Categorie: Hackaday

When building a radio transmitter, unless it’s a very small one indeed, there’s a need for an amplifier before the antenna. This is usually referred to as the power amplifier, or PA. How big your PA is depends on your idea of power, but at the lower end of the power scale a PA can be quite modest. QRP, as lowe power radio is referred to, has a transmit power in the miliwatts or single figure watts. [Guido] is here with a QRP PA that delivers about a watt from 1 to 30 MHz, is made from readily available parts, and costs very little.

Inspired by a circuit from [Harry Lythall], the prototype is built on a piece of stripboard. It’s getting away with using those cheap transistors without heatsinking because it’s a class C design. In other words, it’s in no way linear; instead it’s efficient, but creates harmonics and can’t be used for all modes of transmission. This PA will need a low-pass filter to avoid spraying the airwaves with spurious emissions, and on the bands it’s designed for, is for CW, or Morse, only.

We like it though, as it’s proof that building radios can still be done without a large bank balance. Meanwhile if the world of QRP interests you, it’s something we have explored in the past.

[Yeckel] recently put the finishing touches on an ambitious implementation of a simple D-STAR (Digital Smart Technologies for Amateur Radio) transceiver using some very accessible and affordable hardware. The project is D-StarBeacon, and [Yeckel] shows it working on a LilyGO TTGO T-Beam, an ESP32-based development board that includes a SX1278 radio module and GPS receiver. It even serves a web interface for easy configuration.

What is D-STAR? It’s a protocol used by radio operators for voice that also allows transmitting low-speed data, such as short text messages or GPS coordinates. While voice is out of scope for [Yeckel]’s project (more on that in a moment) it can do all the rest, including send images. That makes beacon-type functions possible on inexpensive hardware, instead of requiring a full-blown radio.

As mentioned, voice is a big part of D-STAR. While [Yeckel] was able to access the voice data, attempts to decode it were unsuccessful. A valiant effort, but we suppose voice decoding isn’t terribly relevant to beacon-type operations like transmitting APRS (Automatic Packet Reporting System).

So far as [Yeckel] is aware, D-StarBeacon is currently the only open-source implementation of a D-STAR radio available on the internet, which is pretty interesting. We’ve seen projects that touch indirectly on D-STAR, but nothing like this.

Watch it go through its paces in the video embedded below. Since the T-Beam is just a microcontroller development board, the user interface comes from an Android app on a mobile phone, which is why you see a phone in the video.

Antenna design is often referred to as a black art or witchcraft, even by those experienced in the space. To that end, [Janne] wondered—could years of honed skill be replaced by bruteforcing the problem with the aid of some GPUs? Iterative experiments ensued.

[Janne]’s experience in antenna design was virtually non-existent prior to starting, having a VNA on hand but no other knowledge of the craft. Formerly, this was worked around by simply copying vendor reference designs when putting antennas on PCBs. However, knowing that sometimes a need for something specific arises, they wanted a tool that could help in these regards.

The root of the project came from a research paper using an FDTD tool running on GPUs to inversely design photonic nanostructures. Since light is just another form of radio frequency energy, [Janne] realized this could be tweaked into service as an RF antenna design tool. The core simulation engine of the FDTD tool, along with its gradient solver, were hammered into working as an antenna simulator, with [Janne] using LLMs to also tack on a validation system using openEMS, an open-source electromagnetic field solver. The aim was to ensure the results had some validity to real-world physics, particularly important given [Janne] left most of the coding up to large language models. A reward function development system was then implemented to create antenna designs, rank them on fitness, and then iterate further.

The designs produced by this arcane system are… a little odd, and perhaps not what a human might have created. They also didn’t particularly impress in the performance stakes when [Janne] produced a few on real PCBs. However, they do more-or-less line up with their predicted modelled performance, which was promising. Code is on Github if you want to dive into experimenting yourself. Experienced hands may like to explore the nitty gritty details to see if the LLMs got the basics right.

