[Elecraft] Internal SWR Readings

[Rick Bates]

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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