Antennas and Feed Lines Flashcards

What is the characteristic impedance of open-wire (ladder) line typically used with balanced antenna tuners?

Answer: 300 to 600 ohms

Open-wire or ladder line has a characteristic impedance typically ranging from 300 to 600 ohms depending on conductor spacing and diameter.

What is the effect of adding a director element to a Yagi antenna in front of the driven element?

Answer: It increases the front-to-back ratio and forward gain

A director, being slightly shorter than the driven element, re-radiates energy forward, increasing forward gain and front-to-back ratio.

What is a loading coil used for in a shortened mobile HF antenna?

Answer: To add inductance and resonant the electrically short antenna

A loading coil adds the inductance needed to resonate an electrically short antenna at the desired operating frequency.

What is the input impedance of a quarter-wave matching section (Q-section) used to transform a 200-ohm antenna to 50 ohms?

Answer: 100 ohms

The characteristic impedance of a quarter-wave matching transformer equals the geometric mean: √(50 × 200) = 100 ohms.

What is the approximate gain of a three-element Yagi antenna relative to a dipole?

Answer: 5 dBd

A well-designed three-element Yagi typically achieves approximately 5 dBd (5 dB gain over a dipole) of forward gain.

Which transmission line type has the lowest loss per unit length at VHF frequencies?

Answer: Open-wire ladder line

Open-wire ladder line has the lowest loss per unit length because its low-density dielectric minimizes dielectric losses, especially at VHF.

What is a collinear antenna array designed to achieve?

Answer: High directivity toward the horizon by stacking vertical elements in phase

A collinear array stacks vertical elements fed in phase, compressing the radiation pattern toward the horizon for increased omnidirectional gain.