AMATEUR RADIO
By GARRY CRATT, VK2YBX
SWR
-
just how important is it?
Do you worry about the SWR (standing wave ratio)
of the antenna system for amateur or CB radio
transmitters? Maybe you have nothing to worry
about. After all, what is really important is just
how efficiently the antenna radiates your signal.
MODEL 111
One of the most common misconceptions held by communications enthusiasts and amateur
operators alike is the importance
above all else of antenna SWR.
Most operators agree that a standing wave ratio of 1:1 is the
ultimate goal when it comes to
antenna performance but is it real1y? A resistive dummy load
presents that ideal ratio but it is
lousy as a radiator of RF power.
To understand the importance
(or unimportance) of SWR, one
needs to think back to some basic
antenna theory. Without going into
the complicated mathematical
analysis, a few facts can be stated.
Firstly, the load for any transmission line, be it open wire feeder or
coaxial cable, can be any device
capable of dissipating RF energy.
This load is the only factor that
determines the standing wave ratio
which exists on the line. If, in the
case of a line having a characteristic impedance of 500, the
load is a 500 resistor, there will be
no standing waves. If the load is not
equal to the characteristic impedance of the line, then there will
be standing waves.
Secondly, there is nothing that
can be done at the transmitter end
of a feed line to change the SWR
which exists on any particular
transmission line. This includes
making the transmission line longer
or shorter!
Standing waves
Basically, standing waves are the
distribution of voltage and current
along the transmission line, and the
subsequent interference pattern
formed by the superposition of the
forward and reflected waves.
Under normal conditions we
refer to VSWR, or voltage standing
wave ratio, simply because it is
easier to make a probe that samples
the electric field in the line than it is
to make a shielded loop that
samples the magnetic field. In any
case, the voltage and current
distributions have exactly the same
shape. They vary only in position,
as the voltage and current standing
Fig.1: the voltage and current
standing waves in a transmission
cable are displaced relative to each
other by a quarter wavelength.
V
84
SILICON CHIP
I
This handy instrument from Dick
Smith Electronics can measure SWR,
RF power & field strength. It is
installed in series with the antenna
feedline.
waves are displaced relative to
each other by a quarter wavelength
(see Fig.1).
If the VSWR is to be made as low
as possible, it is essential that the
load, at the point of connection to
the transmission line, be made as
close to a purely resistive value as
possible. This means that the load
must be resonant at the operating
frequency.
From basic theory, we know that
the nominal impedance of a half
wave dipole is in the vicinity of 700
or so. We also know that a folded
dipole has an impedance of four
times that value (ie, 2800), so it is
relatively easy to use a suitable
feed system having a similar
characteristic impedance.
At VHF and UHF, things get a bit
more difficult, in that theory says
that the characteristic impedance
of a quarter wave groundplane is
around 200, which is quite a poor
match if using (as most amateurs
do) 500 coaxial cable. However, we
also know that a quarter wave
ground plane antenna has quite
useable gain and can provide quite
reasonable results due to its
relatively low angle of radiation.
So while the antenna presents a
poor match to 500 cable, it has
quite good performance as a
radiator.
Theory says that we should try to
match the antenna impedance to
the line impedance, thus eliminating standing waves. This could be
done by using a quarter wave
transformer or a matching circuit
using inductors and capacitors.
However the practicality and the
necessity to go to this trouble can
readily be overlooked, once we
understand more about the consequences of a less than ideal SWR.
In practice, most transmitter output stages are designed to have a
very low source impedance. This is
done to minimise the amount of
energy dissipated in the output
stage itself. Inevitably, this means
that there is quite a mismatch at the
transmitter end of the transmission
line, causing reflected "waves" of
RF energy.
Many amateurs are under the
misapprehension that this energy is
lost, wasted by heating the feedline.
In fact, energy which is reflected
back down the transmission line by
an ante.nna mismatch is rereflected by the transmitter output
stage/feedline mismatch and reappears at the antenna where it is
radiated.
So in practice, as long as the
transmitter is stable and not likely
to be damaged or subject to
overheating due to line mismatch,
the effect of a high VSWR will have
no affect on the radiated power of
the antenna system. In fact, the only affect that should be taken into
account is the propagation delay
between the initial wavefront and
subsequent reflected waves.
In particular, amateur television
operators do need to pay attention
to antenna and feedline characteristics and construction to avoid the
transmission of multiple images or
ghosts.
Measuring VSWR
When it comes to measuring
VSWR, the most accurate method is
to measure both the forward and
reflected transmitter power with
an insertion type wattmeter and
calculate the VSWR from Fig.2a &
Fig.2b. Precisely where the watt-
"'
1· 0
1- 5 2-0
3 -0 4 -0 5 ·0
100
10
15 20 2 530 4 0 50
Fo rward po wer (Wa t ts }
(a )
15 0 200
300 4 00 500
Fig.2a: this graph plots the VSWR for measured reverse powers between .01 W
and 1W. A less than ideal VSWR is generally of little consequence.
20
1B
16
14
12
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10
B
6
4
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1· 0
1·5 2·0
3·0 4 -0 5 ·0
10
15
20 2 5 30
40 50
100
150 200
3 004 00 500
For ward p o w er (Watt s)
Fig.2b: this graph can be used for reverse powers up to 20W. The VSWR
indicates whether the transmitter is being presented with a satisfactory load.
meter should be placed is another
point on which there are several
theories.
Suffice to say, the most convenient place to measure forward and
reflected power is at the transmitter, even though it can be argued
that cable losses will cause the
reflected power to be indicated at a
level lower than actually present.
The most accurate results will be
obtained by measuring the ratio using the shortest possible feedline,
even if this involves moving the
transmitter close to the antenna
during the antenna set-up.
So VSWR is really an indication
as to whether or not the transmitter is being presented with a
satisfactory load. While this is an
important consideration, it has no
bearing upon the performance of
the antenna system.
Further reading
(1). VHF/UHF Manual; G.R. Jessop;
published by the RSGB.
(2). The ARRL Handbook; published
by the ARRL.
(3). Handbook of Coaxial Microwave Measurements; published by
General Radio.
OCT0BER1989
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