Deep Dive: E9C06
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.
Why Other Answers 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.
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 Example
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.
Source & Coverage
Question Pool: 2024-2028 Question Pool
Subelement: E9C
Reference: FCC Part 97.3
Key Concepts
Verified Content
Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E9C topic.