The Ten Minute Transmitter

 Decades ago, a ham by the name of Steve, G4RAW, conceptualized a CW transmitter in a magazine article capable of putting out up to 1 watt of RF power over the air. It was so barebones that even the newly-licensed amateur, with little to no experience, could throw it together, tune it up, and possibly make a QSO in a matter of minutes. No, not hours - minutes. 10 minutes to be exact. This rig is called the Ten Minute Transmitter.

Connected to just 12VDC and a simple dipole antenna, one could easily make a stateside QSO and even a worldwide QSO with just less than a dozen parts. I'm not the only one who has experimented with it, and in fact I was first inspired by the makeRF website article here: https://makerf.com/posts/ten-minute-transmitter. Ciprian, YO6DXE, who has been such an inspiration for new amateurs and old amateurs alike, created it when he first caught wind of amateur radio. He has an article as well as a build video up on YouTube! And of course, we can't forget about Peter, VK3YE, who built this tiny transmitter and added an IRF510 amplifier stage on the output to push out around 5 watts. Did I also mention he made a QSO with it paired with a 1970s-era AM-only shortwave receiver (more info on that below)?

Well, if these guys made it then so can you! The schematic is really simple, and in fact, you don't have to use a BFY51 - you can use the standard 2N2222, 2N3053, etc. NPN transistors! Just remember to add a heatsink or else you'll release the magic smoke demons!!! I wonder if a BD139 would work great...

Building (or Modding) the Ten Minute Transmitter

Here are the parts you will need:

• 1x - NPN transistor (REQUIRES HEATSINK)
• 1x - 100uH inductor (axial type works just fine, 3.3uH may or may not push out more power)
• 1x - 22k resistor
• 2x - 0.01uF/10nF capacitors (ceramic disk are just fine)
• 1x - 1000pF variable capacitor (or you can use whatever variable cap you have on hand. Even a disk ceramic in parallel with a large value of trimmer capacitor works just fine! For example, a 470pF disk and a 120pF trimmer variable capacitor = a range of 470ish to 590ish pF!!!)
• 1x - crystal oscillator with a frequency of your choice (80 to 10 meters - experiment!!!)
• 1x - 10k resistor (see below)

See, isn't it easy to build this thing???

In addition, you'll also want your favorite RF connector (BNC is excellent for this application), a 12V supply, an LED for power indication, a jack for a straight key, and whatever else you would like to add on! This circuit should work well on all the HF bands, and the only thing you'll need to adjust is the crystal frequency and the variable capacitor on the output. Heck, you should be able to put a variable capacitor in series with the crystal to make it that much more frequency agile too!

One modification that I picked up, more so from Peter, is the addition of a 10k resistor across the key contacts to make this circuit constantly oscillate at a super low power level. Let me explain why you may want to do this...

We all know that resistors allow a specific amount of current through. If we were to short the emitter directly to ground, we would get a steady carrier wave at full output power. In essence, current takes the path of least resistance, and wires have very little to no resistance. While in theory the emitter is shorted, this time by a resistor, this only allows a minute amount of "shorting the key" current to ground, resulting in a power output of around a few microwatts instead of the full milliwatt or solid watt power levels.

With this microwatt oscillator, we can utilize the transmitter as a beat frequency oscillator for the frequency band we are interested in. A BFO is important in demodulating SSB and CW signals, and with this in mind, we can use a cheap, generic, unselective, and low-sensitivity AM-only receiver as a partner to this transmitter - even a direct conversion receiver will work wonders! The BFO adds back the missing carrier that SSB/CW lacks and produces a pleasant and audible tone we know as SSB voice or CW pulses.

Whether you build your own receiver, buy one or scrounge up a used one, this should pair significantly well with the transmitter.

Minimal Count != Fun

Although this is a minimal count transmitter, I would only use this in an emergency situation if you were to build it barebones, i.e., without a LPF. For casual everyday operation, you'll want to add a low pass filter. This rejects any unwanted frequencies beyond a certain point. 

Just like with multiple musical instruments playing the same note at one time, crystals produce what are known as overtones. Say you have a 3.5MHz crystal. The second harmonic, or overtone, would be about 7MHz, the third at 10MHz, and so on. While overtones are weaker in output power in comparison to the fundamental frequency, remember that even 10 milliwatts can cause issues across the world on certain days. As amateurs, we are considerate to others, so if you are building this, please build a low pass filter! I recommend a 5 or 7-pole design, as 3-pole designs are cheap and small but do a terrible job at suppression. Also, please get a ham license if you don't have one already!

Why YOU Should Build This!

Building a circuit such as the Ten Minute Transmitter can teach you a lot, but it really teaches you about the foundations of radio and the part that drives all radios - an oscillator. In simplicity, this circuit is nothing but a basic oscillator that uses the transistor as the oscillator stage and the amplifier stage. 

As a Technician, you have the capability to get on HF, but you're limited to CW only on a small portion of 80, 40, 15, and 10. 10 meters is the only place with Tech SSB phone activity. While it is a small portion of the HF band, learning CW straight away is a fun and exciting adventure! I haven't mastered CW yet, but I'm growing confident that I could carry on a 5 to 10 minute CW QSO, albeit with some mistakes. A simple transmitter like this can help you get started.

For those of you who are nay-sayers to QRP, just remember every ham had to start somewhere. My first radio was a 5 watt VHF/UHF handheld, and my first HF rig had only 10W output power on 15, 12, and 10 meters. QRP is fun and exciting, especially when there are people on the bands listening for a CQ call. I've listened in on several QRP conversations and you wouldn't believe how far away some of them were! Despite the masses saying that QRP is a fruitless effort at making contacts, I'm here to say that QRP has been my friend since day one. Still don't believe me? I've got an entire article written up about my experiences as a QRP operator and designer.

The Indian 10-Minute Transmitter

I saw this circuit on a ham from India's (VU2NAN) webpage originally, and thought I'd share it here:

It's literally the 10-Minute Transmitter, but with a cool little tuning/SWR indicator! The capacitor is tuned so the bulb glows as bright as it can on the given crystal frequency. This indicates the most RF current that the little transmitter can produce. Nandu claims to have made a DX contact over 1000km (~600mi) away. Look for 6V low-amperage "wheat" or "rice grain" bulbs.

The SL100 transistor can be replaced with any NPN transistor. You might have to try several transistors to see which one produces the most power output. I'd imagine that this design, if built according to the schematic, would output a bit more than a watt with 15VDC, perhaps 3/4 of a watt at 12V. Remember to put a low pass filter on to filter out the harmonics!!!

With all that said, have you built this transmitter? If so, let me know in the comments below how long it took to build and if you made a QSO on it!