A short vertical can be made electrically resonant by adding a horizontal "top hat." Slide the frequency and the vertical down-lead height, and watch how much flat-top wire you need — the total always adds up to one quarter-wave.
How it works. A quarter-wave (90° electrical) at this frequency is the target. The vertical down-lead supplies part of that; the horizontal flat-top supplies the rest as capacitive "top loading." In this first-order model the two simply add to one quarter-wave, so total wire is constant for a given frequency — shorten the tower and the top hat grows.
Reality check. This is an idealized estimate, not a build spec. Top-hat loading isn't a perfect 1:1 substitute for vertical height, so treat the numbers as a starting point. Real stations don't cut to exact resonance anyway — an antenna tuning unit (ATU) at the base does the final matching. And the deeper reason to add a top hat isn't just resonance: it raises the short vertical's radiation resistance, so ground losses eat less of your power. Formula: λ (ft) = 984 ÷ f (MHz); quarter-wave = 246 ÷ f (MHz).