When your homebrew Yagi antenna only sort-of works, or when your WiFi cantenna seems moody on rainy days, we can assure you: it is not only you. You can stop doubting yourself once and for all after you’ve watched the Tech 101: Antennas webinar by [Dr. Jonathan Chisum].
[Jonathan] breaks it all down in a way that makes you want to rip out your old antenna and start fresh. It goes further than textbook theory; it’s the kind of knowledge defense techs use for real electronic warfare. And since it’s out there in bite-sized chunks, we hackers can easily put it to good use.
The key takeaway is that antenna size matters. Basically, it’s all about wavelength, and [Jonathan] hammers home how tuning antenna dimensions to your target frequency makes or breaks your signal. Whether you’re into omnis (for example, for 360-degree drone control) or laser-focused directional antennas for secret backyard links, this is juicy stuff.
If you’re serious about getting into RF hacking, watch this webinar. Then dig up that Yagi build, and be sure to send us your best antenna hacks.
How do you know that your patch cables are good? For simple jumper wires, a multimeter is about all you need to know for sure. But things can get weird in the RF world, in which case you might want to keep these coaxial patch cable testing tips in mind.
Of course, no matter how high the frequency, the basics still apply, and [FesZ] points out in the video below that you can still get a lot of mileage out of the Mark 1 eyeball and a simple DMM. Visual inspection of the cable and terminations can reveal a lot, as can continuity measurements on both the inner and outer conductors. Checking for shorts between conductors is important, too. But just because the cable reads good at DC doesn’t mean that problems aren’t still lurking. That’s when [FesZ] recommends breaking out a vector network analyzer like the NanoVNA. This tool will allow you to measure the cable’s attenuation and return loss parameters across the frequency range over which the cable will be used.
For stubborn problems, or just for funsies, there’s also time-domain reflectometry, which can be done with a pulse generator and an oscilloscope to characterize impedance discontinuities in the cable. We’ve covered simple TDR measurement techniques before, but [FesZ] showed a neat trick called time-domain transformation, which uses VNA data to visualize the impedance profile of the whole cable assembly, including its terminations.
If you’ve been tearing electronic devices apart for long enough, you’ll know that the old gear had just as many mysteries within as the newer stuff. The parts back then were bigger, of course, but often just as inscrutable as the SMD parts that populate boards today. And the one part that always baffled us back in the days of transistor radios and personal cassette players was those little silver boxes with a hole in the top and the colorful plug with an inviting screwdriver slot.
We’re talking about subminiature intermediate-frequency transformers, of course, and while we knew their purpose in general terms back then and never to fiddle with them, we never really bothered to look inside one. This teardown of various IF transformers by [Unrelated Activities] makes up somewhat for that shameful lack of curiosity. The video lacks narration, relying on captions to get the point across that these once-ubiquitous components were a pretty diverse lot despite their outward similarities. Most had a metal shell protecting a form around which one or more coils of fine magnet wire were wrapped. Some had tiny capacitors wired in parallel with one of the coils, too.
Perhaps the most obvious feature of these IF transformers was their tunability, thanks to a ferrite cup or slug around the central core and coils. The threaded slug allowed the inductance of the system to be changed with the turn of a screwdriver, preferably a plastic one. [Unrelated] demonstrates this with a NanoVNA using a nominal 10.7-MHz IFT, probably from an FM receiver. The transformer was tunable over a 4-MHz range.
Sure, IFTs like these are still made, and they’re not that hard to find if you know where to look. But they are certainly less common than they used to be, and seeing what’s under the hood scratches an itch we didn’t even realize we had.
We love seeing the incredible work many RF enthusiasts manage to pull off — they make it look so easy! Though RF can be tricky, it’s not quite the voodoo black art that it’s often made out to be. Many radio protocols are relatively simple and with tools like gnuradio and PocketSDR you can quickly put together a small system to receive and decode just about anything.
[Jean-Michel] wanted to learn more about GNSS and USB communication. Whenever you start a project like this, it’s a good idea to take a look around at existing projects for designs or code you can reuse, and in this case, the main RF front-end board is taken from the PocketSDR project. This is then paired with a Cypress FX2 development board, and he re-wrote almost all of the PocketSDR code so that it would compile using sdcc instead of the proprietary Keil compiler. Testing involved slowly porting the code while learning about using Python 3 to receive data over USB, and using other equipment to simulate antenna diversity (using multiple antennas to increase the signal-to-noise ratio): Continue reading “GNSS Reception With Clone SDR Board”→
We’ve always been fascinated by things that perform complex electronic functions merely by virtue of their shapes. Waveguides come to mind, but so do active elements like filters made from nothing but PCB traces, which is the subject of this interesting video by [FesZ].
