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

Choices:  
A: Wind torque cancellation  
B: Mechanical balance  
C: Control of phase shift  
D: Minimize losses    

Correct Answer is  C: Control of phase shift  

Correct answer: C — Control of phase shift

Why C is correct:
- In a Yagi, parasitic elements are not driven; their currents are induced by the driven element.  
- Making an element slightly longer than resonance makes it electrically inductive ($X>0$), so its induced current lags. This is used for the reflector.  
- Making an element slightly shorter than resonance makes it electrically capacitive ($X<0$), so its induced current leads. This is used for directors.  
- These lead/lag relationships control the phase of reradiated fields, creating the end-fire pattern and setting forward gain and front-to-back ratio.

Why the others are incorrect:
- A: Wind torque cancellation — Mechanical issue; not why elements are detuned electrically.  
- B: Mechanical balance — Also mechanical; element length changes here are for RF phasing, not balance.  
- D: Minimize losses — Element loss in aluminum/metal rods is already very low; slight detuning is not for reducing loss.

Useful rules of thumb:
- Driven element length near resonance: $L_D \approx 0.47\lambda$ (varies with diameter/factor).  
- Reflector: slightly longer, typically $+2\%$ to $+5\%$ (e.g., $L_R \approx 0.49\!-\!0.50\lambda$) → inductive → current lags.  
- Director(s): slightly shorter, typically $-2\%$ to $-5\%$ (e.g., $L_{Dir} \approx 0.44\!-\!0.46\lambda$) → capacitive → current leads.  
- Typical element spacing: $0.1\!-\!0.25\,\lambda$ (sets coupling and thus phase/gain).

Phase concepts:
- Reactance sign sets current phase:  
  - Inductive: $X_L>0$ → current lags voltage.  
  - Capacitive: $X_C<0$ → current leads voltage.  
- The induced current phase relative to the driven element steers the main lobe toward the directors and provides reflection from the reflector.