Question
What happens to SWR bandwidth when one or more loading coils are used to resonate an electrically short antenna?
Answer Options
- A) It is increased
- B) It is decreased
- C) It is unchanged if the loading coil is located at the feed point
- D) It is unchanged if the loading coil is located at a voltage maximum point
Correct Answer: B
Explanation
An electrically short antenna exhibits a very high inherent Quality factor (Q) because its capacitance is high and its radiation resistance is low. When a loading coil is used to cancel this high capacitive reactance, the resulting resonant circuit (the antenna plus coil) also has a very high overall Q.
Since the Q of a resonant circuit is inversely proportional to its bandwidth (\Delta F \propto 1/Q), adding one or more loading coils to an electrically short antenna forces a high resonant Q, which means the SWR bandwidth is decreased. The antenna becomes highly selective and must be carefully re-tuned for relatively small changes in operating frequency.
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