Here we are presenting a long range one that can cover a reasonable distance of 2 miles and beyond with a one watt RF power with full circuit details,bill of material and testing procedure.
A simple FM transmitter (like Belkin) links your home-entertainment system to a portable radio that can be carried around the house and into the back yard. For example, you can play music on the CD changer in your living room, and listen to it on a portable radio by the back-yard barbeque with a 100 mW,10 meter range car FM transmitter easily available on Ebay,Amazon,Wall Mart,Ross,Target etc. With a little bit of hacking the range can be increased to maximum 100 meters.
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Step 1: Block Diagram & Circuit Details
This FM transmitter has 3 RF stages.
- A (VFO) Variable frequency Oscillator (30 mw) ,
- A class C driver stage (150 mw) as buffer and
- A class C final RF power amplifier (1 Watt )
Basically every FM Transmitter has to have a Voltage Controlled Oscillator (VCO) . This is a high frequency oscillator whose output frequency changes based on the voltage applied at a particular control point. This is a variable frequency oscillator (VFO).Q1 with is associated components form the VFO. The VFO output is fed to Q2. Q2 being a buffer does not load the VFO but amplifies the power only. This output is fed to the final RF power amplifier Q3, the output of which feeds the tuned circuit. Several capacitors C 4,8,9,10 are used on the supply rail for HF filtrations .If one feeds the VFO transistor Q1 directly with a microphone at its base it becomes a FM Transmitter circuit.
At V1 at 12 volt DC it will deliver 1 watt RF power. With Yagi antenna, looking like early days of TV antenna with aluminum pipes at both at transmitter and receiver end looking each other at line of sight distance, the range can be up to 5 KM .
Step 2: Component List
The Q2 pack has to be "TO 92-B" type(slightly bigger than BC547 pack) and not simple TO 92 which is slightly smaller in size ( same like BC547 pack). Moreover please note that the pin configurations are different for these 2 types. In case TO92 pack is used then increase the value of R7 to 56 ohms 1/2 watt failing which it shall burn. But this TO92 pack may affect the range
Q3 has to be 2N3866 with a heat sink for proper range. However 2N 2219 can be used that will but compromise the range drastically
Step 3: Testing
Initially use a simple 75CM single wire standing straight as antenna for getting a range of about 100-200 meters indoor. Similar length telescopic antenna is also OK for testing which will give only about 100-200 meters range . But never go longer than 79 CM antenna wire thinking that it will cover higher range. In fact if you do so the range will fall.
Frequency of the transmitter can be set with in 88 to 108 MHz FM band by adjusting the TR1 (Trimmer 1) of the VFO or by changing the spacing between the Coil L1.
1 Frequency Adjustment:
NOTE: Don’t try to test the unit in evening to night because at that time many powerful FM stations will be active. Test it only at day time. A few people have had trouble with this circuit if not soldered properly. The biggest problem is not knowing if it is even oscillating, since the frequency is outside the range of most simple oscilloscopes. One may require the use of an RF frequency counter which is very expensive. So, to know that it oscillates, and just have to find out at what frequency , the simplest way is to put a cell phone having FM radio ( or any FM radio) in search mode near your transmitter to hear some sound while you tap the microphone. Please note very near the transmitter will have several frequencies responding to the microphone and one will get confused. So go ,at least 30 meters away from the transmitter after initial test as above is verified. There the display gives only one frequency to which it gets best clear sound and all other frequencies giving hissing sound and that is the frequency the transmitter is operating. Adjust the trimmer TR1a very very very (about 1 degree) little clockwise or anticlockwise ,the transmission frequency will change .Then put the cell phone to search again and find the frequency. If it is very near a powerful transmitter you will not get the range. Change the frequency again to go towards 106 MHz where no commercial transmission usually takes place.
2 The distance adjustment, after connecting a Yagi or GP antenna.
Transmission range is adjusted by TR2. For that use a multi meter in 250 mA DC current mode in series with the 12 volt supply and then adjust the trimmer TR2 while the current is maximum. Adjust the current to around 75 mA (at 12 Volt DC supplied by a good adaptor) or the peak current by trimmer 2 to say about 85 mA. From the peak while you turn clockwise current will fall or while you turn anti clockwise it will also fall. And that is the best position of TR2 for full power delivery to the antenna. Please note Q3, round metal body must be fully covered by the black heatsink supplied, without which it will get badly heated up and finally get burnt. In around 100mA at 12 volt it shall cover a good range and shall be warm but beyond that current though it may cover a longer range it shall get very badly heated up ,and is likely to fail.Initially try to touch the heatsink and feel the heat as warm only. If it gets heated up badly switch off and reduce the current.
Important Note:-(Don’t use a metallic screw driver .You have to use a small piece non iron metallic object to work as a screwdriver - this will not alter the frequency while you take your hand near or away from the trimmer that usually happens in a metallic one).Copper or aluminum screw driver with insulated top is preferred.
Step 4: For Long Range
For Long Range use a Yagi antenna
The output is fed to a coaxial cable (generally used for cable TV ) which is nearly matched to the Yagi antenna (though 300 Ohms) impedance of 75 ohms by trimmer TR 2 of the tuned circuit for maximum power delivery to the load ie the Yagi / GP antenna. Transmitter should never be powered without the antenna (ie the load) in which case the total power forms a SWR standing wave ratio on the power transistor Q3 heating it up badly to result in failure.
DESIGN OF YAGI UDA ANTENNA: (YAGI ANTENNA)
Refer my details elsewhere on my postings or here