What is the approximate feed point impedance at the center of a two-wire half-wave folded dipole antenna?
The correct answer is A: 300 ohms. The approximate feed point impedance at the center of a two-wire half-wave folded dipole antenna is 300 ohms. This is approximately 4 times the impedance of a standard half-wave dipole.
A folded dipole consists of two parallel wires connected at the ends, forming a loop. The folding and the two-wire structure increase the impedance compared to a simple dipole. A standard half-wave dipole has about 72 ohms impedance. A two-wire folded dipole has approximately 4 times that, or about 300 ohms. This higher impedance can be useful for matching to certain feed lines or when you need higher impedance. The exact value depends on the wire spacing and diameter, but 300 ohms is the typical value.
Exam Tip
Folded dipole impedance = 300 ohms. Remember: A two-wire half-wave folded dipole has approximately 300 ohms feed point impedance - about 4 times a standard dipole's 72 ohms.
Memory Aid
"**F**olded **D**ipole **I**mpedance = **3**00 ohms (think 'FDI = 300Ω')"
Real-World Application
You build a two-wire folded dipole. The two parallel wires are connected at the ends, creating a loop. At the center feed point, you measure approximately 300 ohms impedance. This is much higher than a standard dipole's 72 ohms, which can be useful for matching to 300-ohm twin-lead or when you need higher impedance.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. 72 ohms is the impedance of a standard half-wave dipole, not a folded dipole. The folding increases the impedance.
Option C: Incorrect. 50 ohms is a common feed line impedance, not the folded dipole impedance. Folded dipoles have higher impedance.
Option D: Incorrect. 450 ohms would be too high. A two-wire folded dipole typically has about 300 ohms impedance.
题目解析
The correct answer is A: 300 ohms. The approximate feed point impedance at the center of a two-wire half-wave folded dipole antenna is 300 ohms. This is approximately 4 times the impedance of a standard half-wave dipole. A folded dipole consists of two parallel wires connected at the ends, forming a loop. The folding and the two-wire structure increase the impedance compared to a simple dipole. A standard half-wave dipole has about 72 ohms impedance. A two-wire folded dipole has approximately 4 times that, or about 300 ohms. This higher impedance can be useful for matching to certain feed lines or when you need higher impedance. The exact value depends on the wire spacing and diameter, but 300 ohms is the typical value.
考试技巧
Folded dipole impedance = 300 ohms. Remember: A two-wire half-wave folded dipole has approximately 300 ohms feed point impedance - about 4 times a standard dipole's 72 ohms.
记忆口诀
**F**olded **D**ipole **I**mpedance = **3**00 ohms (think 'FDI = 300Ω')
实际应用示例
You build a two-wire folded dipole. The two parallel wires are connected at the ends, creating a loop. At the center feed point, you measure approximately 300 ohms impedance. This is much higher than a standard dipole's 72 ohms, which can be useful for matching to 300-ohm twin-lead or when you need higher impedance.
错误选项分析
Option B: Incorrect. 72 ohms is the impedance of a standard half-wave dipole, not a folded dipole. The folding increases the impedance. Option C: Incorrect. 50 ohms is a common feed line impedance, not the folded dipole impedance. Folded dipoles have higher impedance. Option D: Incorrect. 450 ohms would be too high. A two-wire folded dipole typically has about 300 ohms impedance.
知识点
Folded dipole, Feed point impedance, 300 ohms, Two-wire dipole
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.