If you’re planning on working satellites or doing any sort of RF work where the signal lives down in the dirt, you’re going to need a low-noise amplifier. That’s typically not a problem, as the market is littered with dozens of cheap options that can be delivered in a day or two — you just pay your money and get to work. But is there a case to be made for rolling your own LNA?

[Salil, aka Nuclearrambo] thinks so, and he did a nice job showing us how it’s done. The first step, as always, is to define your specs, which for [Salil] were pretty modest: a low noise figure, moderate gain, and good linearity. He also wanted a bandpass filter for the 2-meter amateur radio band and for weather satellite downlinks, and a bias-tee to power the LNA over the coax feedline. The blog post has a detailed discussion of the electrical design, plus some good tips on PCB design for RF applications. We also found the discussion on bias-tee design helpful, especially for anyone who has ever struggled with the idea that RF and DC can get along together on a single piece of coax. Part 2 concentrates on testing the LNA, mostly using hobbyist-grade test gear like the NanoVNA and tiny SA spectrum analyzer. [Salil]’s tests showed the LNA lived up to the design specs and more, making it more than ready to put to work with an RTL-SDR.

Was this more work than buying an LNA? Absolutely, and probably with the same results. But then again, what’s to learn by just getting a pre-built module in the mail?

A dip meter is basically a coil of wire that, when you excite it, you can use to tell if something inside that coil is resonating along. This lets you measure unknown radio circuits to figure out their resonant frequency, for instance. This week, we featured a clever way to make a dip meter with a nanoVNA, which is an odd hack simply because a dip meter used to be a common spare-parts DIY device, while a vector network analyzer used to cost more than a house.

Times have changed, and for the better. Nowadays, any radio amateur can pick up a VNA for less than the cost of all but the cheesiest of walkie talkies, putting formerly exotic test equipment in the hands of untrained mortals. But what good is a fancy-pants tool if you don’t know how to use it? Our own Jenny List faced exactly this problem when she picked up a nanoVNA, and her first steps are worth following along with if you find yourself in her shoes.

All of this reminded me of an excellent series by Mike Szczys, “Scope Noob”, where he chronicled his forays into learning how to use an oscilloscope by running all of the basic functions by working through a bunch of test measurements that he already knew the answer to.

It strikes me that we could use something like this for nearly every piece of measuring equipment. Something more than just an instruction manual that walks you through what all the dials do. Something that takes you through a bunch of example projects and shows you how to use the tool in question through a handful of projects. Because these days, access to many formerly exotic pieces of measuring gear has enabled many folks to have gear they never would have had before – and all that’s missing is knowing how to drive them.

A staple of the radio amateur’s arsenal of test equipment in previous decades was the dip meter. This was a variable frequency oscillator whose coil would be placed near the circuit to be tested, and which would show an abrupt current dip on a moving coil meter when its frequency matched the resonant frequency of what it was testing. For some reason the extremely useful devices seem hard to come by in 2024, so [Rick’s Ham Shack] has come along with a guide to using a nanoVNA in their place.

It’s a simple enough technique, indeed it’s a basic part of using these instruments, with a large sensor coil connected to the output port and a frequency sweep set up on the VNA. The reactance graph then shows any resonant peaks it finds in the frequency range, something easily demonstrated in the video below the break by putting a 20 meter (14 MHz) trap in the coil and seeing an immediate clear peak.

For many readers this will not be news, but for those who’ve not used a VNA before it’s a quick and easy demo of an immediate use for these extremely versatile instruments. For those of us who received our callsigns long ago it’s nothing short of miraculous that a functional VNA can be picked up at such a reasonable price, and we’d go as far as to suggest that non radio amateurs might find one useful, too. Read our review, if you’re interested.

[Kerry Wong], ever interested in trying out and tearing down electrical devices, demonstrates and examines the SV 6301a Handheld Vector Network Analyzer. He puts the machine through its paces, noting that the 7 inch touchscreen is a pretty nice feature for those whose eyesight isn’t quite what it used to be.

What’s a Vector Network Analyzer (VNA)? It’s not for testing Ethernet or WiFi. It’s aimed at a more classical type of “network”. The VNA tests and evaluates characteristics of electrical networks, especially as related to RF and microwave.

