What must the frequency stability of a transmitter be when operating below 148 MHz over one hour?

The frequency stability requirement for transmitters operating below 148 MHz over one hour is indeed two parts per million (ppm). This standard is particularly important in ensuring that the transmission remains within the allotted frequency spectrum and does not interfere with adjacent channel operations. Frequency stability relates to how much the frequency can drift over time; for reliable communication, especially in tightly regulated bands like amateur radio, maintaining stability minimizes the risk of interference with other users.

In practical terms, a stability of two parts per million means that for a 10 MHz signal, the frequency could drift by a maximum of 20 Hz over the course of an hour, which is a very small and manageable deviation. This level of precision is necessary for ensuring that multiple users can operate in close frequency ranges without causing interference, thereby promoting the efficient use of the radio spectrum.

In contrast, options suggesting a different level of frequency stability would not meet the regulatory requirements for operating parameters below 148 MHz. Higher or lower ppm values are either overly stringent or insufficient, potentially allowing for drift that could lead to accidental interference with legitimate communications. Thus, a frequency stability of two parts per million appropriately balances operational reliability with regulatory compliance in the amateur radio context.