Question E9C04:
What happens to the radiation pattern of an unterminated long wire antenna as the wire length is increased?  

Choices:  
A: Fewer lobes form with the major lobes increasing closer to broadside to the wire  
B: Additional lobes form with major lobes increasingly aligned with the axis of the antenna  
C: The elevation angle increases, and the front-to-rear ratio decreases  
D: The elevation angle increases, while the front-to-rear ratio is unaffected    

Correct Answer is  B: Additional lobes form with major lobes increasingly aligned with the axis of the antenna  

To understand an unterminated long-wire (standing-wave) antenna, think of it as many in-phase half-wave sections strung in a line. As the wire gets longer (in wavelengths), the far-field becomes the vector sum of more elements spaced along the wire, which produces more lobes and pushes the strongest lobes toward the wire’s axis (endfire).

Why the correct answer is correct (B)
- As length $L$ increases (in wavelengths), the pattern develops more distinct lobes.
- The strongest lobes move closer to the wire axis (toward endfire). In other words, the main radiation directions are increasingly aligned with the wire.
- Rule of thumb (free space, approximate):
  - Total number of lobes ≈ $2L/\lambda$ (bidirectional symmetry).
  - Lobe maxima occur roughly at angles (measured from the wire axis):
    $$\sin\theta_m \approx \frac{m\lambda}{L},\quad m=1,2,\dots,\left\lfloor \frac{L}{\lambda}\right\rfloor$$
    As $L/\lambda$ grows, $\theta_1$ gets smaller, so the main lobe moves closer to endfire.

Why the other choices are incorrect
- A: “Fewer lobes” is the opposite of what happens; a longer unterminated long wire produces more lobes. Also, the major lobes do not move toward broadside; they move toward the wire’s axis (endfire).
- C: “Elevation angle increases, and F/R decreases” is not characteristic of an unterminated long wire. The dominant effect with increasing length is more lobes and a shift toward endfire (which, over ground, usually corresponds to lower takeoff angles, not higher). Also, “front-to-rear” is not a primary metric here because an unterminated long wire is inherently bidirectional.
- D: Same issue with “elevation angle increases” and the irrelevant “front-to-rear unaffected.” An unterminated long wire remains bidirectional, and the main trend is toward endfire, not higher elevation.

Useful rules of thumb
- Unterminated long wire (standing-wave): bidirectional with multiple lobes; more length → more lobes, main lobes tilt toward endfire.
- Terminated long wire (e.g., Beverage): predominantly unidirectional endfire; termination controls direction rather than creating multiple lobes.
- Approximate relationships (free space):
  - Number of lobes ≈ $2L/\lambda$
  - Lobe angles from the wire axis: $\sin\theta_m \approx m\lambda/L$
  - As $L$ increases, $\theta_1$ decreases → major lobes align more with the antenna axis.