In near-field coupling between an antenna and its feed line, reactive interactions primarily cause impedance to change in the near-field region.

In near-field coupling between an antenna and its feed line, reactive interactions primarily cause impedance to change in the near-field region.

• A. It does not affect impedance.
• B. It causes reactive changes in impedance.
• C. It improves matching automatically.
• D. It eliminates loss.

Answer: (A)

In the near-field region, stored energy in the electric and magnetic fields around the antenna and its feed line dominates the interaction. This near-field coupling is mutual, and it mainly shows up as a reactive mutual impedance. That means the impedance seen at the input is shifted by an imaginary component (inductive or capacitive) due to the exchange of energy between the line and the antenna, rather than being driven by radiated power alone. The real part can change as well, but the defining effect here is the change in the reactance, which is why matching networks must compensate for this reactive part to achieve good power transfer. The idea that there would be no effect, automatic matching, or elimination of loss isn’t accurate, because near-field coupling inherently introduces this reactive change in impedance.

In the near-field region, stored energy in the electric and magnetic fields around the antenna and its feed line dominates the interaction. This near-field coupling is mutual, and it mainly shows up as a reactive mutual impedance. That means the impedance seen at the input is shifted by an imaginary component (inductive or capacitive) due to the exchange of energy between the line and the antenna, rather than being driven by radiated power alone. The real part can change as well, but the defining effect here is the change in the reactance, which is why matching networks must compensate for this reactive part to achieve good power transfer. The idea that there would be no effect, automatic matching, or elimination of loss isn’t accurate, because near-field coupling inherently introduces this reactive change in impedance.