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

Frequency and wavelength share an inverse relationship in radio theory. When you want to lower the resonant frequency of an antenna, such as moving from 3900 kHz down to 3600 kHz, the corresponding wavelength increases. Because an antenna's physical length is directly proportional to its resonant wavelength, you must physically increase the length of the wire to match the new, longer wavelength. Changing the wire diameter or installing it as a sloper might affect bandwidth or radiation pattern, but it will not effectively retune the antenna to a lower frequency.