The Birds and The Bees of SWR Knowledge Part 2

     The operative part of this first part of a previous article/reminder I
sent is that you calculate coaxial cable loss by reading the open or short
ended cable's SWR.  If the cable is lossy, power is lost going up to the
antenna, and in a short or open -ended cable all the power that arrives is
reflected back.  Low loss (in dB) going up the (short?, good quality) coax
cable will have low loss coming back as reflected power, ..., and more
reflected power means to be measured at the input end = higher SWR.  In
this case, a high SWR is a good  thing, a high SWR indicating low losses.

     If, on the other hand,
Understanding SWR by Example

Take the mystery and mystique out of standing wave ratio.
Darrin Walraven, K5DVW

From November 2006 QST © ARRL

It sometimes seems that one of
the most mysterious creatures
in the world of Amateur Radio is
standing wave ratio (SWR). I
often hear on-air discussion of guys bragging
about and comparing their SWR numbers
as if it were a contest. There seems
to be a relentless drive to achieve the most
coveted 1:1 SWR at any cost. But why? This
article is written to help explain what SWR
actually is, what makes it bad and when to
worry about it.
What is SWR?
SWR is sometimes called VSWR, for
voltage standing wave ratio, by the technical
folks. Okay, but what does it really mean?
The best way to easily understand SWR is by
example. In the typical ham station setup, a
transmitter is connected to a feed line, which
is then connected to the antenna. When you
key the transmitter, it develops a radio frequency
(RF) voltage on the transmission
line input. The voltage travels down the feed
line to the antenna at the other end and is
called the forward wave. In some cases, part
of the voltage is reflected at the antenna and
propagates back down the line in the reverse
direction toward the transmitter, much like
a voice echoing off a distant cliff. SWR is a
measure of what is happening to the forward
and reverse voltage waveforms and how they
compare in size.
Let’s look at what happens when a transmitter
is connected to 50 Ω coax and a 50 Ω
antenna. For now, pretend that the coax cable
doesn’t have any losses and the transmitter
is producing a 1 W CW signal. If you were
to look at the signal on the output of the
transmitter with an oscilloscope, what you
would see is a sine wave. The amplitude of
the sine wave would be related to how much
power the transmitter is producing. A larger
amplitude waves means more power. This
wave of energy travels down the transmission
line and reaches the antenna. If the antenna
impedance is 50 Ω, just like the cable, then
all of the energy is transferred to the antenna
system to be radiated. Anywhere on the
transmission line you measured, the voltage
waveform would measure exactly the same
as the sine wave coming from the transmitter.
This is called a matched condition and is what

Take the mystery and mystique out of standing wave ratio.
Darrin Walraven, K5DVW
Table 1
SWR vs Reflected Voltage or Power

VSWR            Voltage                 Power
                Reflected (%)   Reflected (%)
1.0:1           0               0
1.1:1           5               0.2
1.2:1           9               0.8
1.3:1           13              1.7
1.4:1           17              2.8
1.5:1           20              4
1.6:1           23              5.3
1.7:1           26              6.7
1.8:1           29              8.2
1.9:1           31              9.6
2.0:1           33              11
2.5:1           43              18.4
3.0:1           50              25
4.0:1           56              36
5.0:1           67              44.4
10.0:1          82              67

     So, a high SWR into your open-ended (new or old) coaxial cable
indicates most or "all the power reflected" is making it back to the SWR
meter to be read has a high SWR, which means low loss.  If very little
reflected power (or none of it) makes it back to the SWR meter, it reads a
low SWR, but means the cable is very lossy.  Tomorrow, I'll find the "SWR
in dB Loss" chart and give the exact values.