There was a time when oscilloscopes were big and expensive. Now you can get scopes of various sizes and capabilities on nearly any budget. Vector network analyzers — VNAs — haven’t had quite the same proliferation, but NanoVNA may change that. [IMSAI Guy] bought one for about $50 and made a series of videos about it. Spoiler alert: he likes it. You can see one of the several videos he’s posted, below.
NanoVNA is tiny but sweeps from 50 kHz to 900 MHz and has a touch screen. The device uses a rechargeable battery if you need to haul it up to an antenna tower, for example. Just as a quick test, you can see early in the video the analysis of a rubber duck antenna. The device shows return loss as a plot and you can use a cursor to precisely measure the values. It also shows a Smith chart of the reactance.
Antenna tuning at HF frequencies is something that radio amateurs learn as part of their licence exam, and then hone over their time operating. A few basic instruments and an LC network antenna tuner in a box are all that is required, and everything from a bit of wet string to ten thousand dollars worth of commercial antenna can be loaded up and used to work the world. When a move is made into the gigahertz range though it becomes a little more difficult. The same principles apply, but the variables of antenna design are much harder to get right and a par of wire snippers and an antenna tuner is no longer enough. With a plethora of GHz-range electronic devices surrounding us there has been more than one engineer sucked into a well of doom by imagining that their antenna design would be an easy task.
An article from Baseapp then makes for very interesting reading. Titled “Antenna tuning for beginners“, it approaches the subject from the perspective of miniature GHz antennas for IoT devices and the like. We’re taken through the basics and have a look at different types of antennas and connectors, before being introduced to a Vector Network Analyser, or VNA. Here is where some of the Black Art of high frequency RF design is laid bare, with everything explained through a series of use cases.
Though many of you will at some time or other work with these frequencies it’s very likely that few of you will do this kind of design exercise. It’s hard work, and there are so many ready-made RF modules upon which an engineer has already done the difficult part for you. But it does no harm to know something about it, so it’s very much worth taking a look at this piece.
Congratulations and thank you go to Theodore Yapo for authoring the first paper to complete the peer review process for the Hackaday Journal. You can read the standalone paper here; it will be included in the first volume of the Hackaday Journal officially released later this year.
The Hackaday Journal is an open access, peer reviewed journal that seeks to ensure hard-won domain knowledge is preserved and made available for the benefit of all. Before jumping into Ted’s topic, please take a moment to consider submitting your own paper for the journal.
Paper Submissions Wanted
We have other submissions in the pipleline now but we still need more papers to round out the first volume of the Hackaday Journal. Please consider authoring a paper on any creative research, engineering, or entertaining discovery in the areas of interest to the Hackaday community. The full name of our journal is the Hackaday Journal of What You Don’t Know — it will be a tome of infinite appeal to any who seek to broaden their minds in the engineering space. But for that to happen we need you to share your knowledge.
We are in an age of unparalleled opportunity for individuals and small teams to make interesting discovery. You should not need to be working on a degree to have your findings published, but of course students and faculty are encouraged to submit their papers. Do not hesitate to get in touch with us about topics you want to write about.
Scalar Network Analyzer Leakage Correction by Theodore Yapo
Low-pass filter being tested by a Rigol DSA-815 using the phase shifting correction technique
The title of Ted’s paper is a mouthful and the subject material wades into radio frequency knowledge with gusto. We applaud him, and the peer reviewers, for the attention to detail while moving toward publication.
In his work, Ted finds an interesting opportunity to get more performance out of relatively inexpensive bench equipment used to characterize RF components. This task is often reserved for Vector Network Analyzers (VNA) but with a heafty price tag these tools aren’t available to everyone. Spectrum Analyzers with Tracking Generators (SA/TG) have come onto the market, but especially with early versions, there is a leakage problem that causes inaccuracy. Ted found a simple technique that can correct for the leakage.
The solution is based on phase shifting the measurement. Starting with a properly calibrated machine, Ted uses a small board he built to electronically shift the phase of the Tracking Generator where the leakage is a problem. The signal is first measured, then measured again with a phase shift of 180 degrees. This effectively cancels out the error while preserving the signal being measured.
This paper goes into great technical detail in the RF domain. It is worth noting that the Hackaday Journal is open to discovery on multiple topics and levels of complexity. Don’t let what you think is a simple, useful idea go unpublished. We’re interested in a wide range of the simple, the obscure, and the frighteningly technical as long as the ideas of both novel and well supported.
This is Your Journal
You, yes you reading this right now, embody a movement of inventive and curious people working both inside and outside of formal academic structures. This is our time to contribute to the knowledge base of humanity. Pour yourself a refreshing beverage, saddle up your headphones, crack those knuckles, and let the writing process begin. Let us now what we don’t know. Submit your paper now.
If you are working with AC circuits a vector network analyzer (VNA) is quite handy. As an entry to the InnovateFPGA competition for students, [Evgenii Vostrikov], [Danila Nikiforovskii], and [Daniil Smirnov] created a VNA using a DE10-Nano, high-speed analog to digital and digital to analog converters, and a circulator. Most of the details are in the video below, and on the project’s GitHub page.
