Internal SWR Readings
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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
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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: [hidden email] [mailto:[hidden email]] On Behalf Of [hidden email] Sent: Wednesday, November 13, 2013 9:25 AM To: [hidden email] 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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: Internal SWR Readings
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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 <[hidden email]> 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: [hidden email] > [mailto:[hidden email]] On Behalf Of [hidden email] > Sent: Wednesday, November 13, 2013 9:25 AM > To: [hidden email] > 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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Rick WA6NHC
Actually, the SWR is constant along a lossless transmission line. It is
true that the complex impedance varies in cycles of a half-wave as you move up or down the line. If the line has appreciable loss, the measured SWR will decrease the further you get from the antenna. Many years ago, the ham club at Keesler AFB had a tribander that didn't work, although the SWR was very low on all three bands. We finally discovered that it was very low *everywhere* in or out of the ham bands. Coax was wet inside and was probably the longest dummy load in Harrison County MS. Most SWR indicators and power meters aren't real accurate anyway. With the advent of digital displays, the confusion between accuracy and precision has really increased. The only number that really counts is zero return power. 73, Fred K6DGW - Northern California Contest Club - CU in the 2014 Cal QSO Party 4-5 Oct 2014 - www.cqp.org On 11/13/2013 10:00 AM, Rick Bates wrote: > 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). ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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Please lets not allow this to run as long as the care and feeding of a PL259
connector. :oD I accept what you're all saying, in a perfect world. The originating poster stated that he was running through a tuner, which indicates it's far from perfect. That's fairly common, the other tuner reason is to extend the bandwidth of a given antenna (another conversation). The primary question was "Why do they not read the same" and the honest answer is: because they aren't looking at the same location(s) in the network. My take (and I'm not high on technical competency here): Since antennas are rarely (a number approaching zero) balanced or perfectly resistive and feedlines are never really lossless (just low enough to be acceptable) and the meters (expecting to see 50 ohms loads) are generally reading voltages (the results of the complex impedance) by various means, the combinations mean that the complex impedances (resistive and reactive at THAT point) are not the same along the entire run of the feed; meaning the meter will read what is at that point. It is inherently inaccurate, but commonly used. One worst case real world example, the G5RV in multi-band operation. It requires a specific feed length (per band) to be efficient because the feed is part of the matching system. End fed or OCF are about as bad (not balanced at all, impedances run wild). In an ideal world, the feed is simply a garden hose (or fire hose for QRO) to deliver signal to/from the antenna. In reality, it's far more complex than that. I submit that if anyone with a non-1:1 SWR antenna were to add a non x/2 wave section of feed (greater than say 5') and re-measure the SWR at the original location (the input to a tuner) it will be different (might be subtle, but it won't be the same). As mentioned by everyone, the actual causes vary, but are repeatable. Case in point: my center fed (80M EDZ) dipole (current length: 104 meters) is very reactive at the low end of 80 meters (22:1) even through a 4:1 common mode choke (balun). If I add a random section of coax (~42') between the balun and the tuner, the SWR at the tuner falls to 8.xx:1 which allows the KAT500/KPA500 to happily pump out all the power available. The coax and connectors are so low in loss as to be invisible at 80 meters, yet the added feed makes a huge difference. [The same antenna system is already ~8:1 in the upper 75 meter portion, so I plan to add another few meters of wire to accommodate tree(s) reactivity.] The bottom line for this original poster was to have his external meter at the input (as close as possible) of the tuner. In this way the internal K3 meter will observe the match from K3 to amp (just because stuff happens) and the external tuner will show/confirm that the tuner is making an effective acceptable difference in the SWR and also confirm that the amp is truly putting out the stated power (if the meter includes a power meter). Finally, yes, I wish I understood Smith charts. While it simplifies the complex, it is not simple to fully grasp what it is showing without adequate knowledge. So for those of us that can't read a Smith chart (some just don't care), the SWR is the reading we watch and basically ignore the complexity of what we're really doing with empirical results (it works, so we leave it alone). In my case, understanding the math and formulas is the root of the issue. :-\ 73, Rick wa6nhc -----Original Message----- From: Fred Jensen Actually, the SWR is constant along a lossless transmission line. It is true that the complex impedance varies in cycles of a half-wave as you move up or down the line. If the line has appreciable loss, the measured SWR will decrease the further you get from the antenna. Many years ago, the ham club at Keesler AFB had a tribander that didn't work, although the SWR was very low on all three bands. We finally discovered that it was very low *everywhere* in or out of the ham bands. Coax was wet inside and was probably the longest dummy load in Harrison County MS. Most SWR indicators and power meters aren't real accurate anyway. With the advent of digital displays, the confusion between accuracy and precision has really increased. The only number that really counts is zero return power. 73, Fred K6DGW - Northern California Contest Club - CU in the 2014 Cal QSO Party 4-5 Oct 2014 - www.cqp.org On 11/13/2013 10:00 AM, Rick Bates wrote: > 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). ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: Internal SWR Readings
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Guys - please self moderate to limit threads. Except in rare cases, 5-10 replies are more than enough on any thread.
