[Elecraft] K3 - Numeric SWR Display

[Ron D'Eau Claire]

It is best to measure the SWR at the antenna. That ensures the actual SWR on
the feed line (and so feed line losses) are as low as possible. 

Using an SWR bridge designed for 50-ohm lines will do the job. When the SWR
bridge reads something close to 1:1 the reactance will, by definition, be
close zero. 

If the resistive part of the impedance isn't 50 ohms, or if reactance is
present, a match near 1:1 won't be possible. An analyzer is helpful when
happens since it'll tell you what is off, but you can often determine that
by simply checking the best match at another frequency (within the Ham band,
of course). If the lowest SWR is lower at a higher frequency, there is
inductive reactance. If the opposite is true, there is capacitive reactance.


You may know that with a gamma match you adjust the length of the matching
"arm" to hit the right resistive value then, because the gamma arm has
length, and so inductance, you set the capacitor to compensate for that
inductance to bring the reactance to zero. So really there's only two
adjustments to get the reactance to zero and the resistance to 50 ohms. If
you make the arm longer (move the shorting bar or whatever you're doing to
adjust it) you'll raise the resistive value. Shorter will lower it. Each
time you change the arm length you adjust the capacitor for minimum SWR. So
you just work back and forth to reach an acceptably low SWR. 

Exact matches aren't important, especially since you have the KAT3. All you
need to do is avoid too-high an SWR on the feeder. What is too high depends
upon the length of the feed line and frequency, but in most amateur
installations with <100 feet of decent coax there's little to tell between
the losses caused by an SWR of 1:1 and 3:1 on a feed line. For example,
common RG-8 coax with an SWR of 3:1 at 30 MHz only shows about 1 dB of loss.
The loss is less as you reduce frequency and/or coax length. But, to
understand what the SWR on the feed line really is, you need to measure it
at the antenna (load), not at the source (rig). An SWR bridge will indicate
lower at the rig end than at the antenna end due to the losses in the feed
line. 

All the fancy gear, like analyzers, makes life easier and lets us do things
more quickly, but they aren't required to install an effective, efficient
antenna and they cost substantial dollars.

Shoot, when I first got on the air, I and hundreds of thousands of Hams like
me, put out good signals using efficient antennas without so much as an SWR
bridge. Such exotic stuff as an SWR meter was still years in the future for
most Hams! Our rigs offered tuning controls for the output network that
allowed us to adjust them for proper mismatching, often over quite a range
of load impedances. We didn't have to worry about mismatches until the feed
line losses were so high as to throw away significant RF power. 

SWR only became really important with the advent of modern "no tune"
transmitters. These transmitters use fixed-tuned output filters that match
the impedance of the finals to 50 ohms. No tuning is required, but they
require the load impedance to be close to 50 ohms, non reactive, or they
reflect the mis-match to the finals which then run inefficiently and hot.
The output filters as well no longer do their job to reduce spurious
emissions correctly when faced with a severe impedance mis-match.

With the first no-tune rigs, Hams become understandably paranoid about low
SWR. Finals, both tube and transistors, tended let out their smoke very fast
with high SWRs. That happened a lot as Hams brought their first no-tune rig
home and hung it onto the feed line they had been using with the old,
manually tuned output rigs, never realizing the 50 ohm coax was providing a
load far removed from 50 ohms. 

The easiest, fastest solution for many Hams was to add a matching network,
an "antenna tuner", between the rig and the feed line. That's when companies
like MFJ became popular with their lineup of tuners. We were back to
twiddling knobs to adjust the output network, only now it was external to
the rig. 

And that has led to the now-common built-in "antenna tuner" such as the KAT3
that does the same thing but automatically. 

So, once again, we no longer need be concerned with SWR unless it's so high
it results in unacceptable feeder losses. 

We're right back were we started when coax first became popular in the late
1940's! The difference today is that we just push a button and hear the
relays click instead of having to know which knob to turn while watching a
meter to establish a proper match between the finals and the feed line. 

Ron AC7AC