This calculator estimates the inductance required for a loading coil when an antenna is physically shorter than the ideal resonant length at a given frequency.

The formula used is: \[ L = \frac{(L_r - L_a) \cdot 7500}{f^2} \] Where:

• L = required inductance (µH)
• L_r = resonant length = 468 / f (in feet)
• L_a = actual antenna length (in feet)
• f = frequency in MHz

The constant 7500 comes from empirical derivations and approximations based on antenna theory and practical measurements. It provides a reasonable estimate for the inductance needed to electrically compensate for the missing physical length in HF antenna applications.

This is useful for designing mobile antennas, compact HF verticals, or other space-limited installations. Always fine-tune with an SWR meter or antenna analyzer after installation.