Loop Antennas for Use On/Off Metal Ground Planes

TL;DR

This paper introduces a small loop antenna at 433 MHz that maintains its tuning both on and off a large ground plane, enabling versatile RFID applications without complex structures.

Contribution

It presents a novel antenna design and an equivalent circuit model that passively maintains resonance regardless of ground plane presence, simplifying RFID tag design.

Findings

The antenna maintains free-space resonance on and off a ground plane.

The equivalent circuit explains the dual-environment behavior.

Experimental results confirm ground plane detuning elimination.

Abstract

Many applications benefit from the ability of an RFID tag to operate both on and off a conducting ground plane. This paper presents an electrically small loop antenna at 433 MHz that passively maintains its free-space tune and match when located a certain distance away from a large conducting ground plane. The design achieves this using a single radiation mechanism (that of a loop) in both environments without the use of a ground plane or EBG/AMC structure. An equivalent circuit model is developed that explains the dual-environment behavior and shows that the geometry balances inductive and capacitive parasitics introduced by the ground plane such that the free-space loop reactance, and thus resonant frequency, does not change. A design equation for balancing the inductive and capacitive parasitic effects is derived. Finally, experimental data showing the design eliminates ground plane detuning in practice is presented. The design is suitable for active, "hard" RFID tag applications.