What is the most efficient location for a loading coil on an electrically short whip?
Why On an electrically short whip the current tapers from maximum at the base to zero at the tip, and radiated power depends on the area under that current distribution. Putting the loading coil partway up, typically near the center, keeps a higher, more uniform current flowing in the whole lower section instead of letting it taper off immediately, which raises the radiation resistance and efficiency. The tradeoff is that a center coil needs more inductance and has higher loss than a base coil, so in practice center loading is the usual best compromise.
Watch out Base loading, the choice that says as low as possible, is the easiest to build and is common on mobile whips, but it is the least efficient because the current above the coil falls off quickly. Loading at a voltage maximum (the tip) would require an impractically huge coil.
Raise the coil, raise the current: center loading beats base loading for efficiency.