how to calculate the exact frequency of this FM transmitter?

instead of the inductor used i put a piece of wire and made it into a coil

Hopefully you manage to hit a frequency in the range of your radio receiver (88 .. 108 MHz is not a too broad range).

Note that this circuit is primitive ...

... so chances are good the mic audio signal affects frequency somehow and produces an FM-like signal. I guess if it works at all, it will affect all FM radio frequencies. No need to worry about 101.3 in special. :wink:

So you used a 100,000pF ceramic capacitor in place of a 6 to 60pF variable? So I'd say the frequency is 0Hz, because it won't oscillate.

As for the coil, there is no way anyone can tell you the inductance with the information you gave.

This is a rather unstable oscillator, for the reasons given. Pretty much anything you change that is connected to Q2 (including the battery condition) will change the frequency. You can get close by calculating the resonance of C5 and L1, although the frequency will be something lower than that due to the feedback capacitor C6 and the transistor's operating point characteristics.

So the only way to know for sure is either tune a radio into its signal, or use a frequency counter. If you connect an oscilloscope probe or anything else directly to it, you'll lower the frequency.

What a horrid schematic. Terrible place to draw a (wasteful) power LED. And the LED is pointing the wrong way.

mauried:
What are you trying to do with this transmitter?
The frequency is given by the formula 1 / 2 X pi X sqroot (L X C), but as
the circuit has the oscillator as the output stage , its frequency stability will be terrible.
You need some kind of crystal oscillator followed by a multiplier to get a stable frequency.

Actually it may more complex than you think, since its a modified Hartley
oscillator several capacitors are contributing to the overall capacitance.
(Including those of the device itself which are modulated by the input)

Tuning a radio into the output is a fairly sane way to tune this (although
it will also detect harmonics, so a rough calculation based on the component
values is a good safety check.

Like 100,000pf for the resonating capacitor? ;')

polymorph:
Like 100,000pf for the resonating capacitor? ;')

well yea i thought this will affect it big time, i just used because i didn't have the 60pF trimmer but seems it's very wrong !

about the tuning using a radio, what do you mean by that ?

about the inductor, it's 6 rounds, 0.5 cm diameter

Well, it is only about 2000x larger. :'/ What happens if I put mining truck tires on my car? It isn't meaningful any more to ask how fast it can go, because it won't work.

Tuning the radio? I meant tuning your receiver around until you hear the signal from the transmitter.

We need to know a lot more about the inductor to calculate the inductance. Is that 5mm inner or outer diameter? How long is it? Wire gauge?

in fact this frequency in my country is used (can i still be using it and interfere to the station that is using it ?)

Most countries will not allow you to transmit in this band anyway.
If you fill in the location information in your profile everyone can see it and give you better advice.

In general this is a broadcast band and using any frequency is illegal especially using such an unstable circuit.

polymorph:
Well, it is only about 2000x larger. :'/ What happens if I put mining truck tires on my car? It isn't meaningful any more to ask how fast it can go, because it won't work.

Tuning the radio? I meant tuning your receiver around until you hear the signal from the transmitter.

We need to know a lot more about the inductor to calculate the inductance. Is that 5mm inner or outer diameter? How long is it? Wire gauge?

i guess it's better to buy an inductor than build one like that

Grumpy_Mike:

in fact this frequency in my country is used (can i still be using it and interfere to the station that is using it ?)

Most countries will not allow you to transmit in this band anyway.
If you fill in the location information in your profile everyone can see it and give you better advice.

In general this is a broadcast band and using any frequency is illegal especially using such an unstable circuit.

than what is the solution ? how can i choose a specific frequency and make a circuit for that frequency ?

First, tell us what you are trying to accomplish.

polymorph:
First, tell us what you are trying to accomplish.

ok my goal is to go from here to build a system like the walkie talkie or something small similar to the communication system used in racing when the team speak to driver and the driver speak back

i can say after i finished the book i was reading i managed to build a small speaker system, and i can say i have an idea (not very advanced but i am not much confused like when i first started) of what is going on in sound stuff

From the document attached:-

Article 14: Requirement of a Frequency License

  1. Subject to Article 14 (3), no Person may use Radio Frequencies or operate a radio apparatus in Lebanon, or on board of any ship, aircraft or space object that is registered in Lebanon, unless authorized by a Frequency License of the Authority.

