What happens as the frequency of a signal in coaxial cable is increased?
The correct answer is D: The loss increases. As the frequency of a signal in coaxial cable is increased, the loss increases. Higher frequencies experience more loss in coaxial cable due to skin effect (current flows near the surface of conductors at high frequencies) and dielectric losses. This is why low-loss cable is important for VHF/UHF operation. For amateur radio operators, understanding frequency-dependent loss helps explain why feed line selection matters more at higher frequencies and why shorter feed lines are often used for VHF/UHF.
Exam Tip
Higher frequency = more loss in coax. Think 'H'igher 'F'requency = 'H'igher 'F'eed line loss. Skin effect and dielectric losses increase with frequency. Impedance stays constant, but loss increases.
Memory Aid
"Higher frequency = more loss. Think 'H'igher 'F'requency = 'H'igher loss. Skin effect and dielectric losses increase with frequency. Impedance stays constant (50 ohms), but loss increases significantly."
Real-World Application
You use the same RG-58 cable for 2 meters (146 MHz) and 70 cm (440 MHz). At 2 meters, you might have 2 dB loss per 100 feet. At 70 cm, the same cable has about 4 dB loss per 100 feet - double the loss. This is why low-loss cable is especially important for UHF operation, and why feed lines are often kept short at higher frequencies.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Characteristic impedance doesn't decrease with frequency - it's determined by the cable's physical construction (conductor size, dielectric), not frequency.
Option B: Incorrect. Loss doesn't decrease with frequency - it increases. Higher frequencies have more loss due to skin effect and dielectric losses.
Option C: Incorrect. Characteristic impedance doesn't increase with frequency - it's constant, determined by cable construction.
题目解析
The correct answer is D: The loss increases. As the frequency of a signal in coaxial cable is increased, the loss increases. Higher frequencies experience more loss in coaxial cable due to skin effect (current flows near the surface of conductors at high frequencies) and dielectric losses. This is why low-loss cable is important for VHF/UHF operation. For amateur radio operators, understanding frequency-dependent loss helps explain why feed line selection matters more at higher frequencies and why shorter feed lines are often used for VHF/UHF.
考试技巧
Higher frequency = more loss in coax. Think 'H'igher 'F'requency = 'H'igher 'F'eed line loss. Skin effect and dielectric losses increase with frequency. Impedance stays constant, but loss increases.
记忆口诀
Higher frequency = more loss. Think 'H'igher 'F'requency = 'H'igher loss. Skin effect and dielectric losses increase with frequency. Impedance stays constant (50 ohms), but loss increases significantly.
实际应用示例
You use the same RG-58 cable for 2 meters (146 MHz) and 70 cm (440 MHz). At 2 meters, you might have 2 dB loss per 100 feet. At 70 cm, the same cable has about 4 dB loss per 100 feet - double the loss. This is why low-loss cable is especially important for UHF operation, and why feed lines are often kept short at higher frequencies.
错误选项分析
Option A: Incorrect. Characteristic impedance doesn't decrease with frequency - it's determined by the cable's physical construction (conductor size, dielectric), not frequency. Option B: Incorrect. Loss doesn't decrease with frequency - it increases. Higher frequencies have more loss due to skin effect and dielectric losses. Option C: Incorrect. Characteristic impedance doesn't increase with frequency - it's constant, determined by cable construction.
知识点
Coaxial cable loss, Frequency-dependent loss, Feed line losses, High frequency losses
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.