Eric List moderator elecraft.com _..._ > On Nov 13, 2013, at 2:46 PM, Rick Bates <[hidden email]> wrote: > > Please lets not allow this to run as long as the care and feeding of a PL259 > connector. :oD > > I accept what you're all saying, in a perfect world. The originating poster > stated that he was running through a tuner, which indicates it's far from > perfect. That's fairly common, the other tuner reason is to extend the > bandwidth of a given antenna (another conversation). The primary question > was "Why do they not read the same" and the honest answer is: because they > aren't looking at the same location(s) in the network. > > My take (and I'm not high on technical competency here): Since antennas are > rarely (a number approaching zero) balanced or perfectly resistive and > feedlines are never really lossless (just low enough to be acceptable) and > the meters (expecting to see 50 ohms loads) are generally reading voltages > (the results of the complex impedance) by various means, the combinations > mean that the complex impedances (resistive and reactive at THAT point) are > not the same along the entire run of the feed; meaning the meter will read > what is at that point. It is inherently inaccurate, but commonly used. > > One worst case real world example, the G5RV in multi-band operation. It > requires a specific feed length (per band) to be efficient because the feed > is part of the matching system. End fed or OCF are about as bad (not > balanced at all, impedances run wild). In an ideal world, the feed is > simply a garden hose (or fire hose for QRO) to deliver signal to/from the > antenna. In reality, it's far more complex than that. > > I submit that if anyone with a non-1:1 SWR antenna were to add a non x/2 > wave section of feed (greater than say 5') and re-measure the SWR at the > original location (the input to a tuner) it will be different (might be > subtle, but it won't be the same). As mentioned by everyone, the actual > causes vary, but are repeatable. > > Case in point: my center fed (80M EDZ) dipole (current length: 104 meters) > is very reactive at the low end of 80 meters (22:1) even through a 4:1 > common mode choke (balun). If I add a random section of coax (~42') between > the balun and the tuner, the SWR at the tuner falls to 8.xx:1 which allows > the KAT500/KPA500 to happily pump out all the power available. The coax and > connectors are so low in loss as to be invisible at 80 meters, yet the added > feed makes a huge difference. [The same antenna system is already ~8:1 in > the upper 75 meter portion, so I plan to add another few meters of wire to > accommodate tree(s) reactivity.] > > The bottom line for this original poster was to have his external meter at > the input (as close as possible) of the tuner. In this way the internal K3 > meter will observe the match from K3 to amp (just because stuff happens) and > the external tuner will show/confirm that the tuner is making an effective > acceptable difference in the SWR and also confirm that the amp is truly > putting out the stated power (if the meter includes a power meter). > > Finally, yes, I wish I understood Smith charts. While it simplifies the > complex, it is not simple to fully grasp what it is showing without adequate > knowledge. So for those of us that can't read a Smith chart (some just > don't care), the SWR is the reading we watch and basically ignore the > complexity of what we're really doing with empirical results (it works, so > we leave it alone). In my case, understanding the math and formulas is the > root of the issue. :-\ > > 73, > Rick wa6nhc > > -----Original Message----- > From: Fred Jensen > > > Actually, the SWR is constant along a lossless transmission line. It is > true that the complex impedance varies in cycles of a half-wave as you > move up or down the line. If the line has appreciable loss, the > measured SWR will decrease the further you get from the antenna. > > Many years ago, the ham club at Keesler AFB had a tribander that didn't > work, although the SWR was very low on all three bands. We finally > discovered that it was very low *everywhere* in or out of the ham bands. > Coax was wet inside and was probably the longest dummy load in > Harrison County MS. > > Most SWR indicators and power meters aren't real accurate anyway. With > the advent of digital displays, the confusion between accuracy and > precision has really increased. The only number that really counts is > zero return power. > > 73, > > Fred K6DGW > - Northern California Contest Club > - CU in the 2014 Cal QSO Party 4-5 Oct 2014 > - www.cqp.org > >> On 11/13/2013 10:00 AM, Rick Bates wrote: >> >> 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). > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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