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.

[Hans Rosenberg] knows a thing or two about RF PCB design and has provided a three-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.

[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!

PCB design is as much art as science, and we’ve discussed this subject a lot. Here’s our simple guide to rocking RF PCB designs. There’s also a lot of devil in that detail, for example when understanding edge-launch SMA connectors.