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Cheap GHz VNAs!

posted Dec 25, 2020, 2:23 PM by Charles Boling

Sorry I didn't post this in time for you to gift yourself one today. Merry Christmas!

One of the pieces of test equipment that many hams would love to have but don't want to spend the money on -- besides an FM deviation meter (Serious? $200 for uncalibrated hunk o' junk from the 70's? Why doesn't one of my cheap Chinese HTs come with one built into it? It shouldn't be that hard, and it would be useful!), perhaps a topic for another time -- is a good Vector Network Analyzer (VNA).

What's that you say, you don't even know what that is, much less that you'd been asking for one? Well, let's take a closer look. What's a network analyzer? The word "network" may conjure images of Ethernet cables & switches, but in this case it refers to LRC networks -- electrical circuits exhibiting inductance, resistance, and capacitance. The tool is often called an "antenna analyzer", because the most popular use for hams is analyzing antenna systems.

What can you do with it?  LOTS of things, but let's start with a big one: a glorified SWR meter. An SWR meter will indicate a degree of impedance mismatch by showing the amount of power that is reflected back to your transmitter. Anything higher than that coveted 1:1 shows that some power isn't being transferred to the antenna and out into the ether where you want it to go. (A 1:1 doesn't guarantee that the power's going where you want it too -- merely that it's not coming back to you -- but that's enough said about that.)  The classic way to adjust an antenna for the band you want to operate on is to use a meter to check SWR at least at the bottom, middle, & top of the band.  Measure it at multiple points and you can connect the dots to produce a more informative graph.  Wouldn't it be convenient if you could be lazy and just press a button, and watch an SWR graph over any range of frequencies appear? Neato! You could instantly see what an antenna was good for and what it wasn't (as well as if you had a broken coax)

In addition to SWR (which is derived by measuring forward & reflected power) the analyzer can compare timing/phase to compute complex impedances, showing resistive, capacitive, and inductive components so you can understand what's going on "behind the scenes" with the circuit represented by the antenna and easily compensate to create a more appropriate match. A dual-port analyzer isn't limited to measuring reflected signals, but can be attached to the "input" and "output" of an electronic circuit to make similar measurements.  Or, how about the equivalent of a TDR (time domain reflectometer -- something that sends an electrical pulse down a wire and then checks the amplitude/time of the returned signal) that compares behavior at different wavelengths to tell you where that damaged section of coaxial cable is? When you have hardware that can generate a precise signal and just as precisely measure what come back, then the rest is all mathematics, i.e. software, so if you can modify the software, you can add features and make it smarter at will.  Open-source it, so that people who are smarter than you in some areas can contribute, and the possibilities grow.

Traditional VNAs were/are the tools of the well-equipped laboratory, and very expensive -- tens of thousands of dollars.  Within the past couple of decades, I've started to see "antenna analyzers" marketed to hams at more affordable prices.  One class of these are simple self-contained handheld devices that are crude and not that big of a step up from an SWR meter.  Another entry into the market was W6BIG's AIM4170 and similar devices, one of which I bought 13 years ago for $500. It has no display, but must be attached via RS-232 to a computer running MS Windows, where its accompanying software works its magic.  It was a fantastic device when it was introduced, but only went up to 180 MHz. Over the years I've wanted to upgrade to a UHF-capable unit, but the prices around $1k are a deterrent. Besides, having to lug a laptop around with it (and keeping a copy of Windows just for that, when all I normally run is Linux) is a pain.

A NanoVNA showing a Smith chart

I was delighted when KF7ZIM sent me a link to a product known as NanoVNA. It combines the basic RF hardware w/ a low-cost mobile computing board powered by a Li-Ion battery and a full color LCD touchscreen, running Linux and an open-source program to implement the VNA functionality and UI.  It's a lot better than the cruder analyzers from MFJ, etc. -- closer to the power of my AIM but w/ the portability that I want, and best of all, it goes over 1GHz, at last granting me access to 70cm & beyond. All this starting at $50!? Wow!

Since the original NanoVNA was produced, there have been hardware and software revisions, and different companies making them using hardware of varying quality. Versions are now available that can use the 9th harmonic of the 300 MHz clock to make useful measurements up to 2.7 GHz.  Alexander & I bought one towards the low end of the market for $54 shipped.  It was a low-risk way to evaluate the platform, and it was exactly what he needed for his employer, who runs a fleet of trucks w/ CB radios.  There was a bit of a learning curve for us (the kid who wrote the software is Japanese, and there's still a lack of detailed high-quality English documentation) but we got it all set up so that all one has to do is turn it on and press the scan button to get an SWR graph of the CB band (and just a little on either side for context). Any dummy can use it to get an instant picture of the health of the antenna system on a truck.

Most pre-made devices come as kits that include the calibration loads and everything else you need to make it work.  We found that if you care about absolute accuracy you really do want to calibrate it over a narrow range of frequencies that you're graphing, but this is helped by its ability to store profiles with calibration parameters to go with the display settings. When used properly, its measurements compared favorably to my more expensive instrument.

Having a device like this is not only convenient, but educational, encouraging experimentation and helping you learn more about antennas. You may be surprised (sometimes pleasantly!) how some of your antennas behave on frequencies that they weren't necessarily designed for, and something like this can definitely aid in quickly validating (or re-validating) an antenna system and assuring you that it's functioning as intended. If you've never played with one before, or if you're looking to upgrade without spending a lot of money, now's a great time!