What is the highest modulation index permitted at the highest modulation frequency for angle modulation below 29.0 MHz?

Angle modulation, which includes both frequency modulation (FM) and phase modulation (PM), creates sidebands that spread the signal across the radio spectrum. Below 29.0 MHz, where amateur HF bands are quite narrow and crowded, keeping the bandwidth of signals tightly controlled is essential to avoid interfering with adjacent stations. To achieve this, the FCC mandates that for angle modulation below 29.0 MHz, the highest modulation index permitted at the highest modulation frequency is 1.0. The modulation index is the ratio of the frequency deviation to the modulating frequency, and limiting it to 1.0 ensures the resulting signal bandwidth stays within acceptable limits for these lower frequency bands. Values like 0.5 are unnecessarily restrictive, while 2.0 or 3.0 would produce a signal that is too wide, violating bandwidth limitations for standard HF operations.