The DE10-Nano has a dual-core ARM processor and an Altera FPGA in one package. That allows you to use the CPUs where that makes sense and still leverage the FPGA where you need high performance.
The circulator uses an op-amp to allow the test signal to route to the device under test, while steering any reflected signal back to the device for measurement. The design also uses a lock-in amplifier, something we’ve talked about a few times recently. This allows less expensive converters to generate magnitude and phase information.
Judging by the fan in the video, we suspect the setup gets a little toasty. The GitHub page has a lot of Russian on it, so we aren’t sure how much we could puzzle out since our Russian skills were mostly from watching the Adventures of Moose and Squirrel.
[KB9RLW] wanted to build a vector network analyzer (VNA), but then realized he could buy a ready-made one without nearly the cost it would have been only a few years ago. The network in this case, by the way, is an electrical network, not a computer network. You can use a VNA to characterize components, circuits, antennas, and even feed lines at different frequencies. The miniVNA Pro is economical and can exercise circuits from 1 MHz to 3 GHz. You can see the review in the video below.
There are a few ways to actually create a VNA, but in concept, it is a sweep generator, a detector, and a means to plot the response at each frequency in the sweep. So you’d expect, for example, a resonant frequency to show a peak at resonance and a band reject filter to show a low point.
One of the things interesting about the device is that it uses Java software. That means it doesn’t care much what platform you want to use. The software can show two different plots at once, so [Kevin] hooks it to his 20 meter antenna and shows how it can plot the SWR and impedance around the frequency of interest.
The instrument can be USB powered with the same cable you use to connect the PC. However, it also has an internal rechargeable battery. That battery charges on USB and can operate the device with Bluetooth. We can imagine that being handy when you want to climb up a tower and connect it directly to an antenna as long as you stay in Bluetooth range of the PC. There’s also a phone app, so you can go that route, if you prefer and [Kevin] shows the device working with Android. Of course, you could probably rig a Raspberry Pi on your belt and then use WiFi to let someone on the ground remote desktop in to run measurements. A lot of possibilities.
If anything about electronics approaches the level of black magic, it’s antenna theory. Entire books dedicated to the subject often merely scratch the surface, and unless you’re a pro with all the expensive test gear needed to visualize what’s happening, the chances are pretty good that your antenna game is more practical than theoretical. Not that there’s anything wrong with that — hams and other RF enthusiasts have been getting by with antennas that work without really understanding why for generations.
But we’re living in the future, and the tools to properly analyze antenna designs are actually now within the means of almost everyone. [Andreas Spiess] recently reviewed one such instrument, the N1201SA vector impedance analyzer, available from the usual overseas sources for less than $150. [Andreas]’s review does not seem to be sponsored, so it seems like we’re getting his unvarnished opinion; spoiler alert, he loves it. And with good reason; while not a full vector network analyzer (VNA) that will blow a multi-thousand dollar hole in your wallet, this instrument looks like an incredible addition to your test suite. The tested unit works from 137 MHz to 2.4 GHz, so it covers the VHF and UHF ham bands as well as LoRa, WiFi, cell, ISM, and more. But of course, [Andreas] doesn’t just review the unit, he also gives us a healthy dose of theory in his approachable style.
Instrumentation has progressed by leaps and bounds in the last few years, however, the fundamental analysis techniques that are the foundation of modern-day equipment remain the same. A network analyzer is an instrument that allows us to characterize RF networks such as filters, mixers, antennas and even new materials for microwave electronics such as ceramic capacitors and resonators in the gigahertz range. In this write-up, I discuss network analyzers in brief and how the DIY movement has helped bring down the cost of such devices. I will also share some existing projects that may help you build your own along with some use cases where a network analyzer may be employed. Let’s dive right in.
Network Analysis Fundamentals
As a conceptual model, think of light hitting a lens and most of it going through but part of it getting reflected back.
The same applies to an electrical/RF network where the RF energy that is launched into the device may be attenuated a bit, transmitted to an extent and some of it reflected back. This analysis gives us an attenuation coefficient and a reflection coefficient which explains the behavior of the device under test (DUT).
Of course, this may not be enough and we may also require information about the phase relationship between the signals. Such instruments are termed Vector Network Analysers and are helpful in measuring the scattering parameters or S-Parameters of a DUT.
The scattering matrix links the incident waves a1, a2 to the outgoing waves b1, b2 according to the following linear equation: .
The equation shows that the S-parameters are expressed as the matrix S, where and denote the output and input port numbers of the DUT.
This completely characterizes a network for attenuation, reflection as well as insertion loss. S-Parameters are explained more in details in Electromagnetic Field Theory and Transmission Line Theory but suffice to say that these measurements will be used to deduce the properties of the DUT and generate a mathematical model for the same.
General Architecture
As mentioned previously, a simple network analyzer would be a signal generator connected and a spectrum analyzer combined to work together. The signal generator would be configured to output a signal of a known frequency and the spectrum analyzer would be used to detect the signal at the other end. Then the frequency would be changed to another and the process repeats such that the system sweeps a range of frequencies and the output can be tabulated or plotted on a graph. In order to get reflected power, a microwave component such as a magic-T or directional couplers, however, all of this is usually inbuilt into modern-day VNAs. Continue reading “Network Analysers: The Electrical Kind”→
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