Legal FM Transmitter?

[TheNewGuy]

Hello Everyone,
Today, I was scouring the web looking at different FM Transmitter circuits that I could build and I came across **broken link removed**:

For what I'm using, the values are:
**broken link removed**
R1 = 1 kohm
R2 = 10 - 100 ohms
C1 = 22 nF
L1 = 6 turns 32 gauge enamel coated wire (.5 mm thickness), 1/4 diameter, close wound on non-conductive form.
L2 = 5 turns 24 gauge enamel coated wire (1 mm thickness), 1 in. diameter, wound around an "AA" type battery with 2 mm space per turn...
Q1 = 2N3904 (using a different transistor will change the frequency)

It is powered by a 5 volt battery, and it gave me 108 Mhz of frequency (which is the last channel in the FM band)

**Make sure all parts are close to each other so it will oscillate.

I happened to have all the parts for it that he used except the 1,000 Ω resistor, so hoping it would work it subsituted it with a 100,000 Ω resistor. I then breadboarded it out took it to the car and tried to tune the radio to the signal. I could hear music, but the quality stunk, and I could barely make out my music (I knew it was my music because I could change the song), but I was still thrilled it worked.

My dad was impressed, and told me to find a circuit that would have better quality audio (because you could barely make out the music), that wouldn't drift, and that wouldn't get us a fat ticket or fine (I live in the US), we could eventually make it into a PCB and use it.

I've looked at several circuits, one I like a lot is AudioGuru's Mod 4 Miniature FM Transmitter (Link for: schematic and thread), but I'm worried that it would transmit to far and get me in trouble. There are several small ones I find, but I don't want the signal to drift (that is why I like the Mod 4 circuit).

I know there is a cornucopia of information on FM/AM Transmitters, but me not yet being a EE Student yet, I need help understanding all this information and figuring out what schematic would be best for this application.

Thanks everyone,
TheNewGuy

 

[Hero999]

As you've discovered, a simple single transistor FM transmitter gives poor quality, if you used a 1k resistor it probably would've been better because the signal would be stronger but it would still sound poor.

Audioguru's transmitter is better but will probably get you into trouble. You could build it but reduce the length of the aerial to say 150mm (6") and it'll probably be more legal but I can't guarantee that you won't get into trouble.

 

[audioguru]

Why don't you build a Micromitter that is a small FM stereo transmitter that has an attenuator at its output so it is legal? A kit is available.
It uses a crystal for its frequency synthesizer so its frequency does not drift. All the small MP3 to car radio transmitters use the circuit.

 

[TheNewGuy]

I like the micromitter idea AudioGuru! I'll look for a kit, but is this (schematic and **broken link removed** I found) the kind of circuit?

 

[Dragon Tamer]

As you've discovered, a simple single transistor FM transmitter gives poor quality, if you used a 1k resistor it probably would've been better because the signal would be stronger but it would still sound poor.

Audioguru's transmitter is better but will probably get you into trouble. You could build it but reduce the length of the aerial to say 150mm (6") and it'll probably be more legal but I can't guarantee that you won't get into trouble.

it's only illegal if you get cought but i agree that audioguru's thrasmiter would work best just because it's slightly more stable.

 

[audioguru]

audioguru's transmitter would work best just because it's slightly more stable.

No.
The Micromitter's frequency is made from a crystal controlled frequency synthesizer so its frequency stability is absolutely perfect.
It is also stereo while mine drifts its frequency and is mono.

 

[MrDEB]

just buy a berklin2 transmitter. Do the modification to it. There are numerious links to the mod. Us Computer Christmas light freaks do it. My transmitter, note a town with wide open spaces, goes about 2 miles (according to my brother and depending on car radio type.
Toyotas have pretty good eception as do Fords. My Chevy radio , lucky at 1/4 mile.

 

[audioguru]

My son bought a cheap Chinese transmitter from E-bay. It was mono, not stereo and it had horrible distortion and muffled sounds. But its frequency was solid.

