What type of radiation pattern is created by two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase?
The correct answer is D: A figure-eight oriented along the axis of the array. Two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase create a figure-eight pattern oriented along the axis of the array. The pattern has nulls broadside and lobes along the axis.
When two antennas are fed 180 degrees out of phase (opposite phase), they cancel in the broadside direction (perpendicular to the line between them) and reinforce along the axis (in line with them). This creates a figure-eight pattern with the lobes along the axis of the array. The spacing of 1/2 wavelength and 180-degree phase difference creates this characteristic pattern. This is different from in-phase feeding, which creates a broadside pattern.
Exam Tip
1/2λ spacing, 180° phase = Figure-eight along axis. Remember: Two 1/4λ verticals, 1/2λ apart, 180° out of phase create a figure-eight pattern oriented along the array axis (end-fire).
Memory Aid
"**1**/**2**λ, **1**80° = **F**igure-**8** **A**xis (think '1/2λ,180° = F8A')"
Real-World Application
You set up two 1/4-wave verticals 1/2 wavelength apart and feed them 180 degrees out of phase. The antennas cancel in the broadside direction and reinforce along the axis, creating a figure-eight pattern with lobes along the line between the antennas. This is an end-fire array pattern.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. A cardioid pattern requires different spacing and phase (like 1/4 wavelength spacing with 90-degree phase). 1/2 wavelength with 180 degrees creates a figure-eight along the axis.
Option B: Incorrect. The pattern isn't omnidirectional. The 180-degree phase difference creates a directional figure-eight pattern.
Option C: Incorrect. A figure-eight broadside would occur with in-phase feeding, not 180 degrees out of phase. The 180-degree phase creates an end-fire pattern.
题目解析
The correct answer is D: A figure-eight oriented along the axis of the array. Two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase create a figure-eight pattern oriented along the axis of the array. The pattern has nulls broadside and lobes along the axis. When two antennas are fed 180 degrees out of phase (opposite phase), they cancel in the broadside direction (perpendicular to the line between them) and reinforce along the axis (in line with them). This creates a figure-eight pattern with the lobes along the axis of the array. The spacing of 1/2 wavelength and 180-degree phase difference creates this characteristic pattern. This is different from in-phase feeding, which creates a broadside pattern.
考试技巧
1/2λ spacing, 180° phase = Figure-eight along axis. Remember: Two 1/4λ verticals, 1/2λ apart, 180° out of phase create a figure-eight pattern oriented along the array axis (end-fire).
记忆口诀
**1**/**2**λ, **1**80° = **F**igure-**8** **A**xis (think '1/2λ,180° = F8A')
实际应用示例
You set up two 1/4-wave verticals 1/2 wavelength apart and feed them 180 degrees out of phase. The antennas cancel in the broadside direction and reinforce along the axis, creating a figure-eight pattern with lobes along the line between the antennas. This is an end-fire array pattern.
错误选项分析
Option A: Incorrect. A cardioid pattern requires different spacing and phase (like 1/4 wavelength spacing with 90-degree phase). 1/2 wavelength with 180 degrees creates a figure-eight along the axis. Option B: Incorrect. The pattern isn't omnidirectional. The 180-degree phase difference creates a directional figure-eight pattern. Option C: Incorrect. A figure-eight broadside would occur with in-phase feeding, not 180 degrees out of phase. The 180-degree phase creates an end-fire pattern.
知识点
Antenna array, 180 degrees out of phase, Figure-eight pattern, End-fire array
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.