Question E1C09:
[97.307] What is the highest modulation index permitted at the highest modulation frequency for angle modulation below 29.0 MHz?  

Choices:  
A: 0.5  
B: 1.0  
C: 2.0  
D: 3.0    

Correct Answer is  B: 1.0  

To comply with FCC 97.307 for angle modulation (FM or PM) below 29.0 MHz, the modulation index at the highest modulating frequency must not exceed 1.0.

Why B (1.0) is correct:
- The rule limits the angle-modulation index at the highest audio/modulating frequency to $β \le 1.0$ to control occupied bandwidth and prevent excessive sidebands.
- For FM: $β = \dfrac{\Delta f}{f_m}$, so at the highest $f_m$, you must have $\Delta f \le f_m$.
- For PM: the modulation index is the peak phase deviation in radians, limited to 1.0 at the highest $f_m$.

Why the others are incorrect:
- A: 0.5 — This is within the limit but is not the maximum permitted. The question asks for the highest permitted value.
- C: 2.0 — Exceeds the regulatory cap and would create excessive bandwidth.
- D: 3.0 — Even further beyond the allowed limit.

Useful formulas and rules of thumb:
- FM modulation index: $$β = \frac{\Delta f}{f_m}$$
- Carson’s Rule (occupied bandwidth estimate): $$B \approx 2(\Delta f + f_m) = 2 f_m(\,β + 1\,)$$
  - Keeping $β \le 1$ at the highest $f_m$ helps keep $B$ in check.
- Practical check: If your highest audio frequency is 3 kHz, then set deviation $\Delta f \le 3$ kHz so that $β \le 1$ at $f_m = 3$ kHz.