We’ve featured similar “evolutionary” techniques before, including one project that aimed to develop a radio. If you’ve found ways to creatively generate functional hardware from boatloads of mathematics, be sure to let us know on the tipsline!

[GreatScott] has recently been tinkering in the world of radio frequency emissions, going so far as to put their own designs in a proper test chamber to determine whether they meet contemporary standards for noise output. This led them to explore the concept of shielding, and how a bit of well-placed metal can make all the difference in this regard.

The video focuses on three common types of shielding—absorber sheets, shielding tapes, and shielding cabinets. A wide variety of electronic devices use one or more of these types of shielding. [GreatScott] shows off their basic effectiveness by putting various types of shielding in between a noise source and a near-field probe hooked up to a receiver. Just placing a bit of conductive material in between the two can cut down on noise significantly. Then, a software defined radio (SDR) was busted out for some more serious analysis. [GreatScott] shows how Faraday cages (or simple shielding cabinets] can be used to crush down spurious RF outputs to almost nothing, and how his noisy buck-boost designs can be quieted down with the use of the right absorber sheets that deal well with the problematic frequencies in question. The ultimate upshot of the tests is that higher frequencies respond best to conductive shielding that is well enclosed, while lower frequency noise benefits from more absorptive shielding materials with the right permeability for the job.

Shielding design can be a complex topic that you probably won’t master in a ten minute YouTube video, but this content is a great primer if you’re new to the topic. We’ve covered the topic before, too, particularly on how a bit of DIY shielding can really aid a cheap SDR’s performance. Video after the break.

Although we do often see projects that take antiques and replace some or all of their components with modern equipment, we can also sympathize with the view that (when possible and practical) certain antique electronics should be restored rather than gutted. [David] has this inclination for his 1948 GE radio, but there are a few issues with it that prevented a complete, period-correct restoration.

The main (pun intended) issue at the start of this project was safety. The original radio had a chassis that was just as likely as not to become energized, with the only protection being the plastic housing. [David] set up an isolation transformer with a modern polarized power cable to help solve this issue, and then got to work replacing ancient capacitors. With a few other minor issues squared away this is all it took to get the radio working to receive AM radio, and he also was able to make a small modification to allow the radio to accept audio via a 3.5mm jack as well.

However, [David] also has the view that a period-correct AM transmission should accompany this radio as well and set about with the second bit of this project. It’s an adaptation of a project called FieldStation42 originally meant to replicate the experience of cable TV, but [Shane], the project’s creator, helped [David] get it set up for audio as well. A notable feature of this system is that when the user tunes away from one station, it isn’t simply paused, but instead allowed to continue playing as if real time is passing in the simulated radio world.

Although there are a few modern conveniences here for safety and for period-correct immersion, we think this project really hits the nail on the head for preserving everything possible while not rolling the dice with 40s-era safety standards. There’s also a GitHub page with some more info that [David] hopes to add to in the near future. This restoration of a radio only one year newer has a similar feel, and there are also guides for a more broad category of radio restorations as well.

One of the major difficulties in studying electricity, especially when compared to many other physical phenomena, is that it cannot be observed directly by human senses. We can manipulate it to perform various tasks and see its effects indirectly, like the ionized channels formed during lightning strikes or the resistive heating of objects, but its underlying behavior is largely hidden from view. Even mathematical descriptions can quickly become complex and counter-intuitive, obscured behind layers of math and theory. Still, [lcamtuf] has made some strides in demystifying aspects of electricity in this introduction to analog filters.

The discussion on analog filters looks at a few straightforward examples first. Starting with an resistor-capacitor (RC) filter, [lcamtuf] explains it by breaking its behavior down into steps of how the circuit behaves over time. Starting with a DC source and no load, and then removing the resistor to show just the behavior of a capacitor, shows the basics of this circuit from various perspectives. From there it moves into how it behaves when exposed to a sine wave instead of a DC source, which is key to understanding its behavior in arbitrary analog environments such as those involved in audio applications.

