Question
What is the function of a loading coil in an electrically short antenna?
Answer Options
- A) To increase the SWR bandwidth by increasing net reactance
- B) To lower the losses
- C) To lower the Q
- D) To resonate the antenna by cancelling the capacitive reactance
Correct Answer: D
Explanation
An electrically short antenna is shorter than its natural half-wave or quarter-wave resonance. This shortness means the antenna exhibits a net capacitive reactance (Z_A = R - jX_C) at the operating frequency. This reactance causes a high Standing Wave Ratio (\text{SWR}) on the feed line.
To make the antenna electrically full-sized and achieve resonance (X=0), a loading coil (an inductor) must be added. The function of the loading coil is to resonate the antenna by cancelling the capacitive reactance. The coil’s inductive reactance (X_L) is chosen to exactly equal the antenna’s capacitive reactance (X_C), resulting in a purely resistive feed point impedance.
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