SP-2B2C: A video series on the design and construction of a Dual-Band 2 Channel QRP CW rig

My radio projects have involved a build and test effort, often spanning 3, 4 or even 6 months, culminating in one long, detailed blog post which was assembled over many months and a video that serves the dual purposes of showing and explaining the rig followed by an outing to one or more SOTA summits.

One consequence of this is that my video and blog output is quite low. Another is the resulting content is long, detailed, and not necessarily accessible to all readers or viewers. The concept/plan/build/test/box up and, finally, demonstrate approach is akin to building a house over a few years and publishing your account upon completion. So much of the story goes untold — old school thinking in 2021.

Concept

I decided to turn this approach around, in the style of Charlie Morris ZL2CTM who (when in homebrewing mode) pumps out an interesting video at least once a week, in which he shares his thoughts, turns over components in his fingers, sketches out circuits and stages, and involves you in a construction story. In fact Charlie went further and invited his subscribers to suggest or vote on candidate projects, which introduces the risk that he will end up with a project that doesn’t work, a situation which I note Charlie is too smart to have fallen in to.

By opening up a homebrew project, or at least by documenting it stage by stage, taking and incorporating feedback along the way, your viewers come on the journey with you, and the outcome emerges as being kind of our achievement rather than my achievement.

There is also a sense that you (the maker) are your viewer’s surrogate maker, which is a worthy thing, given so many would-be makers lack the time, space, experience, tools, eyesight, dexterity, or freedom to complete a homebrew radio project.

2B2C rig in pocket.

Feeding the YouTube video-monster

There is another reason for taking this approach. After a few years on YouTube I started wanting to know more about how the platform works. Or, how to work it. The motivation to ‘catch more subscribers’ is a thought exercise in itself and I won’t go further on it, as it is different for everyone. Suffice to say that when you go to the trouble of making what you think is a good YT video, you naturally want it to be seen by as many people as possible. That in itself seems logical, human, and not crazily narcissistic.

Guidance on catching more YouTube eyeballs is not difficult to find. Useful clues can be found the videos released by YT themselves to creators. Anyone can view these, and there are no secrets, similar advice can be found on many of the thousands of ‘grow your YT channel’ channels. It basically comes down to a bit of commonsense YT Search Engine Optimisation hygiene, and, mixing it up a bit and keeping on with it until you find your niche, then being consistent. Good and engaging content is king (as if we needed to be told that).

One algorithm input is sustained views. How do you get views? Well, you could mine internet memes, or film yourself doing something totally ridiculous or crazy, or take your clothes off, or annoy Police. The options are endless and most have been tried. Almost nobody watches the same great video every day for a month, so the answer isn’t in making a small number of great videos and expecting them to be watched repeatedly.

The majority of YT creators churn out content as best they can, on a regular basis, typically weekly. And new videos get watched, even if only for a few minutes, generating views, and feeding the voracious YT video monster. Video is a consumable medium. The strategy then is to turn out a good video weekly. With all this in mind I resolved to make a video series about the design and construction of a simple but useful QRP CW transceiver, almost as a byproduct. This post kicks off episode 1 and the 8 part series. The complete transceiver design and build story will be published on YouTube as follows:

Part 1 – Concept
Part 2 – Receiver PCB
Part 3 – Receiver Band Pass Filters
Part 4 – Receiver build & test
Part 5 – Tx PCB, keying, pre-driver, LPFs
Part 6 – Transmitter driver
Part 7 – Transmitter PA and tests
Part 8 – Case, finishing and field test.

Other videos on the rig on SOTA and park outings will be added as they occur.

Promoting solder-melting

The other motivation is to share and promote making, or ‘melting solder’. While blogs are an important record and are critical to convey detail, a picture paints a thousand words and moving pictures moreso. The most powerful, pervasive and pervasive medium is undoubtedly video via YouTube, which has become the world’s goto source for content, how-to’s, and entertainment. And in the best of content, all of these elements come together. So, like it or not, video/YT is now the pre-eminent platform for reaching eyeballs at scale, regardless of content type.

