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Super simple 1 & 2 transistor 3.5 & 7 MHz QRP transmitters

There is a certain elegance in being heard hundreds or thousands of kilometres away with a one or two transistor CW transmitter using a handful of components. The game is to get the most power out with the cleanest signal while not breaking the crystal or burning out the transistor.

The availability of online we controlled receivers like KiwiSDR make it easy to hear your transmitted signal from afar. Another great resource for transmitter testing is the Reverse Beacon Network. Transmitters described here are quick to build but take longer to get contacts due to mostly being crystal controlled and people having to find you (rather than you finding them). However you can get still random contacts without pre-arrangement and there's a lot of satisfaction when someone does reply.

The videos show some of what's possible. So gather the parts and start soldering up a simple transmitter. You'll be glad you did.

Chopping board one transistor 7 MHz CW transmitter

This one's solderless. It makes a great beginners project. Its low power can carry hundreds of kilometres but you will need signifiant persistence to make contacts. The first video describes contruction, the second video demonstrates a contact while the third describes a low pass filter that you can add to clean up its output. You should use this filter (or one like it) on all transmitters described here as their inbuilt harmonic suppression isn't sufficient on their own.

 

Your first transmitter - solderless (3.5 MHz CW)

A lot like the 7 MHz transmitter described just above. Except this one uses more commonly available 3.58 MHz crystals. But if you have a crystal for lower 3.5 MHz frequencies then you can use that and be more likely to make contacts as that's where most CW activity is. The second video describes how to make it transmit AM speech by adding a microphone amplifier / modulator.

 

Frequency agile 3.5 MHz with 1 transistor

The video title isn't very appetising but this is actually a useful contact-getter despite its 200 mW output power. Unlike many simple QRP rigs it's frequency agile. This makes a big difference in getting contacts as you can call people on their frequency. This is possible by using a 3.58 MHz ceramic resonator that can be pulled to lower frequencies to cover the whole CW band segment. Just build the filter (video above) to ensure a clean output.

 

Frequency agile 3.5 MHz: 2 watts with 2 transistors

This builds on from the above to add a BD139 final power amplifier to give about 2 watts power output. This higher power greatly increases the quantity, quality and distance of contacts possible. It means that even if there is fading your signal will be above the noise for longer and your contact will copy more of your transmission. The first video is a live stream where I describe it and show you the circuit. The second video is an even longer live stream where I put it in a case and demonstrate it on air. As this is only a transmitter I'm using a web SDR to receive my (and others') signals.

 

Poxie 2 transistor 7 MHz transceiver

Not just a transmitter but a full transceiver. Yet just 2 transistors. It's basically a marriage between the 1 transistor chopping board 7 MHz QRP transmitter (above) and the 1 transistor reflex direct conversion receiver (described below). It's very minimalist and you will need to be in a quiet room with a crystal earpiece. But you can still get contacts hundreds of kilometres away. The first video describes the Poxie. The other two are long live streams where I demonstrate its use.

 

The ten minute 7 MHz transmitter and more

This has been a really fun project. I start with a Sprat 1 transistor transmitter (like that described above but soldered) then add a FET power amplifier. That's described in the first video. To prove it's not just a workshop curiosity I get outside and make some contacts. The minimalist theme is continued with the receiver where I use a National Panasonic shortwave radio with the transmitter's oscillator running as the BFO. Lastly I try the setup during a field day contest with surprisingly good results.

 

Reflex receiver add-on for simple CW transmitters

If you want to add a simple receiver to a small transmitter like the above to make it a transceiver you can do it with just one transistor provided you have a sensitive earpiece or external amplifier you can plug in. Video below explains how.

 

Not QRP. High power 2 transistor 7 MHz CW transmitter

If you've thought that 2 transistor transmitters are strictly QRP affairs, here's something that will change your mind. Its pretty much the same circuit as described above but I've applied a higher voltage to the final amplifier. That's easier than it used to be thanks to low cost DC to DC converters available on ebay. A two transistor transmitter based on this is basically a modern version of the two stage crystal oscillator / power amplifier transmitter popular from the 1930s.

 

Hand-powered 7 MHz CW transmitter

Based on the Oxo of GM3OXX/G-QRP fame this small transmitter was built into a hand-cranked torch. Cranking vigorously gives around 500 milliwatts output. Sending CW with the other hand is a little tedious but you can get a certain rhythm where the sending speed matches the cranking.

 

One transistor surprisingly high power 7 MHz CW transmitter

Videos describe my experiments with a 7 MHz CW transmitter using just one active transistor. It's based on a design from SV3ORA's website.

 

The parts here might help you with projects like the QRP transmitters described above.

 

Disclosure: I receive a small commission from items purchased through links on this site.
Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.

 

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(c) Peter Parker VK3YE 1997 - 2020.

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