It provides detailed information about properties across a specified frequency range, making it an indispensable tool for advanced work. Tektronix has a resource page that goes into detail about exactly what kinds of things a VNA is good for.

[Kerry] shows off a few different features and sample tests before pulling the unit apart. In the end, he’s satisfied with the features and performance of the device, especially the large screen and sensible user interface.

After all, not every piece of test equipment does a great job at fulfilling its primary function, like the cheap oscilloscope that was a perhaps a little too cheap.

Dipole antennas are easy, right? Just follow the formula, cut two pieces of wire, attach your feedline, and you’re on the air.  But then again, maybe not. You’re always advised to cut the legs a little long so you can trim to the right length, but why? Shouldn’t the math just be right? And what difference does wire choice make on the antenna’s characteristics? The simple dipole isn’t really that simple at all.

If you’ve got antenna questions, check out [FesZ]’s new video on resonant dipoles, which is a deep dive into some of the mysteries of the humble dipole. In true [FesZ] fashion, he starts with simulations of various dipole configurations ranging from the ideal case — a lossless conductor in free space with as close to zero diameter conductors as the MMANA antenna simulator can support — and gradually build up to more practical designs.

We’ve got to admit that we were surprised by how much the wire diameter affects the resonant frequency of these theoretical antennas — the chunkier the wire, the lower the resonant frequency, which is defined as the frequency at which the antenna’s impedance has only a resistive component. On the other hand, material selection plays a role, too, with copper wire being the best choice in terms of loss, followed by aluminum wire and then iron pipe, which is very lossy at small diameters. Luckily, these differences even out with increasing conductor diameter.

The most interesting part of the video for us was the experiments with practical antennas, which he builds from different materials and tests on a LiteVNA — kind of like a NanoVNA on steroids. As expected, wire thickness plays a part in antenna bandwidth — the finer the wire, the narrower the bandwidth — and the measured resonant frequency worked out to be pretty much what it was in simulation. Insulation on the wire had an unexpectedly huge effect too, pushing the resonant frequency down around 25 MHz.

Thanks to [FesZ] for this effective demonstration of designing antennas for the real world.

NFC tags are a frequent target for experimentation, whether simply by using an app on a mobile phone to interrogate or write to tags, by incorporating them in projects by means of an off-the-shelf module, or by designing a project using them from scratch. Yet they’re not always easy to get right, and can often give disappointing results. This article will attempt to demystify what is probably the most likely avenue for an NFC project to have poor performance, the pickup coil antenna in the reader itself.

The tags contain chips that are energised through the RF field that provides enough power for them to start up, at which point they can communicate with a host computer for whatever their purpose is.

“NFC” stands for “Near Field Communication”, in which data can be exchanged between physically proximate devices without their being physically connected.  Both reader and tag achieve this through an antenna, which takes the form of a flat coil and a capacitor that together make a resonant tuned circuit. The reader sends out pulses of RF which is maintained once an answer is received from a card, and thus communication can be established until the card is out of the reader’s range.

Very Few NFC Tags And Readers Are On The Same Frequency

For the majority of tags likely to be experimented by Hackaday readers the RF frequency is 13.56 MHz, and the RF emissions are supposed to be in the magnetic field plane rather than the electric field. There’s nothing complex about the antennas, indeed it’s easy enough to make one yourself by winding a suitable coil and tuning it with a small variable capacitor. The RF properties of the antenna can be explored with instruments as simple as a signal generator and an oscilloscope, or if you’re a radio amateur old enough to have picked one up, a dip meter. For the purposes of this article I’m using a NanoVNA because of its extreme convenience, and I’ve set it to measure SWR on port 1 with a sweep between 10 MHz and 20 MHz. I’m loosely coupling it to the NFC antennas I’m testing by means of an RF pickup coil, one turn of wire about 10mm diameter soldered to a coaxial connector and secured with a bit of glue. When I place the pickup coil over an NFC tag, I’m rewarded with a sharp peak on the VNA from infinity down to near 1:1 SWR. This works well with most reader coils and with lower power NFC tags that simply contain a memory chip, but my VNA doesn’t provide enough energy to measure those tags with higher power integrated circuits such as bank cards, a public transport card, or my passport.

