How does radiation resistance of a base-fed whip antenna change below its resonant frequency?
Why Radiation resistance measures how effectively the antenna converts current into radiated power, and for a short vertical it scales with the square of the antenna's electrical length: roughly 40*pi^2*(h/lambda)^2 ohms for a short monopole over a good ground. Operating below the resonant frequency makes the whip a smaller fraction of a wavelength, so h/lambda drops and the radiation resistance falls, often to just a few ohms or less. That low resistance is why loaded short verticals lose so much power in coil and ground losses, since those losses are then comparable to or larger than the radiation resistance.
Watch out The idea that it becomes imaginary confuses radiation resistance with the feed point reactance: an electrically short whip does develop a large capacitive (imaginary) reactance below resonance, but radiation resistance itself stays a real number.
Radiation resistance follows (height/wavelength) squared: shorter in wavelengths means lower R.