Late Night NanoVNAing: Band Pass Filter Analysis For Friends That Don’t Like To Share
Lets start with the fact that there is no S2 port on the NanoVNA. There is a “1” port. And there is a “0” port. Or lets talk about that S12 measurements tell you how well a transmission goes from port 2 to port 1. NOT how well the power of the transmission goes from port 1 to port 2 as you would expect by looking at the numbers. Or in the case of the nanoVNA, S12 tells you how well the transmission goes from port 1 to port 0. This is just the beginning of the mental scramble that envelops band pass filter assessment.
S parameters….They are rather confusing, and I am just starting to peel back the vector network analysis onion. But what is starting to intellectually coalesce for me is the extraordinary interplay between capacitance, inductance, and resistance that corrals sinusoidal electricity. The elegance of harnessing RF.
A friend sent me a collection of handmade test cables to use with my nanoVNA a few years ago. At the time I did not appreciate the significance of these, and sadly broke one using it for satellite work instead. Now with quite a bit of desktop analytical gear (oscilloscope, signal generator, nanoVNA, hf/vhf/uhf antenna analyzers) and a deepening understanding of radio, these cables are prize possessions.
Another friend led me to the “NanoVNA Saver” freeware as a slick nanoVNA interface. You can find it here: https://github.com/NanoVNA-Saver/nanovna-saver. He spent a lot of time showing a group of us how to use the cheap, but incredibly powerful, little device.
At the time, my knowledge base was not developed enough to explore the nuances of electrical diagnoses — I was only beginning my journey. Those who know me in the real world may have an appreciation for why I would be drawn to laboratory analytics however. And now, with the failure of a second band pass filter recently, I am considering what it would take to start building my own. And here we are, listening (via the SDS100 scanner) to the bars closing on a Saturday night, and testing my band pass filters. And you know what? It makes a lot more sense now to me than it did when I first started this journey!
This is the 80M band pass filter that was showing a high SWR at its last multi-op use. The top right corner shows the S11 VSWR curve. To the right of that is the S11 return loss curve. The S21 parameter (forward transmission) is in the bottom right corner. The gray band in the center of the dip is the 80M ham band frequencies. The SWR looks a bit funky…
Here is a closer look at the SWR curve. In the upper phone section of the band, the VSWR almost reaches 3:1.
This is in contrast to the plot displayed by the 40M band pass filter. For what it’s worth, I believed that I let out the magic smoke on this filter as well, however it looks ok on analysis.
It seems to me that the S11 return loss and VSWR curves are much more satisfactory. Here is a close-up of the VSWR, well under 1.3:1 across the entire band.
So what direction is this taking me? I am going to start messing with various L, C, and R combination circuits and see how exactly I too can control RF. I may try this next: https://www.youtube.com/watch?v=jBNcTn9BEy0 and https://www.youtube.com/watch?v=Uaj8oLpj0XI.
And so for my friends that like to play together, but not too much together, the ham radio band pass filters are our very best allies! Who needs to share a radio when you can just block out their signals? Enjoy your next multi-ops…(I’m looking at you Field Day!)
Always yours,
KM1NDY