Immediately, the VNA pinpoints one of the problems inherent to mass-produced NFCs, that the resonant frequency is rarely exactly on 13.56 MHz. In writing this article I found that both cards and readers appear to resonate anywhere between 13.5 and 15 MHz, with the majority being measured at about 14 MHz. In practice most readers provide more than enough energy so the tag can still be energised despite the resulting inefficiency, but for any NFC tag system to work at maximum efficiency it should have both reader and tag adjusted to resonate at the 13.56MHz frequency of communication.

The Simple But Clever Tech In Your Bank Card

Most tags, and the cheapest reader modules, have very little effort put in to tuning them to resonance, but one of the more interesting tags I examined for this piece, a bank card subjected to a teardown by a hackerspace friend, shows a very clever approach to automated tuning. A bank card is a standard chip card made from two laminated layers of plastic, with the chip contacts appearing in the front face. Upon dismantling it can be seen that the chip and its contacts are on a small piece of plastic about 10 mm by 10 mm that can be lifted clear of the card.

This module can be read by a card reader, but only when it is placed directly on the antenna rather than with any part of the whole card in proximity to the reader as would happen in a shop. To ensure the small chip module can be energised by a reader over the whole surface of the card, the rear half of the card is a printed circuit board that is simply a tuned circuit with a large coil and an ingenious variable capacitor made from a row of small PCB plates. The coil is half-and-half round the edge of the card and closely round the chip, allowing it to pick up the field over a large area and couple the resulting energy closely into the chip. It’s tuned during manufacture by cutting a trace connecting the capacitors, at a guess this will be an automated process. Measuring its resonance it turns out to be a little higher than 13.56 MHz, but since that measurement was made on a dismantled card with no chip in place it’s likely that the resonant point will have been moved upwards.

Tuning An NFC Reader For Maximum Smoke

Turning to the readers, the more expensive devices have a built-in variable capacitor and will have been factory-tuned to 13.56 MHz, while the cheap modules normally have a fixed capacitor and resonate at a higher frequency. Experience with these cheaper modules suggests that they will usually interact with the simpler cards such as the ubiquitous MiFare Classic, but that they are unable to provide enough energy to power the smarter cards such as the MiFare DESfire tags. Adjusting the antenna on the module for resonance at 13.56 MHz improves the efficiency to the extent that the higher-power tags can be read, for example in the picture is a cheap reader module prepared by a hackerspace friend. He used an RF pickup coil and an oscilloscope to measure the amplitude of the 13.56 MHz carrier, and adjusted the tuned circuit until a point of maximum amplitude had been reached. In this case he wound his own coil and removed wire from it turn by turn to find the maximum, but the same result could just as easily be done with the PCB coil and a small trimmer capacitor. This cheap reader now works with DESfire cards that previously required a far more expensive module, making the process well worth the effort.

So while much of the technological magic in an NFC tag lies in its digital electronic package it’s worth remembering that making it all work is still a firmly analogue antenna. A bit of old-fashioned RF tweaking work with your ‘scope and a signal generator can transform their performance for the better.

Those of us who have bought cheap TinyVNA devices for our RF experimentation will be used to the calibration procedure involving short-circuit, 50 Ω, and open terminations, followed by a direct connection between ports. We do this with a kit of parts supplied with the device, and it makes it ready for our measurements. What we may not fully appreciate at the level of owning such a basic instrument though, is that the calibration process for much higher-quality instruments requires parts made to a much higher specification than the cheap ones from our TinyVNA. Building a set of these high-quality parts is a path that [James Wilson] has taken, and in doing so he presents a fascinating discussion of VNA calibration and the construction of standard RF transmission line components.

We particularly like the way that after constructing his short, load and open circuit terminations using high-quality SMA sockets, he put a custom brass fitting 3D printed by Shapeways on the end of each to make them easier to handle while preserving their RF integrity. If we’d bought a set of terminations looking like these ones as commercial products we would be happy with their quality, but the real test lay in their performance. Thanks to a friend he was able to get them tested on instruments with much heftier price tags, and found them to be not far short of the simulation and certainly acceptable within his 3 GHz range.

Curious about VNAs at the affordable end of the spectrum? We took a look at the TinyVNA, which while it is something of a toy is still good enough for lower frequency measurements.