short after
all.
You can fit three complete wavelengths on a foot of
transmission
line because the wavelength of 3000 megahertz is approximately
4 inches.
Notice that the voltage changes along the line because the
transmission
line is long compared to the 3000 megahertz wavelength.
Figure 2.
Transmission Lines can be LONG or SHORT.
4.
When is a transmission line short?
A
transmission line is called short when its length is short
compared to the wavelength of the source frequency.
Let's use our 1-foot
transmission line again. This time let's couple the line to a source whose
frequency is 60 hertz, as shown in Part B of Figure 2. The wavelength of 60
hertz is approximately 3,100 miles.
Notice that the voltage is constant
because the transmission line is so short.
5.
We are concerned with long lines.
In this learning event we will concern ourselves only with long
lines.
By long, of course, we are referring to electrical length of the
transmission line, not its physical length.
Where is the dividing point
between long and short? No definite value can be given because a line does
not change suddenly from short to long. As a rough approximation, consider
a line shorter than one sixteenth wavelength as a short line. Keep in mind,
however, that we mean electrical length when we say a line is short or long.
Actually, a transmission line only a few inches in length may be a long line
at ultra-high frequencies.
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