The Madagascar Mighty Mite

Finished mighty mite on copper board
The completed (yet unboxed) MMM.

Over the last few years, there have been a spate of postings from homebrewers taking inspiration from the Soldersmoke podcast to whip up various incarnations of the Michigan Mighty Mite, a very simple rock-bound QRPp transmitter. I’m a little late to the party, but here’s my story.

The compelling logic to make one of these was that I had the parts on hand and a free weekend. The choice of crystal determines the operating frequency, and although I have a lot of crystals in the junque box, most of them came out of microprocessor-based systems and don’t correspond to amateur radio frequencies (which, come to think of it, is a good thing). However, I did have about ten 3.579 Mhz colorburst crystals in the box, presumably extracted from old television sets or other projects. I also have a pile of 32khz crystals, likely from digital clocks. I’ll have to think of another project for those.

blank and white schematic of mighty mite circuit
Mighty Mite Schematic

Every site I look at on the internet has the same picture for the circuit, so it is duely reproduced here. In my iteration, I added a power indictor LED. A few people had mentioned that their 27 ohm resistor between the transistor emitter and ground got hot, so I opted to parallel three 82 ohm resistors for a bit more power handling. The 3.5mm jack uses the tip and ground, and I added Pete Juliano’s suggested 0.1uF cap across them to filter out the contact closure.

The output of the MMM is expected to contain a lot of harmonics, and this was the case. This sine wave output looks more like a cardiac patient in need of medication:

oscilloscope trace of unfiltered MMM output is periodic, but does not look like a smooth sine wave
Ventricular tachycardia.

Clearly, too gunky to put on the air, but it’s got the expected periodicity. I added a low pass filter based on an online table from the GQRP, which claims to knock down 2nd harmonic by 50dB, which should be 10dB better than good enough for government work.

With the LPF in place, I got the expected sinusoidal output, with power output around 200mW with 13.8V supply.

sinusoidal oscilloscope trace

I had been hoping for a bit more juice — there aren’t a lot of other hams in my neighborhood. I had originally built the circuit with a 2n2222 transistor and wondered if anything else I had on hand might work better. I thought a 2N4401 was a likely candidate, but it yielded similar output, measured at 3.24Vrms, so 210mW.

Photo of earlier version with air variable capacitor and a 47k resistor serving as dummy load.
Development version with variable capacitor

I decided to be content with this output level, and left the 2n4401 in place. For prototyping purposes, I had built the MMM with an old air variable capacitor, but didn’t want to tie it up in this project. I tuned up the circuit for maximum smoke and then backed off very slightly to give a bit of margin for stability, without really losing anything significant in power output. At that point, I measured the capacitance as 125pF and just replaced it with equivalent fixed capacitors (100+15+10pF in parallel).

I had built the MMM with the transitor standing up above the board, so passively cooled by air. I measured its temperature during keydown, but to do so, had to clip the probe in place, so that clip probably added to the heat dissipation. Nonetheless, I’d say that the transistor is not in danger of overheating even with extensive keydown. I measured with both the 2n2222 and 2n4401, and both leveled out at a temperature increase of about 7 celsius degrees after two and half minutes of keydown. At five minutes, the temperatures were stable.

FWIW, here’s my sketchy layout for Manhattan Indian Ocean Island style construction:

Component and island layout on PCB
handwritten capacitor and inductor values for the LPF filter
The MMM lowpass filter

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