*Question E9D12:
What is the purpose of making a Yagi’s parasitic elements either longer or shorter than resonance?*  

*Correct Answer is  C: Control of phase shift*  

### AI Response: (may contain errors)  

To understand why the correct answer is "Control of phase shift," let's break down the role of parasitic elements in a Yagi antenna:

1. **Purpose of Parasitic Elements**:  
   - In a Yagi antenna, parasitic elements (directors and reflectors) are used to shape the radiation pattern and increase gain.
   - These elements are not directly connected to the feedline but interact with the driven element through electromagnetic coupling.

2. **Control of Phase Shift**:  
   - By making parasitic elements either longer or shorter than their resonant length, you can control the phase of the currents induced in them.
   - **Reflectors** are typically longer than the resonant length, causing them to have a lagging phase shift, which reflects energy back towards the driven element.
   - **Directors** are shorter than the resonant length, causing a leading phase shift, which directs energy forward, enhancing gain in that direction.

3. **Why Other Choices Are Incorrect**:
   - **A: Wind torque cancellation**: This is unrelated to the electrical function of the elements. Wind torque is a mechanical consideration, not an electromagnetic one.
   - **B: Mechanical balance**: While mechanical balance is important for antenna stability, it is not the reason for adjusting the length of parasitic elements.
   - **D: Minimize losses**: Loss minimization is not directly achieved by adjusting the length of parasitic elements. Losses are more related to material properties and construction quality.

In summary, adjusting the length of parasitic elements in a Yagi antenna is primarily for controlling the phase shift of induced currents, which in turn shapes the radiation pattern and improves antenna performance.