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 <[email protected]> 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:[email protected] > > 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:[email protected] > > 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:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
