[Elecraft] Internal SWR Readings

Wed Nov 13 20:15:58 EST 2013

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 <happymoosephoto at gmail.com> 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).
> 
> 
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