 

[Dragon Tamer]

My son bought a cheap Chinese transmitter from E-bay. It was mono, not stereo and it had horrible distortion and muffled sounds. But its frequency was solid.

those Chinese mess everything up!

Sorry I got that wrong about your transmitter, I haven't really had a lot of time to read the entire thread.

The thing that I would be worried about would be the 2 inductors being mounted too close together, unless they're supposed to be. Then the other question, should the inductors be mounted on different axis to prevent "cross induction"? (I hope that I got the terminology right)

 

[audioguru]

The inductors in my FM transmitter are 0.5" apart so, they do not affect each other.

 

[Mishael]

Dont you need to be an HAM radio operator to TX on FM? or am i just crazy

 

[mneary]

TX on many frequencies is legal in the US for anyone if your power is low enough and you don't interfere with anyone. A skilled person may build a few devices for his own use. (CFR47, part 15.23b).

As a rule of thumb you are probably legal if you can't be heard beyond 100 meters using a decent radio.

The actual regulation specifies field strength, harmonics, and special frequencies where you may not ever operate (such as distress beacons, etc.)

 

[Mishael]

right you cant get on the analog cell band, no TX on special use frequencies. thats not a standard lesson in the class i took then

 

[Dragon Tamer]

The inductors in my FM transmitter are 0.5" apart so, they do not affect each other.

that's good to know now i know how far apart i must put the inductors when i make your circuit (it can be made on a breadboard right?)

I was actually talking about the inductors in the circuit shown. i tried making the circuit using the exact parts specified (except the inductor gauge, i used 22 hookup wire not 26 or 34) and i could barely make out my songs over all of the static.

 

[audioguru]

that's good to know now i know how far apart i must put the inductors when i make your circuit (it can be made on a breadboard right?)

No. An FM transmitter or radio works at 100MHz which is too high for the long wires and millions of connections on a breadboard. A 1MHz AM transmitter might work on a breadboard.

I made my FM transmitter on a compact stripboard layout. The wires are very short with low capacitance and low inductance.

The wires on a breadboard act like antennas and pickup all kinds of interference.

 

[TheNewGuy]

No. An FM transmitter or radio works at 100MHz which is too high for the long wires and millions of connections on a breadboard. A 1MHz AM transmitter might work on a breadboard.

I made my FM transmitter on a compact stripboard layout. The wires are very short with low capacitance and low inductance.

The wires on a breadboard act like antennas and pickup all kinds of interference.

Along with the 10,000 Ω resistor, that is probably another reason why that small FM Transmitter stunk badly, the fact that is was on a breadboard.

When I can, I'm going to try using a smaller resistor and see what kind of results I get (maby a 100 Ω).

 

[Dragon Tamer]

No. An FM transmitter or radio works at 100MHz which is too high for the long wires and millions of connections on a breadboard. A 1MHz AM transmitter might work on a breadboard.

I made my FM transmitter on a compact stripboard layout. The wires are very short with low capacitance and low inductance.

The wires on a breadboard act like antennas and pickup all kinds of interference.

That would certainly explain why none of my FM transmitters work.

Along with the 10,000 Ω resistor, that is probably another reason why that small FM Transmitter stunk badly, the fact that is was on a breadboard.

When I can, I'm going to try using a smaller resistor and see what kind of results I get (maby a 100 Ω).

You could always replace the resistor with a pot of just about any value equal to or above the specified resistance.

 

[TheNewGuy]

When using stripboard, you just connect the components together with wire?

 

[Hero999]

Yes but you need to make the circuit as small and as neat as you possibly can, otherwise all the stray capacitances in the wiring will stop it from working.

 

[kolisson]

hi,

try out this one:
https://www.electro-tech-online.com/custompdfs/2010/02/SPK-TFM-101020SPECIFICATION.pdf

works fine

klaus