If Xa is zero, then the impedance of the antenna is purely resistive.
The resonant frequency will move in the opposite direction where the reactive impedance was (capacitive or inductive).
Uh, a constant Xa typically means a change in frequency...what's the question?
Impedance matching typically chooses an Xa so that Xa =0 so you can match the real Ra to Z0. As in a basic EM/Tline text. Is that your question, or something else? Typically you design a circuit so you cancel out Xa of the antenna, thus delivering max power to the antenna. Sorry if this is too basic, I'm not sure what you're asking beyond this.