Question E9D11:
Why do most two-element Yagis with normal spacing have a reflector instead of a director?  

Choices:  
A: Lower SWR  
B: Higher receiving directivity factor  
C: Greater front-to-side  
D: Higher gain    

Correct Answer is  D: Higher gain  

Correct answer: Higher gain

Why that’s correct
- With normal element spacing (about $0.1$–$0.2\,\lambda$), a two-element Yagi configured as a driven element plus a reflector (parasitic made slightly longer than resonant) produces stronger constructive interference in the forward direction than the driven+director case.  
- The reflector’s induced current lags, and when combined with the space phase across the spacing, the total forward-phase addition is closer to in-phase, giving slightly higher forward gain for typical “normal” spacings.
- In practice and modeling, a 2-element DE+reflector commonly yields about 0.5–1 dB more gain than DE+director at the same spacing/diameter.

Why the other choices are incorrect
- Lower SWR: Not inherently. Feedpoint impedance and SWR depend on exact lengths, spacing, and the chosen matching network. Both versions often need a match (gamma, hairpin, etc.).
- Higher receiving directivity factor: RDF differences are minimal for a two-element Yagi; the main benefit driving the choice is forward gain, not RDF.
- Greater front-to-side: Two-element arrays have limited F/S. A reflector often improves front-to-back more than F/S, but the principal reason for using a reflector (with normal spacing) is higher forward gain.

Useful rules of thumb and relationships
- Element tuning:
  - Reflector length: $L_R \approx (1.03$–$1.07)\,L_{\text{res}}$ (inductive)
  - Director length: $L_D \approx (0.95$–$0.98)\,L_{\text{res}}$ (capacitive)
- Spacing: $s \approx 0.1$–$0.2\,\lambda$
- Typical 2-element forward gain:
  - DE + reflector: $G \approx 4.5$–$5.0$ dBi
  - DE + director: $G \approx 3.5$–$4.5$ dBi
- Phase-addition idea for forward field:
  $$E_{\text{fwd}} \propto E_{DE} + E_{par}\cos(\Delta\phi), \quad \Delta\phi \approx k s + \phi_I$$
  where $k = 2\pi/\lambda$, $s$ is spacing, and $\phi_I$ is the parasitic current’s phase shift. For a reflector at normal spacing, $\Delta\phi$ is closer to 0, so $\cos(\Delta\phi)$ is larger, giving higher gain.