*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)  

The correct answer to the question about the purpose of making a Yagi’s parasitic elements either longer or shorter than resonance is:

**C: Control of phase shift**

### Explanation of the Correct Answer:
- **Control of phase shift**: In a Yagi antenna, the parasitic elements (reflector and directors) are not directly connected to the feedline but influence the radiation pattern of the antenna. By adjusting the length of these elements, you can control the phase relationship between the driven element and the parasitic elements. This phase shift is crucial for optimizing the antenna's directivity and gain. Longer elements tend to resonate at lower frequencies, while shorter elements resonate at higher frequencies, allowing for fine-tuning of the antenna's performance.

### Why the Other Choices are Incorrect:
- **A: Wind torque cancellation**: While wind torque is a consideration in antenna design, the length of parasitic elements does not specifically address this issue. Wind torque is more related to the overall structure and mounting of the antenna rather than the tuning of parasitic elements.

- **B: Mechanical balance**: Mechanical balance refers to the physical stability of the antenna structure. Adjusting the lengths of parasitic elements does not inherently provide balance; rather, it is the overall design and mounting that ensure stability against environmental factors.

- **D: Minimize losses**: While minimizing losses is important in antenna design, the length of parasitic elements primarily affects the phase and impedance characteristics rather than directly minimizing losses. Losses are more related to the quality of materials and connections used in the antenna.

### Useful Formulas or Rules of Thumb:
- The resonant frequency of a dipole antenna can be approximated by the formula:
  
  $$ f_r = \frac{c}{L} $$

  where:
  - $f_r$ = resonant frequency in hertz
  - $c$ = speed of light (approximately $3 \times 10^8$ m/s)
  - $L$ = length of the antenna in meters

- For Yagi antennas, the typical lengths of the elements can be adjusted by about 5% to 10% from their resonant lengths to achieve the desired phase shift and gain characteristics.

This understanding of how parasitic elements affect phase shift is crucial for optimizing Yagi antenna performance in amateur radio applications.