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audioguru fm transmitter

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If this is a standard FM band transmitter then I would say they are air wound. I would wind about 10 turns of 22AWG enameled copper wire on a 4 to 5 mm former. Then remove the former. It is about the size of the coils in the transmitter I have.

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Hi Windoze,
My FM transmitter is rock solid stable in its RF frequency, has a range of more than 2km to my sensitive car radio and home stereo tuner, sounds very clear due to its low distortion and pre-emphasis (treble boost like radio stations have) and still works fine (although with reduced RF power) when its 9V battery runs down to 6V. ;D

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hi audioguru!
in am any transmitter or receiver or any other transistor circuit for that matter,
does the collector supply of the transistor matter?
how does oe decide how much supply to chose?
5V ,6V,12V?
what can happen if it exceeds or lowers below the desired value?

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in am any transmitter or receiver or any other transistor circuit for that matter,
does the collector supply of the transistor matter?

Of course the supply voltage for a transistor circuit matters. The transistor will fail if its max voltage or power dissipation are exceeded.

how does one decide how much supply to chose?
5V ,6V,12V?
what can happen if it exceeds or lowers below the desired value?

The supply voltage determines the power output. The power dissipated by the parts must be calculated and parts are selected that can handle the calculated values. The circuit's biasing must be designed for the circuit to operate at all expected supply voltages.
In my FM transmitter I used a low-dropout 5V regulator for the RF oscillator and mic preamp because this regulator allows the circuit to operate well when the 9V battery runs down to 6V.

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hi audioguru
but the coils of both pictures are different which one i will make ?
in first colour picture there are ten turn of coils
in second picture it shows 9 turns on 5mm former and 1mm wire.
what is the exact turns and former for coils.
some another questions
I found IC 2931A 5.0 and IC 2931T 5.0 in TO-220 package. It is ok ?

i m using all capacitors ceremics except 100uf. it will OK?

and i m using trimmers 1-27 pf.

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but the coils of both pictures are different which one i will make ?
in first colour picture there are ten turn of coils
in second picture it shows 9 turns on 5mm former and 1mm wire.
what is the exact turns and former for coils.

It won't make much difference, but your tuning caps are with a low value so make the 10-turn coils so that the frequency adjustment goes low enough.
The coil in the pic has a 3mm former and so does the coil in the sketch.

I found IC 2931A 5.0 and IC 2931T 5.0 in TO-220 package. It is ok?

I used the "Z" case that is in a TO-92 small transistor case. You can use the big "T" TO-220 case version if you want.

i m using all capacitors ceremics except 100uf. it will OK?

I always use metalized plastic film caps for audio coupling since they cause very low distortion and come in a 5% tolerance. Ceramic coupling caps cause some distortion and have very wide tolerance. The cap for the pre-emphasis must have a 5% accuracy for the sound to be correct.

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Can u give me the all calculations of FM Transmitter

There are hundreds of FM transmitter projects on the internet. I just copied the best features of a few of them, then added a low dropout voltage regulator and some pre-emphasis.

I calculated North America's 75us of pre-emphasis by using 75u/470 ohms= 160n so I used 150nF.
I calculated Europe and Australia's 50us of pre-emphasis by using 50u/470= 106n so I used 100nF. 

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thanks audioguru

the RF stage of your transmitter (mod4) wants special tuning like 4W fm transmitter (two transistor 2219)
or your`s circuit rf stage is tuned?

i m using C7 = 5pf except 4.7 it will ok?

thanks to windoze killa also.

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the RF stage of your transmitter (mod4) wants special tuning like 4W fm transmitter (two transistor 2219) or your`s circuit rf stage is tuned?

Both Q2 and Q3 are tuned RF stages.

i m using C7 = 5pf except 4.7 it will ok?

It will be fine.

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hi audio guru

i built your circuit and it is working.

i check it by field strength metre. I joint both circuits antenna together. (both circuits have 5cm antenna)

your circuit shows 3 to 4 volts in digital volt metre.

and previosly i built a one transistor fm transmitter which shows 5 to 6 volts. from same field strength metre to digital volt metre.

two transistors have more out put power then y my metre shows 3 to 4 volts. ?

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Don't connect the antenna of your field strength meter to the antenna of the transmitter, and the transmitter's antenna should be about 80cm long. The antenna on the field strength meter should also be about 80cm long and have a fairly low impedance at 100MHz.

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I made a simple field strength indicator for adjusting the tuning of the output stage. My FM transmitter caused strong RF all over my lab which made measuring its field strength very difficult.
The tuning for the output stage was very broad and when tuned for a peak at the center of the FM band then the power was about half at each end of the band.

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but my question is y my metre shows 3-4 volts. in two transistor (by joint antenna )
and 5-6 volts in one transistor.
when i was made 4W (two transistor 2219) transmitter when my metre shows
10-12 volts from 9volts duraplus battery. and 16-17 volts from 12 volt supply.
in joint antennas of field strenght metre and transmitter. y?

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When the antennas are the wrong length and your meter is not the same impedance as an antenna, then you aren't measuring RF power output, you are measuring the voltage swing that a transmitter can put into whatever impedance is the meter.

Make each antenna the proper length with a fairly long distance between so they don't interfere with each other.

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hi audioguru
i measure your transmitter with 1 metre distance between both antennas.
field strength metre and your transmitter.
and other one or two transistor fm transmitter circuit i have.
but your circuit is generating more RF power as compare to them.
and stability is good also.
but i judge that during transmitting Q3 is producing very littile amount of heat is it right?
i want to tune it around 80 MHZ, how many turns of coil do i make?

as you know my trimmers are 1-27pf.

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hi audioguru
i measure your transmitter .... during transmitting Q3 is producing very littile amount of heat is it right?

Yes, my transmitter's Q3 produces a little heat with a brand new 9V alkaline battery. I can't remember exactly but the supply current was about 55mA so the power from the battery was 495mW. The preamp and oscillator doen't use much supply power so most must be used by Q3, 230mW as heat and 230mW as output I guess. If Q3 is operating in class-C then the output power is more.

i want to tune it around 80 MHZ, how many turns of coil do i make?

A calculation would probably be in error due to unknown stray capacitance. Try 11 or 12 turns on the coils.

as you know my trimmers are 1-27pf.

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The RF amplifier transistor Q3 might already operate a little in class-C if the oscillator signal is strong enough. The base-emitter junction could act like a rectifier and charge C12 so that Q3 is cutoff a little.
If R8 is connected to ground instead of to the positive supply then Q3 will operate in pure class-C because it will be cutoff and some of its base signal turns it on.

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:)i think one of those simple to biuld peaking circuits would be handy for tuneing it for peak performance . look at the talking electronics site  they use the peaking circuit  for some of the fm transmitter bugs

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