Designing Custom HF Antennas for RFID Applications

By Akshay Bal

How to build and tune high-frequency antennas that can operate at 13.56 MHz.

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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).

In most cases, when the energy reaches the end of the cable, you will want as much of that energy as possible to transfer into the load. The antenna (in the case of a transmitter) or the input RF stage (in the case of a receiver for a transmitter) offers the highest power efficiency, while for a receiver it provides the best noise performance.

To ensure this optimum energy transfer, you will need to match the characteristic impedance of the cable to the impedance and resistance of the load. So for a 75-watt antenna or receiver input, you will need to use 75-watt coaxial cable; for a 50-watt antenna, you will need to use 50-watt cable; and so on. This, then, is an area in which impedance matching is quite important, because if the cable and antenna (or receiver) impedances do not match, some of the RF energy reaching the end of the cable will not be transferred into the load—rather, it will be reflected back along the cable, toward the source.

This can set up standing waves in the cable (another cause of power loss, and possibly cable damage), and it is important to ensure that the transmitter output stage will feed as much RF energy as possible into the cable’s input impedance. There can even be an advantage in deliberately mismatching the impedances (that is, having the transmitter impedance much lower than the cable), to minimize power loss in the final stage and ensure that if RF is reflected back from the antenna end, most of it will be bounced back up again.

Small changes in capacitance can make large differences to the matching, so it is easy to miss the window when using trial-and-error methods. This is particularly the case with larger antennas, for which only small picofarad capacitance values are required for matching. The formulas used in these cases usually provide inexact values, as there are many variables involved—for instance, changing the length of a capacitor’s legs. They are a starting point for trial-and-error work with your antenna analyzer.

The most common case for measuring and examining SWR is when installing and tuning transmitting antennas. When a transmitter is connected to an antenna by a feed line, the impedance of the antenna and feed line must match exactly in order for maximum energy transfer from the feed line to the antenna to be possible. The impedance of the antenna varies based on many factors, including the antenna’s natural resonance at the frequency being transmitted, the antenna’s height above the ground and the size of the conductors used to construct the antenna.

When an antenna and feed line do not have matching impedances, some of the electrical energy cannot be transferred from the feed line to the antenna. Energy not transferred to the antenna is reflected back toward the transmitter. It is the interaction of these reflected waves with forward waves that causes standing wave patterns. Reflected power has three main implications in radio transmitters: RF energy losses increase, distortion on the transmitter can occur due to reflected power from the load, and damage to the transmitter can result.

Matching the impedance of the antenna to that of the feed line can also be accomplished using a custom-designed antenna tuner. The tuner can be installed between the transmitter and the feed line, or between the feed line and the antenna. Both installation methods will allow the transmitter to operate at a low SWR; however, if the tuner is installed at the transmitter, the feed line between the tuner and the antenna will still operate with a high SWR, causing additional RF energy to be lost through the feed line.

Keeping these factors in mind, if you are designing an HF RFID antenna, consider making a prototype with a one-inch copper tape. This tape must be laid across a platform to which it can be adhered. Clean the surface and make sure it does not have any sort of unwanted material on it, which may cause problems when the antenna is being tuned. The antenna’s range may vary depending on the tuning of the given antenna size and the dielectric of the material to which it is attached.

In order to make an HF antenna that can be attached to an HF reader, lay out a copper tape on a flat surface material in a rectangular shape. When laying the copper tape out, be sure that you have two openings to which the co-axial cable can be attached; this, in turn, will be attached to the reader output. Make sure that the tape is completely adhered to the material’s surface. In order to match the antenna to 50 ohms, you will first need to determine its resistance and reactance in order to tune it. Connect an RG 174 wire to the antenna analyzer and the antenna itself.

After tuning the antennas and determining the required capacitive and inductive values, ensure that the antenna resonates at 13.56 MHz, and that the R value is close to 50 and the impedance value is 0, while the SWR ration should be close to 1. When this antenna is connected to an HF reader at 13.56 MHz, you can then begin reading tags at a distance of a few inches from the antenna’s surface. These custom-designed antennas can, therefore, be used for various RFID applications as needed.

Akshay Bal is an industry-recognized RFID innovator with extensive experience designing and implementing RFID and logistics systems for Fortune 1,000 companies and U.S. government agencies. His contributions in the field of HF antenna design and building passive UHF RFID solutions have seen organizations successfully utilize and adapt RFID technology throughout Europe, the Americas and the Asia-Pacific Economic Cooperation (APEC). He holds a master’s degree in engineering from California’s San Jose State University.