A Practical Look At Chokes For EMI Control

March 15, 2020 by Dan Maloney 6 Comments

Radio frequency electronics can seem like a black art even to those who intentionally delve into the field. But woe betide the poor soul who only incidentally has to deal with it, such as when seeking to minimize electromagnetic interference. This primer on how RF chokes work to reduce EMI is a great way to get explain the theory from a practical, results-oriented standpoint.

As a hobby machinist and builder of machine tools, [James Clough] has come across plenty of cases where EMI has reared its ugly head. Variable frequency drives are one place where EMI can cause problems, and chokes on the motor phase outputs are generally prescribed. He used an expensive choke marketed as specific for VFD applications on one of his machines, but wondered if a cheap ferrite core would do the job just as well, and set to find out.

A sweep of some ferrite cores with a borrowed vector network analyzer proved unsatisfying, so [James] set up a simple experiment with a function generator and an oscilloscope. His demo shows how the impedance of a choke increases with the frequency of the test signal, which is exactly the behavior that you’d want in a VFD – pass the relatively low-frequency phase signals while blocking the high-frequency EMI. For good measure, he throws a capacitor in parallel to the choke and shows how much better a low-pass filter that makes.

We love demos like this that don’t just scratch an intellectual itch but also have a practical goal. [James] not only showed that (at least in some cases) a $13 ferrite can do the same job as a $130 VFD choke, but he showed how they work. It’s basic stuff, but it’s what you need to know to move on to more advanced RF filter designs.

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Get Your Weather Images Straight From The Satellite

March 14, 2020 by Al Williams 15 Comments

[Josh] has a series called Ham Radio Crash Course and a recent installment covers how you can grab satellite images directly from weather satellites. This used to be more of a production than it is now thanks to software defined radio (SDR). Josh also has another project using a 3D printer to make an antenna suitable for the job. You can see the video below.

The software is the venerable WXtoImg program. This is abandonware, but the community has kept the software available. The program works on Linux, Windows, and Mac. The satellites in question operate around 137 MHz, but that’s easily in the range of even the cheap SDR dongles. [Josh] shows how to use a virtual audio cable on Windows to connect the output of the radio to the input of the WXtoImg program. Under Linux, you can do this with Pulse or Jack very easily without any extra hardware.

There’s some setup and calibration necessary for the software. You’ll also need the current orbital data and the program will tell you when you can find the next satellite passing overhead. Generally speaking you’ll want your antenna outside, which [Josh] solved by taking everything outdoors and having some lunch during the pass. It also takes some time to post-process the data into images and audio.

We know this isn’t new. But we did like [Josh’s] clear and up-to-date guide. We remember watching NOAA 15 as it started to lose its electronic mind.

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Raising The Titanic’s Radio Room

March 4, 2020 by Dan Maloney 149 Comments

For some reason, of all the ships that have sailed the oceans, it’s the unlucky ones that capture our imagination. Few ships have been as unlucky as the RMS Titanic, sinking as she did on the night of April 15, 1912 after raking across an iceberg on her maiden voyage, and no ship has grabbed as much popular attention as she has.

During her brief life, Titanic was not only the most elegant ship afloat but also the most technologically advanced. She boasted the latest in propulsion and navigation technology and an innovation that had only recently available: a Marconi wireless room, used both for ship-to-shore and ship-to-ship communications.

The radio room of the Titanic landed on the ocean floor with the bow section of the great vessel. The 2.5-mile slow-motion free fall destroyed the structure of the room, but the gear survived relatively intact. And now, more than a century later, there’s an effort afoot to salvage that gear, with an eye toward perhaps restoring it to working condition. It’s a controversial plan, of course, but it is technologically intriguing, and it’s worth taking a look at what’s down there and why we should even bother after all these years.

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Dog-Harnessing The Power Of Walkie Talkies

March 4, 2020 by Kristina Panos 12 Comments

[javier.borquez] likes to take his dog to the hang out at the dog park around dusk. But once the sun goes down and [Rusio]’s off the leash, running amok with the other dogs, it’s almost impossible to keep track of him.

Sure, there are probably glow-in-the-dark or lighted collars out there, but if you go commercial, chances are good that someone else’s dog will be wearing the same thing. Besides, what’s the fun in buying something that you can do a better job making yourself? With this dog distance indicator harness, you don’t even have to program anything. Instead, it uses a cheap pair of modified walkie talkies to show green LEDs on the harness while the dog is in range, and red when it isn’t.

Although [javier]’s pupper is the best pupper yes he is, [Rusio] can’t be expected to hold down the button and bark his location. His walkie talkie uses a 555-based frequency generator and a glued-down button to speak at 1 kHz.

