Question E9D03:
What is the most efficient location for a loading coil on an electrically short whip?  

Choices:  
A: Near the center of the vertical radiator  
B: As low as possible on the vertical radiator  
C: At a voltage maximum  
D: At a voltage null    

Correct Answer is  A: Near the center of the vertical radiator  

To maximize efficiency in a short vertical whip, place the loading coil where it carries less current but still leaves enough radiating length above it. That balance is achieved near the middle of the radiator.

Why the correct answer is correct (A: Near the center of the vertical radiator)
- Current in a short monopole is highest at the base and decreases toward the tip. Coil loss is dominated by ohmic loss:
  $$P_{\text{coil}} = I_{\text{coil}}^2 R_{\text{coil}}$$
- Moving the coil up from the base reduces the current through it, reducing loss. Placing it near the center also leaves a substantial length above the coil with appreciable current, increasing radiation.
- Net effect: higher radiation resistance and lower coil loss, so better efficiency than base loading and more practical than very high placement.

Why the other choices are incorrect
- B: As low as possible (base loading)
  - Puts the coil at the current maximum, maximizing $$I^2 R$$ loss and minimizing efficiency.
- C: At a voltage maximum (near the tip)
  - Although current is low there, little radiator remains above the coil to radiate, and a larger inductance is required at very high voltage, increasing losses and risking arcing. Not the most efficient placement.
- D: At a voltage null (the feedpoint/base)
  - Equivalent to base loading; suffers the same high-loss problem as choice B.

Useful formulas and rules of thumb
- Small-antenna efficiency:
  $$\eta \approx \frac{R_r}{R_r + R_{\text{ground}} + R_{\text{coil}}}$$
- Coil loss:
  $$P_{\text{coil}} = I_{\text{coil}}^2 R_{\text{coil}}$$
- Radiation resistance of a short monopole of height $h \ll \lambda$:
  $$R_r \approx 160\pi^2 \left(\frac{h}{\lambda}\right)^2 \ \text{ohms}$$
- Practical rule of thumb:
  - Loading efficiency ranking: top loading (capacitive hat) > center loading > base loading.
  - For coils, placing the coil near the middle (often around 0.6–0.7 of the height) is a good compromise for efficiency.