### I. Introduction

### II. Antenna Design

*ɛ*

*= 4.4, tanδ = 0.02). Each antenna consists of the ground plane, a feed structure, and a radiating element. The size of the ground plane is 100 mm × 50 mm with a clearance of 20 mm × 50 mm for setup. The feed structure is a parallel resonator that includes a feed line, a shorting line, and a shunt capacitor line. The left-loop circuit in the feed structure includes the shorting line and the shunt capacitor line. Because of the distributed inductance and its lumped capacitance, the left-loop circuit is considered an LC-resonance circuit. The resonance frequency of this circuit is usually controlled near the operating frequency of the radiating element to achieve impedance bandwidth enhancement [9–11]. Therefore, the size and value of the shunt capacitor of this loop remain the same in antenna 1 and antenna 2. The left-loop circuit measures 5.5 mm × 0.3 mm, where C*

_{r}_{1}= C

_{2}= 8 pF. The right-loop circuit in the feed structure includes the shunt capacitor line and the feed line. It measures 5.5 mm × 3.2 mm for antenna 1, whereas it measures 1 mm × 3.2 mm for antenna 2.

### III. Operating Mechanism

*I*

_{1}. Next, it couples to the connected left-loop circuit to induce loop current

*I*

_{2}. Energy is mainly delivered to the radiating element through the shorting line, as in conventional PIFA. Thus, in the PIFA, using PIE technology, RF energy is delivered from the feed to the circuits of

*I*

_{1}and

*I*

_{2}, successively, and then to the radiating element. The overlapping part of the two loop circuits is recorded as

*D*, determining the coupling between

*I*

_{1}and

*I*

_{2}.

*Z*elements are lumped elements representing the impedance on a circuit line.

*Z*

*is determined by the right-loop circuit of the feed structure but excludes*

_{a}*D*.

*Z*

*is determined by the left-loop circuit of the feed structure but excludes*

_{b}*D*.

*Z*

*is determined by the distributed inductance and lumped capacitance of overlapping line*

_{c}*D*. Therefore, the voltage value on

*D*can adjust the mutual coupling strength between

*I*

_{1}and

*I*

_{2}[13].

*D*can cause series LC resonance at specific frequencies. This is called “parasitic resonance.” At the frequency of parasitic resonance,

*Z*

*is zero, and the induced voltage on*

_{c}*D*is shorted to zero. Thus,

*I*

_{2}is not efficiently generated, and energy cannot be delivered to the radiating element. Therefore, an efficiency null is generated, accompanied by a narrowed bandwidth.