Deep Dive: E9D08
The correct answer is B: SWR bandwidth decreases. As the Q of an antenna increases, the SWR bandwidth decreases. High Q means the antenna is very selective (narrow bandwidth) - it only has low SWR over a small frequency range. Q (quality factor) is related to bandwidth: Q ≈ f₀ / bandwidth, where f₀ is the resonant frequency. High Q means narrow bandwidth. An antenna with high Q has a sharp resonance - SWR is low only very close to resonance and rises quickly as you move away. This means you have less frequency range where the SWR is acceptable. Low Q antennas have wider bandwidth - they maintain good SWR over a larger frequency range. This is a fundamental trade-off in antenna design.
Why Other Answers Are Wrong
Option A: Incorrect. SWR bandwidth doesn't increase with Q - it decreases. High Q means narrow bandwidth. Option C: Incorrect. High Q doesn't reduce gain. Gain and Q are different parameters. High Q might actually be associated with certain high-gain designs, but Q itself doesn't reduce gain. Option D: Incorrect. High Q doesn't cause more common-mode current. Common-mode current is about feed line issues, not antenna Q.
Exam Tip
High Q = Decreased SWR bandwidth. Remember: As antenna Q increases, SWR bandwidth decreases. High Q means narrow bandwidth (very selective), low Q means wide bandwidth.
Memory Aid
**H**igh **Q** = **D**ecreased **B**andwidth (think 'HQ = DB')
Real-World Example
You have a high-Q antenna (maybe a loaded vertical with Q=200). The SWR bandwidth is very narrow - maybe only 10 kHz where SWR is below 2:1. You switch to a lower-Q antenna (maybe a full-size dipole with Q=20). Now the SWR bandwidth is much wider - maybe 200 kHz. High Q = narrow bandwidth, low Q = wide bandwidth.
Source & Coverage
Question Pool: 2024-2028 Question Pool
Subelement: E9D
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 E9D topic.