*Question E9H02:
Which is generally true for 160- and 80-meter receiving antennas?*  

*Correct Answer is  A: Atmospheric noise is so high that directivity is much more important than losses*  

### AI Response: (may contain errors)  

For 160- and 80-meter receiving antennas, the correct answer is:

**A: Atmospheric noise is so high that directivity is much more important than losses**

### Explanation:

1. **Why the correct answer is correct:**
   - **Atmospheric Noise:** On the 160- and 80-meter bands, atmospheric noise is significant due to natural sources like thunderstorms and cosmic noise. This noise can overwhelm weak signals.
   - **Directivity Importance:** A directional antenna can help focus on the desired signal while reducing the reception of noise from other directions. This makes directivity more crucial than minimizing losses in the antenna system.

2. **Why the other choices are incorrect:**
   - **B: They must be erected at least 1/2 wavelength above the ground to attain good directivity**
     - **Height Requirement:** While height can improve performance, achieving 1/2 wavelength height is often impractical for these low-frequency bands due to the large physical size required. Directivity can still be achieved with lower heights using specific antenna designs.
   - **C: Low loss coax transmission line is essential for good performance**
     - **Transmission Line Losses:** While low-loss coax is generally beneficial, the impact of coaxial cable losses is less critical compared to the benefits gained from improved directivity in these bands. The noise level is the limiting factor, not the transmission line loss.

3. **Useful Rules of Thumb:**
   - **Directivity vs. Losses:** In high-noise environments, improving the signal-to-noise ratio through directivity is often more effective than focusing solely on minimizing losses.
   - **Antenna Height:** For lower frequency bands, practical antenna height is often a compromise, and other design factors like directivity and noise rejection become more important.

Understanding these principles helps in designing and choosing antennas that perform well in the presence of high atmospheric noise, which is typical for the 160- and 80-meter bands.