Think of a guitar string. You pluck it and it vibrates at a certain frequency. That is the resonant mechanical frequency of the guitar string.
With an antenna we are not looking for a mechanical frequency, but an electrical resonant frequency, but it is still the same concept, just applied a little differently.
Let me try and explain further, there are two types of "reactance", and the combination of those determines the antennas resonant frequency. The two types of reactance are inductive and capacitive.
If you take an antenna and add inductive reactance it is like adding length to the antenna, or adding length to the guitar string in the example above, they resonate at a lower frequency.
If you add capacitive reactance it is like shortening the the antenna or the guitar string in the example above. This is seen on the guitar when holding the strings when plucking them. Both of these cases leads to raising the resonant frequency.
In short, when working with antennas, resonance is the frequency at which inductive and capacitive reactances are in balance, or add up to 0.
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