Amateur Extra E9F: Transmission line behavior
These are the 12 official questions in group E9F of the 2024–2028 Amateur Extra question pool, with the correct answer marked and a plain-English explanation of why it's right. The real exam asks one question from this group.
- E9F01Question 1
What is the velocity factor of a transmission line?
- AThe ratio of its characteristic impedance to its termination impedance
- BThe ratio of its termination impedance to its characteristic impedance
- CThe velocity of a wave in the transmission line multiplied by the velocity of light in a vacuum
- DThe velocity of a wave in the transmission line divided by the velocity of light in a vacuum Correct
Why: Velocity factor = speed in the line ÷ speed of light in a vacuum.
- E9F02Question 2
Which of the following has the biggest effect on the velocity factor of a transmission line?
- AThe characteristic impedance
- BThe transmission line length
- CThe insulating dielectric material Correct
- DThe center conductor resistivity
Why: The dielectric has the biggest effect on velocity factor.
- E9F03Question 3
Why is the electrical length of a coaxial cable longer than its physical length?
- ASkin effect is less pronounced in the coaxial cable
- BSkin effect is more pronounced in the coaxial cable
- CElectromagnetic waves move faster in coaxial cable than in air
- DElectromagnetic waves move more slowly in a coaxial cable than in air Correct
Why: Waves move slower in coax than in air, so it's electrically longer.
- E9F04Question 4
What impedance does a 1/2-wavelength transmission line present to an RF generator when the line is shorted at the far end?
- AVery high impedance
- BVery low impedance Correct
- CThe same as the characteristic impedance of the line
- DThe same as the output impedance of the RF generator
Why: A shorted half-wave line repeats the short: very low impedance.
- E9F05Question 5
What is microstrip?
- ASpecial shielding material designed for microwave frequencies
- BMiniature coax used for low power applications
- CShort lengths of coax mounted on printed circuit boards to minimize time delay between microwave circuits
- DPrecision printed circuit conductors above a ground plane that provide constant impedance interconnects at microwave frequencies Correct
Why: Microstrip is precise PC traces over a ground plane at microwave frequencies.
- E9F06Question 6
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?
- A7.0 meters
- B8.5 meters
- C10.6 meters Correct
- D13.3 meters
Why: λ = 300 ÷ 14.1 ≈ 21.3 m; half is about 10.6 m.
- E9F07Question 7
How does parallel conductor transmission line compare to coaxial cable with a plastic dielectric?
- ALower loss Correct
- BHigher SWR
- CSmaller reflection coefficient
- DLower velocity factor
Why: Parallel-conductor line has lower loss than plastic-dielectric coax.
- E9F08Question 8
Which of the following is a significant difference between foam dielectric coaxial cable and solid dielectric coaxial cable, assuming all other parameters are the same?
- AFoam dielectric coaxial cable has lower safe maximum operating voltage
- BFoam dielectric coaxial cable has lower loss per unit of length
- CFoam dielectric coaxial cable has higher velocity factor
- DAll these choices are correct Correct
Why: Foam differs from solid dielectric in several ways. All the choices are correct.
- E9F09Question 9
What impedance does a 1/4-wavelength transmission line present to an RF generator when the line is shorted at the far end?
- AVery high impedance Correct
- BVery low impedance
- CThe same as the characteristic impedance of the transmission line
- DThe same as the generator output impedance
Why: A shorted quarter-wave line looks like a very high impedance.
- E9F10Question 10
What impedance does a 1/8-wavelength transmission line present to an RF generator when the line is shorted at the far end?
- AA capacitive reactance
- BThe same as the characteristic impedance of the line
- CAn inductive reactance Correct
- DZero
Why: A shorted eighth-wave line looks inductive.
- E9F11Question 11
What impedance does a 1/8-wavelength transmission line present to an RF generator when the line is open at the far end?
- AThe same as the characteristic impedance of the line
- BAn inductive reactance
- CA capacitive reactance Correct
- DInfinite
Why: An open eighth-wave line looks capacitive.
- E9F12Question 12
What impedance does a 1/4-wavelength transmission line present to an RF generator when the line is open at the far end?
- AThe same as the characteristic impedance of the line
- BThe same as the input impedance to the generator
- CVery high impedance
- DVery low impedance Correct
Why: An open quarter-wave line looks like a very low impedance.