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Long range FM transmitter

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mstechca

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T1 and the components around it make a basic FM transmitter. R4 and R5 make a voltage divider, but C2 can allow R4 to be electrically bypassed if it is the correct value. Any components up to the coupling capacitor C9 appears to be a weird setup in my opinion because a standard FM transmitter setup will have the input at the NPN's base and the output at the collector.

L1 and L2 make transformer coupling. I don't use transformer coupling personally.

I think VC1 and VC2, L3, L4 and C7 make one or more filters to prevent interference from happening at the wrong stations.

The 100K resistor limits the signal, but I think 100K is too high.

I don't use a varicap.
 

audioguru

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mstechca said:
T1 and the components around it make a basic FM transmitter.
T1 is just a VHF Colpitts oscillator.

Any components up to the coupling capacitor C9 appears to be a weird setup in my opinion because a standard FM transmitter setup will have the input at the NPN's base and the output at the collector.
The varicap diode's capacitance plus C9 in series with it determines the RF frequency. Since the varicap diode's capacitance changes with voltage changes, then VC1 tunes the RF frequency and the input audio modulates the varicap diode's voltage providing pure FM.

L1 and L2 make transformer coupling. I don't use transformer coupling personally.
L2 has fewer turns than L1 so the transformer matches the high impedance at the collector of T1 to the low impedance of the base of T2.

I think VC1 and VC2, L3, L4 and C7 make one or more filters to prevent interference from happening at the wrong stations.
VC1 adjusts the RF frequency of the oscillator by changing the capacitance of the varicap diode.
VC2 adjusts the RF power output level. Lower resistance equals higher power.
L3, VC1 and VC2 match the impedance of the collector of T2 to the antenna.
L4 is just an RF choke with C7 a bypass cap to keep high power RF away from the supply for T1.

The 100K resistor limits the signal, but I think 100K is too high.
R2 doesm't limit or reduce the audio signal since the varicap diode is reverse-biased and therefore is a very high impedance, and the tuned circuit at the collector of T1 is also a very high impedance.
If R2 was less than 100k then it would reduce the sharp tuning.

I don't use a varicap.
Try it. It allows an FM transmitter to make pure FM without the AM added by simpler circuits. :lol:
 

Nigel Goodwin

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pike said:
man that thing looks dodgy (crap). I doubt you'll get any range from that. The first stage should be a pre-amp for the mic but it will work...just not as effectively as say compared to this circuit.
Actually the design is far better than the ones you suggest!, for a start it generates proper FM (which the others don't), and has a properly coupled RF power amplifier stage. It's range will probably be far greater than the previously mentioned designs - it's output power is probably around 1W, making a range of 50-60 miles easily possible!.

The actual audio gain is probably fairly low, as the mike feeds the varicap directly, but it's quite possible that it's intended for NBFM, and NOT WMFM. In any case, a simple audio preamplifier could be added to increase it's deviation for WBFM.
 

audioguru

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I designed my FM transmitter with enough audio gain (too much, actually) to pickup voices from across a room. I had to keep telling my wifey to "watch what you're saying".
If I speak to it up close then it has an extremely high FM deviation (and probably AM over-modulation) causing severe distortion in radios. :cry:
I had to attenuate the signal from my audio generator way down in order to test its pre-emphasis and low distortion at normal deviation. :lol:
 

saurabhppawar

New Member
Hello
I have the same circuit of FM tx , i wan realize it . Im not understanding the specifications of L3 & L4. What they exactly are ? What mi supposed to tell the shopkeeper if i wan but them?
 

saurabhppawar

New Member
Hello
I have the same circuit of FM tx , i wan realize it . Im not understanding the specifications of L3 & L4. What they exactly are ? What mi supposed to tell the shopkeeper if i wan but them?
 

audioguru

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The project tells you exactly how to make the coils. But I think it was a very bad translation from a foreign language so it is difficult to understand.
 

audioguru

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Of course the circuit will work.
Its oscillator is the same as hundreds of FM transmitters except it is tuned and modulated by the varicap diode.
The RF amplifier is an ordinary one.
The oscillator transistor and varicap diode were used in a car radio I worked with in 1964 when there were few FM radio stations for this circuit to cause interference.

Its frequency will drift all over the place, its sound will be very muffled and its power will get the user into trouble with the law.
 

audioguru

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L3, L4 and the other coils are easy to make. The instructions in the project tell you how.
Don't you have the entire article about this simple old transmitter project?
 

audioguru

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I saw the article many years ago. I don't know where it is today, maybe on a website in India.
Why don't you use a modern project?
 

Space Varmint

New Member
Of course the circuit will work.
Its oscillator is the same as hundreds of FM transmitters except it is tuned and modulated by the varicap diode.
The RF amplifier is an ordinary one.
The oscillator transistor and varicap diode were used in a car radio I worked with in 1964 when there were few FM radio stations for this circuit to cause interference.

Its frequency will drift all over the place, its sound will be very muffled and its power will get the user into trouble with the law.
OK I suppose I could see where the varactor is used in the oscillator. Something I would never consider. Well, I'm sure you agree, it has much to be desired.
 
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