22C:118 RFID Honors Project

Antenna Design

One of the limiting factors in low frequency passive RFID is reading distance. Maximum reading distance is determined by frequency, power, and signal interference. Typical reading distance is only a few centimeters. Because increasing power and frequency is not always practical a common solution to increasing reading distance is modify the antenna being used.

The magnetic induction type antenna used for low frequency RFID is constructed from multiple turns of magnetic wire in a loop. Generally a bigger radius of the loop will result in a greater reading distance. The limit to this rule is that eventually resistance and electro-magnetic interference will become to great for a large antenna to function correctly.

The main challenge of antenna design is to determine the optimal number of turns. The number of turns determines the magnetic inductance of the loop. There are many formulas that approximate how many turns are required to achieve a specified inductance. For example this equation gives N turns based on loop radius a, the loop height h, the loop width b, and magnetic inductance L.

Unfortunately this equation is only an approximation for ideal conditions. In practice the number of turns can change based on the properties of wire being used and the shape of the antenna being used.

A more practical approach to antenna design is to measure the inductance during construction and adding turns until the specified inductance is reached. Unfortunately measuring inductance accurately requires a highly specialized and expensive inductance meter. A cruder approach is to match the resonant frequency of and LCR circuit and then use this formula to find the inductance.

An LCR circuit is an inductor with inductance L, a capacitor with capacitance C, and a resistor all placed in series. An oscilloscope can be used to examine how this circuit responds to different frequencies produced by a sine wave generator. The peak response will be at the frequency of natural resonance of the circuit. This frequency can then be used in the formula above to calculate L.

The picture below shows the resonance frequency being found for an antenna. The green shows the generated sine wave and the yellow shows the circuit response. The highest peak in the spectral graph shows the fundamental frequency in near prefect resonance.

Below is a large antenna that was constructed with this method shown with the smaller antenna that came with the kit. Both antennas have an inductance of about 900 uH. The smaller antenna has a read range of about 8.5 cm. The larger antenna is 12 cm in diameter and has a read range of about 10 cm. This is not a large difference but even larger antennas that are more carefully tuned could show better results.