Electronic Circuits and Projects Forum

30 guage wire is awful darn small for a VHF inductor. I have never heard of using an iron core for such a high frequency.

I used 9 turns of 1mm enamelled wire tightly wound on a 3mm former. The end wires have exactly 0.4" spacing and I spaced these air-core inductors about 2mm above the board. Even though I used Veroboard, my layout was tight without any extra track lengths so my wiring was extremely short, like a pcb.
My Walkman radio usually works very well but overloads causing the transmitter to be all over its dial when closer than about 10m to this powerhouse FM transmitter unless I switch the radio's input attenuator to "local".

For Mod4, I reduced the gain of the preamp for even lower distortion, more pre-emphasis boost and simplicity. I added a low-dropout 5V regulator for powering the preamp and RF oscillator.

It works great, but sorry, it is still too cold outside to measure its range.

I agree with everyone else, you shouldn't leave a core inside your coils, this will massively increase the inductance - they should be air-cored coils.

I'm also not suprised your board doesn't work, it's very badly laid out! - at VHF layout is critical, your design is spread all over the place. In particular the coils, they are no where near the transistors - this is probably the most critical part of the circuit!.

Hopefully Audioguru might post his veroboard layout?.

Incidently, I've thrown dozens of these little transmitters together in the distant past, I always just wrapped four turns or so of thick copper wire (inside core of TV coax) around a pencil as a removable former - you can adjust the frequency by spreading the coil apart.

What is the output voltage of adapter that you are using?

Place the components as close as possible. Avoid long wires. See component placeement in my varactor tuned FM transmitter below.

Have you checked whether the capacitors are of nearly same value that your want? If not check them using LCR meter or cap. meter in your multimeter (if present). Even polyester capacitors have large tolerance. Last week I got few 6.8nF polyester caps and when I measured their value, they were all measuring between 33nF to 46nF!!!! Thats no where near 6.8nF.
The case is even worse with ceramic capacitors. I once read tolerance that was mentioned on the packing bag of caramics. It was 80% !!!!!
So its always better to measure and then use capacitors when working with radio circuits.
Your circuit may not be working due to this also.

Originally Posted by t_anjan

I didn't know that the transistor needed to be close too. Could you please give me the reason for this?

Because you are adding extra capacitance and inductance into the circuit with the long wires, this circuit is extemely crude, there are many components which affect it's operating frequency - not just the LC tuned circuit.

VHF inductors are so easy to make that I have never purchased any. Your purchased inductors might have built-in capacitance that is far too high.

Sorry, the 0.1uH value is not measured, just a value and construction recommendations stolen from another website's project.
Likewise, most of the small caps that I used weren't measured, but stolen from old radios. My cap that is marked "30" is probably somewhere near 30pF.

Your layout has parts so far apart that the capacitance and inductance of the wiring probably far exceeds the values in the circuit.

When I find my son's digicam, I'll post pics of my transmitter's tight layout and coils.

The cap that is chosen for different parts of the world is for the pre-emphasis, treble boost that FM radio and TV stations use, then the radios and TVs in the same part of the world have a coresponding treble cut to reduce hiss. America invented FM radio and decided on a lot of treble boost (big mistake, but they can't change it) but when Europe and Australia got FM, they reduced the amount of treble boost, and their radios and TVs are also different from American ones.
Therefore all the simple FM transmitter circuits on the web that don't have pre-emphasis don't have any treble when heard on an FM radio.