm resistors (brown - black - green)
* 1 capacitor 0.1 uF (0.1)
* 1 nF capacitor 47 (0.047)
* 1 capacitor 4.7 pF (479)
* 2 pF capacitors 22 (22)
* 1 variable capacitor 1.5 pF ... 15
* 2 transistor BF 246 (F246A)
* 1 red LED
* 1 audio cable (minijack)

Wednesday, July 2, 2014

The Complete FM Bug



FM BUG Circuit

Corporate espionage is reaching new heights in sophistication. The latest information to be released shows the depths firms will go to pry into a rival firm's operations.
By using the latest in electronic bugging, they have stolen information, secrets and even formulas known only to the inventors themselves.
Take the example of one firm:Leaks from Top Management level remained a mystery until, one day, a bug was discovered inside the Managing Director's office.Sitting prominently on his desk was a gift box of imported cigars!Cleverly concealed in the lower part of the box was a miniature FM transmitter . . all a gift from a phony sales rep.This is just one of the many bugging devices available on the eaves-dropping market. The range includes pen and pencil holders, trophies, framed pictures and office furniture with false bottom drawers.
These products are readily sold to fledgling companies, eager to nestle into big brother's market.
And for a while these bugging devices worked. Few firms knew of their existence, and even less on how to sniff them out.
But that has all changed now. If a corporation suspects a leak at any level, the first thing they order is an investigation into security. Not only personnel, but information and electronic security.
Debugging has grown into big business. Most large security organisations have a section concentrating on electronic surveillance including bugging and debugging.
They use scanners to detect hidden devices and can locate absolutely anything, anywhere, and on any frequency.
It was only after the firm above had commissioned a scan of the entire floor, that the cigar box was discovered. Its innocence had deceived everyone. And cost them a small fortune!
Bugging of this kind is completely illegal and we don't subscribe to this type of application at all.
But the uses for our SUPER-SNOOP FM WIRELESS MICROPHONE can be harmless, helpful and a lot of fun.
Our unit is both compact and very sensitive and can be used to pick up even the faintest of conversations or noises and transmit them 20 or so metres to any FM receiver.
When you build the FM BUG you will see why we consider the design to be very clever. We have used only low priced components and they are all easy to obtain.
No air trimmer capacitor is required as the coil is squeezed slightly to obtain the desired frequency. This has allowed us to fit the bug into a tooth-brush case so that it can be carried around or placed on a shelf.
If it is set between two books it will be hidden from view or as a supervision accessory it can be placed on a small child, etc. The transmitted signal will over-ride the background noise and the output will be clean. If the child wanders beyond the range of the transmitter, the background noise will come up and signal that the tot is out of range.
As an added bonus, you can listen to the chatterings and squabbles as the children amuse themselves in the back yard.
It is also great for picking up the first signs of a child awakening from his afternoon sleep or it can be used as an indicator from a bed-ridden patient.
The great advantage of the bug is the absence of wires. And since it draws only about 5-10 milliamps, the pair of AAA cells will last for many months.
The success of this FM BUG is the use of TWO transistors in the circuit. To create a good design, like this, each transistor should be required to perform only one task. In any type of transmitter, there is a minimum of two tasks.
One is to amplify the signal from the microphone and the other is to provide a high frequency oscillator.
The amplified microphone signal is injected into the oscillator to modify its frequency and thus produce a FREQUENCY MODULATED oscillator. If an aerial is connected to the output of the oscillator, some of the energy will be radiated into the atmosphere.
To increase the output of our design, an RF amplifier would be needed but this gets into legal technicalities with maximum transmitting power.
It may be of Interest to know that a record distance of 310 miles was achieved with a 350 micro-watt transmitter in the USA, some 15 years ago. This equates to an astounding ONE MILLION miles per watt!
In simple terms, an RF amplifier becomes a LINEAR amplifier.
We have opted for sensitivity and the first transistor is employed as a pre-amplifier. This will enable you to pick up very low-level sounds and transmit them about 20 to 50 metres.
The only critical component in the FM BUG is the oscillator coil. When I say critical, I am referring to its effect on the frequency. Its critical nature only means it must not be touched when the transmitter is in operation as this will detune the circuit completely.
It is the only component which needs to be adjusted or aligned and we will cover its winding and formation in detail.
The oscillator coil is made out of tinned copper wire and does not need any insulation. This is not normal practice but since the coil is small and rigid, the turns are unable to touch each other and short-out.
The coil is made by winding the tinned copper wire over a medium-size Philips screw-driver. The gauge of wire, the diameter of the coil and the spacing between turns is not extremely important and it will be adjusted in the alignment stage. However when the project is fully aligned, it must not be touched at all.
Don't be over-worried at this stage. Just follow the size and shape as shown in the diagram and everything will come out right in the end.

The coil has 5 turns and is wound on a 3.5mm shaft. To be more specific, it has 5 loops of wire at the top and each end terminates at the PC board. The coil must be wound in a clock-wise direction to fit onto the board and if you make a mistake, rewind the coil in the opposite direction.
Construction is quite straight-forward as everything is mounted on the printed circuit board. The only point to watch is the height of some of the components. The electrolytic must be folded over so that the board will fit into the case.
Positioning of the parts is not as critical as you think as the final frequency is adjusted by squeezing the coil together or stretching it apart.
However it is important to keep the component leads as short as possible and the soldering neat due to the high frequencies involved. The components must be soldered firmly to the board so that they do not move when the transmitter is being carried.
Even the poorest of soldering will work but who wants to see poor soldering on a project?
The soldering may not affect the resulting frequency but poor layout of the components certainly will.
All the resistors must be pressed firmly against the PC board before soldering and the two transistors must be pushed so that they are as closes as possible to the board.
Some BC 547 transistors will not work in the circuit. Maybe the frequency is too high. SGS BC 547 transistors did not work at all. The other two types: f BC 547 and Philips BC 547 worked perfectly.
All the small-value capacitors are ceramic as they are not critical in value and do not need to be high stability. But you must be careful when identifying them. It would be a very simple mistake to buy a 56p instead of 5p6 because there is no difference in the size. 22n may be identified with 223 or 22n or .022. A capacitor marked 22k will be a 22p cap and will not be suitable. The 1n capacitor may be marked 1n or .001 or 102. These are all the same value. The value 101 or 103 is NOT 1n so be careful, the caps may be about the same size. The rule is: don't use a capacitor unless its markings are clear and you are sure of the value.