There’s some math underlying all of these explanations, of course, but it’s not overwhelming like a third-year electrical engineering course might be. For anyone looking to get into signal processing or even just building a really nice set of speakers for their home theater, this is an excellent primer. We’ve seen some other demonstrations of filtering data as well, like this one which demonstrates basic filtering using a microcontroller.

Not every project has to be complicated– reinventing the wheel has its place, but sometimes you find a module or two that does exactly what you want, and the project is more than halfway done. That the kind of project [mircemk]’s Simple Retro Style VFO is — it’s a variable frequency oscillator for HAM and other use, built with just a couple of modules.

The modules in question are the SI5351 Clock Generator module, which is a handy bit of kit with its own crystal reference and PLL to generate frequencies up to 150 MHz, and the Elecrow CrowPanel 1.28inch-HMI ESP32 Rotary Display. The ESP32 in the CrowPanel controls the SI5351 module via I2C; control is via the rest of the CrowPanel module. This Rotary Display is a circular touchscreen surrounded by a rotary display, so [mircmk] has all the inputs he needs to control the VFO.

To round out the parts count, he adds an appropriate connector, plus a power switch, red LED and a lithium battery. One could include a battery charger module as well, but [mircmk] didn’t have one on hand. Even if he had, that still keeps the parts count well inside the single digits. If you like video, we’ve embedded his about the project below; if not the write up on Hackaday.io is upto [mircmk]’s typical standard. 

People have been using the SI5351 to make VFOs for years now, but the addition of the round display makes for a delightfully retro presentation.

Thanks to [mircmk] for the tip.

If you own a handheld transceiver of any type then the chances are it will come with a “rubber duck” style antenna. These flexible rubber-coated antennas are a compromise in performance, being significantly smaller than a wavelength at their frequency of operation. [OM40ET] has an interesting video in which he investigates this by tearing down a cheap rubber duck antenna and an even cheaper fake.

These antennas usually have a flexible upper section and a bulge at the bottom over the connector. The fake one has nothing in the bulge except the antenna wire and thus has a very high SWR, while the “real” one has a loading coil. This coil is an interesting design, because it’s designed such that the antenna has two resonant points at the 2 metre and 70 centimetre amateur bands. On paper it’s a tapped coil fed at the tap through a capacitor for matching, but a more detailed appraisal will tell you that the two halves of the coil are designed to return those two resonances. It’s still a not-very-good antenna, but the fact that it works at all is something.

If you want something better at VHF and haven’t got much space, all is not lost. We recently featured a VHF magnetic loop.

The magnetic loop antenna is a familiar sight in radio amateur circles as a means to pack a high performance HF antenna into a small space. It takes the form of a large single-turn coil made into a tuned circuit with a variable capacitor, and it provides the benefits of good directionality and narrow bandwidth at the cost of some scary RF voltages and the need for constant retuning. As [VK3YE] shows us though, magnetic loops are not limited to HF — he’s made a compact VHF magnetic loop using a tin can.

It’s a pretty simple design; a section from the can it cut out and made into a C shape, with a small variable capacitor at the gap. The feed comes in at the bottom, with the feed point about 20 % of the way round the loop for matching. The bandwidth is about 100 MHz starting from the bottom of the FM broadcast band, and he shows us it receiving broadcast, Airband, and 2 meter signals. It can be used for transmitting too and we see it on 2 meter WSPR, but we would have to wonder whether the voltages induced by higher power levels might be a little much for that small capacitor.

He’s at pains to point out that there are many better VHF antennas as this one has no gain to speak of, but we can see a place for it. It’s tiny, if you’re prepared to fiddle with the tuning its high Q gets rid of interference, and its strong side null means it can also reduce unwanted signals on the same frequency. We rather like it, and we hope you will too after watching the video below.