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