The complete FM BUG

The switch is mounted on the PC board with its three terminals fitted into the large holes.
The final items to add to the board are the two AAA cells. These come with the kit and we have chosen them for slenderness so that they can be fitted side-by-side.
It is very difficult to solder to the zinc case but if you roughen the surface with a file and use a large, HOT, soldering iron, the job can be done very quickly. Use a piece of tinned copper wire to join the positive of one to the negative of the other. At the other end, solder longer lengths of wire so that they can be connected directly to the PC board. Make sure the positive terminal connects to the plus on the PC board.

Top and bottom of the FM BUG PC board

AAA cells are also obtainable at photographic shops. The only alternative is an 'N' cell which is nearly as thin as an AAA cell but only half the length.
The terminal marked A on the board is the antenna output. For a frequency of 90MHz, the antenna should be 165cm long. This is classified as a half-wave antenna and provides one of the most effective radiators. If you find the antenna gets in the way you can opt for a quarter-wave antenna and this will be 83cm long. If you only require to transmit 10 to 20 metres the antenna can be as short as 42cm or even as low as 5 or 10 cm.
The most suitable length will depend on the sensitivity of the FM radio used to pick up the signal and the obstructions between the transmitter and receiver. It will be a good experiment for you to 'cut' your own antenna and determine which is the most suitable for your application.
The circuit consists of two separate stages. The first is an audio pre-amplifier and the second is a 90MHz oscillator.
The first stage is very simple to explain. It is a self-biasing common-emitter amplifier capable of amplifying minute signals picked up by the electret microphone. It delivers these to the oscillator stage. The amplification of the first stage is about 70 and it only operates at audio frequencies. The 22n capacitor isolates the microphone from the base voltage of the transistor and allows only AC signals to pass through. The transistor is automatically biased via the 1M resistor which is fed from the voltage appearing at the collector. This is a simple yet very effective circuit. The output from the transistor passes through a 2.2u electrolytic. This value is not critical as its sole purpose is to couple the two stages.
The 47k, 1n, 470R and 22n components are not critical either. So, what are the critical components in this circuit?
The critical components are the coil and 47p capacitor. These determine the frequency at which the bug will transmit. In addition, the effective capacitance of the transistor plays a deciding factor in the resulting frequency.
This stage is basically a free-running 90MHz oscillator in which the feedback path is the 5p6 capacitor.
When the circuit is turned on, a pulse of electricity passes through the collector-emitter circuit and this also includes the parallel tuned circuit made up of the oscillator coil and the 47p capacitor. This pulse of electricity is due to the transistor being turned on via the 47k resistor in the base circuit.
When ever energy is injected into a tuned circuit, the energy is firstly absorbed by the capacitor. The electricity will then flow out to the coil where it is converted to magnetic flux. The magnetic flux will cut the turns of wire in the coil and produce current and voltage which will be passed to the capacitor.
In theory, this current will flow back and forth indefinitely, however in practice, there are a number of losses which will cause the oscillations to die down fairly quickly.
If a feedback circuit is provided for the stage, the natural RESONANT frequency of the coil/capacitor combination will be maintained. The 5p6 provides this feedback path and keeps the transistor oscillating.
The 5p6 feeds a small sample of the voltage appearing at the collector, to the emitter and modifies the emitter voltage. The transistor sees its base-to-emitter voltage altering in harmony with the resonant frequency of the tuned circuit and turns the collector on and off at the same frequency.
Thus there is a degree of stability in the oscillator frequency.
The actual frequency of the stage is dependent upon the total capacitance of the circuit and this includes all the other components to a minor extent.
Once the basic frequency of 90MHz is set, the variations in frequency are produced by the changes in effective capacitance of the transistor. This occurs when its base voltage is increased and reduced. The electret microphone picks up the sound waves which are amplified by the first transistor and the resulting frequency is passed to the base of Q2 via the 2.2u electrolytic.
This alters the gain of the transistor and changes its internal capacitance. This junction capacitance modifies the oscillator with a frequency equal to the sound entering the microphone thus FREQUENCY MODULATING the circuit. A short length of antenna wire is connected to the collector of the oscillator via a coupling capacitor and some of the energy of the circuit will be radiated to the surroundings.
Any FM receiver will pick up this energy and decode the audio portion of the signal.
When the FM BUG is complete, checked and ready for insertion into its case, there is one slight adjustment which must be made to align it to the correct frequency.
As we have said, the only critical component is the oscillator coil. It is the only item which is adjustable.
Since we are working with a very high frequency, the proximity of your hand or even a metal screw-driver will tend to de-tune the oscillator appreciably.
For this reason you must use a plastic aligning stick to make the adjustment. Any piece of plastic will do. A knitting needle, pen barrel or plastic stirring stick can be used.
Place the bug about a metre from the FM radio and switch both units on. Tune the radio to an unused portion of the band and use the alignment stick to push the turns of the coil together. Make sure none of the turns touch each other as this will short out the operation of the oscillator.
All of a sudden you will hear the background noise diminish and you may even get feed back. This amount of adjustment is sufficient. Place the BUG in its case and tape up the two halves.
The fine tuning between radio and transmitter is done on the radio. Peak the reception and move the BUG further away. Peak the fine tune again and move the BUG into another part of the house and see how far it will transmit.
If the bug fails to operate, you have a problem. Simple digital tests will not fix it nor will ordinary audio procedures. The frequency at which the BUG operates is too high.
You have to use a new method called comparison.
This involves the comparing of a unit which works, with the faulty unit.
This means it is ideal for a group of constructors to build a number of units and compare one against the other.
This will not be possible with individual constructors and they will have to adapt this fault-finding section.
The first fact you have to establish is the correct operation of the FM receiver.
If you have another BUG and it is capable of transmitting through the radio you know the radio is tuned to the correct frequency. Otherwise you will have to double-check the tuning of the dial and make sure the radio is switched to the correct setting.
The next stage is to determine if the BUG is functioning AT ALL. The only voltage measurements you can make are across the collector-emitter terminals of the first transistor (1 v to 1.5v) and across the collector-emitter terminals of the second transistor (1.3v to 1.5v) These values won't tell you much, except that the battery voltage is reaching the component.
Tune the radio to about 90MHz and lay the radio antenna very close to the antenna of the BUG. Switch the BUG on and off via the slide switch. You should hear a click in the radio if the BUG is on a frequency NEAR 90MHz. Move the turns of the aerial coil together or apart with a plastic stick as you switch the unit ON and OFF.
If a click is heard but no feed-back, the oscillator will be operating but not the pre-amp stage. This could be due to the electret microphone being around the wrong way, the transistor around the wrong way, a missing component or an open 2.2u electro.
If the fault cannot be located, compare your unit with a friend's. You may have made a solder bridge, connected the batteries around the wrong way, made the coil too big or used the wrong value capacitor for one of the values.
If all this fails, put the unit aside and start again.

