Question E9C05:
What is the purpose of feeding an off-center-fed dipole (OCFD) between the center and one end instead of at the midpoint?  

Choices:  
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 is  A: To create a similar feed point impedance on multiple bands  

To create a similar feed point impedance on multiple bands

Why this is correct:
- Off-center feeding samples a point along the wire where the current is substantial on several harmonics. This yields a similar, moderate feed-point resistance on multiple HF bands (both odd and even harmonics), making multiband operation practical with one matching transformer (e.g., 4:1 or 6:1).
- A common OCFD feed is about 1/3 of the total length from one end, giving roughly 200–300 Ω on several harmonics, which transforms well to 50 Ω.

Why the other choices are incorrect:
- B: To suppress off-center lobes at higher frequencies  
Radiation lobes at higher frequencies are set primarily by the antenna’s electrical length, not the exact feedpoint. Off-center feeding does not “suppress” lobes; the pattern complexity still increases with frequency.
- C: To resonate the antenna across a wider range of frequencies  
An OCFD still resonates at discrete harmonics, not across a continuous wide range. Broadbanding requires techniques like thicker elements, folded elements, or special matching networks, not merely moving the feedpoint.
- D: To reduce common-mode current coupling on the feed line shield  
Off-center feeding increases asymmetry and tends to aggravate common-mode currents unless you add a good choke/balun. It does not reduce them by itself.

Useful formulas and rules of thumb:
- Half-wave dipole length (feet):
  $$L_{\frac{1}{2}\lambda} \approx \frac{468}{f_{\text{MHz}}}$$
- Center-fed half-wave dipole impedance (free space):
  $$Z_{\text{center}} \approx 70\,\Omega$$
- Typical OCFD feedpoint (about 1/3 from an end):
  $$Z_{\text{OCF}} \approx 200\text{–}300\,\Omega$$
  Use a transformer so that:
  $$\frac{Z_{\text{OCF}}}{\text{ratio}} \approx 50\,\Omega$$
  e.g., 4:1 or 6:1.
- Harmonic behavior:
  - Center-fed dipoles present low impedance mainly on odd harmonics.
  - OCFDs are chosen so the feedpoint sees usable impedance on both even and odd harmonics, enabling multiband use.