*Question G9D07:
Which of the following describes a log-periodic antenna?*  

*Correct Answer is  A: Element length and spacing vary logarithmically along the boom*  

### AI Response: (may contain errors)  

A log-periodic antenna is a type of directional antenna that is designed to operate over a wide range of frequencies. Here's why the correct answer is correct and why the other choices are incorrect:

1. **Correct Answer: Element length and spacing vary logarithmically along the boom**

   - **Explanation**: In a log-periodic antenna, both the lengths of the elements and the spacing between them change in a logarithmic fashion along the boom. This design allows the antenna to maintain consistent performance over a broad frequency range. The logarithmic variation is key to achieving the wide bandwidth characteristic of log-periodic antennas.

2. **Incorrect Choices**:

   - **B: Impedance varies periodically as a function of frequency**
     - **Explanation**: While the impedance of any antenna can vary with frequency, the defining feature of a log-periodic antenna is not periodic impedance variation. Instead, it is the logarithmic variation of element lengths and spacing that defines this type of antenna.

   - **C: Gain varies logarithmically as a function of frequency**
     - **Explanation**: The gain of a log-periodic antenna does not vary logarithmically with frequency. Instead, the design aims to provide relatively constant gain across its operating frequency range. The logarithmic aspect refers to the physical structure, not the gain.

   - **D: SWR varies periodically as a function of boom length**
     - **Explanation**: The standing wave ratio (SWR) is not a function of boom length in a log-periodic antenna. The SWR is more related to how well the antenna is matched to the transmission line over its frequency range. The boom length does not cause periodic SWR variation.

In summary, the defining characteristic of a log-periodic antenna is the logarithmic variation in element length and spacing, which allows it to cover a wide frequency range effectively.