1 - 470R
1 - 10k
1 - 22k
1 - 47k
1 - 1M
1 - 5.6p ceramic = 5p6
1 - 22p ceramic or 27p or 33p
1 - 47p ceramic
1 - 1n ceramic = 1,000p or 102
1 - 22n ceramic = .022 or 223
1 - 2.2u 16v or 25v
2 - BC 547 transistors
1 - mini slide switch spdt.
1 - electret microphone (insert)
2 - AAA cells
10cm tinned copper wire
2 - metres aerial wire
1 - FM BUG PC board

Saturday, June 28, 2014

FM Transmitter

Single transistor Miniature FM transmitter with VCO


This simple transmitter allows you to broadcast on FM radio band (VHF) 87.5 - 108 MHz. It consists of a simple oscillator with silicon planar RF PNP transistor. Directly to the oscillator an antenna is connected. Due to the large amplitude of RF voltage is sufficient antenna length of about 5-10 cm. I used insulated 7cm long copper wire 1mm diameter. I eliminated the tuning capacitor, which is usual for most bugs and miniature transmitters, because this greatly complicates the tuning. From my own experience I know that if you get closer to such capacitor, the operating frequency is changed. That's why I chose to use the voltage tuning using the Voltage Controlled Oscillator (VCO). Instead of tuning capacitor the varicap (capacitance diode) is used, which changes its capacity by changing the reverse DC voltage. We can tune the operating frequency by changing the DC voltage using the trimmer P1. Varicap also provides frequency modulation.

Tuning: Set P1 to the center. Turn on the FM radio and tune it to an unoccupied frequency in the 87.5 - 108 MHz band. You will hear a noise. Turn on the transmitter and the first tune the operating frequency of roughly by stretching turns in the coil L1. Then fine-tune the frequency using P1. Proper tuning is indicated by the radio getting silent. You can then connect audio source to the input (such as cassette player, CD or MP3 player, record player, audio output of PC or laptop, etc.). It is also possible to tune while already connected to the signal source. The circuit can be powered from 5V USB port available on your PC or laptop.

Inductor L1 is airborne and has six turns of 0.5 mm diameter wire wound on 3 mm diameter. Varicap is arbitrary, which covers the range of about 5-20pF, such as BB105, KB105, KB109. I used the varicap KB109G made by Tesla with yellow paint on the cathode. The transistor is a high-frequency planar PNP type, for example, BF970, BF979, or simmilar. You can also use a transistor with different type of case. The disadvantage of the circuit is sensitivy to changes in supply voltage (it is changing the varicap voltage and thus the operating frequency). The antenna is connected directly to the oscillator, so if you touch it or placing it near the conductive object, the frequency shifts. At its simplicity, however, the circuit works surprisingly well and the range is about 20 to 100 meters. You can use power supply of 5-12V or a battery. There should be no ripple in the supply voltage, otherwise it may be heard in the receiver.

     Warning: Broadcasting on VHF-FM band may be illegal in your country. Author does not take any responsibility for your possible legal penalties for illegal broadcast or due to abuse of the bug for illegal purposes! Everything you do at your own risk.

The schematic of the Single transistor Miniature FM transmitter with VCO

88-108 MHz FM covert listening device (bug)


This is a simple listening bug. The signal can be tuned on any FM radio. The first transistor (in the circuit diagram on the left) works as an oscillator (in Colpitts connection), the frequency depends on: trimmer capacitor, inductor (with 4 turns wound on 5mm diameter, no core), varicap and capacitor between collector and emitter of the first transistor. Low frequency signal from the electret microphone affects varicap voltage and thus its capacity. Varicap affectc the oscillator frequency and thus modulates the carrier wave. The second transistor acts as an amplifier and also contributes to separation of the antenna from the oscillator, thereby improving the frequency stability.

How to use the bug: Turn on the FM radio and connect the bug to voltage 9-12 VDC and try to tune the radio frequency bugs. If the bug is near the radio and the radio is well tuned, you can hear feedback whistling. Range of this bug is about 20 to 100m (66 to 330 feet). The antenna is cca 10 - 30cm (1/3 - 1 feet) wire.

  Warning: Broadcasting on VHF-FM band may be illegal in your country. Author does not take any responsibility for your possible legal penalties for illegal broadcast or due to abuse of the bug for illegal purposes! Everything you do at your own risk.

5W1.5W 88-108Mhz FM Transmitter 14MHz (20m) AM Transmitter




This transmitter is designed to transmit sound (music, speech, ...) at frequencies 88-108MHz with a frequency modulation (FM). Its RF power is about 1.5 W. The first transistor is used as an RF oscillator. Varicap allows the oscillator frequency shifting and thus its frequency modulation and frequency tuning via potentiometer. Varicap may not be the BB105, it can be BB409, BB109G, KB109G or other type. The second transistor is the power output stage. The output signal goes through a filter to remove harmonics and then it enters antenna, eg dipole or Yagi antenna (it has better directivity). Power transistor is on the heatsink with min. 100 cm2 area. Coils are air, wire diameter of 0.6 mm wound on 5 mm.

     Warning! Operating this transmitter without permission is illegal.

1.5W 88-108Mhz FM power Transmitter schematic (note: "z" means number of turns)

Saturday, June 21, 2014

Simple FM Receiver


Frequency modulation is used in radio broadcast in the 88-108MHz VHF band. This bandwidth range is marked as FM on the band scales of radio receivers, and the devices that are able to receive such signals are called FM receivers. The FM radio transmitter has a 200kHz wide channel. The maximum audio frequency transmitted in FM is 15 kHz as compared to 4.5 kHz in AM. This allows much larger range of frequencies to be transferred in FM and thus the quality of FM transmission is significantly higher than of AM transmission.

