Which frequency band has the smallest first Fresnel zone?
The correct answer is A: 5.8 GHz. The frequency band with the smallest first Fresnel zone is 5.8 GHz. The Fresnel zone size is inversely proportional to frequency - higher frequencies have smaller Fresnel zones.
The first Fresnel zone is an elliptical region between transmitter and receiver that must be clear of obstructions for optimal signal propagation. The radius of the first Fresnel zone is proportional to the square root of (distance × wavelength). Since wavelength = speed of light / frequency, higher frequencies have shorter wavelengths, resulting in smaller Fresnel zones. At 5.8 GHz (the highest frequency listed), the wavelength is shortest, so the Fresnel zone is smallest. This is important for microwave communications where Fresnel zone clearance is critical.
Exam Tip
Smallest Fresnel zone = Highest frequency. Remember: Higher frequencies have smaller Fresnel zones. 5.8 GHz (highest) has the smallest Fresnel zone.
Memory Aid
"**S**mallest **F**resnel **Z**one = **H**ighest **F**requency (think 'SFZ = HF', 5.8 GHz)"
Real-World Application
You're setting up a 5.8 GHz microwave link. The first Fresnel zone is very small (maybe a few feet in radius), so you need precise clearance of obstructions. At 900 MHz, the Fresnel zone would be much larger (maybe 20-30 feet), requiring more clearance. The higher frequency means a smaller Fresnel zone that's easier to clear but more sensitive to obstructions.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. 3.4 GHz has a longer wavelength than 5.8 GHz, so it has a larger Fresnel zone, not smaller.
Option C: Incorrect. 2.4 GHz has an even longer wavelength, resulting in a larger Fresnel zone than 5.8 GHz.
Option D: Incorrect. 900 MHz has the longest wavelength of the options, so it has the largest Fresnel zone, not the smallest.
题目解析
The correct answer is A: 5.8 GHz. The frequency band with the smallest first Fresnel zone is 5.8 GHz. The Fresnel zone size is inversely proportional to frequency - higher frequencies have smaller Fresnel zones. The first Fresnel zone is an elliptical region between transmitter and receiver that must be clear of obstructions for optimal signal propagation. The radius of the first Fresnel zone is proportional to the square root of (distance × wavelength). Since wavelength = speed of light / frequency, higher frequencies have shorter wavelengths, resulting in smaller Fresnel zones. At 5.8 GHz (the highest frequency listed), the wavelength is shortest, so the Fresnel zone is smallest. This is important for microwave communications where Fresnel zone clearance is critical.
考试技巧
Smallest Fresnel zone = Highest frequency. Remember: Higher frequencies have smaller Fresnel zones. 5.8 GHz (highest) has the smallest Fresnel zone.
记忆口诀
**S**mallest **F**resnel **Z**one = **H**ighest **F**requency (think 'SFZ = HF', 5.8 GHz)
实际应用示例
You're setting up a 5.8 GHz microwave link. The first Fresnel zone is very small (maybe a few feet in radius), so you need precise clearance of obstructions. At 900 MHz, the Fresnel zone would be much larger (maybe 20-30 feet), requiring more clearance. The higher frequency means a smaller Fresnel zone that's easier to clear but more sensitive to obstructions.
错误选项分析
Option B: Incorrect. 3.4 GHz has a longer wavelength than 5.8 GHz, so it has a larger Fresnel zone, not smaller. Option C: Incorrect. 2.4 GHz has an even longer wavelength, resulting in a larger Fresnel zone than 5.8 GHz. Option D: Incorrect. 900 MHz has the longest wavelength of the options, so it has the largest Fresnel zone, not the smallest.
知识点
Fresnel zone, Frequency, Wavelength, Microwave propagation
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.