3-9
A transmission line is electrically long when its physical length is long compared to a quarter-
wavelength of the energy it is to carry. You must understand that the terms "short" and "long" are relative
ones. For example, a line that has a physical length of 3 meters (approximately 10 feet) is considered
quite short electrically if it transmits a radio frequency of 30 kilohertz. On the other hand, the same
transmission line is considered electrically long if it transmits a frequency of 30,000 megahertz.
To show the difference in physical and electrical lengths of the lines mentioned above, compute the
wavelength of the two frequencies, taking the 30-kilohertz example first:
Now, computing the wavelength for the line carrying 30,000 megahertz:
Thus, you can see that a 3-meter line is electrically very short for a frequency of 30 kilohertz. Also,
the 3-meter line is electrically very long for a frequency of 30,000 megahertz.
When power is applied to a very short transmission line, practically all of it reaches the load at the
output end of the line. This very short transmission line is usually considered to have practically no
electrical properties of its own, except for a small amount of resistance.