Here’s a simple FM receiver with minimum components for local FM reception. Transistor BF495 (T2), together with a 10k resistor (R1), coil L, 22pF variable capacitor (VC), and internal capacitances of transistor BF494 (T1), comprises the Colpitts oscillator. The resonance frequency of this oscillator is set by trimmer VC to the frequency of the transmitting station that we wish to listen. That is, it has to be tuned between 88 and 108 MHz. The information signal used in the transmitter to perform the modulation is extracted on resistor R1 and fed to the audio amplifier over a 220nF coupling capacitor (C1).

You should be able to change the capacitance of the variable capacitor from a couple of picofarads to about 20 pF. So, a 22pF trimmer is a good choice to be used as VC in the circuit. It is readily available in the market. If you are using some other capacitor that has a larger capacitance and are unable to receive the full FM bandwidth (88-108 MHz), try changing the value of VC. Its capacitance is to be determined experimentally.

The self-supporting coil L has four turns of 22 SWG enamelled copper wire, with air core having 4mm internal diameter. It can be constructed on any cylindrical object, such as pencil or pen, having a diameter of 4 mm. When the required number of turns of the coil has reached, the coil is taken off the cylinder and stretched a little so that the turns don’t touch each other.

Capacitors C3 (100nF) and C10 (100µF, 25V), together with R3 (1k), comprise a band-pass filter for very low frequencies, which is used to separate the low-frequency signal from the high-frequency signal in the receiver.You can use the telescopic antenna of any unused device. A good reception can also be obtained with a piece of isolated copper wire about 60 cm long. The optimum length of copper wire can be found experimentally.

The performance of this tiny receiver depends on several factors such as quality and turns of coil L, aerial type, and distance from FM transmitter. IC LM386 is an audio power amplifier designed for use in low-voltage consumer applications. It provides 1 to 2 watts, which is enough to drive any small-size speaker. The 22k volume control (VR) is a logarithmic potentiometer that is connected to pin 3 and the amplified output is obtained at pin 5 of IC LM386. The receiver can be operated off a 6V-9V battery.

VHF FM Transmitter MAX2606


VHF FM Transmitter MAX2606

If you want to be independent of the local radio stations for testing VHF receivers, you need a frequency-modulated oscillator that covers the range of 89.5 to 108 MHz — but building such an oscillator using discrete components is not that easy. Maxim now has available a series of five integrated oscillator building blocks in the MAX260x series which cover the frequency range between 45 and 650 MHz. The only other thing you need is a suitable external coil, dimensioned for the midrange frequency.

Stereo FM Transmitter with BA1404



Stereo FM Transmitter with BA1404

A high quality stereo FM transmitter circuit is shown here. The circuit is based on the IC BA1404 from ROHM Semiconductors. BA1404 is a monolithic FM stereo modulator that has built in stereo modulator, FM modulator and RF amplifier. The FM modulator can be operated from 76 to 108MHz and power supply for the circuit can be anything between 1.25 to 3 volts. In the circuit R7, C16, C14 and R6, C15, C13 forms the pre-emphasis network for the right and left channels respectively. This is done for matching the frequency response of the FM transmitter with the FM receiver. Inductor L1 and capacitor C5 is used to set the oscillator frequency. Network C9,C10, R4,R5 improves the channel separation. 38kHz crystal X1 is connected between pins 5 and 6 of the IC. Composite stereo signal is created by the stereo modulator circuit using the 38kHz quartz controlled frequency.

FM Transmitter with 2N2218



FM Transmitter with 2N2218

Here's simple FM transmitter circuit using medium power 2N2218 transistor. Micropohone is of electret type that connects to two input terminals and the antenna should be a copper wire from 15 to 40 cm. Below is schematic circuit of the fm transmitter.

Phone Spy Transmitter


Phone Spy Transmitter

Here is a very simple telephone broadcaster transmitter which can be used to eavesdrop on a telephone conversation. The circuit can also be used as a wireless telephone amplifier. One important feature of this phone transmitter is that the circuit derives its power directly from the active telephone lines, and thus avoids use of any external battery or other power supplies.

1.5V FM Transmitter


1.5V FM Broadcast Transmitter

The objective of this 1.5V FM Broadcast Transmitter design is to provide a simple low-power transmitter solution for broadcasting audio from various audio sources. This transmitter accepts stereo input via two 470K resistors. Since there is no audio level control on the input, the audio level out from the source needs to be adjusted. Or, you can just add a 10k as an input level control. Transmitter's frequency, as built is tunable via spreading or compressing the coil to the desired frequency, and the coil can be glued down. If you want to make one that's tunable, it might be easiest to reduce the 18 pf capacitor and put a small trimmer capacitor in parallel with the inductor (across the reduced value capacitor). Voltage variable capacitors would be an nice alternative to a mechanical variable capacitor but they don't offer much tuning range with only a 1.5V power supply.

Coil less FM Transmitter


Coilless FM Transmitter

The RF oscillator using the inverter N2 and 10.7Mhz ceramic filter is driving the parallel combination of N4 to N6 through N3.Since these inverters are in parallel the output impedance will be low so that it can directly drive an aerial of 1/4th wavelength. Since the output of N4-N6 is square wave there will be a lot of harmonics in it. The 9th harmonics of 10.7Mhz (96.3Mhz) will hence be at the center of the FM band. N1 is working as an audio amplifier. The audio signals from the microphone are amplified and fed to the varicap diode. The signal varies the capacitance of the varicap and hence varies the oscillator frequency which produce Frequency Modulation.

1W Long Range FM Transmitter

1W Long Range FM Transmitter

Long range, very stable, harmonic free, FM transmitter circuit which can be used for FM frequencies between 88 and 108 MHz. With good antenna transmitter can cover 5km range. It has a very stable oscillator because it uses LM7809 voltage regulator which is a 9V stabilized power supply for T1 transistor. Frequency adjustment is achieved by using the 10K linear potentiometer. The output power of this long range RF transmitter is around 1W but can be higher if you use transistors like KT920A, BLX65, BLY81, 2N3553, 2SC1970 or 2SC1971.

