I have bought a cheap fm transmitter on informatic store.
It work fine but sometime have noise and I understand the neighboord station, it's very enoying when you listen music.
I just sold a little antenna on the circuit just this delete the noise !!
I'm not electronic professional just hacker.
Sorry by advance my english, i'm french and i just want to share you my little experience, because I read alot on instructable :-)
Step 1: Found the Right Location to Connect Antenna
Open your fm transmitter
Take copper wire or simple wire.
Turn on another radio at the same frequency of your transmitter and play music.
Try to find the tank circuit, touch with the wire each parts of transmitter circuit and you will hear the better transmission on your another radio.
On some circuit you can see "ant" or "antenna"
On my case l6 and c26 is the good location to add antenna but is too small to sold something.
Step 2: Solder
I finally solder into jack output i work fine.
Step 3: Close Fm Transmitter and Test It.
I tested in my car is work very well for better transmission touch the new antenna with metal of your cars. I didn't ear any noise or neighboord station.