A wire dipole has a resonant frequency of 3600 kHz. How can you change its resonant frequency to 3900 kHz?

To raise the resonant frequency of an antenna from 3600 kHz to 3900 kHz, the operating wavelength must decrease. This is due to the inverse relationship between frequency and wavelength: as one goes up, the other goes down. Since a dipole's physical length is tied to its resonant wavelength, raising the frequency requires you to make the antenna physically shorter. Adjusting the wire diameter or changing the installation to a sloper affects other characteristics like impedance and bandwidth, but cutting the wire shorter is the standard method for tuning to a higher frequency.