See the last section of the document for:-
Radio Experiment License
Amateur Radio License

I don't think they will allow you to make a transmitter to do this. Also see "type approved in Lebanon".

_Library_Files_Uploaded (219 KB)

Grumpy_Mike:
I don't think they will allow you to make a transmitter to do this. Also see "type approved in Lebanon".

but why what is the reason, if it is just like the FPV used on RC airplanes ?

Because that radio is carefully designed by trained engineers, extensively tested, and approved.

polymorph:
Because that radio is carefully designed by trained engineers, extensively tested, and approved.

so is it possible to build a small talkie walkie similar a bit to the one used in the racing world ?
and what about the toys i guess it's very low frequency that doesn't cover much more than 20meters or something ?

firashelou:
but why what is the reason, if it is just like the FPV used on RC airplanes ?

Because some one who does not know what they are doing can cause havoc interfering with vital communications of the Police and Military. Governments do not need that extra hassle.
If you do know what you are doing you can apply for type approval which means your design is independently tested to make sure there is no interference outside the band you are using and that it is being manufactured in a professional way. Of course you pay for that.
If you want to learn then there is always the Amateur or Ham license but there are restrictions on the use you can put to that.

RC airplanes use a specific band and the equipment has to be type approved. Imagine you were flying your plane and it was brought down by someone messing about with an FM transmitter. You would not be pleased and it could kill someone.

so is it possible to build a small talkie walkie similar a bit to the one used in the racing world ?

No in your country you are not permitted.

what about the toys i guess it's very low frequency

You guess wrong.

doesn't cover much more than 20meters or something

Range is not a consideration for most countries, as yours and mine, it is the act of making and using an unlicensed transmitter. Again there are bands for type approved equipment and that includes toys.

Grumpy_Mike:
Because some one who does not know what they are doing can cause havoc interfering with vital communications of the Police and Military. Governments do not need that extra hassle.
If you do know what you are doing you can apply for type approval which means your design is independently tested to make sure there is no interference outside the band you are using and that it is being manufactured in a professional way. Of course you pay for that.
If you want to learn then there is always the Amateur or Ham license but there are restrictions on the use you can put to that.

RC airplanes use a specific band and the equipment has to be type approved. Imagine you were flying your plane and it was brought down by someone messing about with an FM transmitter. You would not be pleased and it could kill someone.

aha ok your right
ok i have a question which i didn't get the answer i am looking for which is :
how to define the frequency i want to work on ? is it by making calculations and defining what cap, resistors and inductors i must use or .. ?
and still didn't get how a walkie talkie can't interfere with another walkie talkie or even other devices ? well so far i know there is some sort of encryption, so how is that done ? and what components ?

i remember when i was a kid i got a walkie talkie for about 10$ but extremely powerful according to a toy, it was very similar to the old big real ones and can cover a very big distance, + we used to hear people talking sometimes but very few moments that this happened !

how to define the frequency i want to work on ?

It depends on the frequency range you are working in.

is it by making calculations and defining what cap, resistors and inductors

For low frequencies say less than 30MHz you can do this although the value of all these components will change with temperature with careful design you can keep it down to manageable proportions.
For higher frequencies it is more complex and a simple free running oscillator will not be stable enough. There are different techniques like heterodyning, crystal oscillators and mixers, and the use of phase locked loops to name but three techniques.

Grumpy_Mike:

how to define the frequency i want to work on ?

It depends on the frequency range you are working in.

is it by making calculations and defining what cap, resistors and inductors

For low frequencies say less than 30MHz you can do this although the value of all these components will change with temperature with careful design you can keep it down to manageable proportions.
For higher frequencies it is more complex and a simple free running oscillator will not be stable enough. There are different techniques like heterodyning, crystal oscillators and mixers, and the use of phase locked loops to name but three techniques.

alright instead of a simple oscillator made of conductor and 60pf, it is more efficient to use the crystal oscillators for example