*Question T9B11:
Which of the following types of feed line has the lowest loss at VHF and UHF?*  

*Correct Answer is  C: Air-insulated hardline*  

### AI Response: (may contain errors)  

To determine which type of feed line has the lowest loss at VHF and UHF, we need to consider the characteristics of each option.

### Correct Answer: C - Air-insulated hardline

- **Why it's correct**: 
  - Air-insulated hardline has a very low loss due to its construction. It typically uses a solid dielectric and is designed to minimize signal attenuation. The air insulation reduces dielectric losses, making it ideal for high-frequency applications like VHF and UHF.

### Why the other choices are incorrect:

- **A: 50-ohm flexible coax**:
  - While 50-ohm coaxial cable is commonly used and has decent performance, it generally has higher losses compared to air-insulated hardline, especially at VHF and UHF frequencies. The flexible nature also means it may have a thicker dielectric, which contributes to higher losses.

- **B: Multi-conductor unbalanced cable**:
  - This type of cable is not optimized for VHF and UHF frequencies. It can suffer from higher losses due to its construction and is more susceptible to interference and signal degradation, making it less suitable for these frequency ranges.

- **D: 75-ohm flexible coax**:
  - Although 75-ohm coax is often used for television and some radio applications, it typically has higher losses than 50-ohm coax at VHF and UHF frequencies. It is not designed for the same power handling capabilities as 50-ohm coax or air-insulated hardline.

### Useful Formulas and Rules of Thumb:

- **Loss in Coaxial Cable**: The loss in coaxial cable can be approximated using the formula:

$$
L = \frac{10 \cdot \log_{10}(P_{in}/P_{out})}{d}
$$

where:
- $L$ = loss in dB
- $P_{in}$ = input power
- $P_{out}$ = output power
- $d$ = length of the cable in feet

- **General Rule**: For VHF and UHF applications, always prefer hardline or low-loss coaxial cables to minimize signal loss. 

In summary, air-insulated hardline is the best choice for low loss at VHF and UHF frequencies due to its superior construction and materials.