How does radiation resistance of a base-fed whip antenna change below its resonant frequency?

Radiation resistance is the portion of an antenna's feed point impedance that represents the power actually radiated into space. For a given antenna structure, this value is heavily dependent on the antenna's electrical length. As the operating frequency is lowered below the antenna's natural resonant frequency, the antenna becomes electrically shorter. An electrically shorter antenna captures and radiates less electromagnetic energy, causing its radiation resistance to steadily decrease. This is a critical concept for mobile HF operators. When operating a fixed-length whip on lower frequencies like 80 meters, the radiation resistance drops to just a few ohms, meaning that even small amounts of ground or coil resistance will waste a massive percentage of your transmitted power.