got me thinking of a series of articles Jerry Sevick wrote for CQ magazine about twenty years ago showing how to do impedance-matching using pieces of coax cut and soldered together, often reversing the center conductor and sheath but I don't remember it in any detail. I tried googling and found a couple of papers
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but still can't reconstruct the understanding I got from those CQ articles twenty years ago. Can anybody explain how that wifi antenna works, or have a link with some introductory explanation of how stub and series matching works?
I saw that paper. It addresses wide-band transformers wound on ferrite. I'm talking about frequency-dependent transformers that are typically built from bits of coax at certain fractions of a wavelength, cut and soldered together in certain configurations. You're probably familiar with closed and open quarter-wave stubs, for example. That sort of thing, but more general; 4:1 and 9:1 impedance transformers made from pieces of coax, used at a fixed frequency or a narrow band. Sevick's articles went into it in depth. The antenna I gave the link to is made the same way. Take a look at it.
Assuming you mean the first link you posted, that's a typical "coaxial collinear." The points where center and outer reverse are feedpoints to a string of dipoles; the half-wave (accounting for the propagation velocity of the line used to make them) forces the feed voltages to be in-phase. The currents that result are nearly in phase. The parallel combination of all the feedpoints is generally a fairly low impedance, but not 50 ohms, and some matching is required. It's also useful to decouple the feedline from the antenna.
As for impedance matching...you can match between any two dissipative impedances using line segments (or between any two non-dissipative impedances using a lossless line segment). A Smith chart is (for many) a nice way to visualize the matching, and there are computer programs that automate the calculations of a Smith chart and free you to concentrate on the visualization part. I use WinSmith, but there are several others that will do the job, too. I guess I view it rather like the old saw, "give a man a fish and you feed him for a meal; teach him to fish and you feed him for a lifetime" (or at least give him an excuse to waste weekends trying to catch them...). I'd rather point you to tutorials on use of Smith charts or the like than to tell you how to do some specific matching job. Along the way you may come to understand that you can control the bandwidth of the matching over quite a range, too...and you can mix line segments with lumped components as well.
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