*Question E9D09:
What is the function of a loading coil in an electrically short antenna?*  

*Correct Answer is  D: To resonate the antenna by cancelling the capacitive reactance*  

### AI Response: (may contain errors)  

A loading coil is used in an electrically short antenna to make it resonate at the desired frequency. Here's how it works:

### Correct Answer: D
- **To resonate the antenna by cancelling the capacitive reactance**:  
  An electrically short antenna typically has capacitive reactance because it is shorter than a quarter wavelength. A loading coil, which is an inductor, is added to the antenna to introduce inductive reactance. The purpose is to cancel out the capacitive reactance, thereby bringing the antenna to resonance. At resonance, the reactance is zero, and the antenna can efficiently radiate or receive signals.

### Why Other Choices Are Incorrect:
- **A: To increase the SWR bandwidth by increasing net reactance**:  
  This is incorrect because increasing net reactance would not improve the SWR (Standing Wave Ratio) bandwidth. In fact, the goal is to reduce reactance to zero at resonance, not increase it.

- **B: To lower the losses**:  
  While a loading coil can help the antenna operate more efficiently by achieving resonance, its primary function is not to lower losses. In fact, adding a coil can introduce some additional losses due to its resistance.

- **C: To lower the Q**:  
  The Q factor, or quality factor, of an antenna is related to its bandwidth. A loading coil does not inherently lower the Q; it is used to achieve resonance. Lowering the Q would actually broaden the bandwidth, which is not the primary function of a loading coil.

### Useful Formula:
- **Resonance Condition**:  
  At resonance, the total reactance $X$ is zero:  
  $$X = X_L + X_C = 0$$  
  where:
  - $X_L$ is the inductive reactance of the loading coil
  - $X_C$ is the capacitive reactance of the antenna

By adding a loading coil with the appropriate inductive reactance, you can cancel out the capacitive reactance and achieve resonance.