Wes, As someone involved in the design and manufacture of couplers for VSWR measurement for the aviation industry (admittedly some 30 odd years ago!), I would say that you are 100% correct, although I suspect that in amateur gear number 4 in your list is probably the biggest culprit of all. We had people on the production tweaking bridges to maximise directivity and it was a job that required some skill, e.g. bending leads of matched zero bias Schottky diodes until the spec was achieved - admittedly this was in the days before the large scale adoption of lead-less components, which must have made things a bit easier.
73 Stephen G4SJP On 3 September 2014 22:12, Wes (N7WS) <w...@triconet.org> wrote: > Oh dear me! > > If I take a lossless 50-ohm line and terminate it in 100 ohm and measure > the VSWR using an ideal bridge/coupler/VNA/etc that is calibrated for a > 50-ohm system, I will measure 2:1 SWR no matter how long the line is, from > zero to infinity. The transformed Z will change with length, but the SWR > will not. That's why one can draw a circle of constant SWR on a Smith > Chart. Any point on the circle will have a different Z from another, but > they all have the same SWR. > > If you change line length and the SWR reading changes, then: 1) the line > has loss, 2) the line Z and the SWR meter Z are different, 3) the source > match is poor, 4) the bridge/coupler directivity is poor, or 5) all of the > foregoing. With most ham stuff, it's 5. > > Wes N7WS > > On 9/3/2014 1:19 PM, Jeffrey Otterson wrote: > >> Unless your antenna is exactly the same impedance as your feedline at the >> desired frequency (pretty unlikely) then the feedline is going to >> transform the antenna impedance based on distance from the antenna. The >> exception to this is feedline lengths that are perfect multiples of a half >> wave, electrically (that is to say, accounting for the velocity factor) >> >> Any other length will result in a transformed impedance, and corresponding >> different VSWR. >> >> You can demonstrate this by changing the feedline length and watching the >> VSWR change. Try adding some small fraction of a electrical wavelength of >> coax at 40M and see what your meter shows. You might be surprised. >> >> TLDR; try adjusting the length of your coax and see if the readings >> change. >> >> Jeff n1kdo >> >> >> >> >> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to eastbrantw...@gmail.com > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
