A couple of days with a NanoVNA

[dosw]

Since I'm a gadget addict I got a NanoVNA a few days ago, for about $59 through Amazon. It came with two SMA-Male to SMA-Male cables, an SMA Female to SMC Female adapter, and three "standards" (open, short, and 50 ohm load), as well as a little stylus, and a lanyard with a guitar pick on it (presumably to use in place of the stylus).

 

I wasn't expecting a lot, but I have to say, this thing is pretty cool, for the price. It's not professional-quality analysis gear, but for a hobbiest it's great.

 

The first thing you should do when putting it to use is to calibrate it for the band you wish to analyze. You use the "standards" that it comes with to perform the calibration. I followed that up by putting an antenna on it, and on my RTL-SDRv4, and verifying its signal generation was accurate. It was close enough without any tweaking.

 

Next, I used it to test some antennas. At first I just confirmed that the MXTA26 is very well tuned, out of the box, centered on 465MHz. And then I put the NA771G on it:

 

In that graph, you can see that I have it configured to scan the range from 420MHz to 480MHz. I've set a marker at 462.2MHz just by dragging it into place with the stylus. I could have set it more precisely through the menu. The S11 LOGMAG represents return loss. The lower the dip on the graph, the better (less reflection/return means more signal is radiating, which is what an antenna is supposed to do). The blue line here indicates SWR. And as you can see for the NA771G, it is 1.055:1 at 460.2MHz. At 480MHz (I don't have a marker set there), it was about 1.27:1. The green curve is the Smith diagram. And the purplse is the |Z|, coming in at 52.2 ohms at 460.2MHz. There are many graph views, I selected four of them. But they include logmag, phase, delay, smith, swr, resistance, reactance, |Z|, polar, linear, real, imaginary, Q factor, conductivity, susceptivity, |Y|, series C, series L, Parallel R, Parallel X, Parallel C, Parallel L ... most of which I don't comprehend.

 

One thing that I learned was how great it is to be able to look in realtime at the various curves. It made it really easy to observe changes as I added and removed the ground plane rods from my Nagoya GPK-01 (Ground plane kit) when the antennas were mounted on a camera tripod where there's no good inherent ground plane. And it was really useful to be able to observe what happens when I take a half centimeter off of an antenna. In the case of the NA771G, if one were wanting to tune it (which you can't), you might decide to subtract a fraction of a centimeter from its length, and you can see that on the chart. I used the SWR and Logmag curves to tune my Laird TE B4505CN antenna, for example, as I recognized its SWR dipped a little further to the left of the GMRS band. By trimming its base element down to about 10" (taking about 2cm off), I moved the dip to 465MHz. And knowing which direction to go was really only possible by looking at these curves and observing what they do as I move the element in and out a little with its set screws.

I also tested the pass-through mode using a VHF/UHF diplexer, verifying the cutoff ranges for each of the diplexer's ports.

 

A good feature is the ability to plug it into a computer and command the device, and observe its graphs, right on the larger screen of a computer. On my Linux laptop the NanoVNASaver application was already available for Ubuntu install through the apt and apt-get package manager tools without having to compile from source (one dot-release behind the Git repository). With the free NanoVNASaver software I can specify a scan range and set up multi-pass averaging. And the software can step through a scan range to achieve much finer resolution than the device natively supports. For example, if I use the native 101 data points the device supports to scan from 144-480MHz, that's one sample every 324kHz. Not very good resolution. But if I tell the external software to take 3240 samples, it will step through the range 100 samples at a time, and get me resolution finer than I can reasonably use on a full size monitor.

 

Another interesting option, through the NanoVNCSaver software, I'm able to see feed line length and attenuation. I can test a 50 foot run of LMR400, and a 50 foot run of RG58, and see loss per frequency. That's really interesting information for a hobbiest.

 

Some negatives: If you're completely in standalone mode (no computer), it's only got 100 sample points in a given scan. So if you're trying to optimize a dual-band antenna, you would want to concentrate on 2m, and then on 70cm, going back and forth one at a time, rather than trying to show both on the screen while tuning an antenna. This is where using a computer with a much higher sampling rate is important. Its screen it pretty tiny, too. Having spent the $59, I kind of am on the fence as to whether I should have spent $89 to get the larger screen. On the other hand, adding a computer to the mix, even an inexpensive low-powered laptop, the screen becomes irrelevant, and sample-points become virtually infinite. Another negative is the lack of instructions. It comes with a single page showing where to find the various menu options, but no other explanation. It took some youtube videos and online reading to figure it out. And without a microSD card you're missing out on being able to save screenshots (though you can take a screenshot through the computer's software), and limited on number of calibration configurations you can store. But most of those shortcomings can be overcome with an external computer and/or a cheap microSD card.

 

For my purposes this device is fantastic. With the Surecom SR102, I could see the SWR of a given antenna setup. With this device, I can see which direction things need to go in tuning antenna setups. And I can verify a diplexer is working as intended. 

[WRUE951]

the Nano is a must have for the tool chest, especially for the price.  I tune all my antennas with the Nano before deploying and they always check out perfect on the SWR meter. Couple of friends even have use a Nano to tune multiplexers.