Of course, it’s not quite that simple. A PCB is more than just copper, of course, and the properties of the substrate have to be taken into account when designing these elements. To demonstrate this, [FesZ] used an online tool to design a bandpass filter for ADS-B signals. He designed two filters, one using standard FR4 substrate and the other using the more exotic PTFE.
He put both filters to the test, first on the spectrum analyzer. The center frequencies were a bit off, but he took care of that by shortening the traces slightly with a knife. The thing that really stood out to us was the difference in insertion loss between the two substrates, with the PTFE being much less lossy. The PTFE filter was also much more selective, with a tighter pass band than the FR4. PTFE was also much more thermostable than FR4, which had a larger shift in center frequency and increased loss after heating than the PTFE. [FesZ] also did a more real-world test and found that both filters did a good job damping down RF signals across the spectrum, even the tricky and pervasive FM broadcast signals that bedevil ADS-B experimenters.
Although we would have liked a better explanation of design details such as via stitching and trace finish selection, we always enjoy these lessons by [FesZ]. He has a knack for explaining abstract concepts through concrete examples; anyone who can make coax stubs and cavity filters understandable has our seal of approval.
For as useful as computers are in the modern ham shack, they also tend to be a strong source of unwanted radio frequency interference. Common wisdom says applying a few ferrite beads to things like Ethernet cables will help, but does that really work?
It surely appears to, for the most part at least, according to experiments done by [Ham Radio DX]. With a particular interest in lowering the noise floor for operations in the 2-meter band, his test setup consisted of a NanoVNA and a simple chunk of wire standing in for the twisted-pair conductors inside an Ethernet cable. The NanoVNA was set to sweep across the entire HF band and up into the VHF; various styles of ferrite were then added to the conductor and the frequency response observed. Simply clamping a single ferrite on the wire helped a little, with marginal improvement seen by adding one or two more ferrites. A much more dramatic improvement was seen by looping the conductor back through the ferrite for an additional turn, with diminishing returns at higher frequencies as more turns were added. The best performance seemed to come from two ferrites with two turns each, which gave 17 dB of suppression across the tested bandwidth.
The question then becomes: How do the ferrites affect Ethernet performance? [Ham Radio DX] tested that too, and it looks like good news there. Using a 30-meter-long Cat 5 cable and testing file transfer speed with iPerf, he found no measurable effect on throughput no matter what ferrites he added to the cable. In fact, some ferrites actually seemed to boost the file transfer speed slightly.
Ferrite beads for RFI suppression are nothing new, of course, but it’s nice to see a real-world test that tells you both how and where to apply them. The fact that you won’t be borking your connection is nice to know, too. Then again, maybe it’s not your Ethernet that’s causing the problem, in which case maybe you’ll need a little help from a thunderstorm to track down the issue. Continue reading “Ferrites Versus Ethernet In The Ham Shack”→
[Hans Rosenberg] knows a thing or two about RF PCB design and has provided a three-part four-part video demonstration of some solid rules of thumb. We will cover the first part here and leave the other two for the more interested readers!
The design process begins with a schematic diagram, assuming ideal conductors. Advanced software tools can extract the resistive, inductive, and capacitive elements of the physical wiring to create a parasitic model that can be compared to the desired schematic. The RF designer’s task is to optimize the layout to minimize differences and achieve the best performance to meet the design goals. However, what do you do when you don’t have access to such software?
[Hans] explains that at low frequencies, return current flows through all paths, with the lowest resistance path taking most of the current. At higher frequencies, the lowest inductance path carries all the current. In real designs, a ground plane is used instead of an explicit return trace for the lowest possible impedance.
You really wouldn’t design an RF circuit like this.
[Hans] shows the effect of interrupting the signal return path on a physical test PCB. The result is pretty bad, with the current forced to detour around the hole in the ground plane. A nanoVNA shows a -20 dB drop at 4 GHz, where the ground plane has effectively become an antenna. Energy will be radiated out, causing signal loss, but worse, it will create an EMC hazard with an unintended transmission.
Additionally, this creates an EMC susceptibility, making the situation worse. Placing a solder blob to bridge the gap directly under the signal trace is all that’s required to make it a continuous straight path again, and the performance is restored.
Floating planes are also an issue in RF designs, causing signal resonance and losses. One solution is to pull back the planes near the signal or stitch them to the ground plane with vias placed closely on either side of the signal trace. However, such stitching may slightly affect transmission line impedance and require tweaking the design a little. The next two parts of the series expand on this, hammering home the importance of good ground plane design. These are definitely worth a watch!
