Which of the following is an advantage of using a horizontally polarized as compared to a vertically polarized HF antenna?
Why A vertical monopole works against ground: the return currents flow through the soil near the base, and lossy earth absorbs a significant part of the power unless you install an extensive radial system. A horizontal dipole is a self-contained, balanced radiator that does not rely on earth as part of the antenna, so ground conduction losses are much smaller (though ground still affects the pattern and takeoff angle). That is why a horizontal antenna at a decent height often outperforms a ground-mounted vertical on HF despite the vertical's low-angle pattern.
Watch out Feed point impedance and radiation resistance actually go the other way: a quarter-wave vertical shows about 35 ohms versus roughly 70 ohms for a halfwave dipole, so the horizontal antenna has the higher values. Radials are a vertical-antenna feature and are not part of a horizontal dipole at all.
Verticals borrow the dirt for their other half; horizontal dipoles do not, so they lose less in the ground.