Dual-band 2 channel QRP CW transceiver

‘SP-2B2C’ is a project to design, build and document in a video series the design and construction of a two band (40 and 20m) crystal locked (channelised) QRP CW transceiver. The rig is entirely scratch-built, from ‘borrowed’ designs, circuit elements and ideas. It is a compact, neat pocket rig that will provide more than adequate service as a simple parks and portable rig, but it comes into its own for SOTA where the crystal-locked channels will not be a major impediment to making contacts.

Frequency control is provided by separate dedicated and trimmed 7MHz and 14MHz crystal oscillators with buffers and with a fixed transmit offset. The receiver is a conventional Direct Conversion design with strong band pass filtering and an SA612 mixer, followed by a dual op amp for audio filtering and gain, and an LM386 for headphone or speaker listening. It has ample gain both in the shack and on a windy summit.

The transmitter duplicates a parts of the popular QCX and MTR radios using a high speed logic gate as a digital driver, to three BS170 FETs in parallel for a full 5 watts on both bands. Keying for a straight key is done using discrete components. The receiver draws about 50mA and the transmitter up to 0.8A on key down. Band switching is done with two miniature telecom relays.

Sketching the physical and electronic design options.

Schematic

2b2c-p1Download

Case

The aluminium sheet and angle case was made to closely match that of a previous rig, SP-X. I wanted these two rigs to look like big brother and little brother. The 2B2C case measures 52mm wide, 105mm long and 32mm high. That’s 2 inches wide, 4 1/4 inches long, and 1 1/4 inches high.

Case sketches.

Critique

The videos have plenty of on-air snippets so it is easy to get a sense of performance. They openly address the design and build challenges encountered so I won’t repeat them here, other than to note a few observations on the design that the astute or experienced viewer will notice.

The first is the 40m receiver note — some of the callers come back at around 300 to 500Hz, not the 700Hz that is usual. This is because the transmit offset is done by pulling the crystal low with a diode switched capacitor (around 22pF on 40m). There is also a series C for trimming the crystal (30pF) to the desired frequency, and these two Cs interact. As this trimmer is closed to pull the crystal down, the transmit offset reduces due to the diminishing effect of the 22pF capacitor. The best compromise I could get was to have the oscillator on 7022.3kHz which delivers around 4 to 500Hz transmit pull.

7022.3kHz is an odd frequency. If the caller nets exactly, they sound a 500Hz note. But I suspect some operators call on 7022, and some on 7022.5, so the caller’s CW note is unpredictable. A solution is to go on eBay and buy two 7023 crystals, and parallel these for (hopefully) more swing, and also, a bit more transmit pull. I might do that, because it would be much better to transmit on 7023.0, not 7022.3, and callers should respond consistently on the whole kHz.

The other obvious limitation is the DC receiver bandwidth, which is probably as much as 8kHz wide. That means strong signals can capture the receiver anywhere in this range. The best QSOs are had on a clear band. The crystal-clear tinkling sound of CW on a DC receiver is a delightful thing, but it wears thin in a crowded band.

Apart from these points I am happy with the usability and performance of this simple CW rig, and look forward to trying it on a SOTA activation soon, once COVID lockdowns are lifted, where I think it will shine!

2B2C was spotted at VE6JY on 14060 CW.

Acknowledgments

Thanks go to the authors/designers of the various receiver/transmitter circuit blocks I have copied, in particular, Steve Webber KD1JV, creator of the MTR series, and Hans Summers G0UPL whose QCX is an ongoing inspiration. I’ve used KD1JV bandpass filters, and the same RF driver and PA as is in both these designs.

The keying is from RSGB Homebrew columnist Eamon EI9GQ.

Inspiration is drawn constantly from Hayward, Campbell and Larkins’ Expermiental Methods in RF Design (EMRFD).

Finally the SOTA crowd is a global source of genuine interest and know-how in low power portable radios and their operation, and continues the fine amateur radio tradition of operating minimal QRP radios from the mountains and the fields.

From EMRFD.