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Designing Custom HF Antennas for RFID Applications

How to build and tune high-frequency antennas that can operate at 13.56 MHz.
By Akshay Bal
Jan 11, 2018

The purpose of this article is to explain, in detail, the process involved in building and tuning a high-frequency (HF) RFID antenna, which would operate at 13.56 MHz. Some concepts will be explained regarding how an antenna works, and how to tune one to make it work at a given frequency. Various tools are involved in this process, such as an antenna analyzer, a voltage standing wave ratio (VSWR) meter, HF RFID readers and so forth. The procedure outlined below can help you understand the process of making and testing an HF RFID antenna.

A loop antenna is a tuned LC circuit for a particular frequency, which is 13.56 MHz for the purpose of our application. When the inductive impedance (XL) is equal to the capacitive impedance (XC), the antenna will be at resonance. This is when you can have the antenna read multiple tags.

Akshay Bal
One of the most important factors in designing these antennas is impedance matching, which is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source in order to maximize power transfer and minimize reflections from the load. Impedance matching is used in the design of RF circuitry to provide the maximum possible transfer of power between a source and its load.

The concept of impedance matching was originally developed for electrical power, but can be applied to any other field in which a form of energy (not necessarily electrical) is transferred between a source and a load. For optimum performance, the antenna and its feeder coaxial cable must have an impedance of 50 ohms. Matching changes the impedance of a resonant loop to 50 ohms, and the accuracy of the matching is checked by the VSWR (< 1:1.2) on the VSWR meter.

The maximum transfer of power, from a source to its load, occurs when the load impedance is equal to the complex conjugate of the source impedance. Therefore, the primary objective in any impedance-matching scheme is to force a load impedance to match the complex conjugate of the source impedance so that maximum power can be transferred to the load.

To minimize interference, you will need to use coaxial cable to connect the antenna to a transmitter or receiver. Coaxial cable behaves as a transmission line at radio frequencies; as a result, it has its own characteristic impedance. This simply means that because of the inductance-to-capacitance (L/C) ratio of the cable, RF energy tends to move along it with a particular ratio between the electric and magnetic fields (that is, voltage to current).

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