Question
What is the purpose of feeding an off-center-fed dipole (OCFD) between the center and one end instead of at the midpoint?
Answer Options
- A) To create a similar feed point impedance on multiple bands
- B) To suppress off-center lobes at higher frequencies
- C) To resonate the antenna across a wider range of frequencies
- D) To reduce common-mode current coupling on the feed line shield
Correct Answer: A
Explanation
A standard half-wave dipole has a low impedance (about 70 \Omega) only at its fundamental resonant frequency. As the frequency is increased to multiples (e.g., 3F or 5F), the feed point impedance changes drastically to hundreds or even thousands of ohms. The Off-Center-Fed Dipole (\text{OCFD} or Windom) addresses this limitation.
By moving the feed point to a point that is 1/3 or 1/4 of the way from one end (e.g., 33\% or 25\% of the length), the antenna places the feed point near a high-current point for several different harmonic bands. The primary purpose of feeding an \text{OCFD} off-center is to create a similar feed point impedance (typically 200 \Omega to 300 \Omega) on multiple bands, making it easier to match with a 4:1 balun and a single feed line.
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