Inexpensive yet invaluable tools for the RTO, and why I don’t take Pi into the field, by Jefferson Thomas

This write-up is primarily regarding a small device called the nanoVNA.

The NanoVNA is a Vector Network Analyzer (Antenna Analyzer on steroids.) The primary website for it is:

nanovna.com

It is a small (palm-of-your-hand) device – about the size of a deck of cards, roughly. It is an opensource device that has many capabilities far beyond what the typical RTO will need. It does a ton of different things, but so as not to scare off any more readers than necessary I will list only these: measures and plots SWR of an antenna system, determine the length or velocity factor of a coax cable and it can find the distance to a break or damage to a cable.

It is self-contained, runs off of a small Lithium battery for hours (USB rechargeable) with a small (2.4″, 3.2″ or 4″) touchscreen, but can also be connected to a computer or tablet for enhanced capabilities.

It will scan frequency ranges of 10KHz-1.5GHz. The Version 2 Plus4 models up to 4GHz (with firmware update.)

While many of our radios will display the current SWR at the frequency we have our rig tuned to, that is only of limited value in that it can take some extra work to determine the antenna system’s resonant value (even more work if it has multiple resonant frequencies, as they usually do.) If you are adjusting the antenna, the irritations compound with each adjustment. As the VNA shows the graphical plot in realtime this cuts your work down to a small fraction of time involved. It is also a great tool to help “tune” your manually-adjusted antenna-tuner (really a “matcher”) if that’s the route you went (silly you.)

My NanaVNA cost me…$45 less than a year ago. Newer and better ones will run up to about $150, depending on model.

Commercial VNA’s for industry will easily cost you $10,000 and up. This is an opensource product with many different people building and selling them. Is it the quality that you would get for 200X more? Uh, no, but this gets you more than ANY other product available to us, including the traditional antenna analyzer.

Mine has helped me immensely determine differences between using ladder-line vs. coax feedlines, properly sizing antennas (using the 936/freq equation is JUST a starting point), effects of different insulators, effects of the terrain on an otherwise similar antenna configuration and many more.

There are now a number of different models with varying capabilities. There are a few considerations that seem common to them.

They typically use SMA connections. As SMA connections have a limited lifespan regarding attaching/detaching, (rated at about 500 reconnects) it is beneficial to purchase a couple male-female SMA connectors to put on the NanoVNA and leave them there. That way it isn’t the NanoVNA’s connector itself that wears out. As you need to calibrate a VNA EVERY time you change ANY measuring parameter (like the frequency range being used) you could wear out the connectors in short order. (A calibration requires at least three reconnections before going on to the test – so that’s 125 tests.)

You would also want to purchase various SMA-to-whatever-connection-types-you-use. (e.g., SMA-BNC or PL-259 adapters.) Ideally, you would also get a “standard” for whatever connector you use. That is a very accurate 50 ohm load used specifically for calibrations, and is manufactured with very tight tolerances and specifications, much more so than just a “terminator.” Any reasonably close load will actually work for our purposes – I use my Elecraft 20W Dummy Load (comes in kit-form) that is pretty accurate, and actually set up so that you can measure transmitted power with a basic voltmeter as an added bonus. The NanoVNA comes with a SMA standard, but that won’t help you for your BNC connected stuff.

Another issue, the NanoVNA can come in cased versions, and some as “sandwich boards.” The sandwich-board versions are cheaper but much less weather-resistant (read: not.)

Another issue is that the USB3 cable that comes with it MAY be a power-only cable, and NOT good for data-comms, requiring you to supply your own if you wish to connect it to a computer for use of computer software or firmware updates.

Important note: If you plan on testing rubber-duck type antennas, remember that these antennas use the radio body as the groundplane. Therefore you have to build a suitable mount for the antenna that has a groundplane – it looks like a jungle antenna. Another way to test it is to directly attach the rubber duck to the nanoVNA as if it was a HT and hold it as such to test it. Using the USB cable to connect it to a computer in such a case will NOT work as it has a major effect on the groundplane created by the NanoVNA body.

Good manuals to get you going on the NanoVNA are:

-and-

Good videos can be found at:

In particular # 312-314.

Extra functionality can be had through the use pc/tablet software. I like VNA Saver from

It runs on Windows (at least as far back as ver. 7), Mac in development I believe, and supposedly on Linux. I wasn’t able to get it going on my Linux system, but as I run Win7 in a VM on my Linux machine, and Win8.1 on my field tablet that hasn’t been an issue.

As much software I use is unfortunately Windows-only (for my Uniden scanner, Anytone HTs, Baofeng-1701s to name a few) it makes sense – to me at least – to use a Windows tablet in the field. I hate Windows in general, and Microsoft specifically. However I have not found a good Linux tablet or distribution that will run properly on a tablet yet, let alone install and run a virtual machine with windows, or even WINE to handle that Windows-only software. It also allows me to run  full versions of Fldigi, Flrig, and Flmsg.

Some might bring up using a Raspberry Pi in the field. I know many others do, like OH8STN (Julian, the Survival Tech Nord) at OH8STN.ORG and my hats off to them. Several years ago I worked hard on getting Fldigi running on the RPi2b. Not many of us were successful but I was able to – after several hours of installation AFTER figuring out the procedure. And we still had to run it over VNC from another tablet. While these days it easy easy to get Fldigi, JS8-Call and other good radio software running on the Pi, you still need to use something to see it. If I have to bring a tablet to see what I’m doing on the R-Pi, then why not just get rid of the R-Pi? The Pi could only run the Windows software through either WinEMU (another HUGE pain to get going) or through an X86 emulator. The only one of those I know of was Exagear Desktop by Eltechs – which is now defunct. That’s too bad, as it was an outstanding product for the Pi and easy to set up, but again, if I need another tablet (or even the Pi touchscreen) than why have the Pi at all with all of it’s problems with the filesystem sitting on a flaky SD card or an external harddrive. Too many things to go wrong – and will. Not to mention having to haul along another power-supply system for it. Don’t get me wrong, I think the Pi is a great product and I have a number of them and spent a good deal designing ways for us to use them, like at https://www.americanpartisan.org/2020/01/r-pi-otp-dryad-true-hardware-rng-how-to/ .I just don’t think it’s for the field. Pi in the field will always become a mess eventually.

As a last quick note, I recommend RTOs get a copy of an Antenna Modeling Software package. Many are free, or have limited evaluation versions. I have used EZNEC from www.eznec.com. I believe I actually got it along with my ARRL Antenna Handbook. While they do only “model” the propagation, they seem to be quite accurate. I’ve used it to see likely effects on adjusting orientation of my Inverted-V antennas. It can save a lot of time adjusting things that will have literally no perceivable effect. Unfortunately, it too is Windows only.

Jefferson Thomas