When [Jephthai] wanted to build his own Yagi antenna, he turned to MMANA software for antenna modeling. This is an antenna analysis program that uses the moment method to calculate parameters for different antenna geometries. After building the Yagi, the predicted tuning and impedance matched the real antenna nicely. But what about the radiation pattern? To test that, he used a NanoVNA and a clever test setup.
He needed a test spot out of the antenna’s near field so he set up his workstation 18 feet away from the test antenna which was on a mount that could rotate. On the edge of the workstation table — affixed with painter’s tape — is a NanoVNA connected to a laptop.
If you’re going to fail, you might as well fail ambitiously. A complex project with a lot of subsystems has a greater chance of at least partial success, as well as providing valuable lessons in what not to do next time. At least that’s the lemonade [Josh Johnson] made from his lemon of a low-cost vector network analyzer.
For the uninitiated, a VNA is a versatile test instrument for RF work that allows you to measure both the amplitude and the phase of a signal, and it can be used for everything from antenna and filter design to characterizing transmission lines. [Josh] decided to port a lot of functionality for his low-cost VNA to a host computer and concentrate on the various RF stages of the design. Unfortunately, [Josh] found the performance of the completed VNA to be wanting, especially in the phase measurement department. He has a complete analysis of the failure modes in his thesis, but the short story is poor filtering of harmonics from the local oscillator, unexpected behavior by the AD8302 chip at the heart of his design, and calibration issues. Confounding these issues was the time constraint; [Josh] might well have gotten the issues sorted out had the clock not run out on the school year.
After reading through [Josh]’s description of his project, which was a final-year project and part of his thesis, we feel like his rating of the build as a failure is a bit harsh. Ambitious, perhaps, but with a spate of low-cost VNAs coming on the market, we can see where he got the inspiration. We understand [Josh]’s disappointment, but there were a lot of wins here, from the excellent build quality to the top-notch documentation.
Any grizzled electronic engineer will tell you that RF work is hard. Maintaining impedance matching may be a case of cutting wires to length at lower frequencies, but into the low centimetre and millimetre wavelengths it becomes a Dark Art aided by mysterious and hugely expensive test equipment beyond the reach of mere mortals. A vector network analyser or VNA may be beyond the reach of many, but [Tomasz Wątorowski] is here to tell us about how with some resistors, mathematics, and a bit of lateral thinking its functions may be replicated with a more modestly equipped bench.
It’s not a method for the faint-hearted as the mathematics are of the variety that you probably learned as an undergraduate but let slip from your memory with thanks after the course ended. The method involves measuring the return loss both with and without a resistor of known value in series with the antenna, these figures allow the real and imaginary components of the antenna’s impedance to be calculated. There is a further piece of work though, this method doesn’t determine whether the antenna is capacitive or inductive. Repeating the measurement with either a capacitive or inductive matching network allows this to be determined, and the value of the appropriate matching component to be calculated.
The sun may be spotless, but that doesn’t mean it isn’t doing interesting things. A geomagnetic storm is predicted for this weekend, potentially giving those at latitudes where the Northern Lights are not common a chance to see a cosmic light show. According to SpaceWeather.com, a coronal hole, a gap in the sun’s atmosphere that can let the solar wind escape, is about to line up with Earth. The last time this hole was on the Earth-facing side of the sun, the resultant storm gave aurora as far south as Colorado. So if you’re in any of the northern tier states, you might want to find somewhere with dark skies and a good view to the north this weekend.
It’s not only space weather that’s in the news, but weather-weather too. Hurricane Dorian will probably make landfall as a Category 4 storm, probably along Florida’s Atlantic coast, and probably in the middle of the night on Monday. That’s a lot of uncertainty, but one thing’s for sure: amateur radio operators will be getting into the action. The Hurricane Watch Net will activate their net for Dorian on Saturday afternoon at 5:00 PM Eastern time, ready to take reports from stations in the affected area. Not a ham? You can still listen to the live feed once the net activates.
Hams aren’t the only ones getting geared up for Dorian, though. Weather satellite enthusiasts are pointing their SDRs at the sky and grabbing some terrifyingly beautiful pictures of Dorian as it winds up. Some of the downloaded images are spectacular, and if you’ve got an SDR dongle and a couple of pieces of coat hanger wire, you too can spy on Dorian from any number of satellites.
Speaking of which, over on r/RTLSDR, someone has done a little data mining and shown that NOAA 15 is still very much alive. u/amdorj plotted the scan motor current draw and found that it steadily decreased over time, possibly indicating that the bearings aren’t as worn as previously thought. We recently covered the story of the plucky satellite that’s almost two decades past its best-by date; here’s hoping our report on its death was greatly exaggerated.
We’ll finish up today with something not related to space. As Al Williams recently covered, for about fifty bucks you can now score a vector network analyzer (VNA) that will do all sorts of neat RF tricks. The NanoVNA sounds like a great buy for anyone doing RF work, but its low price point and open-source nature mean people are finding all kinds of nifty uses for it. One is measuring the length of coax cables with time-domain reflectometry, or TDR. Phasing antenna arrays? the NanoVNA sounds like the perfect tool for the job.
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.
