Chirp and My Michigan Mighty Mite

I’ve been doing lots with my recently brewed Michigan Mighty Mite.  By lots I mean listening to my Tecsun PL-600 with SSB turned on and twiddling the BFO knob to find the tone being emitted by this new transmitter.  What I noticed early on is that the tone emitted started as what sounded like a pretty pure tone would quickly degrade into a warble before settling out at a slightly lower tone which became mostly stable.  After I little research I confirmed that this is what I had heard referred to as “chirp.”  That all makes sense — a variability to the tone emitted might sound like a like a bird’s chirp.

But what was causing it?  It sounds like a loss of power, so I looked into the stability of the power supply, a 9v heavy duty battery.  Watching the voltage across the battery, I saw an approximately 1v drop.  That looks like a culprit!  (Right?)  So, I proceeded to try using a more substantial 7.6v battery pack I made out of 1.2v AA NiMH batteries.  Did that resolve the issue?  Well, if by the dropping voltage, yes.  The voltage dropped less with the NiMH battery pack.  However, I was sad to find, the chirp was still there.  Next idea!

Maybe the transistor is overheating?  After all, I’m using a plastic case 2N2222a TO-90 case transistor.  Everyone says to use a metal can packaged transistor.  So, I figured it could be the transistor for many reasons.  I tried a different 2n2222a.  No dice.  The chirp was still there.  I tried a 2N3904.  Still there.  A MPS A14… Wait a minute, the sound changed.  It still has a chirp, but it is delayed.  So, I learned that the transistor has SOME effect on the chirp.  Next.  2N4401.  Now the chirp is shorter in duration and softer (it doesn’t get as quiet before returning to a tone).  More effects while only changing the transistor.  9287 — Wait, virtually no chirp.  That’s different.  Still there, but greatly diminished.  Am I on to something?  Maybe.  Next, I got some BD139s from Mouser and tried those since they can handle a lot more heat.  What!?  Still a chirp.  One time I held the BD139 body while keying to see if it was getting warm and something surprising happened — no chirp.

Here’s what my bench looks like right now:

Five transistors laid out on postit notes labeled with their part numbers and their effects on chirp.
These are the transistors I tested in my Michigan Mighty Mite to see if the transistor affected the chrip. The BD139 is in-circuit.

From: http://arcarc.xmission.com/PDF_Electronics/Transmitter%20Faults.pdf

CHIRPING
Chirping only applies to CW (Morse) transmitters. Keying chirps are quick changes in the frequency of a transmitter and occur each time the telegraphy key is closed. […]

In CW mode, if the stage being keyed is too close to the oscillator, then the oscillator may shift slightly off frequency each time the key is closed. These quick changes in frequency sound very much like a canary chirping, hence the term ’chirping‘. Chirping can also occur if the voltage regulation to the oscillator stage is inadequate.

So what’s going on when I hold the transistor?  Well, lots of little things.  I’m not sure which one is causing the change. One thing I realized right after that was that my key is conductive and connected into my circuit.  And, remember, in the Michigan Mighty Mite (MMM, from now on), the circuit is little more than an oscillator…  Hmmm, “if the stage being keyed is too close to the oscillator, then the oscillator may shift slightly off frequency each time the key is closed”  So, by keying and touching the transistor and key I’m altering the electrical (capacitive and possibly feedback) properties of the oscillator.  How can it not chirp?  It’s like licking a tuning fork or something!?  Maybe that isn’t a fair analogy.  I wouldn’t know.  But, it made me smile for a minute 🙂

So, how do I remove the chirp with my hands off the circuit?  I did some reading about Hartley oscillators, which the MMM is styled after, and found that the position of the tap in the inductor in the tank circuit decides how much feedback is delivered to the oscillator.  Then I remembered that the air core inductor I wound on a 1.25″ form for the MMM wasn’t center tapped — it was tapped after like 1/3 of its turns.  So, the orientation of the inductor affects the feedback and therefor stability of the oscillator.  I tried the inductor both ways.  No change.

