*Question G4E05:
Which of the following most limits an HF mobile installation?*  

*Correct Answer is  C: Efficiency of the electrically short antenna*  

### AI Response: (may contain errors)  

In an HF mobile installation, the most significant limitation is the efficiency of the electrically short antenna. Here's why:

1. **Correct Answer: Efficiency of the electrically short antenna (C)**
   - Mobile antennas are often much shorter than a quarter wavelength, especially on lower HF bands like 75 meters. This makes them electrically short.
   - Short antennas have lower radiation efficiency because they have higher reactance and lower radiation resistance, leading to more power being lost as heat rather than being radiated.
   - The efficiency of an antenna can be expressed as:
     $$\text{Efficiency} = \frac{R_r}{R_r + R_l}$$
     where:
     - $R_r$ = radiation resistance
     - $R_l$ = loss resistance
   - In electrically short antennas, $R_r$ is small, making the efficiency low.

2. **Why the other choices are incorrect:**
   - **“Picket fencing” (A):** This term refers to rapid signal fading due to movement, typically associated with VHF/UHF frequencies, not HF.
   - **The wire gauge of the DC power line to the transceiver (B):** While important for ensuring adequate power delivery, it is not the primary limiting factor in HF mobile installations. Properly sized wires can mitigate this issue.
   - **FCC rules limiting mobile output power on the 75-meter band (D):** The FCC does not impose specific power limits on mobile operations that would be more restrictive than those for fixed stations, so this is not a limiting factor.

Understanding these factors helps in optimizing HF mobile installations by focusing on improving antenna efficiency, such as using loading coils or capacitive hats to increase effective length.