Read this: It is possible to use the impedance matching concepts to construct vertical antennas substantially shorter than the 14 wavelength at which the antenna is resonant. By adding an inductance in series with the antenna, a so-called loading coil, the capacitive reactance of this antenna can be cancelled leaving a pure resistance which can then be matched to the transmission line. Sometimes the resulting resonant frequency of such a system (antenna plus matching network) is described using the construct of electrical length and the use of a shorter antenna at a lower frequency than its resonant frequency is termed electrical lengthening.

What could be coupled with an antenna to form pure resistance?
What is the answer? (If it cannot be answered, return "unanswerable")
inductance