Which of the following improves the efficiency of a ground-mounted quarter-wave vertical antenna?
Why A ground-mounted quarter-wave vertical uses the earth as the other half of the dipole, so the return currents flow through lossy soil. Efficiency is radiation resistance divided by the total of radiation resistance plus loss resistance, and poor ground can add many ohms of loss to a feedpoint resistance of only about 36 ohms. Laying down a system of buried or elevated radial wires gives those currents a low-resistance metallic path instead of dirt, cutting the loss term and raising efficiency.
Watch out Shortening the radiating element makes things worse: it lowers radiation resistance, so the fixed ground and coil losses eat a larger share of the power. Isolating the coax shield from ground affects common-mode current and feedline radiation, not ground-loss efficiency.
Efficiency = Rrad / (Rrad + Rloss). Radials shrink Rloss; more radials, more efficiency.