What factors determine the radiation resistance of an antenna?

The radiation resistance of an antenna is a function of its physical geometry and its surrounding environment. The primary physical factor is the electrical length of the elements, which is influenced by the physical length-to-diameter ratio of the conductor used. Additionally, the environment plays a massive role. The proximity of the antenna to the ground and other nearby conductive objects introduces mutual coupling and reflections that dynamically alter the feed-point impedance and radiation resistance. Distractors involving sunspots or time of day confuse antenna impedance characteristics with ionospheric propagation factors. Furthermore, radiation resistance is definitely not a universal physical constant; it varies wildly depending on antenna design.