Question E9C08:
What is a folded dipole antenna?  

Choices:  
A: A dipole one-quarter wavelength long  
B: A center-fed dipole with the ends folded down 90 degrees at the midpoint of each side  
C: A half-wave dipole with an additional parallel wire connecting its two ends  
D: A dipole configured to provide forward gain    

Correct Answer is  C: A half-wave dipole with an additional parallel wire connecting its two ends  

Correct answer: C — A half-wave dipole with an additional parallel wire connecting its two ends.

1) Why C is correct
- A folded dipole is made by taking a regular half-wave dipole and adding a second, parallel conductor connected at both ends, with the feedpoint at the center of one conductor.
- This arrangement keeps the same half-wave resonant length and radiation pattern as a standard dipole but raises the feedpoint impedance and broadens bandwidth.
- For equal-diameter wires, the input impedance is approximately:
  $$Z_{in} \approx n^2 \times 73\,\Omega$$
  For a two-wire folded dipole ($n=2$):
  $$Z_{in} \approx 4 \times 73 \approx 292\,\Omega \ (\text{often taken as } \sim 300\,\Omega)$$

2) Why the other choices are incorrect
- A: “A dipole one-quarter wavelength long”  
  A folded dipole is not quarter-wave; it is a half-wave radiator. A 1/4-wave center-fed dipole is not the standard resonant dipole configuration and is not what “folded dipole” means.

- B: “A center-fed dipole with the ends folded down 90 degrees at the midpoint of each side”  
  That describes a bent or drooped dipole, not a folded dipole. A folded dipole requires an additional parallel conductor connected at both ends, forming a continuous loop.

- D: “A dipole configured to provide forward gain”  
  A single folded dipole has essentially the same broadside pattern and gain as a regular half-wave dipole. Forward gain comes from arrays (e.g., Yagis) using directors/reflectors, not from the folded dipole alone.

3) Useful formulas and rules of thumb
- Half-wave dipole (and folded dipole) total length:
  $$L_{\text{feet}} \approx \frac{468}{f(\text{MHz})}$$
- Folded dipole input impedance (equal-diameter conductors):
  $$Z_{in} \approx n^2 \times 73\,\Omega \quad (\text{two-wire} \Rightarrow \sim 300\,\Omega)$$
- Feeding: commonly used with 300 Ω balanced line, or via a 4:1 balun when interfacing to coax. The radiation pattern is the same as a half-wave dipole (no inherent forward gain).