# another FM transmitter project

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by hannobisschoff@gmail.com, Apr 14, 2011.

1. ### audioguruWell-Known MemberMost Helpful Member

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My FM transmitter was designed for a 9V battery, not a 12V battery. Its output transistor will have a current that is too high for a little 2N3904 and will overheat.

The high available current from a battery does not force all its current into a circuit.
My car has a 12V 840A battery. The starter motor draws 840A when it is cold. But the clock in the car does not use 840A, it uses only 1mA (0.001A).

With a new 9V battery, the output transistor in my FM transmitter draws about 50mA. Its collector load is an LC tuned circuit so its collector voltage is an average of 9V. Then it heats with 9V x 50mA= 450mW which is very warm.
If it has a 12V supply then the output transistor will probably try to conduct 200mA which is too high for a little 2N3904 transistor. It will heat with about 12V x 200mA= 2.4W so a power transistor with a little heatsink will be needed.

You should learn how to calculate electronic equations. It is simple.
You should learn how to design electronic circuits. It is simple.

2. ### NepalimanMember

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It is simple because you can do hahaha but i've no idea how to do so learning from you every little idea. My academic field is language. I am doing Master Degree in Nepali language. But i am interested in electronics. It is difficult to understand such equation. Thanks for your continuous help guru

3. ### audioguruWell-Known MemberMost Helpful Member

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Most of us learned the basics of electricity and electronics in high school then we learned the details in university.
Most of us talk about electronics with people with our same level of education.

I am not a teacher and usually I do not want to teach. But some people here are teachers.

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5. ### JMWMember

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6. ### audioguruWell-Known MemberMost Helpful Member

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7. ### NepalimanMember

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Wao! Inspirable.

8. ### NepalimanMember

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1) The suitable length of FM's antenna is 80cm. I have a 80cm telescopic antenna. I stand this antenna at the top of my house and kept FM transmitter in my room. So i used 3 meter long wire to connect antenna and transmitter. But i think here is the total length of the antenna has become 3.80 meter. Is it harmful for long range transmission? ( I think, all liscensed FM station has long wire to connect the real antenna, which is in too heighty tower.)

2) Look this transistor details:
RF power transistor:- BFS55= 40 watt/12 volt/1amp./Driver 1 watt

A) In this details, what is mean by "Driver 1 watt"? It means- BFS55's input power should be 1watt RF?? If it, should we make driver pre- amplifier with 3866 for input of BFS55??

B) If i use BFS55 (40 W) instead of 2n3866 (5 W), can transmitter transmit 8 times long range?

Last edited: Apr 8, 2012
9. ### audioguruWell-Known MemberMost Helpful Member

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Radio stations use coax shielded cable up to their antenna. Then their antenna is the correct length. Your antenna is too long which reduces your range.

The BFS55 is a low power high frequency transistor. Its maximum allowed heating is only 0.25W so its maximum RF output is only 0.125W or less.

Last edited: Apr 8, 2012
10. ### NepalimanMember

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Sorry that was wrong transistor. Look
this one
transistor
details:
RF power
transistor:
SD1274=
30
watt/12
volt/1 watt Drive
A) In this
details,
what is
mean by
"1 watt Drive"? It
means-
SD1274's
input
power
should be
1watt
RF?? If it,
should we
make
driver pre-
amplifier
with 3866
for input
of
SD1274??
B) If I use
SD1274 (30
of 2n3866
(5 W), can
transmitte
r transmit
6 times
long range?
C) Generally, Is high watt transistor mean long range transmission?

11. ### audioguruWell-Known MemberMost Helpful Member

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The SD1274 transistor has power gain that is higher at frequencies lower than 136MHz.
You can calculate how much power gain it has at 100MHz.

It has MINIMUM power gain or TYPICAL power gain. You do not know how much power gain your transistor has.

If its minimum power gain is 10dB at 100mHz then for 30W output then its input power (drive) must be 3W. The 2N3866 can produce the 3W drive power.

If the antenna is not directional then 10 times the power doubles the range.

12. ### NepalimanMember

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In your 3904 based transmitter, there is a pre-emphasis stage made by Q1. I think it is for voice amplify.
-Does it play the role for long range transmission?
-Why C4 should be different as different country? Which value of C4 is suitable for Nepal?

Last edited: Apr 10, 2012
13. ### audioguruWell-Known MemberMost Helpful Member

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FM radio stations add pre-emphasis that boosts high audio frequencies. Then all FM radios have de-emphasis that is the opposite that cuts high audio frequencies including hiss picked up during the transmission and reception. They cancel perfectly so the audio has a flat response from 50Hz to 15kHz but hiss is reduced.

Pre-emphasis makes the sound very clear (not muffled), it does not increase the range.

My FM transmitter has far range to a very sensitive FM tuner (to my expensive home stereo and to my high quality car radio) with nothing in between (across a large river valley) and when the transmitting and receiving antennas are 80cm long and are parallel.

In North America and some other areas the boost (+3dB) begins at 2133Hz which is well below the highest voice frequencies (14kHz) and begins at 3200Hz for the different amount of pre-emphasis/de-emphasis used in Europe, Australia and in other areas because they introduced FM radio later and decided on having less pre-emphasis/de-emphasis.

