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Some More Simple FM Transmitters

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audioguru

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A good FM radio is made as sensitive as is possible. Adding an RF amplifier just adds noise and it will be overloaded by strong stations.
A cheap FM radio will also be overloaded by the RF amplifier's amplified strong stations.
 

transistor495

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You can improve your reception by trying a 4 x 75 cm cubical quad antenna. I tried it. Great reception! Greater than a Yagi antenna. It's especially for fm.

I've tried a BF495 rf amp long time before. It also worked well with a Yagi antenna. I don't have the circuit. I think EFY have it.
 
I don't want to light a candle under people's testicles here, but there are way too many folks who don't know enough (and that's why hams write exams and get licenses) about rf, yet keep going to higher and higher powered transmitters. Not only is it illegal, but it is illegal for a reason, you are causing interferences over tv, and other stations in the area, on emergency two radios etc. Please consider this before making your first electronics project ever a transmitter because it's "cool".

Audioguru I built yours for fun on my protoboard, and it worked suprisingly well (for a rf circuit on proto), well done.
 

audioguru

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Audioguru I built yours for fun on my protoboard, and it worked suprisingly well (for a rf circuit on proto), well done.
I am afraid to use my FM transmitter because my FM dial is full of stations. I tested it for only 1 hour after I made it to measure its range.

I used the same frequency as a low power foreign language radio station on the other side of my city so I was causing interference, then the RF cops might come looking for me.

Another guy in my city made my FM transmitter and reported that he saw the RF cops going down his street in their van with the rotating antenna on top. Maybe they were looking for me!
 
Audio guru ,allow me to ask

if i do not use ur resistor values as u suggest in transmitter #3
but using mine as below, same circuit diagram.,will it works?

i using 2N3904 too.
R 1 = 36k
R 2 =100k
R 3 = 70K
R 4 = 150
R 5 = 300
C 1 = 2.2pF
C 2 = 2.7nF
C 3= 22 mico F
c4 = 27nF
R 6 = 390

may i ask how u know the adjustment of the value resistors?
sorry i just new in electronic and wish to know more ,thx a lot ^0^
 

audioguru

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Audio guru ,allow me to ask

if i do not use ur resistor values as u suggest in transmitter #3
but using mine as below, same circuit diagram.,will it works?
The 3rd transmitter does not have R5 and R6 so don't know which circuit you are talking about.

The parts have common values so why change them?

may i ask how u know the adjustment of the value resistors?
You need to learn about electronics to see why certain parts values are used.
 

Tails

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Hi I'm not an expert in electronic but I have a question: In the circuits can I use another transistor than (for example) the 2N2222?
 

audioguru

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Hi I'm not an expert in electronic but I have a question: In the circuits can I use another transistor than (for example) the 2N2222?
Philips' datasheet for a 2N2222 in a metal package lists its minimum transistion frequency at 250mHz so it will oscillate at 100MHz perfectly.
A BC547 and BC548 are 150MHz so they are also fine.
A 2N3904 is 300MHz so it is fine.
Their current gain are almost the same so nothing in the circuit needs changing.
There are many more.
 
I have some more transmitters (FM) for you to view...
Regarding the second transmitter, it looks the designer here is using a 4.7 pF feedback to the collector, is that feedback needed to maintain the gain or for some other reasons? The 0.01 UF seems it will short at certain frequencies and therefore block all those frequencies from being coming out to the antenna. Am I getting this right?. But what does happen if some noise is coming from the DC power supply +9V, would that affect the performance at all? My last question is to the designer of the last circuit: how much did you calculate your gain for the transmitter. I found it to be around 2 for a 100 Ohms antenna:p
 

audioguru

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Regarding the second transmitter, it looks the designer here is using a 4.7 pF feedback to the collector, is that feedback needed to maintain the gain or for some other reasons?
The 4.7pF capacitor provides positive feedback from collector to emitter so that the transistor oscillates near 100MHz on the FM broadcast band.

The 0.01 UF seems it will short at certain frequencies and therefore block all those frequencies from being coming out to the antenna. Am I getting this right?.
No.
The 0.01uF capacitor bypasses the base of the RF oscillator to ground at RF frequencies since it is a common-base transistor oscillator.
But its value is way too high since it reduces frequencies at 3200Hz and above from modulating the oscillator.
All FM radio stations use pre-emphasis (treble frequencies boost) and all FM radios have de-emphasis (treble frequencies cut) to reduce hiss. Since this transmitter cuts high audio frequencies instead of boosting them then it will sound very muffled when heard on an FM radio.

My FM transmitter has pre-emphasis and its base to ground capacitor has a value of only 470pF (21 times less than 0.01uF). It sounds great.
 
how did you figure out the frequency of the transmitter to be 100 Mhz, is that from the LC circuit? If you want to increase the range can you just another another amplfying stage?
 

audioguru

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how did you figure out the frequency of the transmitter to be 100 Mhz, is that from the LC circuit?
The LC circuit, stray capacitance in parallel with it including anything conductive near the antenna, the capacitance of the transistor (also affected by voltage), the value of the positive feedback capacitor and the stray capacitance in parallel with them determine the frequency.

If you want to increase the range can you just another another amplfying stage?
Of course adding an RF amplifier will increase the range.
Using an impedance matching circuit to match the RF amplifier to the antenna will also increase the range.
 
Thanks for the reply, did you have a look at the tracking transmitter I believe was posted by Colin? does the antenna on the circuit picks up signal, which turns on the LED?
 

audioguru

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Thanks for the reply, did you have a look at the tracking transmitter I believe was posted by Colin? does the antenna on the circuit picks up signal, which turns on the LED?
The tracking transmitters are on another thread.
A transmitter transmits, it doesn't receive.

One of Colin's tracking transmitters uses a flashing LED to turn the transmitter on and off over and over.
Another of Colin's tracking transmitters uses two transistors as a slow oscillator to turn the transmitter and maybe an LED on and off over and over.
 

audioguru

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The microphones of the circuit 1 and 3 should be in series with a resistor of 4.7K~10K as the #2 circuit is.
No.
Circuit #2 uses an electret mic that needs to be powered with the resistor from the positive voltage.
Circuits #1 and #3 use a dynamic mic (coil and magnet) that generates a signal and does not need to be powered. The resistor will cause its diaphragm to move and then maybe the dynamic mic might be damaged or will not work.
 
Audioguru, I'm putting together your transmitter circuit, but I hope to use it with a portable music player, so (as you have already said somewhere), will probably want to cut out everything left of C3, just whacking in the line-in straight to that cap. But from what I can gather, the pre-amp was where the pre-emphasis was done. How would I keep pre-emphasis in there? My somewhat basic electronics skills would say to try and make the pre-amp have a gain of unity, would making R4=R5 suffice? I guess I'm not sure how C2 and C4 will affect things (or even if I'm calculating the pre-amp gain correctly)?

What modifications would you advise?
 

audioguru

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If you change R4 or R5 then the preamp transistor will not be biased correctly and it will be a rectifier with severe distortion.

You probably need two resistors as a left and right channels mixer at the input so use them with a resistor to ground at the input of the preamp transistor's C1 to make an attenuator.
 
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