Antenna Reflection Problem

hi all, for antenna reflection, Γ = (Za-Zo)/(Za+Zo). Assume Zo = 50Ω, Za = Ra + jXa.
What will be observed when Xa = 0? change of frequency?

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.