What is the effect of adding a terminating resistor to a rhombic or long-wire antenna?
The correct answer is B: It changes the radiation pattern from bidirectional to unidirectional. Adding a terminating resistor to a rhombic or long-wire antenna changes the radiation pattern from bidirectional to unidirectional. The resistor absorbs the backward-traveling wave, creating a unidirectional pattern.
Without termination, a long-wire antenna is bidirectional - it radiates in both directions along the wire. The terminating resistor (matched to the antenna's characteristic impedance) absorbs the wave traveling toward the end, preventing reflection. This eliminates the backward lobe, making the antenna unidirectional. The resistor must be the correct value (typically equal to the antenna's characteristic impedance) and must be able to dissipate the power. This is why terminated long-wire antennas are popular for point-to-point communications.
Exam Tip
Terminating resistor = Bidirectional to unidirectional. Remember: Adding a terminating resistor to a rhombic or long-wire antenna changes the pattern from bidirectional to unidirectional by absorbing the backward wave.
Memory Aid
"**T**erminating **R**esistor = **B**idirectional **T**o **U**nidirectional (think 'TR = BTU')"
Real-World Application
You have a long-wire antenna that radiates in both directions. You add a terminating resistor (matched to the antenna impedance) at the far end. The resistor absorbs the wave traveling toward it, preventing reflection. Now the antenna only radiates in one direction (toward the feed point) - it's unidirectional. This is useful for point-to-point communications.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. The terminating resistor doesn't reflect standing waves back. It absorbs the traveling wave, preventing reflection and eliminating standing waves.
Option C: Incorrect. The terminating resistor doesn't change polarization from horizontal to vertical. It changes the pattern from bidirectional to unidirectional, but polarization stays the same.
Option D: Incorrect. The terminating resistor doesn't decrease ground loss. It changes the radiation pattern directionality, not ground losses.
题目解析
The correct answer is B: It changes the radiation pattern from bidirectional to unidirectional. Adding a terminating resistor to a rhombic or long-wire antenna changes the radiation pattern from bidirectional to unidirectional. The resistor absorbs the backward-traveling wave, creating a unidirectional pattern. Without termination, a long-wire antenna is bidirectional - it radiates in both directions along the wire. The terminating resistor (matched to the antenna's characteristic impedance) absorbs the wave traveling toward the end, preventing reflection. This eliminates the backward lobe, making the antenna unidirectional. The resistor must be the correct value (typically equal to the antenna's characteristic impedance) and must be able to dissipate the power. This is why terminated long-wire antennas are popular for point-to-point communications.
考试技巧
Terminating resistor = Bidirectional to unidirectional. Remember: Adding a terminating resistor to a rhombic or long-wire antenna changes the pattern from bidirectional to unidirectional by absorbing the backward wave.
记忆口诀
**T**erminating **R**esistor = **B**idirectional **T**o **U**nidirectional (think 'TR = BTU')
实际应用示例
You have a long-wire antenna that radiates in both directions. You add a terminating resistor (matched to the antenna impedance) at the far end. The resistor absorbs the wave traveling toward it, preventing reflection. Now the antenna only radiates in one direction (toward the feed point) - it's unidirectional. This is useful for point-to-point communications.
错误选项分析
Option A: Incorrect. The terminating resistor doesn't reflect standing waves back. It absorbs the traveling wave, preventing reflection and eliminating standing waves. Option C: Incorrect. The terminating resistor doesn't change polarization from horizontal to vertical. It changes the pattern from bidirectional to unidirectional, but polarization stays the same. Option D: Incorrect. The terminating resistor doesn't decrease ground loss. It changes the radiation pattern directionality, not ground losses.
知识点
Terminating resistor, Long-wire antenna, Unidirectional pattern, Rhombic antenna
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.