# need help with fm transmitter

#### MentorEnigma

Oct 6, 2017
1
Hi,
how can i make a coil of the same value of the inductor in this transmitter circuit and what type of antenna is used ? will it be different if higher frequency ?

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
There are pages on the web which will allow you to determine the inductance of a coil of wire of a certain gauge, formed into a helix of a certain diameter and length.

My guess is 1.5 turns of 0.5mm wire with a coil diameter of 6mm.

#### Audioguru

Sep 24, 2016
3,656
An ordinary FM transmitter transmits from 88MHz to 108MHz. Its coil is about 100nH with 5 to 9 turns of wire on a 3mm inside diameter former. But this circuit uses a lower value coil so its turns are fewer and its frequency is much higher. Do you have a radio that will receive it?
C7 is not 103pF instead it is a ceramic disc capacitor that is marked 103 which is 10000pF (0.01uF).
C3 is not 102pF instead it is a ceramic disc capacitor that is marked 102 which is 1000pF (0.001uF).
I think C6 and C8 should be at least 22pF.
The length of the antenna should be calculated to be a quarter wavelength. The transmitting and receiving antennas should be physically parallel.

This very simple circuit will have problems:
1) There is nothing to tune its frequency. Usually C1 is a trimmer capacitor.
2) Its tuned circuit connects directly to the antenna so the frequency will change if anything moves towards or away from the antenna which changes the tuning capacitance.
3) Its frequency will change as the battery voltage runs down because it is missing a voltage regulator.
4) It will sound bad (muffled) because it is missing pre-emphasis.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
An ordinary FM transmitter transmits from 88MHz to 108MHz. Its coil is about 100nH with 5 to 9 turns of wire on a 3mm inside diameter former. But this circuit uses a lower value coil so its turns are fewer and its frequency is much higher. Do you have a radio that will receive it?

It calculates as 277MHz. My guess is that a factor of 10 is error is hiding there somewhere (because 277/sqrt(10) ≈ 88)

This very simple circuit will have problems:
1) There is nothing to tune its frequency. Usually C1 is a trimmer capacitor.

Often in the really cheap circuits, the inductor is squeezed or stretched to change the frequency. My guess is that the original circuit had an inductance about 10x what is shown here and the dimensions would be more similar to what you suggest (and indeed, that is what I was expecting). In that case, the closely wound coil would give you the bottom of the band, and stretching it out would move you up the band.

2) Its tuned circuit connects directly to the antenna so the frequency will change if anything moves towards or away from the antenna which changes the tuning capacitance.

In fairness, the circuit suggests there is a buffer between the tuned circuit and the antenna.

3) Its frequency will change as the battery voltage runs down because it is missing a voltage regulator.
4) It will sound bad (muffled) because it is missing pre-emphasis.[/QUOTE]

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