When setting up your amateur radio station, it’s important to know the characteristics of the feed lines you use in your antenna system. For example, did you know that the physical length of a coaxial cable transmission line is shorter than its electrical length? The reason for this is that electrical signals move more slowly in a coaxial cable than in air.
QUESTION: Why is the physical length of a coaxial cable transmission line shorter than its electrical length? (E9F03)
ANSWER: Electrical signals move more slowly in a coaxial cable than in air
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. The dielectric materials used in the transmission line is one of the biggest factors that determine the velocity factor of a transmission line.
QUESTION: What is the velocity factor of a transmission line? (E9F01)
ANSWER: The velocity of the wave in the transmission line divided by the velocity of light in a vacuum
QUESTION: Which of the following has the biggest effect on the velocity factor of a transmission line? (E9F02)
ANSWER: Dielectric materials used in the line
Here are some typical velocity factors:
Solid polyethylene dielectric coaxial transmission line: 0.66Foam polyethylene dielectric coaxial transmission line: 0.8Air-insulated, parallel-conductor, or open-wire, feedline: 0.98
QUESTION: What is the approximate physical length of a solid polyethylene dielectric coaxial transmission line that is electrically 1/4 wavelength long at 14.1 MHz? (E9F05)
ANSWER: 3.5 meters
A 1/4-wavelength at 14.1 MHz is approximately 5.3 m, but the velocity factor of a solid polyethylene dielectric coaxial transmission line is about 0.66, so the physical length will be 5.3 m x 0.66, which is 3.5 meters.
QUESTION: What is the approximate physical length of a foam polyethylene dielectric coaxial transmission line that is electrically 1/4 wavelength long at 7.2 MHz? (E9F09)
ANSWER: 8.3 meters
A 1/4-wavelength at 7.2 MHz is approximately 10.4 m, but the velocity factor of a foam polyethylene dielectric transmission line is about 0.8, so the physical length will be 10.4 m x 0.8, which is 8.3 meters.
QUESTION: What is the approximate physical length of an air-insulated, parallel conductor transmission line that is electrically 1/2 wavelength long at 14.10 MHz? (E9F06)
ANSWER: 10.6 meters
A 1/2-wavelength at 14.1 MHz is approximately 10.63 m, but the velocity factor of an air-insulated, parallel conductor transmission line is nearly 1.0, so the physical length will be nearly equal to the electrical length.
In general, coaxial cable transmission lines with a foam dielectric have a higher velocity factor than coaxial cables with a solid dielectric. There are other differences, too. A coaxial cable with a foam dielectric has lower safe operating voltage limits and lower loss per unit of length than a coaxial cable with a solid dielectric.
QUESTION: Which of the following is a significant difference between foam dielectric coaxial cable and solid dielectric cable, assuming all other parameters are the same? (E9F08)
ANSWER: All these choices are correct
Foam dielectric has lower safe operating voltage limitsFoam dielectric has lower loss per unit of lengthFoam dielectric has higher velocity factor
Arguably, feed line loss is one of the most important characteristic of a transmission line. Obviously, the lower the feed line loss, the stronger the signal your antenna will radiate. Most amateurs use coaxial cable for antenna feed lines, but you should also consider open-wire feedlines or ladder lines. These feed lines have lower losses than most coaxial cable, and certainly have lower loss than small-diameter coaxial cable such as RG-58 at high frequencies.
QUESTION: How does ladder line compare to small-diameter coaxial cable such as RG-58 at 50 MHz? (E9F07)
ANSWER: Lower loss
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, and this property could be used match a capacitive load to a transmitter. A 1/8-wavelength transmission line presents a capacitive reactance to a generator when the line is open at the far end. This property could be used to match an inductive load.
QUESTION: What impedance does a 1/8-wavelength transmission line present to a generator when the line is shorted at the far end? (E9F10)
ANSWER: An inductive reactance
QUESTION: What impedance does a 1/8-wavelength transmission line present to a generator when the line is open at the far end? (E9F11)
ANSWER: A capacitive reactance
A length of transmission line has very different characteristics, depending on whether or not the line is open or shorted at the far end. A 1/4-wavelength transmission line presents a very low impedance to a generator when the line is open at the far end. A 1/4-wavelength transmission line presents a very high impedance to a generator when the line is shorted at the far end. On the other hand, a 1/2-wavelength transmission line that is shorted at the far end presents a very low impedance.
QUESTION: What impedance does a 1/4-wavelength transmission line present to a generator when the line is open at the far end? (E9F12)
ANSWER: Very low impedance
QUESTION: What impedance does a 1/4-wavelength transmission line present to a generator when the line is shorted at the far end? (E9F13)
ANSWER: Very high impedance
QUESTION: What impedance does a 1/2-wavelength transmission line present to a generator when the line is shorted at the far end? (E9F04)
ANSWER: Very low impedance