Antennas and Transmission Lines Flashcards
7 cards from real GROL practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Antennas and Transmission Lines flashcards as text
What is the primary function of a balun at a dipole antenna feedpoint?
Answer: To convert between the balanced antenna and the unbalanced coaxial feedline, preventing common-mode currents on the shield
A balun (BALanced-to-UNbalanced transformer) prevents RF current from flowing on the outside of the coax shield, which would cause the feedline to radiate and distort the antenna pattern.
A quarter-wavelength matching transformer (Q-section) is used to match two impedances Z1 and Z2. What must its characteristic impedance equal?
Answer: The geometric mean: the square root of (Z1 × Z2)
A quarter-wave transformer matches two impedances when its characteristic impedance equals √(Z1 × Z2), the geometric mean, and is physically one quarter wavelength at the operating frequency.
A shorted stub connected in parallel with a transmission line at the correct position is used to:
Answer: Cancel the reactive component of the load impedance so the remaining impedance is purely resistive
A shorted stub presents a reactance equal in magnitude but opposite in sign to the load's reactive component, canceling it and leaving a purely resistive impedance at the stub junction.
How does the electrical length of a coaxial cable section compare to its physical length?
Answer: Electrical length equals the physical length divided by the velocity factor
Because signals travel slower than light inside coaxial cable (by the velocity factor), the cable appears electrically longer: electrical length = physical length ÷ velocity factor.
What condition on a transmission line produces a voltage standing wave maximum (antinode)?
Answer: Where the forward and reflected traveling waves add together in phase (constructive interference)
A voltage antinode occurs where the forward and reflected waves arrive in phase and add constructively, creating a point of maximum voltage amplitude on the standing wave pattern.
What is the characteristic impedance of typical open-wire parallel feedline (ladder line or window line)?
Answer: 300 to 600 ohms
Open-wire parallel feedline has a high characteristic impedance of 300–600 ohms, determined by conductor spacing and diameter, and offers very low loss compared to coaxial cable.
How does skin depth in a conductor change as the operating frequency is increased?
Answer: Skin depth decreases as frequency increases, concentrating current in a thinner surface layer
Skin depth decreases inversely with the square root of frequency, so at higher frequencies current is confined to a progressively thinner surface layer, raising effective conductor resistance.