Question E9C07:
What is the approximate feed point impedance at the center of a two-wire half-wave folded dipole antenna?  

Choices:  
A: 300 ohms  
B: 72 ohms  
C: 50 ohms  
D: 450 ohms    

Correct Answer is  A: 300 ohms  

To a first approximation, a half-wave folded dipole made from two parallel, equal-diameter wires has a feedpoint impedance 4 times that of a single-wire half-wave dipole.

Key rule:
$$Z_{\text{folded}} \approx n^2 \, Z_{\text{dipole}}$$
- For a two-wire folded dipole, $n=2$.
- A center-fed half-wave dipole in free space has $Z_{\text{dipole}} \approx 72$–$73\ \Omega$.

So:
$$Z_{\text{folded}} \approx 2^2 \times 72\ \Omega \approx 288\ \Omega \approx 300\ \Omega$$

Why 300 ohms is correct (Choice A):
- It’s the standard value for a two-wire, equal-diameter, half-wave folded dipole, matching the 4× impedance increase over a ~72–73 Ω dipole.

Why the other choices are incorrect:
- 72 ohms (Choice B): That’s the typical feedpoint impedance of a single-wire half-wave dipole, not a folded dipole.
- 50 ohms (Choice C): Common coax impedance, but not the inherent impedance of a two-wire folded dipole.
- 450 ohms (Choice D): Often associated with ladder line, but a standard two-wire folded dipole does not present ~450 Ω at its feedpoint. Values near 450 Ω would require different geometry (e.g., unequal wire diameters/spacing or more conductors).

Useful rules of thumb:
- Half-wave dipole (single wire, center-fed): $Z \approx 72$–$73\ \Omega$ (free space).
- Two-wire folded dipole (equal wires): $Z \approx 4 \times 72 \approx 300\ \Omega$.
- In general: $Z_{\text{folded}} \approx n^2 Z_{\text{dipole}}$ for $n$ equal, closely spaced, in-phase conductors.