Standing Wave Ratio (SWR)(VSWR)
To anyone who has an interest in any form of radio the term
Standing Wave Ratio (SWR) should be familiar, but what exactly is it? The
true term is Voltage Standing Wave Ratio, or VSWR but for simplicity we'll
stick to SWR.
Since SWR is most often used in radio circles as an
indication of how well your antenna system is working probably the biggest
misconception is that your SWR should be as close to 1:1 as possible,
otherwise " you won't get out very well." A 1:1 SWR implies a perfect match
between all elements of the antenna system. When the load impedance matches
the line and antenna impedance there will be no Standing Waves. The only
problem is that it is possible to have a low SWR and still have some very
serious things wrong with your antenna system. Other misconceptions such as
a high SWR causing television interference, or other unwanted problems are
often heard and can cause unnecessary worry.
So what is it?
SWR is a measure of a transceiver’s output power verses
the portion of that power reflected back to the transceiver by the antenna
system. If the antenna system is working well, most of the transmitted power
will be radiated by the antenna with very little power reflected back to the
transceiver.
If an antenna system is not working well, the power
reflected by the antenna will travel back through the transmission line and
arrive at the output of the transceiver. Modern day semiconductor
transceivers do not handle this reflected power well. If the reflected power
is sufficiently high, it can severely damage the transceiver’s power output
transistors. To avoid damage, manufacturers now design protective circuits into
the power output stage. The protective circuit reduces the transceiver’s
output until the magnitude of the reflected power is below that which would
cause damage to the transceiver. In terms of SWR ratios, transceiver’s
typically operate at their full nominal output power
at SWR values of less than 1.5:1 For SWR values greater than 1.5:1 the
transceiver’s protective circuitry reduces the power to avoid damage.
To get maximum power into an antenna the transmitter
and antenna has to have the same impedance or 'match'. As there is usually
some distance between the transmitter and antenna we require feed line to transfer the power
between them. If this feed line has no loss and is of the same impedance as
the transmitter and antenna then we should have a perfect match or 1:1 SWR and all
power will be radiated from the antenna. In practice however, this case is rarely
achieved as the feed line, be it coax or ladder line etc... will have losses
and therefore the antenna system will
always reflect some power back to the transceiver.
Many proprietary SWR meters are calibrated to read FORWARD power as
well as REFLECTED power. They may actually be measuring voltage, and
therefore
have the scales calibrated in power.
If we assume that the SWR meter contributes no errors, the
FORWARD reading is the SUM of the forward power and the reflected power. As
a result, it is greater than your actual power output. The REFLECTED power
reading is that amount of power which was not initially absorbed by the
antenna and has been sent back down the feed line.
The first number in the SWR ratio is greater than 1 in
most cases. From a practical point of view, SWR numbers in the range from
1:1 to 1.5:1 are considered to be very good, meaning that the antenna is radiating most of
the power sent to it.
The conclusion that can be drawn from this is that the
threshold SWR, as seen by the transceiver, is about 2:1. An SWR above 2:1
will result in a power loss that will be noticeable at the receiving end. An
SWR of less than 2:1 will not create a noticeable drop in power. In terms of
perfection, tuning your antenna system for an SWR of less than 1.5:1 will
result in full output power from your transceiver and negligible reflection
losses at the antenna. Spending a lot of time trying to reduce your SWR from
1.5:1 to a perfect match of 1:1 is generally not time well spent. From a
practical stand point, an SWR of 1.5:1 is indistinguishable from a perfect
match of 1:1.
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