Then, I remembered something I never should have forgotten: The tuning capacitor.  Doh!!!  I know, it’s a totally noob thing to do.  I did it.  I confess.  So, I tuned the tuning capacitor with the BD139 in place and BAM!  I got a stable pitch (to my ear at least.  I’ll check it on the oscilloscope soon).  That was such sweet music!  Now, I went back and tested all my previous transistors with this current arrangement and used the tuning capacitor.  I found I could make all of the transistors sound a little better than before (with all this experience), and, while there IS variation between different transistors, the BD139 IS still the best.  This leads me to the hypothesis that temperature is a factor.  Am I sure?  Nope.  It is just a likely factor.  If I figure it out 100% for sure later, I’ll share my findings.  For now, it is more stable in my circuit.

My next step is to build this transmitter in a compact and robust package that I can take on the road.  I need to test the limits of my ability to transmit!

Disclaimer: I’m likely to be amazed if I can receive my transmissions just 20 miles away from the town I work in, Orono, ME.  Why?  I’ve never transmitted on anything other than VHF/UHF so far.  My plan is to leave a device recording the output of my Tecsun PL-600 tuned to 7.030MHz and drive to work.  Then, while in Orono, transmit a sequence of code.  Cross my fingers, and when I get home that night, seek to that time of the day and listen!

Low power operation and homebrew

I’ve always been fascinated with leveraging the benefits of computing and electronic communications without consuming too many resources in the process.  I feel, deep inside, that if it isn’t sustainable, that it will likely go away.  Think Rome.

With that said, I recently built a Michigan Mighty Mite transmitter for 40m according to the outline here: http://www.qsl.net/wb5ude/kc6wdk/transmitter.html  Honestly, to be sure I understood the schematic which has some quirks (at least to my lightly trained eye), I had to read lots of other blogs and watch some videos.  The information about winding the air core inductor was the weakest.  I think some of the lightness of documentation is because it will most likely work somewhat even if you mess it up pretty badly.  So, this marks my first ever homebrew transmitter!

Michigan Mighty Mite transmitter wired on a breadboard.
This is the first homebrew transmitter I’ve built. It has been an adventure.

Testing it involved my first homebrew dummy antenna (or dummy load, depending on your vernacular) a pair of 100 ohm resistors in parallel inside a 6oz tomato paste can.

A pair of 1/4 watt 100 Ohm resistors in parallel connected to alligator clips at the bottom of a 6 ounce tomato paste can
This is my first run at assembling a dummy load to prevent this transmitter from radiating its signal far.  No, the leads are not touching the can 🙂

Then my first homebrew low pass filter to attenuate the higher frequency harmonics.  I followed the recipe for 40m here: http://www.gqrp.com/Datasheet_W3NQN.pdf

ceramic capacitors and hand wound ferrite torroid inductors arranged as a low pass filter for 7MHz ham radio operation
Simple LC harmonic, low pass, harmonic filter for 40m

The one site on the Michigan Mighty Mite I encountered suggested the input voltage be between 10 and 14v.  Mine didn’t work when I first assembled it on the breadboard so I wondered if maybe my 9v battery wasn’t enough to start the oscillator (noobs, huh?). I was pretty sure that wasn’t true, but not understanding the theory enough yet, I couldn’t rule it out.  So I threw together a homebrew LM317 based linear power supply.  I set the voltage to 10.7v and it still didn’t work.

Then I got down to the business of triple checking my connections and trying a little rearranging of some connections to shake things up a bit.  When I triple checked every connection and made sure I was following the schematic 100%, sure enough, I got it to work.  One homebrew project turned into four.  Now I have four interchangeable modules that can be put to use in other projects and I’ve learned a lot while refreshing some of my skills like reading datasheets, soldering, and translating schematics to a breadboard.

Bottom line: this was an adventure and, since I knew it would take some hard work and time, I did not get frustrated.  I just set my mind to work through each challenge and learn from their solutions.  This was a HUGE success for me.

What’s more, I feel a strong push to follow this super low power ethos and see where it takes me.  I’m used to thinking of low power as laptop vs. desktop.  Raspberry Pi vs. Laptop.  Baofeng vs 100watt desktop transceiver.  The thing I was reminded of here is that fundamental, empowering, and exciting communications can take place with mW and mA from small dc power supplies (ex. very small solar panels or 9v heavy duty batteries).

Here’s the parting shot of my fully prototyped transmitter

A breadboarded Michigan Mighty Mite transmitter with an external breadboarded LC filter feeding a dummy load in a tomato paste can all keyed by a keyer made on a slice of maple sapling with a strip of flattened copper pipe
Building this was an adventure. I’m so glad I tried the mighty mite first.