4 Transistor FM Transmitter



4 Transistor FM Transmitter

This circuit provides an FM modulated signal with an output power of around 500mW. The input microphone pre-amp is built around a couple of 2N3904 transistors (Q1/Q2), and audio gain is limited by the 5k preset trim potentiometer. The oscillator is a colpitt stage, frequency of oscillation governed by the tank circuit made from two 5pF ceramic capacitors and the L2 inductor. 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.

1 Watt FM Transmitter Amplifier


1 Watt FM Transmitter Amplifier

This is a 1 Watt FM Transmitter amplifier with a good design that can be used to amplify a RF signal in the 88 – 108 MHz band. It is very sensitive if you use good RF power amplifier transistors, trimmers and coils. It has a power amplification factor of 9 to 12 dB (9 to 15 times). At an input power of 0.1W the output will be 1W. You must choose T1 transistor depending on applied voltage. If you have a 12V power supply then use transistors like: 2N4427, KT920A, KT934A, KT904, BLX65, 2SC1970, BLY87. At 18 to 24V power supply you must use transistors like: 2N3866, 2N3553, KT922A, BLY91, BLX92A. You may use 2N2219 at 12V but you will get an output power of 0.4W maximum.

18W FM Transmitter


18W FM Transmitter

Here's FM transmitter for commercial FM band that provides 18 watts of power. Since the electronic diagram is too large we decided to divide it into two parts. The first part is the actual FM transmitter while the second part is 18W RF amplifier. The circuit should be built on an epoxy printed circuit board with the upper face components reserved for interconnecting tracks and the bottom solder to the ground plane. If powered by 14V and 2.5A transmitter outputs 15W of power, whereas 18V and 3.5A will provide 18W. BB110 variable capacitor connected to the collector of transistor BF199 adjusts the transmission frequency of the circuit. 2K2 potentiometer serves as fine tuning. Once the output frequency is adjusted amplifier variable capacitors must be adjusted for maximum output power one stage at a time. All adjustments must be made with 50 Ohm dummy load connected to the output of transmitter.

FM Wireless Transmitter


fm mike

FM Interference Filter

fm filter

Friday, June 20, 2014

Single Quad Loop For FM Radio DXing



FM Antenna

The quad antenna have a gain of 1.4 dB over a dipole and also operate over a relatively wide frequency. Quad antenna dipole to form 1/4 lambda each side makes a square. American Radio Amateur (HAM) Clarence C. Moore, W9LZX developed this system in 1939 for the Missionary Radio Station HCJB at Quito of Ecuadaor (South America). The altitude of the station was over 10,000 feet in Andes. The station was operated in 25 m band SW with TX power of 10 KW. The band width of a single dipole is quite narrow. The quad loop is having high gain and less corona discharges etc. The half lambda folded dipole impedance is 288 Ohms (300 Ohms) and this quad loop is having 125 Ohms feed impedance. Due to low impedance of the quad, there is no need of any matching, for a general FM receiver system.

Connecting a folded dipole (gain : 2.14 dB, 1.45 m long and 300 Ohms) to a FM receiver of 75 Ohms input, without any matching, its efficiency becomes to 65% (VSWR-4) and a 125 Ohms quad at 75 Ohms receiver without matching it will get 95% efficiency (VSWR-1.66) with an extra gain of 1.4 dB over a folded dipole. Due to this 3.54 dB gain from a quad loop there will be a terrific FM Radio reception. Using a split dipole, having 75 Ohms feed impedance, there will be correct match of 75 Ohms FM receiver system. But one of the element will be isolated and it makes static and lightning problems. if we ground the cable braid (shield), the entire quad or foled dipole antenna system get grounded and it avoid the threat from static or lightning effects.

FM Radio allocation in India is from 88 MHz to 108 MHz. So the mid frequency is 98 MHz. The equation for getting the wave length of the conductor is (300 x 0.95) / 98 MHz. i.e. 2.908163 meters. If we divide this value by 4 we can get the Quarter Lambda length. i.e. 72.70 cm, we can take it as 73 cm or 74 cm. Due to skin effect of VHF frequency the element should be a tube having more than 4 mm dia or use 3/8th tube for getting good efficiency.

Connection To Radio

Connect with proper connector as per your FM Radio. If there is no external antenna input in your Radio, then connect the Ground (Shield) to Battery negative and other line to the telescopic aerial.



FM Radio Rectangle Super Gainer (Moxon Antenna)


It is not much popular antenna but much old design!! The original name of this antenna is Two Element Driven Arrays. In 1952 Les Moxon published this Genius Design in QST July Issue. It is a rectangular shaped two elements with a closed spacing of 0.18 lambda. The ends of the two element are folded in 90 degree face to face with a critical spacing of each other. This rectangle beam is popular among ham radio operators as Moxon Antenna. Few hams are using it for HF (SW) bands. It is a directional type antenna with a wide angle of 136 degree typical aperture and with a very good band width. (The Radiation pattern is like Kidney shape, and it is the same in reciprocity) . Due to the bend at the ends of each element and due to the critical spacing of each tips it is a capacity loaded, and it yields the wide bandwidth and low SWR levels. It is a low take of angle type of 14 degree or low typically, and it pick ups maximum stations from planes. Therefore I decided to make this antenna in FM Radio band to receive the spectrum of 88 MHz to 108 MHz.

One of my SWL Murali (School Teacher), who is a good listener of MW-SW-FM bands, asked to make an antenna which gives directivity, wide angle and very high gain for his own use. For this purpose I converted the basic design to 3 meter BC band radio use. I made this rectangle beam (Moxon) with 3/8 th aluminum tube. For the critical spacing of each element tips, I made hilum insulators as a prototype. The feed point is connected with a simple cable TV connector called F-Connector which is economical and easily available at the local market.

While testing, if it is pointing to eastern direction it will pick the signals from East and also it will pick the signal from South East and North East due to its wide angle aperture. More over the beauty of this antenna is the two elements will give 9 dBi + gain. Typically this antenna gives 7 to 14 dBi depending upto the accuracy of the construction and it can be a wide angle of 100 degrees to 136 degrees, the F/B ratio can be 30 dB to 40 dB. This antenna should be mounted at least 1 lambda of the operating frequency above the ground level. i.e., 3 meter (10 ft). For excellent performance, the height should be 25 to 30 ft. and the surrounding clearance should be maximum. Do not test this antenna near to any metallic objects and that will reduce its performance drastically.

