Well, actually, I'm not sure I really understand what is the meaning of matching impedance, even for an ordinary antenna, but I can figure out the following: if you send a signal to the antenna, then there is no reflection whenever the impedance is matched (say to 50 Ohm). I can also figure out that AT A GIVEN FREQUENCY, you can match the impedance by adding (or cutting) some length of coaxial cable, or with a suitable RF transformer.
But now comes my real problem: I did an ultra-wideband self-grounded bowtie antenna, (400MHz - 3GHZ), and I am even less sure that I understand what is the meaning of "matching impedance" for these antennas.
Assuming it is, as previously that there are no reflections, how to do such a matching ? all the matching methods I know depend upon the frequency of the signal, but here, you have no defined frequency.
EDIT: It is tempting to think (and probably to answer) that it is not possible to do an ultra-wide band matching. Yet, let take a very expensive double ridge horn antenna. They claim that the antenna is roughly flat in the huge range of 500MHz - 18 GHz (say). So, how do they match their antenna ?