than 1, and is written as a ratio such
as 1:1 or 1.5:1 or 3:1, and so forth.
SWR can also be calculated as
the ratio of the feed line Z0 and the
load impedance — whichever is
greater than 1. Since you already
know the feed line’s Z0 and can
measure the load’s impedance, this is
a lot more convenient than trying to
measure maximum and minimum
voltage inside the line:
Another convenient way to
measure SWR is by using forward
power (Pf) traveling from the
generator to the load and reflected
power (Pr) traveling in the opposite
direction:
Figure 3 shows a chart that
converts any combination of forward
and reflected power into an SWR
value.
Effects of SWR (Radio
and Non-Radio)
Why do we care so much about
SWR? A clue was provided earlier in
explaining that if the characteristic
impedance of a line does not match
the impedance of whatever load it is
attached to, some of the power
applied to the line is reflected at the
load. Since we would like all of our
expensive transmitter output power
to be put to work as a radiated
signal, it would be good to minimize
reflected power at the load. SWR is
just a convenient way to measure the
quality of the impedance match
between our line and the load. The
lower the SWR, the better the match!
Non-radio folks also care about
SWR — especially with regard to high
speed digital data. Data signals may
“just” be voltage levels corresponding
to 0 and 1, but it takes very high
frequency components to make the
sharp edges and narrow pulses our
designs require.
A 100 Mbit/s data stream
contains signal components in excess
of 1 GHz! At those frequencies,
every wire and PCB (printed circuit
board) signal trace has to be treated
as a type of parallel-conductor
transmission line because it is.
If the Z0 of a PCB trace does not
match the output impedance of
whatever circuit is generating the
signal or the input impedance of
whatever is receiving the signal,
severe ringing, overshoot,
undershoot, or multiple false
transitions and glitches can occur.
Search for application notes on
“signal integrity” for detailed
information about what PCB
designers must do to control the data
paths at today’s signaling rates.
Video signals — particularly
analog video — can suffer from
impedance mismatches, resulting in
ghost images and distorted pictures.
The solution is to understand when
transmission line considerations apply
and terminate the signal traces in
appropriately.
Measuring Impedance
and SWR
To make accurate measurements
at RF, instruments must be designed
for that purpose. Low frequency
multimeters simply can’t be used.
Nevertheless, inexpensive versions
are available that don’t cost a lot and
still provide useful information. You
just have to know where to shop!
SWR Meters
The most common transmission
line instrument in a ham or CB
station is the basic SWR meter shown
in Figure 4A. The meter is used by
setting the CAL control for a full-scale
reading for forward power (FWD),
then switching to the reflected (REF)
position to read SWR. The meter
sensitivity varies with frequency,
requiring readjustment when using
different bands; accuracy is low
compared to a lab instrument.
Available online and from CB shops
for under $30, these meters give a
good idea whether SWR is high or
not, and are useful in monitoring
output power while operating or
when adjusting an antenna.
More advanced meters like the
Daiwa CN-101 in Figure 4B provide
simultaneous power and SWR
measurements with a crossed-needle
meter. The unit displays both forward
and reflected power with
January 2016 17
FIGURE 4. Inexpensive SWR meters (A) are useful up to about 30 MHz at power
levels up to 100 watts. The more expensive meter (B) displays forward and reflected
power simultaneously along with SWR on a crossed-needle display.
Skin Effect
Above about 1 kHz, AC currents flow in an increasingly thin layer along the surface of
conductors. This is the skin effect ( https://en.wikipedia.org/wiki/Skin_effect). It occurs
because eddy currents inside the conductor create magnetic fields that push current to the
outer surface of the conductor. At 1 MHz in copper, most current is restricted to the
conductor’s outer 0.1 mm, and by 1 GHz, current is squeezed into a layer just a few μm thick.
Vmax SWR = Vmin
1 +√ Pr / Pf SWR = 1 - √ Pr / Pf
Z0 Zload SWR = or Zload Z0
whichever is greater than 1