For receiving FM BC Bands you can connect any good quality and low loss 75 ohms coaxial cable to the driven elements at the middle point of this antenna. No matching is required like balun, gamma, hair pin etc. and therefore no question of matching loss. Refer the following figures for getting specific ideas about the antenna and its construction. A well constructed rectangle beam antenna is equivalent to a four element yagi antenna.

In the next part we will reveal some antenna engineering about rectangle beam for 2 metre ham band operation.

FM bc band - mesurements for 100 Mhz
Fig. 1. FM bc band - mesurements for 100 Mhz. This measurements are for 3/8 Aluminium tube (9.5 mm OD)

FM bc band - mesurements for 100 Mhz
Fig. 2. Rectangle beam (Moxon) plot

Thursday, June 19, 2014

1.3W VHF RF Amplifier 2SC1970 88-108 MHz



    1.3W VHF RF Amplifier 2SC1970 88-108 MHz

    1.3W VHF RF Amplifier 2SC1970 88-108 MHz


    This RF power amplifier is based on the transistor 2SC1970 and 2N4427. The output power is about 1.3W and the input driving power is 30-50mW. It will still get your RF signal quit far and I advice you to use a good 50 ohm resistor as dummy load. To tune this amplifier you can either use a power meter/wattmeter, SWR unit or you can do using a RF field meter.

    1.3W VHF RF Amplifier 2SC1970 88-108 MHz

    RF Amplifier Assembly

    Good grounding is very important in a RF system. I use bottom layer as Ground and I connect it with the top with wires to get a good grounding. Make sure you have some cooling at the transistor. In my case I put the 2SC1970 close to the PCB to handle the heat. With good tuning the transistor shouldn't become hot.

    RF Amplifier Printed Circuit Board

    You can download a pdf file which is the black PCB. The PCB is mirrored because the printed side side should be faced down the board during UV exposure. To the right you will find a pic showing the assembly of all components on the same board.This is how the real board should look when you are going to solder the components. It is a board made for surface mounted components, so the copper is on the top layer. I am sure you can still use hole mounted components as well.
    Grey area is copper and each component is draw in different colors all to make it easy to identify for you. The scale of the pdf is 1:1 and the picture at right is magnified with 4 times. Click on the pic to enlarge it.

    Low-Pass Filter

    Some of you might want to add a low-pass filter at the output. I have not added any extra low pass filter in my construction because I don't think it is needed. You can easy find several homepages about low pass filter and how to build them.

    1.3W VHF RF Amplifier 2SC1970 88-108 MHz

Homebrewed Off-Center Fed Dipole



Building A Homebrewed Off-Center Fed Dipole Scanner Antenna.

Aluminum/copper tubing construction:

You will need to check the fit of the tubing with the T connector and the caps while you are at the store. One combination that fits nicely is 3/4" copper pipe with 3/4" CPVC fittings (not to be confused with 3/4" PVC fittings which will be too large). The tubing/connector is held in place with 2 stainless steel sheet metal screws for connecting the balun to each element.

Find a "U" bolt to fit your mast. Drill two holes in the support pipe to fit the U bolt.The support pipe is 18" from the "T" to the mast.

Remember, bandwidth increases as diameter of the elements increases. I think, if I remember correctly, at the hardware store, that a few CPVC fittings will fit copper tubing perfectly!

Some say that the 18" element on top mounted works best,Some like the 48" element on top.It does'nt matter,it works the same.

If you use the copper tubing,be sure to paint it with some good,non-conductive paint.I used to paint mine light grey. -Have fun! (Teraycoda)

For an alternate/temporary mounting option, drill a hole in one of the end caps and put in an eye bolt with a nut on the underside of the cap to secure. Be sure to secure this end cap to the copper tubing somehow, perhaps with an additional small stainless sheet metal screw. Be sure that the eye bolt itself doesn't make electrical contact with the tubing. Also, drill a small weep hole in the bottom end cap to allow any moisture to escape that may accumulate inside. Use the eye bolt and some rope to pulley the antenna up high in a tree, or use a hook to hang it somewhere. Give careful consideration to safety and grounding depending on your particular usage scenario. (Qdude)

Variation for Off-Center Fed Dipole Using Simple Wire and 75 to 300 ohm TV Balun Transformer

Offset dipole scanner antenna.jpg

Electrically, this version is the same as the one using copper tubing (above) but can be assembled quickly and is quite portable. While not as broadbanded as an OCFD using copper tubing or other metal with a larger diameter, the OCFD made from simple wire turns in great receive performance in all the commonly scanned bands, as reported here on RR in multiple message threads.

The legs/ends of the dipole are simple bell wire and shown here coiled up. Uncoil them and hang them vertically; doesn't matter if the long or short leg is at the top... works the same either way. The wire terminal lugs shown at the end of the legs of the dipole antenna should NOT be connected electrically to the wires - just crimp them on over the wire insulation. They are used as convenient hangers for the antenna, and not meant for electrical connection. Obviously, the lugs at the TV transformer/balun ends of the wire should be stripped before crimping on the terminal lugs to ensure contact with the antenna wires when you attach the TV transformer. Ensure the 75 ohm coax feedline that you connect from the balun/transformer runs away from the antenna at as near a 90 degree angle as possible.

Monday, April 28, 2014

Using 555 as FM transmitter

Using 555 as FM transmitter Circuit Diagram
The integrated circuit 555 has no limits, this FM transmitter circuit, the IC 555 is designed as an stable multivibrator as usual. But the tension control pin is used to connect a piezoelectric element instead of the capacitor disk. The piezoelectric element generates a voltage and the output pin is connected to an antenna wire 30 inches for the transmission of signals.Just tap the piezo element and you can hear the sound on an FM radio station. The range is very short.

