Why do antennas couple?

[Microfielder]

Why does antenna loading actually occur? In placing an object in the reactive near field the antenna resonance frequency shifts, but why? Is there a 'physical' connection (inductance/capacitance in the space between them) that causes the antenna to appear longer altering the resonance frequency?
Thank you

 

[VEBill]

If you think about the equivalent circuit, you're probably adding capacitance (probably lossy capacitance) to ground.

 

[Higgler]

when your antenna moves electrons, they push the electrons on the piece of metal next to it. electrons repel each other, that's the force to move electrons on the nearby metal. Your antenna moves electrons up and down, or closer and further from the nearby metal piece.

Now if any antenna's electron movement affects another, then the electrons moved on the piece of metal affect your antenna's electrons the same way, hence the change in your antenna.

 

[rflink]

If you look carefully at the antenna Field expressions in near field you will find that the expression is complex.

The capacitance and inductance that you were looking for emenate from these imaginary fields=expressions (you can translate these fields into impedance).
Much like a capacitor and inductor, the energy is stored in the Electric and magnetic field, in this case surrounding the antenna.

As you go further from the antenna into Far-Field, these fields decay rapidly and you are left with the propagating fields alone.

Hope it answered your question.

Heylal
RF-Link

http://www.rflinkcalc.com