Question
Which of the following is a common way to adjust the feed point impedance of an elevated quarter-wave ground-plane vertical antenna to be approximately 50 ohms?
Answer Options
- A) Slope the radials upward
- B) Slope the radials downward
- C) Lengthen the radials beyond one wavelength
- D) Coil the radials
Correct Answer: B
Explanation
An ideal quarter-wave vertical antenna over a perfectly conductive ground plane has a feed point impedance near 36 \text{ ohms}. However, most amateur antennas use finite ground planes or elevated radials, which can shift this impedance.
A common technique to raise the feed point impedance to match a 50 \text{ ohm} coaxial cable is to slope the radials downward. By bending the radials down by about 45^\circ, the impedance naturally increases to approximately 50 \text{ ohms} or slightly higher, providing a much better match without requiring a separate matching device.
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