Your antenna is a FIXED size, so therefore it's "natural" frequency is also FIXED.
Your transmitter will transmit on whichever frequency you select.
In order to "match" the transmitter to the antenna, you have the coupling unit which is what will be adjusting capacitance and inductance to make sure no power is reflected back from the antenna.
More information here
Antenna (radio) - Wikipedia, the free encyclopedia)
Happy reading.
I am an ex merchant navy radio officer and to match our antennas over a wide HF frequency range we had a cabinet the size of a refrigerator with winding handles, this was our version of your coupling unit.
I now work as an ATCO at Scottish Control, so just push my transmit button to talk to you guys, leaving the engineers to worry about the systems.