Guest Post: Lake Thunderbird (K-2792) with a homebrew transmitter!

Activating (sort of) Lake Thunderbird (K-2792) with a homebrew transmitter

Sam Duwe WN5C

When I dove into radio a couple years ago a few sub-hobbies caught my attention: QRP, portable ops, CW, and homebrew. Of course, these all fit nicely together, but in my mind there was a huge leap between soldering an unun and a building a radio. But why not try? What’s the worst that could happen by melting solder and then sitting at a picnic table? This is how I built a simple transmitter and kind of activated a park.

The Michigan Mighty Mite

Nearly everyone has heard about the Michigan Mighty Mite (MMM), a QRPp transmitter popularized by the Solder Smoke blog. There are countless YouTube videos and posts across the internet. It’s very simple: a single transistor, a variable cap, a coil, a crystal and some resistors and a cap. Supposedly one can get up to half a watt of output (I couldn’t). But with a small purchase from Mouser one can oscillate. That seemed pretty cool.

I hadn’t touched an iron until I started playing radio. But I’ve been drawn to homebrew projects. I built a regenerative receiver last year which was very rewarding. I’ve also put together kits (a QCX mini and a TR-35). But my dream has always been to construct a transmitter/receiver combo, or a transceiver. I thought a good place to start was the MMM.

I built the transmitter based on the common schematic for the 40-meter band. The MMM is crystal controlled but I opted to solder in a socket and buy a handful of crystals, so I have the luxury of operating on 7056, 7040, and 7030 kHz. I made a few other improvements, too. The first was to build a low pass filter to attenuate harmonics. Second, although I haven’t finished it yet, the switch on the right will be to choose between multiple crystals. And third, I added a BNC jack to connect a receiver, with a transmit switch. When not in use the transmitter will dump into a dummy load. This receiver switching idea was lifted from the design of the MMM that QRP Guys produces.

When I tested the transmitter at home the best I could get with my charged Bioenno 3 Ah battery was about 300 mW output. The filter is reducing things somewhat, but maybe I need to look into a different transistor or rewind the coil. But I was able to get a 339 signal report from Illinois (no sked) in the midst of distance lightning crashes, so I had a little confidence going forwards. School is out for me this summer, so I decided to head to the park.

Lake Thunderbird State Park (K-2792)

I activate Thunderbird all of the time, but only rarely on 40 meters. I’ve been doing that more though because I’m slowly realizing that DX (relatively speaking) isn’t everything and it’s nice to connect with the local guys. I arrived at about 9:00 AM and set up the rig. I chose my QCX mini (tuned for 20 meters) as the receiver because it’s less sensitive out of band and I didn’t want to overload my ears. A 3 Ah Bioenno battery powered everything, and I used my CW Morse key.

My antenna is an compact 40-meter EFHW based on HB9EAJ’s design. It’s cut for 20 meters with a 17-meter band link and has a 34 uH loading coil with an additional 6-ish feet of wire. It’s not as efficient as a full-length EFHW but it sure is easier to deploy. I was able to use my throw weight/line to get it up about 30 feet up into a tree, and have it slope down to the picnic table overlooking the lake.

I pulled out my 7056 kHz crystal. My wattmeter said, even after fussing with the capacitor, that I was going to have to be comfortable with only 200 mW of output. The conditions were okay but not great. Within a few moments of CQing I was spotted (poorly) by the RBN and my friend and Elmer, Perry (N5PJ), called me back. He’s 30 miles away but reported that the “9” in 229 was real; I was transmitting cleanly!

In the next hour of pounding away on the straight key I made contacts in northeastern Oklahoma, north Texas, Missouri, and Kansas. A few of the signal reports were good, conditions were up and down. But it was working! Manually switching between TX and RX wasn’t as difficult as I thought, although I have never worked so hard for POTA QSOs.

After about an hour and a half I had racked up five contacts. My call had fallen off of the RBN and things weren’t looking good. I decided to finish the activation on 20 meters with my TR-35 at 5 watts which I did in short order. The bonus that Randy N5ILQ was also unknowingly activating very closely (just beyond the bridge in the previous picture). We had a booming 17-meter (same) park-to-park QSO.


Like Thomas showed, QRPp is totally doable and very fun. I think that if I were at the park on a different day, or began earlier, I would have completed the activation using the MMM. But in this case 10 contacts wasn’t my yardstick for success; instead, making any contacts with a transmitter and antenna I built from scratch was a win. I’ve had many memorable POTA outings, but this tops them all.

I’d like to build a 20-meter version to extend the net a bit further. I also think that experimenting with other transmitter designs would be fun. And if I build a 40-meter receiver I could realize my dream of having a complete homebrew kit and look even nuttier at a park bench.

This has been a fun project and deployment. I can’t recommend enough building a MMM and trying it out, and if you’re in a 300-mile radius of me I hope to hear you on the air!

