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2016 Extra Class Study Guide: E9F - Transmission lines

Posted: 04 Mar 2016 07:50 AM PST
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E9F Transmission lines: characteristics of open and shorted feed lines;
1/8 wavelength; 1/4 wavelength; 1/2 wavelength; feed lines: coax versus
open-wire; velocity factor; electrical length; coaxial cable dielectrics;
velocity factor

The physical length of a coaxial cable transmission line is shorter than
its electrical length because electrical signals move more slowly in a
coaxial cable than in air. (E9F03) The term we use to quantify the
difference in how fast a wave travels in air versus how fast it travels in
a feedline is velocity factor.

The velocity factor of a transmission line is the velocity of the wave in
the transmission line divided by the velocity of light in a vacuum. (E9F01)
Put another way, velocity factor is the term for the ratio of the actual
speed at which a signal travels through a transmission line to the speed of
light in a vacuum. (E9F08) The dielectric materials used in the line
determines the velocity factor of a transmission line. (E9F02)

The typical velocity factor for a coaxial cable with solid polyethylene
dielectric is 0.66. (E9F04) That makes the approximate physical length of a
solid polyethylene dielectric coaxial transmission line that is
electrically one-quarter wavelength long at 14.1 MHz about 3.5 meters.
(E9F05)The approximate physical length of a solid polyethylene dielectric
coaxial transmission line that is electrically one-quarter wavelength long
at 7.2 MHz is 6.9 meters. (E9F09)

The velocity factor of air-insulated, parallel conductor transmission lines
is a lot closer to 1 than the velocity factor for coaxial cable. The
approximate physical length of an air-insulated, parallel conductor
transmission line that is electrically one-half wavelength long at 14.10
MHz is 10 meters. (E9F06)

While having a higher velocity factor is not really such a big advantage,
open-wire or ladder line feedlines do have other advantages. For example,
ladder line has lower loss than small-diameter coaxial cable such as RG-58
at 50 MHz. (E9F07)

Sometimes we use various lengths of coax to match an antenna system or to
filter out frequencies. A 1/8-wavelength transmission line presents an
inductive reactance to a generator when the line is shorted at the far end.
(E9F10) A 1/8-wavelength transmission line presents a capacitive reactance
to a generator when the line is open at the far end. (E9F11)

A 1/4-wavelength transmission line presents a very low impedance to a
generator when the line is open at the far end. (E9F12) A 1/4-wavelength
transmission line presents a very high impedance to a generator when the
line is shorted at the far end. (E9F13)

A 1/2-wavelength transmission line presents a very low impedance to a
generator when the line is shorted at the far end. (E9F14) A 1/2-wavelength
transmission line presents a very high impedance to a generator when the
line is open at the far end. (E9F15)

All of these choices are correct when talking about significant differences
between foam-dielectric coaxial cable and solid-dielectric cable, assuming
all other parameters are the same (E9F16):

Foam dielectric has lower safe operating voltage limits
Foam dielectric has lower loss per unit of length
Foam dielectric has higher velocity factor


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