___________________________________________________ Principles of Transmission Lines
30 kilohertz. On the other hand, the same transmission line is considered
electrically long if it transmits a frequency of 30,000 megahertz.
3-35. 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--
Given:
v
λ=
f
Where:
λ = wavelength
v = velocity of RF in free space
f = frequency of transmission
Hz = cycles per second
Solution:
300 x 106 meters/second
λ=
30 x 103 cycles/second (Hz)
10 x 103 meters/cycle
λ=
λ=
10,000 meters, or approximately 6 miles for a
complete wavelength
3-36. Now, computing the wavelength for the line carrying 30,000
megahertz--
v
λ=
f
300 x 106 meters/second
λ=
30,000 x 106 cycles/second (Hz)
1
λ=
meter/cycle
100
λ=
.01 meter or approximately .03 foot for a
complete wavelength
3-37. 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.
3-38. 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.
3-39. However, the picture changes considerably when a long line is used.
Because most transmission lines are electrically long (because of the distance
from transmitter to antenna), the properties of such lines must be considered.
Frequently, the voltage necessary to drive a current through a long line is
3-9
Previous Page
