Thursday, 1 August 2013

Standing Wave Ratio

Recently heard on the 2 meter band: "My Standing Wave Ratio currently shows
a 1 to 1.5 . "Now, an antennetuner is really required to improve my antenna
network in order to obtain a better value!". In case the OM realizes his idea,
his station will produce a weaker instead of stronger RF signal. In fact, the loss
at s=1.5 is 0.18 dB. With the incorporation of an antennetuner or other
antenna adaptation circuit an additional loss of 1 dB will be added.
At least remarkable !
When explaining the theory related to the "Standing Wave Ratio", authors
often use nice graphs e.g. showing the radiated and reflected power versus
the SWR. Of course those relations do make sense, however the question
"What does this mean for my transmitted signal at the receiving station?"
is neglected in most cases. The graph shown in this article shows the
relation between losses (indicated as a in dB) and the SWR (indicated as s).
By the way, the graph is a presentation of the formula:
a = -10.log (4s/(s+1)2)
Now take a look at the graph:
A power loss of 1 dB appears at a SWR of s=2,7 (!) A 1 dB difference is
hardly recognizable in the HAM radio bands during regular radio traffic
as we use it. The difference between signals at a perfect SWR (s=1) and
a value between s=1 and s=2 cannot be recognized. Also between s=2
and s=3 the antenna system performs fine. A higher degree of awareness
might be necessary when the SWR exceeds s=3. Now the amount of
reflected power becomes serious. We do not want to operate a chronic
overloaded RF Power Amplifier. Another observation: in order to lower the
S-meter deflection at the receiving station by 1 S-point (which is equivalent
to a power loss of 6 dB) we really have to force things into the wrong direction:
Starting at s=14 (!!) we can see the S-meter at the receiving station moving
to a 1-point-lower S-value. Very important when signals just above the
noise level are observed of course, but when taking regular transmissions
into consideration it hardly does make sense. In fact the accuracy offered
by most SWR meters is far too good. A device with e.g. 3 LEDs should do
fine; s smaller than 2 (green), smaller than 3 (yellow ) and greater than 3
(red) provides sufficient information. However, e.g. antenna builders do
use the accuracy of one number after the dot.



Example of a simple, 1 LED, SWR indicator (according to the balanced-bridge principle)

More important than getting a better SWR, is improving the attenuation
of the antenna (coaxial) cables. In the case your SWR measuring device
shows a value of 1 to 2 (or better) and you still want to improve your
station’s performance, you better spend your money and time on improving
your antenna cabling and stay away from your aerial !

Source: (1) FUNK amateur November 2002, seite 1155 (2) Rothammels Antennenbuch, A. Krichke

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