Over in [javier]’s walkie, there’s a resistor in place of the speaker to keep the talkie parts working. There’s also a half-wave bridge rectifier that charges a capacitor when [Rusio] is within range, and a resistor that drains it when he’s outside the 6-8 meter range. The rectifier’s output goes to a second 555 set up as a Schmitt trigger, which tells a transistor to turn the red LEDs on instead.

If you got stuck on the idea of hearing your dog talk to you over distances, here’s a Bluetooth Babelfish collar.

Multi-Band Receiver On A Chip Controlled By Arduino

March 2, 2020 by Tom Nardi 18 Comments

The Silicon Labs Si4735 is a single-chip solution for receiving AM, FM, and shortwave radio. With a bit of hacking, it even supports single sideband (SSB). All you’ve got to do is provide it with a suitable control interface, which [Ricardo Lima Caratti] has done with his recent project.

Using an Arduino Pro Mini, a handful of buttons, and a standard TFT display, [Ricardo] has put together a serviceable little receiver with a fairly impressive user interface. We especially like the horizontal bars indicating the signal to noise ratio and received signal strength. The next evolution would be to put this whole rig into some kind of enclosure, but for now he seems content to control the action with a handful of unlabeled buttons on a piece of perfboard.

Of course, the presentation of this receiver isn’t really the point; it’s more of a proof of concept. You see, [Ricardo] is the person who’s actually developed the library that allows you to control the Si4735 from your microcontroller of choice over I2C. He’s currently tested it with several members of the official (and not so official) Arduino family, as well as the ESP32.

The documentation [Ricardo] has put together for his MIT licensed Arduino Si4735 library is nothing short of phenomenal. Seriously, if all open source projects were documented even half as well as this one is, we’d all be a few notches closer to world peace. Even if you aren’t terribly interested in adding shortwave radio reception to your next project, you’ve got to browse his documentation just to see where the high water mark is.

We actually first heard about this library a few days ago when we covered another receiver using the Si4735 and [Ricardo] popped into the comments to share some of the work he’d been doing to push the state-of-the-art forward for this promising chip.

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A Patch Antenna Is Just A Rectangle, It Should Be Easy To Design, Right?

March 1, 2020 by Jenny List 20 Comments

If a grizzled RF engineer who bears the soldering-iron scars of a thousand projects could offer any advice, it would be that microwave antennas are not a field to be entered into lightly. Much heartache is to be saved by using an off-the-shelf design, and only the foolhardy venture willingly down the stripline into the underworld of complex microwave resonances.

But every would-be microwave designer has to start somewhere, and for [Adam Gulyas] that start came with a 2.4 GHz patch antenna. His write-up is a fascinating tale of the challenges and pitfalls of creating something which is deceptively simple at first sight but which becomes significantly more complex as he characterizes his design made real as a PCB.

The process started with a set of calculations to derive the patch dimensions and a bit of PCB work adding a stripline feed. This was produced on a PCB, a normal 1.6mm thick FR4 fiberglass board. When hooked up to a VNA its impedance was all wrong. Further, it had a resonance at the required frequency but also unexpected ones at 3.7 and 4.6 GHz. Simulation of the design also yielded a different resonance from the one calculated, and discussing it with others yielded the conclusion that the feed might be at fault. He ended up using an inset feed, with a co-axial cable emerging away from the edge of the patch, and was able to achieve a far better result.

We can all learn something from [Adam]’s write-up, and we salute him for staying the course to get the design to a usable point. It would be interesting to see the same antenna produced from a more consistent dielectric material than generic FR4. Meanwhile, if you are interested in microwave RF design, take a look at Michael Ossmann’s primer on the subject.

Software Defined Everything With Mike Ossmann And Kate Temkin

February 21, 2020 by Jenny List 7 Comments

Software defined radio has become a staple of the RF tinkerer, but it’s likely that very few of us have ever taken their software defined toolchain outside the bounds of radio. It’s an area explored by Mike Ossmann and Kate Temkin in their newly published Supercon talk as they use GNU Radio to do some things that you might find unexpected.

For most people, a software defined radio is a device. An RTL-SDR dongle perhaps, or the HackRF that a popular multi-tool for working in the radio frequency realm. But as they explain, the SDR hardware can be considered merely as the analogue front end, being just the minimal analogue circuitry coupled with a digitiser. The real software-defined part comes — as you might expect — in the software

Kate and Mike introduce GNU Radio Companion — the graphical UI for GNU Radio — as their tool of choice and praise it’s use as a general purpose digital signal processing system whether or not that includes radio. Taking their own Great Scott Gadgets GreatFET One USB hackers toolkit peripheral as an input device they demonstrate this by analysing the output from a light sensor. Instantly they can analyse the mains frequency in a frequency-domain plot, and the pulse frequency of the LEDs. But their bag of tricks goes much deeper, exploring multiple “atypical use cases” that unlock a whole new world through creative digital signal processing (DSP).

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