How to make a Long range FM Transmitter

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1 Watt  –FM Transmitter  circuit. CKT

Range 2 KMs guaranteed with crystal clear sound on Yagi Antenna  

Q1 BC 547 /548

TO92B-pack

R1 10K
Q2 C2570,or C3355 in TO92B-pack R2 15K
Q3 2N3866+Heat sink must R3 4K7
C1 2.2uF/50V R4 4K7
C2 1KPF disk ceramic R5 82E
C3 10PF disk ceramic R6 1K
C4 1KPF disk ceramic R7, 1/2 watt 22E
C5 10PF disk ceramic R8 1K
C6 1KPF disk ceramic L1,20SWG,4mm,dia 4 Turn+1T
C7 15PF disk ceramic L2,20SWG,4mm,dia 7Turn
C8 1KPF disk ceramic L3,20SWG,4mm,dia 7Turn
C9 1KPF disk ceramic L4,20SWG,4mm,dia 5Turn
C10 10KPF disk ceramic Battery/Adopter 12V
C11 15PF disk ceramic Adjust Current by Tr1 < 80 mA
TR1(Trimmer) 22PF Antenna GP/Yagi
TR2(Trimmer) 22PF Co-axial cable 10 meters
Microphone Condenser Type Adjust Tr2 for the range 2 Kms

If 2N 3866 is not available one can use 2N2219 but the range shall fall to a great extent

Q2 has to be TO92B package. Using  TO92 the transistor will fail. Please check the datasheet of both their pin layout are different

If you have no experience on soldering get it soldered by any TV/tape/radio mechanic

Please note L1 has to be tapped at 1 turn as seen on the video image.

Pre-assembly work

Using the heatsink for the power transistor 2N 3866

IMG_1054

Place a screwdriver at the cut area so that it is made little wider

IMG_1056

Then swiftly pass the transistor, watching the notch to fall in the cut place and then remove the screwdriver. Now the transistor is fitted to the heatsink

IMG_1057

IMG_1059

This is how the plane PCB looks like

IMG_1061

Then make the holes little wider for trimmers  Tr1 & Tr2  and the coil holes

IMG_1067

So that the trimmers shall go smoothly into their holes

IMG_1073 IMG_1072

The coil ends must be properly removed for the enamel coating so that copper is clearly visible that makes the soldering good

IMG_1066

Now before doing the soldering try placing all the resistors first and then bending their leads and then do soldering. First resistors because they are all of the same height while sleeping on the board. Thereafter the next higher height component and so on.

IMG_1114

While putting the power transistor for soldering make sure not to flush it to the board but a little gap

IMG_1070

But not like this fully sitting on the PCB

IMG_1071

A DC socket may be glued here with Febi quick instead of hanging by wires

IMG_1060

While fully assembled it looks like this

IMG_1132

Solder the DC socket points as shown , the center pin being DC +ve and the side pin being DC -VeIMG_1136

IMG_1135 IMG_1133

Use a 75 CM straight wire at antenna point.

OPERATION

CIRCUIT DESCRIPTION& OPERATION:

This FM transmitter has 3 RF stages. A (VFO) Variable Frequency Oscillator (30 mW), a class C driver stage (150 mW) as a buffer and a class C final power amplifier (1 Watt )

(VFO) Variable Frequency Oscillator (30 mW)
a class C driver stage (150 mW)
a class C final 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 its 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 an FM

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 at each other at the line of sight distance, the range can be up to 5 KM.

Initially, use a simple 75CM single wire standing straight as an antenna for getting a range of about 100-200 meters indoor. Similar length telescopic antenna is also OK. A telescopic antenna for testing will give only about 100 meters range. But never go longer than 75 CM thinking that it will cover a higher range. In fact, if you do so the range will fall.

The frequency of the transmitter can be set within 88 to 108 MHz FM band by adjusting the TR1 (Trimmer 1) of the VFO or by changing the spacing between the Coil  L1 but you have to be below 90 MHz

  • Frequency Adjustment:( must be below 90 MHz)

NOTE: Don’t try to test the unit in the evening or at 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. Very near the transmitter will have several frequencies responding to the microphone. So go, at least 30 meters away from the transmitter after the initial test as above is verified. There the display gives 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 higher MHz where no commercial transmission takes place but do not go beyond 90 MHz because the Q1 has a very low Ft.

  • The distance adjustment, after connecting a Yagi or GP or Dipole antenna.

Transmission range is adjusted by TR2. For that use a multimeter 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 the adaptor) or the peak current by trimmer 2 to say about 75 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, the round metal body must be fully covered by the black heat sink, without which it will get badly heated up and finally get burnt. In around 75 mA at 12 volts it shall cover a good range and Q3 heat sink 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. Feel the heat of this during the operation. If it heats up badly, then either your frequency setting by Tr1  is too high or the Tr2 adjustment is mismatched, then switch off immediately as the current must be much higher.

Important Note:-(Don’t use a metallic screwdriver.Y ou 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 screwdriver with the insulated top is preferred.

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Advisor on electronics electrical hardware

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