100MHZ VHF FM Transmitter


This transmitter circuit has been built and tested and it has a range of a few feet if no antenna is connected.
  • The input transducer is an electret microphone. R1 provides DC power for the FET pre-amplifier built into the electret microphone.
  • Audio signals cause changes in the FET current and this causes the voltage across R1 to vary.
  • C1 couples the audio AC output from the microphone to the next amplifier stage. This coupling capacitor blocks DC potentials.
  • R2 and R3 form a voltage divider providing a 6 Volt line (50% of the power supply).
  • C2 is a decoupling capacitor. It guarantees that there are no AC voltages at point A in the circuit.
  • R4 ensures that the DC voltage is 6 Volts at the non inverting input (pin 3) of the op amp.
  • R5 and R6 control the gain of the op amp.
  • Gain = 1 + Rf / R1 = 1 + 220 / 3.3 = 67.7
  • In a normal op amp circuit, R5 would be connected to ground and a -12V supply line would be needed.
  • In this circuit, R5 is connected to point A and the -12 Volt line is not needed.
  • The op amp is a 741 but almost any pin compatible device should work.
  • R7 couples the op amp audio output to D1 and provides DC reverse bias across D1.
  • R7 is large to prevent RF signals escaping from the oscillator circuit.
  • D1 is a 5 amp rectifier diode. It is being used as a variable reactance diode. When the diode is reverse biased, virtually no current flows but there is some capacitance. If the reverse bias voltage is increased, the capacitance decreases. This makes a voltage controlled capacitor. The DC output of the op amp reverse biases the diode. The audio output from the op-amp varies the bias voltage and causes the capacitance of the diode to change. This changes the transmission frequency. In this way, Frequency Modulation is produced. Various diodes were tried and this one worked best. It is possible to buy varactor diodes which would work better.
  • D1, C3 and C4 with the coil form a tuned circuit which resonates at about 100 MHz in the middle of the FM radio band.
  • The MPF102 (available from Farnell Electronics) is connected as a source follower with a useful current gain.
  • The source current flows through one turn of the four turn coil. This current induces a voltage across the whole coil and this is fed to the gate. This is positive feedback and the circuit oscillates at the resonant frequency of the tuned circuit.
  • This is a Hartley oscillator with modified tuning capacitors to allow frequency modulation with the varactor diode.
  • C4 is a fixed 22 pF capacitor. The circuit would be easier to get working with a trimmer capacitor covering 5 to 30 pF.
  • C5 is a decoupling capacitor. It prevents RF voltages on the FET drain.
  • R8 limits the DC current through the FET and ensures the device operates inside safe limits.
  • D2 is a polarity protection diode. It protects the circuit if the power supply is connected the wrong way round.
  • C6 and C7 are decoupling capacitors. C7 works well at low (audio) frequencies and C6 works well at VHF radio frequencies.

VHF FM Power Meter

VHF Power Meter
This is a actual accessible apparatus that will acquaint you if your transmitter's oscillator is alive properly. If RF arresting is transmitted the LED will illuminate. Besides that, it will additionally accord you a quick beheld way to analysis how abundant ability is actuality transmitted. I awful acclaim that you accept the transmitter and ability beat on the aforementioned PCB. If you like to agreement a lot, you will acknowledge this bargain but evidently accessible addition.

TX200 VFO/VCO FM transmitter.

 200mW High Quality FM Transmitter With TX-200 - schematic
 FM Transmitter With TX-200 Part List

Here is the latest and abundantly bigger TX200 VFO/VCO FM transmitter. The best able transmitter to date that can be angry into aerial allegiance stereo PLL based 200mW FM transmitter. It is a absolute ambit for transmitting your music about the abode and yard. TX200 uses alone two coils; one in the oscillator and the added one in the 200mW VHF amplifier so it should be adequately accessible for anyone to build. It additionally includes congenital pre-emphasis and C5 for added complete quality. While accumulating the transmitter affliction charge be taken to accomplish abiding that C1 is anon affiliated to L1 and C9 to L2. These caps annihilate the distortions anatomy the DC accumulation and advance the complete affection greatly. 9V voltage accumulation is additionally actual important because it provides the exact bulk of accepted to Q1 to aftermath loud and bright complete quality. I achievement that you'll accept as abundant fun as I had while architecture this transmitter.

This power amplifier can extend 1-2W、88-108MHZ FM transmitter power to 10-15W.It uses single pipe c class amplifier and low-pass filter.It has higher conversion efficiency andstrong suppersion of considered wave. Principle Figure The circuit is showed as above.It adopts high-power transmitting tube C1972 and parameters are as follows:175MHZ,4A,25W,power gain ≥8.5db.As the pictures'parameters,the work center frequency is about 98MHZ.When the input radio-frequency power is about 2W,the rating output can reach 15W.The center frequency of former stage adjusts part of components to make sure that any frequency port among 88~108MHZ can reach rating value when it outputs.

USB FM Transmitter

usb fm transmitter circuit diagram 
To keep the fm transmitter circuit simple as well as compact, it was decided to use a chip made by Maxim Integrated Products, the MAX2606. This IC from the MAX2605-MAX2609 series has been specifically designed for low-noise RF applications with a fixed frequency. The VCO (Voltage Controlled Oscillator) in this IC uses a Colpitts oscillator circuit. The variable-capacitance (varicap) diode and feedback capacitors for the tuning have also been integrated on this chip, so that you only need an external inductor to fix the central oscillator frequency.

It is possible to fine-tune the frequency by varying the voltage to the varicap. Not much is demanded of the inductor, a type with a relatively low Q factor (35 to 40) is sufficient according to Maxim. The supply voltage to the IC should be between 2.7 and 5.5 V, the current consumption is between 2 and 4 mA. With values like these it seemed a good idea to supply the circuit with power from a USB port. 

A common-mode choke is connected in series with the USB connections in order to avoid interference between the circuit and the PC supply. There is not much else to the circuit. The stereo signal connected to K1 is combined via R1 and R2 and is then passed via volume control P1 to the Tune input of IC1, where it causes the carrier wave to be frequency modulated. Filter R6/C7 is used to restrict the bandwidth of the audio signal. The setting of the frequency (across the whole VHF FM broadcast band) is done with P2, which is connected to the 5 V supply voltage.

The PCB designed uses resistors and capacitors with 0805 SMD packaging. The size of the board is only 41.2 x 17.9 mm, which is practically dongle-sized. For the aerial an almost straight copper track has been placed at the edge of the board. In practice we achieved a range of about 6 metres (18 feet) with this fm transmitter usb. There is also room for a 5-way SIL header on the board. Here we find the inputs to the 3.5 mm jack plug, the input to P1 and the supply voltage. The latter permits the circuit to be powered independently from the mains supply, via for example three AA batteries or a Lithium button cell. Inductor L1 in the prototype is a type made by Murata that has a fairly high Q factor: minimum 60 at 100 MHz.

