# how to calculate the exact frequency of this FM transmitter?

hello guys, i have a schematic of FM transmitter so i build it on breadboard but i got some components that are different from the schematic, so i guess this changes the frequency which should be 102.3 MHZ, in fact this frequency in my country is used (can i still be using it and interfere to the station that is using it ?) so i need to calculate the new frequency which i should check to hear the sound from my transmitter, how can i do that ?

the original schematic in attachment file

my changes are : instead of R5 47K i used 50K instead of 4.7uF i used 2.2uF instead of of 220K i used 200K instead of variable capacitor C5 i used a ceramic 0.1uF instead of the inductor used i put a piece of wire and made it into a coil as an antenna i used a piece of wire about 17cm attached to the + of the inductor

i assume there is connection between R2, R3 and the base of transmitter is that right ? and a connection between the LED, 330ohm and the base ot the second transmitter ?

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.

Nice thought, but you really don't want a stable frequency for wideband FM, you have to shift the frequency +/-200kHz or so according to the modulation input, 0.1% or so, far more than a crystal can be "bent". Narrowband FM can be done with crystals (deviation is more like 2.5kHz), wideband you need either PLL or DDS techniques or an modulated LC oscillator like this.

The frequency is set by the tank circuit, C6/L1, since C6 is a trimmer anyway that should give a wide range of adjustment.

Without a spectrum analyser you'll have to use a radio receiver to tune this transmitter.

Note that this circuit is primitive and the antenna will affect the tuning so you must tune with the antenna you will be using.

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. ;)

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:-

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

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.