FM transmitter help

Discussion in 'Wireless & RF Design' started by davidGG, Jan 5, 2014.

  1. davidGG

    Thread Starter Member

    Dec 22, 2012
    I've made a FM transmitter from this schematic:

    The problem is that ....
    Let's say I am tuning the capacitor and I begin broadcasting at 100Mhz (I can hear my music). As soon as I remove my ALL PLASTIC screwdriver from the rotor of the tuning capacitor, the frequency of the tank circuit changes. It seems as if when I am tuning the capacitor, I become the antenna or the reactance of my body is affecting the tank.

    I've tried so many things. I'm not even frustrated anymore, I'm angry :mad:.

    During my first prototype I used a regular breadboard then I moved onto a PCB board but I still have the same problem.

    So I take the FM transmitter to my car and tune the capacitor until I hear something on 89.5Mhz. When I hear my music and remove my screwdriver from the capacitor rotor, I get the same thing, a change in frequency.
    Now this is what really pisses me off, and it only happens in my car, not on my clock radio. So now I have to go up or down a few frequencies to find my broadcast, but as soon as I move my body or the transmitter, the frequency changes AGAIN!

    What can I do? I am aware that I can just buy a premade FM transmitter but my goal is to learn something.

    I appreciate all help.

    Also, something I've considered is removing the tuning capacitor and just use a regular fixed value capacitor. Will this solve all my problems? I haven't tried it yet since I would have to de-solder the tuning capacitor. I've wasted at least 16 hours doing this (past three days) and don't want to waste any more time without getting some advice.
    I bought this tuning capacitor from a HAM radio website...why would they make the rotor conductive?
    Last edited: Jan 5, 2014
  2. ScottWang


    Aug 23, 2012
    Could you attach the pictures for your prototype or PCB board and the plastic screwdriver, maybe you can show them in some different angles.
  3. 4pyros

    New Member

    Jan 3, 2014
    Well then you have learned something, poor circuit poor performance.
    Is your variable cap directly connected to the coil?
    You may have to shield it in a metel box with a small hole in it for your screw driver.
  4. davidGG

    Thread Starter Member

    Dec 22, 2012
    Sure, when I get home tonight I will upload them.

    I did a little bit of research and I came across the concept of frequency drifting.
    From here:
    The second circuit shown "One transistor Circuit" is almost the same as mine and it is stated that it drifts a lot.

    I take it that single transistor FM transmitters are prone to drifting? Should I try building a transmitter with more than one transistor to reduce sensitivity?
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Hey, what do you expect for these one-transistor wonders. Voltage variations cause them to drift, temperature change causes them to drift, where your hand is causes them to drift, so does the phase of the moon...

    Get a "real" transmitter; you know, one with a crystal in the oscillator or a frequency synthesizer, an oscillator stage, buffer stage, final stage, more than one turned circuit between the collector of the final and the antenna; the list goes on...
    panic mode likes this.
  6. davidGG

    Thread Starter Member

    Dec 22, 2012
    Any suggestions?
  7. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    Basically buy a commercial FM transmitter. Belkin makes a couple for example.
  8. WBahn


    Mar 31, 2012
    But doing that defeats the stated purpose, which is learning how they work.

    @OP: As the gist of the thread has already indicated, the circuit you are currently working with has serious shortcomings and, among these, is a very high sensitivity to very minor things. There is a reason, after all, the "real" transmitters are a LOT more complicated than the one you are using -- the one you are using is too difficult to control! So learn what you can about the concepts of FM transmission from the simple circuit, but accept that it probably isn't going to perform too well or be worth spending a whole lot of time trying to get it to perform well. Instead, make note of the ways in which it performs poorly and then move on to the next stage of improvement and make note of how much better it performs and try to understand the concepts behind the improvements.
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
    what type of tuning capacitor are you using? some are very "springy" and will change a little with torque. also, you should have the adjustable side of the cap connected to the battery side of the coil, to reduce effects when adjusting.
  10. radiohead

    Senior Member

    May 28, 2009
    MikeML is right that all those things will de-tune a one-transistor transmitter among other things. The reason is the transmitters don't have any PLL or other circuitry to prevent the circuit from drifting.

    The metal box may help, but the TX will still drift as the battery dies. There are some transmitters that are surprisingly stable at

    The ultra-simple transmitters were never meant to be anything beyond a learning point to demonstrate the concepts of creating an FM signal. They produce many spurious signals (harmonics) as well. You will need a spectrum analyzer to find the center frequency with most simple transmitters as most that claim to transmit in the 88-108 band are simply using one of the harmonics which would dramatically reduce the range you could receive the harmonic.

    Here's a little light reading on PLLs I found for you.