The Smith chart was invented by Phillip Smith and presented in close to its current form in 1937, see. The chart has many numbers printed in quite small font and with signs dropped as there is not room. It takes effort to master but fundamentally it is quite simple combining a polar plot used for plotting \(S\) parameters directly, curves that enable normalized impedances and admittances to be plotted directly, and scales that enable electrical lengths in terms of wavelengths and degrees to be read off. Mastering the Smith chart is essential to entering the world of RF and microwave circuit design as all practitioners use this as if it is well understood by others. I find that the potting is worth it to protect the board and components and keeping the toroid windings in place.The Smith chart is a powerful graphical tool used in the design of microwave circuits. There will be some change in characteristics after potting but nothing significant. Be sure to use clear epoxy, definitely don't use anything with metal fillers. I pot the assembled and tuned matching network with Gorilla Glue 5 Minute two part clear epoxy. And I find it easier to swap out toroids than to try to remove or add turns while the toroid is mounted on the board. This makes it easier to play with spacing to get a targeted inductance. Rather than adding or subtracting turns on one toroid I wind two toroids with different number of turns. Often you have to decide whether to use n turns or n + 1 turns on a toroid. The calculated inductance for toroids is not always close to actual value due to winding spacing and toroid material variance. There are various discussions on accuracy but it will get you in the right ball park. Premeasure the inductors and capacitors with the nanoVNA at the frequency of interest before using in the matching network. The nanoVNA measured high reactance impedances better than standard antenna analyzers designed mainly for SWR measurements. The readings were affected not accurate and consistent. There was just too much RFI from the laptop and power supply that was being picked up by the antenna. Some of my initial measurements where performed with the nanoVNA connected to a computer using nanoSaver software. When measuring ZL impedance use the nanoVNA by itself and use its file saving capability to retain the. If needed, I could remove the 10 pF and substitute a 5 pF or 15 pF capacitor to fine tune the match. I mounted a 47 pF NP0 ceramic SMT capacitor in the BNC connector itself, followed by a 15 pF and 10 pF parallel NP0 SMT capacitors on the protoboard that the matching network was built on.
The mounted BNC showed around 7 pF capacitance, leaving about 71 pF of additional capacitance. I also measured the capacitance of the BNC connector using the nanoVNA and verifying with an autozeroing small capacitance meter. This made me decide to use a set of parallel capacitors so I could fine tune the value that will provide a close match, this turned out to be a good decision. Large changes in capacitor value will prevent the match to be close to the chart center, no matter the value of the inductor. Changes in the capacitor value keeps the network from moving along the 50 ohm constant resistance curve to the center. The most sensitive value seemed to be the capacitor. Initially I looked at how the match varies with changes in the component values.