Sorry, I know nothing about Nepal.

14. ### NepalimanMember

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A) In your 3904 transmitter, i know we have to adjust both trimmer with listening FM radio for best tune or performance. When both trimmer are tuned perfectely, both trimmer got equal value? (like VC1=15 PF & VC2=15 PF?)
B) May I replace Q2 with 2n3866 without modifying other components? Or which modification is necessary?

15. ### audioguruWell-Known MemberMost Helpful Member

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The oscillator has a trimmer capacitor that adjusts the exact radio frequency. The RF amplifier has a trimmer capacitor that has a broad frequency range and it is peaked for the maximum range.
The stray capacitance of the circuit board, transistors and antenna affect the tuning so the trimmer capacitors are adjusted differently.

The 2N3866 transistor is completely different to a 2N3904. Its DC current gain is a wide range from only 10 to 200 but the 2N3904 has a narrow range from 100 to 300. Then many 2N3866 transistors will work very poorly in my circuit that is biased for a 2N3904.
So more complicated biasing for a 2N3866 must be used.

My 2N3904 output transistor has low input power from the oscillator. If a 2N3866 transistor is biased properly and is used as an output transistor then its output power will be almost the same as my 2N3904 transistor unless its input power from the oscillator is increased. So the oscillator must have a driver transistor added.

For higher output power then a higher power supply voltage must be used for the output transistor.

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16. ### NepalimanMember

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Dear guru,
1) In some advanced transmit circuits,, what is mean by- "RF output antenna with 50 ohm load"? For 50 ohm load, may i connect RF output to the ground with 50 ohm resistor?

2) In datasheet of 2n3866:-
a) Its Total Power Diss. is 5.0 watt but why it can produce only 1 watt or lower in FM transmitter?
b) in 2n2222, What is this mean-
T amb< 25 o C= 500mW
T case< 25 o C= 1.2 W

3) Generally, if 1 watt=2 km transmission, can we say this- 2watt= almost 4 km and 5 watt= almost around 9/10 km transmission?

17. ### audioguruWell-Known MemberMost Helpful Member

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No.
The coaxial cable feeding the antenna and the antenna itself have an impedance at the radio frequency of 50 ohms. FM radio and TV coaxial cable and antennas are usually 75 ohms. A resistor simply wastes power.

a) In Philips 2N3866 datasheet, the maximum power is 5W when the supply voltage is higher than 28V and the current is higher than 107mA. Most of the power might be heat.

In the 400MHz circuit they show the supply voltage is only 28V and the current is only 79mA which when multiplied give a total power of 2.2W. Then they say the efficiency is 45% so the output transmit power is 1W.
At 100MHz the current is higher so the total power is 3W. The efficiency is better at 60% so the output transmit power is 1.8W.

These powers are when the heatsink is perfect (maybe it is cooled with an air conditioner) so it does not get warm.

b) The 2N2222 in the old metal case can dissipate a maximum of 500mW when it is in free air that is 25 degrees C. Then its chip will be at its maximum allowed temperature of 200 degrees C. When the metal case is cooled to 25 degrees C somehow then the transistor can dissipate more power, maybe 1.2W.

Maybe.
If the transmitter has an omni antenna then the power is distributed all around. About 10 times the power goes twice as far.
If the transmitter has a very directional dish antenna up high then twice the power goes almost twice as far.

Last edited: Apr 18, 2012
18. ### panic modeWell-Known Member

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this is called dummy load and it is used when testing transmitter for example when tuning circuit and you don't have antenna. running without load would cause reflections (SWR) which could destroy your transmitter. therefore using dummy load allows you to proceed with testing without broadcasting signal and risking death of the transmitter. in normal operation this is not what you would use because all transmitter power would be turned into heat.

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19. ### NepalimanMember

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But coaxial cable has two wire, inner and outer. How should i have to connect it with transmitter and antenna?

In transmitter of Audioguru, there are two big capacitor (100uf) in input and output of voltage regulator. But why in same place C9 and C10 (1000pf) capacitors in parallel is necessary?

20. ### audioguruWell-Known MemberMost Helpful Member

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On my FM transmitter, I simply used an 80cm piece of wire hanging vertically out my window of my home without coax cable.
My car radio has a coax cable with its center wire connected to the 80cm whip antenna and the shield connected to the metal of the car.

A 100uF electrolytic capacitor is a good filter for low frequencies. The datasheet for the LM2931 low-dropout regulator IC insists it is necessary for stability. Its inductance is too high at 100MHz to be any good.
A 1000pf ceramic capacitor is an excellent filter at 100MHz.

Last edited: Apr 21, 2012
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21. ### NepalimanMember

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-May I modify your (audioguru) 3904 FM transmitter by adding Q4 of 2n3866 as RF output for 1 watt?

-Q3 (2n3904) can be a driver for 2n3866 for 1 watt? If not, please give me a list of transistors that can be a driver for 3866.

Last edited: Apr 25, 2012