16 thoughts on “Guest Post: Lake Thunderbird (K-2792) with a homebrew transmitter!”

  1. Absolutely fantastic! This made my day, thanks so much for posting this story. And your construction job is beautiful. I don’t think there is any greater ham radio experience than making contacts with something you built yourself from scratch. (Many years ago I built a 160 meter QRP rig with VFO by piecing together a couple of W1FB designs. Everything from scratch including the circuit board and cabinet. Quite a thrill to make Top Band contacts with a 1.5 watt HB rig!)

    1. Thanks William, I recently started leafing through W1FB’s books. I think I need to invest in some more desoldering tools first so I can scavenge my way to an excellent junk box! Then the sky’s the limit.

  2. You sound like my doppelgänger. I haven’t attempted a scratch-built transmitter yet, but it’s on the ever expanding list. Way to go!

  3. My wife has a theory… that all that RF exposure is having its effect on me. (I think she meant that in a good way). Regardless, I took it as a thumbs up enthusiastic approval encouraging my hobby. I’m a glass half full kinda guy….

    I absolutely loved your report Sam! You have the gears spinning in my mind… “what’s left of it.” (She remarks). Build a homebrew, whats the worst that could happen? (It ignites into a shower of sparks, and burns my finger tips? Naw… )

    Thanks Sam for the inspiration! Really!

    72 de W7UDT

    1. Thanks Rand. There are a lot of good resources, but I’ve been binging on VK3YE’s website/videos for inspiration. It’s all very very cool. He has some good tutorials on building from a schematic, which helped me practically start to connect my understanding of a schematic to what it actually should do/how it works. Then the classic articles start to make more sense. Have fun!

  4. Please pardon this complete newbie question, but how is your setup configured to use the single antenna with a transmit rig and a receive rig?

    Do you use some sort of splitter or switch?

    1. From the picture in the post it appears he’s running the antenna into a BNC port on the transmitter where a double pole switch mounted on the panel routes the antenna lead to a receiver BNC port during receive. The QCX-Mini, acting as the receiver, is connected to this port.

      1. Yup. When I flip to transmit the receiver is completely disconnected from the antenna and transmitter, but I do get a sidetone because it’s close enough. On receive the transmitter is disconnected from the antenna. But not just disconnected, it runs to the dummy load so I don’t accidently cook my transistor transmitting with no antenna.

  5. My first station was a Globe Chief 90a transmitter and a Heathkit HR-10B receiver. I switched the antenna by hand. With a knife switch, actually. (Right out of the cartoons, but the kid who sold me the TX did it that way, so I did, too. About 25 cents off the Radio Shack pegboard, as I recall. And when I QRT’d I could lift the lever midway, opening both circuits, and believe I was protected from lightning.)

    Just one of several perfectly doable radio techniques that hams today consider positively impossible. (Manual antenna tuners being a more recent example.)

    A few years ago I built the NS-40 transmitter kit (great kit, costs 25$, pumps up to 10W out depending on power in) and was back to rx-tx ops. And again, until I assembled a Pacific Antenna auto-switch, was switching manually. I smiled from ear to ear, recapturing those days.

    My point is, assuming such old op-dependent tech is somehow “impossible” keeps us off the air in situations where we could totally be on the air and having fun if we just didn’t listen to the cool kids. What was done in the past, can almost always be done today. (The exception is where prohibited by law now, such as spark gap.)

    Thanks for putting homebrew on the air, Thomas! I admire your skills. Here in the Northwest I doubt I’d raise anyone with 300mW, especially with a portable antenna, but maybe if I went up into the mountains…

    ‘Nother great post!

    1. Thank for the memories! I still have the little knife switch that I used with my Novice rig, proudly displayed with my Novice straight key. Not sure where it came from; we didn’t have anything like Radio Shack around here back then. (Station was a Knight Kit T-60 and a Hallicrafters S-40A.)

      1. Yeah, what was up with those knife switches? Was it to avoid arc’ing, or what? I never learned. Seems I used toggle switches for most things back then.

        But it sure passed the Warner Bros’ test! I still have mine, too. Unused since that day.

    2. I’ve been thinking about your post all day, the part of totally doable techniques that seem impossible inconveniences today, but if we relearn these inconveniences is really opens the door on lots of news ways (or opportunities) to experience the hobby. And this applies to way more than radio! I think that’s why I like minimalist QRP so much. This was my first try with manually switching TX/RX and it wasn’t a thing after a few minutes. Anyway, thanks for your comment!

      1. You and me both, brother! I seriously believe that the more light-switch convenient our tech got, with almost guaranteed communications over tremendous distances (amplifiers, etc), the less fun it got. To say nothing of the expense, that kept lots of people out of it. (Especially kids.)

        So like you, I’m deep into seat-of-the-pants QRP these days. It’s really put the fun back in. Not least because now I can build much of my equipment, so the whole “black box” thing is greatly attenuated.

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