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Ac Applied to a Transmission Line | Up
Electronics Technician Volume 07-Antennas and Wave Propagation | Next
Shielded Pair |
conditions. Maximum power is absorbed by the load
when ZL = Z0. If a line has no standing waves, the
termination for that line is correct and maximum power
transfer takes place.
You have probably noticed that the variation of
standing waves shows how near the rf line is to being
terminated in Z0. A wide variation in voltage along
the length means a termination far from Z0. A small
variation means termination near Z0. Therefore, the
ratio of the maximum to the minimum is a measure
of the perfection of the termination of a line. This
ratio is called the STANDING-WAVE RATIO (swr)
and is always expressed in whole numbers.
For
example, a ratio of 1:1 describes a line terminated in
its characteristic impedance (Z0).
Voltage Standing-Wave Ratio
The ratio of maximum voltage to minimum voltage
on a line is called the VOLTAGE STANDING-WAVE
RATIO (vswr). Therefore:
The vertical lines in the formula indicate that the
enclosed quantities are absolute and that the two values
are taken without regard to polarity, Depending on
the nature of the standing waves, the numerical value
of vswr ranges from a value of 1 (ZL = Z0, no standing
waves) to an infinite value for theoretically complete
reflection. Since there is always a small loss on a
line, the minimum voltage is never zero and the vswr
is always some finite value. However, if the vswr
is to be a useful quantity. the power losses along the
line must be small in comparison to the transmitted
power.
voltage. Since power is proportional to the square
of the voltage, the ratio of the square of the maximum
and minimum voltages is called the power stand-
ing-wave ratio. In a sense, the name is misleading
because the power along a transmission line does not
vary.
Current Standing-Wave Ratio
The ratio of maximum to minimum current along
a transmission line is called CURRENT STAND-
ING- WAVE RATIO (iswr). Therefore:
This ratio is the same as that for voltages. It can be
used where measurements are made with loops that
sample the magnetic field along a line. It gives the
same results as vswr measurements.
TRANSMISSION MEDIUMS
The Navy uses many different types of TRANS-
MISSION MEDIUMS in its electronic applications.
Each medium (line or waveguide) has a certain
characteristic impedance value, current-carrying
capacity, and physical shape and is designed to meet
a particular requirement.
The five types of transmission mediums that we
will discuss in this topic include PARALLEL-LINE,
TWISTED PAIR, SHIELDED PAIR, COAXIAL
LINE, and WAVEGUIDES. The use of a particular
line depends, among other things, on the applied
frequency, the power-handling capabilities, and the
type of installation.
Power Standing-Wave Ratio
Parallel Line
The square of the vswr is called the POWER
One type of parallel line is the TWO-WIRE OPEN
STANDING-WAVE RATIO (pswr). Therefore:
LINE, illustrated in figure 3-9. This line consists of
two wires that are generally spaced from 2 to 6 inches
apart by insulating spacers. This type of line is most
often used for power lines, rural telephone lines, and
telegraph lines. It is sometimes used as a transmission
This ratio is useful because the instruments used to
line between a transmitter and an antenna or between
detect standing waves react to the square of the
an antenna and a receiver. An advantage of this type
3-6
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