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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.