Question E9B11:
What is a disadvantage of decreasing the number of wire segments in an antenna model below 10 segments per half-wavelength?  

Choices:  
A: Ground conductivity will not be accurately modeled  
B: The resulting design will favor radiation of harmonic energy  
C: The computed feed point impedance may be incorrect  
D: The antenna will become mechanically unstable    

Correct Answer is  C: The computed feed point impedance may be incorrect  

Correct answer: C — The computed feed point impedance may be incorrect.

Why C is correct:
- NEC and similar wire-antenna models use the Method of Moments, which approximates the continuous current distribution along a wire with piecewise-constant (or simple) basis functions on short segments.
- If you use too few segments (e.g., fewer than about 10 per half-wavelength), the current distribution is poorly approximated, especially near the feedpoint and discontinuities. This directly corrupts the calculation of feedpoint current and voltage, so the computed feedpoint impedance $Z_{in} = V/I$ can be significantly wrong.

Why the other choices are incorrect:
- A: Ground conductivity is set by the ground model parameters (e.g., Sommerfeld/Norton ground) and is not fixed by wire segmentation density. While coarse segmentation can affect many results, it does not “model” ground conductivity itself.
- B: “Favor radiation of harmonic energy” is a physical antenna characteristic, not a numerical segmentation artifact. Under-segmentation doesn’t make the real antenna radiate more harmonics; it just makes the numerical solution less accurate.
- D: Mechanical stability is a physical/structural issue, unrelated to numerical segmentation in a computer model.

Useful rules of thumb:
- Wavelength: $\lambda = \dfrac{c}{f}$.
- Minimum segmentation density: at least 10 segments per half-wavelength (≈20 per wavelength). Many practitioners prefer 20–40 segments per wavelength for better accuracy.
- Segment length guideline: $\ell_{\text{seg}} \lesssim 0.05\lambda$; use shorter segments where currents change rapidly (feedpoints, loads, bends).
- Place the source on a single, short segment at the feedpoint for stable $Z_{in}$ results.

Bottom line: Too-few segments produce an inaccurate current distribution, leading most noticeably to incorrect feedpoint impedance.