What feature of an HF radio affects its ability to communicate long distances?

Output power is a critical factor that affects an HF radio's ability to communicate over long distances. In amateur radio, increasing the output power allows the radio signal to propagate further. This is particularly important in HF (high-frequency) bands, where signals can traverse vast distances, especially during favorable propagation conditions.

In HF communication, the energy of the transmitted signal is a primary determinant of how far it can travel. A stronger signal is less likely to be absorbed or interfered with by the atmosphere and other environmental factors, leading to better reception and communication clarity on the receiving end.

While features like input impedance, carrier frequency stability, and modulation depth are important in their own right, they do not primarily dictate how far a signal can travel. Input impedance affects the efficiency of the transmission system, frequency stability may influence overall signal quality, and modulation depth relates to how well the information is conveyed within the signal. However, when it comes to sheer distance capabilities, output power is the most significant factor.