Question E9C10:
Which of the following describes a Zepp antenna?  

Choices:  
A: A horizontal array capable of quickly changing the direction of maximum radiation by changing phasing lines  
B: An end-fed half-wavelength dipole  
C: An omni-directional antenna commonly used for satellite communications  
D: A vertical array capable of quickly changing the direction of maximum radiation by changing phasing lines    

Correct Answer is  B: An end-fed half-wavelength dipole  

Correct answer: B — An end-fed half-wavelength dipole

Why this is correct:
- A Zepp (short for Zeppelin) antenna is a half-wavelength radiator fed at one end, historically hung from airships.
- It uses end feeding, typically via a quarter-wave matching section (open-wire line). The radiator itself is about a half wavelength long and has a high end-feed impedance.
- Radiation pattern is similar to a half-wave dipole: broadside to the wire, not omni-directional.

Why the other choices are incorrect:
- A: “Horizontal array with phasing lines” describes a phased array/beam, not a single end-fed half-wave.
- C: “Omni-directional for satellites” typically refers to turnstile, eggbeater, or verticals; a Zepp is not omni and not specific to satellites.
- D: “Vertical array with phasing lines” again describes a phased vertical array, not a Zepp.

Useful formulas and rules of thumb:
- Half-wave radiator length (free space):
  $$L_{\text{rad}} \approx \frac{\lambda}{2} \approx \frac{468}{f_{\text{MHz}}}\ \text{ft}$$
- Quarter-wave matching section (Zepp/J-pole style), adjusted by velocity factor (VF):
  $$L_{\text{stub}} \approx \frac{\lambda}{4}\cdot \text{VF} \approx \frac{246}{f_{\text{MHz}}}\cdot \text{VF}\ \text{ft}$$
- End-feed impedance of a half-wave is high (typically $2\text{–}5\ \text{k}\Omega$); the quarter-wave matching section transforms this to a lower value suitable for the feedline.