Which of the following determines ground losses for a ground-mounted vertical antenna operating on HF?
Why A ground-mounted vertical works against the earth, so return currents flow through the soil near the base and dissipate power as heat. How much is lost depends on how well the soil conducts, which is set by soil conductivity (and moisture/mineral content). Poor, dry, sandy soil has high loss; salt marsh or seawater is nearly lossless, which is why adding many buried radials helps by carrying the return current in copper instead of dirt.
Watch out Take-off angle is tempting because ground quality does affect the far-field pattern and low-angle radiation, but that is a pattern effect, not what creates the I-squared-R loss at the antenna base. SWR just describes the impedance match on the feed line and can even look better when losses are worse.
Ground loss lives in the dirt: conductivity rules. Bad soil? Bury more radials.