Thursday, April 21, 2011

Micro FM Transmitter

Recently, Mark VandeWettering - K6HX of fame, posted his very interesting and success build of Tetsuo Kogawa’s Micro FM transmitter.

Following Mark's great inspiration, I decided to commit the circuit to a Micro PCB using small SMT components.

This project build was implemented using DipTrace PCB Software and the Homebrew Toner Transfer Method. Similar to my other Amateur Radio projects, my goal for this was to build the device as small as I my abilities allowed. The results, is a circuit board .5 x .8 inches, with about a third of that dedicated to the 1/8 inch audio connector. Some of the original circuit component values were change to accommodate the allotted PCB space and my junk box resources.

As Designed with DipTrace                   DipTrace 3D View

Printable Artwork

With a 9 Volt Battery and a twist of the Trimmer Cap, the circuit came alive on the FM Broadcast Band.

Initial testing indicates very good stability and it does not suffer from hand capacitance. It's DX has been tested to greater than 30 feet. I have not yet tested for expected battery life/usage, but so far one 9 Volt Battery has been running for greater that 4 hours (see UPDATE below), while listening to my phone tunes on the FM radio. I have not had to re-tune the receiver - it is very stable.

Mounted and Ready for Use

Click the images for expanded view.

The vertical component on the back edge is a 2.2p capacitor used as a short stub antenna, this simple small consistent load helps stabilize the frequency and provide a greater transmit range. The antenna length is just the original length of the component lead. Power (input or) output has not been measure (yet), but my similar constructed 30 meter beacons have typical measure 43nW into a 50 ohm load, but of course, this project is not terminated into a proper load and therefore the effective radiated power is unknown.

With a simple male-to-male stereo audio cable, the transmitter can be used with any PC or Phone style audio output.

The resulting circuit works very well, it is not stereo, but for now my Droid Phone has never sounded better on FM :-)

Thanks Mark for the build inspiration, and Tetsuo for the circuit design.

At about 24 hours mark the output frequency shifted up about 2kHz.

For the Die Hard Test; A single 9 Volt Battery has now been powering this transmitter continuously for more than 96 hours.

At 96 hours I stopped the Die Hard Test to experiment with other aspects of this Transmitter. I think it is safe to say this Transmitter is very gentle on its battery.

An interesting observation: My HTC Droid Phone has an FM Receiver APP that can be used to listen to FM radio. Which I inadvertently turned it on, the audio was redirected to the audio output jack which was connected to the Micro FM Transmitter. In effect transcribing the Phone received FM station to yet another FM Broadcast Band frequency - as received on another FM receiver (my the FT-817). I think this may be border line illegal.

The MFMT is now even smaller, and available see:

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  1. Great project! I've been mucking about with DipTrace for a few days now and am cautiously optimistic. Would you be willing to share your resulting files?

    Thanks for the great post in any case - that battery life is fantastic!

  2. Successful implementation of a circuit on a small scale. I love the miniaturization Eldon. Great job!