coax stub and series section line transformers

Discussion in 'Electronic Design' started by kell, Jun 16, 2007.

1. kellGuest

Looking at this wifi antenna design:
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
http://www.tuc.nrao.edu/~demerson/weller/weller.pdf
http://www.qsl.net/aa3rl/tlcalc1.html
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?

2. ehsjrGuest

Maybe this will help:
http://www.highfrequencyelectronics.com/Archives/Feb04/HFE0204_Sevick.pdf

Ed

4. kellGuest

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.

5. Tom BruhnsGuest

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.

Cheers,
Tom