Question E9D05:
Approximately how long is a Yagi’s driven element?  

Choices:  
A: 234 divided by frequency in MHz  
B: 1005 divided by frequency in MHz  
C: 1/4 wavelength  
D: 1/2 wavelength    

Correct Answer is  D: 1/2 wavelength  

To a first approximation, a Yagi-Uda antenna uses a half-wave resonant dipole as its driven element. Thus, the driven element is about 1/2 wavelength long.

Why D is correct:
- The driven element in a typical Yagi is a center-fed dipole resonant at the operating frequency, so its length is approximately $0.5\lambda$. In practice it’s often slightly shorter (about $0.47$–$0.49\lambda$) due to end effects and coupling to the parasitic elements, but “approximately 1/2 wavelength” is the standard exam answer.

Why the others are incorrect:
- A: $234/f_{\text{MHz}}$ (feet) is the rule of thumb for a quarter-wave wire element (or each leg of a dipole). A Yagi’s driven element is not a quarter-wave; it’s a half-wave dipole.
- B: $1005/f_{\text{MHz}}$ (feet) is the rule of thumb for a full-wave loop (total perimeter), not a dipole element.
- C: $1/4$ wavelength describes a monopole over a ground plane (or one leg of a dipole), not the length of a Yagi’s driven dipole.

Useful formulas and rules of thumb:
- Wavelength: $\lambda \text{(meters)} \approx \dfrac{300}{f_{\text{MHz}}}$
- Half-wave dipole (total length): $L \text{(feet)} \approx \dfrac{468}{f_{\text{MHz}}}$ or $L \text{(meters)} \approx \dfrac{143}{f_{\text{MHz}}}$
- Quarter-wave wire: $L \text{(feet)} \approx \dfrac{234}{f_{\text{MHz}}}$
- Full-wave loop (perimeter): $L \text{(feet)} \approx \dfrac{1005}{f_{\text{MHz}}}$

In a Yagi, parasitic elements are adjusted around 1/2 wavelength (reflector ~5% longer, directors ~5% shorter), with the driven element near 1/2 wavelength.