Take care when you solder filter choke L2, since the connections on both sides are very close together. The supply voltage is connected to this, so make sure that you don’t short out the USB supply! Use a resistance meter to check that there is no short between the two supply connectors before connecting the circuit to a USB port on a computer or to the batteries.

P1 has the opposite effect to what you would expect (clockwise reduces the volume), because this made the board layout much easier. The deviation and audio bandwidth varies with the setting of P1. The maximum sensitivity of the audio input is fairly large. With P1 set to its maximum level, a stereo input of 10 mVrms is sufficient for the sound on the radio to remain clear. This also depends on the setting of the VCO. With a higher tuning voltage the input signal may be almost twice as large (see VCO tuning curve in the data sheet). Above that level some audible distortion becomes apparent. If the attenuation can’t be easily set by P1, you can increase the values of R1 and R2 without any problems.

Measurements with an RF analyzer showed that the third harmonic had a strong presence in the transmitted spectrum (about 10 dB below the fundamental frequency). This should really have been much lower. With a low-impedance source connected to both inputs the bandwidth varies from 13.1 kHz (P1 at maximum) to 57 kHz (with the wiper of P1 set to 1/10).

In this usb fm transmitter circuit the pre-emphasis of the input is missing. Radios in Europe have a built-in de-emphasis network of 50 μs (75 μs in the US). The sound from the radio will therefore sound noticeably muffled. To correct this, and also to stop a stereo receiver from mistakenly reacting to a 19 kHz component in the audio signal, an enhancement circuit Is published elsewhere in this issue (Pre-emphasis for FM Transmitter, also with a PCB).

MP3 FM Transmitter Parts List
Resistors (all SMD 0805)
R1,R2 = 22kΩ
R3 = 4kΩ7
R4,R5 = 1kΩ
R6 = 270Ω
P1 = 10kΩ preset, SMD (TS53YJ103MR10 Vishay Sfernice, Farnell # 1557933)
P2 = 100kΩ preset, SMD(TS53YJ104MR10 Vishay Sfernice, Farnell # 1557934)
Capacitors (all SMD 0805)
C1,C2,C5 = 4μF7 10V
C3,C8 = 100nF
C4,C7 = 2nF2
C6 = 470nF
L1 = 390nF, SMD 1206 (LQH31HNR39K03L Murata, Farnell # 1515418)
L2 = 2200Ω @ 100MHz, SMD, common-mode choke, 1206 type(DLW31SN222SQ2L Murata, Farnell #1515599)
IC1 = MAX2606EUT+, SMD SOT23-6 (Maxim Integrated Products)
K1 = 3.5mm stereo audio jack SMD (SJ1-3513-SMT
CUI Inc, DIGI-Key # CP1-3513SJCT-ND)
K2 = 5-pin header (only required in combination with 090305-I pre-emphasis circuit)
K3 = USB connector type A, SMD (2410 07 Lumberg, Farnell # 1308875)

Modified MAX2606 FM transmitter

max2606 fm trasmitter
The MAX2606 covers the VHF band, although the frequency can only be varied by approximately ±3 MHz around the midrange frequency set by the coil L. The inductance values shown in the table can serve as starting points for further experimenting.

The SMD coils of the Stettner 5503 series are suitable for such oscillators. In Germany, they are available from Bürklin (, with values between 12 nH and 1200 nH. You can thus directly put together any desired value using two suitable coils. If you want to wind your own coils, try using 8 to 14 turns of 0.5-mm diameter silver-plated copper wire on a 5-mm mandrel. You can make fine adjustments to the inductance of the coil by slightly spreading or compressing the coil.

The circuit draws power from a 9-V battery. The BC238C stabilises the voltage to approximately 4 V. Although the MAX2606 can work with a supply voltage between +2.7 V and +5.5 V, a stabilised voltage improves the frequency stability of the free-running oscillator. The supply voltage connection Vcc (pin 5) and the TUNE voltage (pin 3) must be decoupled by 1-nF capacitors located as close as possible to the IC pins. The tuning voltage TUNE on pin 3 may lie between +0.4 V and +2.4 V. A symmetric output is provided by the OUT+ and OUT– pins. In the simplest case, the output can be used in a single-ended configuration. Pull-up resistors are connected to each of the outputs for this purpose. You can use a capacitor to tap off the radio signal from either one of these resistors. Several milliwatts of power are available. At the audio input, a signal amplitude of 10 to 20 mV is enough to generate the standard VHF frequency deviation of ±40 kHz.

MAX2606 FM Transmitter


With MAX2606 we can make a small mono fm transmitter. It is build with MAX2606 and covers at least 20m with 1.5 m long copper wire antenna. You can use this transmitter as an oscillator but change the 1000pF capacitor from the antenna with 15pF.MAX2606 transmitter output power is -10dBm, that means something around 100µW (micro-watts) = 0.0001W. Of course this is very low, that’s why we recommend you to use an auxiliary power amplifier if you want to build a more powerfull transmitter based on MAX2606.

UPC1651 FM Transmitter

 upc1651 fm transmitter

This fm transmitter is build with UPC1651 which is a silicon monolithic integrated circuit especially designed as a wide band amplifier covering the HF band through UHF band. UPC1651 has a high power gain: 19 dB at 500 MHz, low voltage operation 5V in a small package.

The coil has 5 turns of 0.5 mm copper wire with 4 mm coil diameter. You can use this as a low power fm transmitter or with a fm power amplifier to obtain a higher rf power. If you want to use it as a fm bug use a 70 cm long wire antenna.

Simple FM Transmitter

This is the schematic of the FM Transmitter 
  1. 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.
  2. C5 is used for tuning. This transmitter operates on the normal broadcast frequencies (88-108MHz).
  3. Q1 and Q2 can also be 2N3904 or something similar.
  4. You can use 1/4 W resistors mounted vertically instead of 1/8 W resistors.
  5. You may want to bypass the battery with a .01uf capacitor.
  6. An antenna may not be required for operation.
For more info -