Rick, I believe you're confusing SWR and impedance. If a transmission line is lossless, the SWR will be the same along the entire length of the line.
If a transmission line is lossy, the SWR will get progressively lower as you move closer to the transmitter. In an extreme case (such as 1000 feet of old RG-58 or worse, at high enough frequencies), there will be so much loss in the line that EVERY antenna will look like 50 ohms back at the transmitter and the SWR will be 1:1 there! So the transmitter end of the line is the LEAST useful end to look at SWR. However, the COMPONENTS of the transformed antenna (or load) impedance DO vary along the line. And It is the antenna IMPEDANCE that repeats every 1/2-wavelength back toward the transmitter from the antenna end -- not the SWR. At all other points along the transmission line, unless the antenna impedance is purely resistive and exactly the same as the characteristic impedance of the transmission line the load impedance seen along the line will be a combination of resistive (real) and reactive (imaginary) parts. Those two components of impedance continually change in amplitude with distance (in wavelengths) along the line, repeating every half wavelength along the line. So if you have an impedance meter (rather than an SWR meter), you will, in fact, see those values of R and X vary from point to point along the line. But many, many, many combinations of R and X can all result in the same SWR. A very good way to see how R and X vary with distance back from the antenna along the transmission line -- even though the SWR remains constant -- is to understand what a Smith chart displays. One reason practical SWR meters don't read the same value of SWR on a line -- even at the same point on the line -- is because different circuit designs may have different amounts of error, or inaccuracy, for different amplitudes and combinations of R and X. And the greater the mismatch between the antenna (or load) impedance and the characteristic impedance of the transmission line, the larger those swings in R and X along the line will be. Thus, I'm not surprised that the SWR monitor in my K3 often shows a different SWR than my Kenwood wattmeter at essentially the identical point on the transmission line to my antenna. Even the world-renowned Bird wattmeter doesn't always do so well! There are many good treatments of SWR, transformed antenna impedance along a transmission line, and the use of Smith charts on the web and in many antenna books. Bud, W2RU On Nov 13, 2013, at 1:00 49PM, Rick Bates <happymooseph...@gmail.com> wrote: > For some reason this comes up often on ham lists. > > The SWR meter measures the match AT THAT point in the network. The SWR > varies along the feedline (which is why 1/2 wave feed sections are often > desired, so you can get an accurate antenna feed point reading). > > You can take ten different meters, each a meter apart from each other on the > cable and see ten different measurements. > > The most important reading is the one AT the transmitter (or at input of the > tuner, if used); the rest are superfluous information; nice to know but not > critical. > > 73, > Rick wa6nhc > > -----Original Message----- > From: elecraft-boun...@mailman.qth.net > [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of pastor...@verizon.net > Sent: Wednesday, November 13, 2013 9:25 AM > To: elecraft@mailman.qth.net > Subject: [Elecraft] Internal SWR Readings > > Hi, it's Mark Griffin, KB3Z. I was wondering how reliable the SWR reading is > on the K3? I have an external SWR/Power Meter hooked up between my K3, > Antenna Tuner and my Amp. Sometimes there seems to be some major differences > between the K3 and the External Meter. And it always is the fact the the K3 > gives a higher reading then the External meter. Has anyone experienced this > also and if so can one Sync up there Internal SWR meter on the K3? Thanks! > Mark ______________________________________________________________ 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
