A Simple FM Transmitter

Here is a low component count FM transmitter. It sacrifices some features, like the ability to adjust the frequency, for simplicity’s sake. The build method is fairly common with amateur radio but we don’t see it around here too much. Each component gets a 5mm-by-5mm copper clad pad which is super glued to the ground plate as an insulator. There’s even a pictorial example of this method if you need some help with visualization.

One of the schematics included in the article shows how to incorporate a condenser microphone into the unit. We guess that makes it pretty easy to add an FM ‘bug’ to your arsenal of covert listening devices. Just make sure to check your local laws before building and using this. We’re not sure what the FCC would think of it here in America so we’re hoping some well-informed readers will educate us with a comment.

[Thanks Bart]

34 thoughts on “A Simple FM Transmitter

  1. Well, this kind of device is pretty much flat out illegal to operate anywhere in Europe. You aren’t too likely to get caught due to the low power. However all it takes is only one person who files a complaint about interference on their radio for a van with triangulation equipment to show up.

    Stuff that transmits on frequency bands not licensed by the operator is illegal, especially on commercial bands. And if you use it as a “bug” you can be sure to get a very stiff fine or even criminal prosecution.

    If you want to play with radio, either get an amateur radio license or use one of the unlicensed bands. It is much better for everyone involved.



  2. We have FM transmitters for mp3 players and the like here in the states. However, they are usually VERY low power (sometimes it can’t make it to the car radio antenna) and so the FCC restriction is really on the output power.

  3. I have never seen that kind of board making technique before, it looks like a good way to make more rugged prototypes for cheap.

    As for FCC regulation of FM broadcasting etc. I have no idea if this is legal by the book, but I would think that as long as the broadcast power is low there should be no problems. Similar devices are sold to broadcast iPods to FM.

  4. Amateur Radio operator here.

    Basically, the FCC doesn’t care what it transmits so long as it’s in an unlicensed band. Unfortunately, these bands are so crowded that it’s very difficult to get much range in the power you’re allowed to transmit at.

    A simple google search should reveal what american unlicensed bands are. s/bands/frequencies/ if you need to.

  5. Anyone building a transmitter must desire for someone to receive the signal. The most readily available inexpensive FM receivers are for the broadcast band. After siting Tetsuo Kogawa’s webpages it’s clear he is an activist/archaist/ radio pirate, and the intent is to use this transmitter on the broadcast bands; “These workshops also provoke those involved to consider the technical, political, and social ramifications of electromagnetic broadcasts. By building transmitters the workshop participants inevitably deconstruct broadcasting, challenging their own notions of what broadcasting is now and whiat it could be.”

    Anyway when it come to unlicensed radio transmissions in the USA in any band the FCC expects the transmitted signal to be of good quality.

  6. I remember building some of these for friends many decades ago, and then seeing the outcome. Like the guy who promptly put it in a teddy bear to prove that his ex-wife was getting laid now that she was single. Yawn. Maybe if he’d had access to craigslist in the 1970’s, he could have found someone else to spread herpes with.

    You know what? If you’re finding yourself wanting to spy on an ex, or bug an apartment, or any of a thousand stalker-like behaviors because you need “closure” or “have to know” or whatever…

    It’s a very good sign that 1) You’re way, way, way creepier than you give yourself credit for – no, really, I mean it – you’re messed up and just haven’t realized how messed up you are, 2) You really don’t need that relationship and 3) I can guarantee that you will do nothing but twist your guts up because he/she/it/they have moved on and you haven’t.

    It’s almost the same with pirate radio. You can build it, and about 3 guys will listen to it over the course of 4 months before you screw up and your gear gets hunted down and impounded.

    Seriously – you’d probably have more political and social impact by becoming a mime.

  7. Request to the management: Can the name of the commenter be put on the top of the comment so I can skip certain people?
    Because scrolling down each comment then up to read it is a bit annoying.

  8. The problem with this transmitter is it cannot be used with FM radios with a PLL digital tuner, as they require exact center frequencies to “lock on” to the signal. Since accurately tuning this transmitter is out of the question, only radios with analog tuners will be able to tune into its signal properly. I suppose there is also frequency drift to consider too.

    However, it is a nice old-school style FM transmitter that should work well on any old style FM radio in an area that isn’t plastered with radio stations.

    I live in San Diego, CA where the FM band is completely full from one end to the other, either by local, Mexican, or LA stations. Such things like this never work well here. There is no such thing as an empty spot here.

    I learned all this when I made my own FM transmitter years ago.

  9. Howdy. This was a great post, and the comments are also very informative…. this is the kind of stuff I like to hear. I’ll definitely be looking into this later.

    and @bob
    was that really necessary?

  10. Jesus fucking christ people, it’s just a tiny little radio transmitter. The world is not going to cave-in from something like this. Nor will your local radio station go bankrupt. Nor will you see a mysterious “delivery” van appear in front of your house. Build it, have fun with it, move on.

  11. I’m going to make a couple of these go to my work and block every station i don’t like.
    Listening to wuss R&B music is not helping me concentrate while i’m constructing electrical closets with PLC.

    I just hope i won’t get busted or fined for this stuff.

  12. Another radio amateur chiming in with 2c worth

    This simple but robust method of building a circuit is an ideal opportunity to learn some useful stuff.

    If you provide the circuit with a dummy load to avoid it transmitting while you fiddle, it’s an opportunity to experiment with component choices to see what effects they have on frequency-stability… you’ll learn more effectively that way than you will with any book.
    If you’ve got a frequency-counter, a scope, or both, there’s a lot of fun to be had seeing what matters and what doesn’t.

  13. To make an on-topic remark
    This would have been so much cleaner(and current), if SMD parts were used, it only takes a few parts and you’ll have enough room to manually solder and you don’t have issues with the connecting wires doing their own (parasitic) thing.

  14. Jan you’re not right. This isn’t so simple as it looks.

    In general the low power FM transmitters are also allowed in EU. But it depends from state to state if they implemented respective european norm(EN 301 357-2) into its local law. The european norm is here quite long time so my info is that only in Belarus and Greece this low power transmitters are not allowed.

    As i can speak for law in Czech republic those low power FM transmitters are allowed from half of the year 2006 with max transmittedd power 50nW.
    It is always good to check actual state;)

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