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Recap of the Constraints Governing the Design of Antennas for an NFC Device

In the interest of understanding, let us begin with a few terms of the vocabulary specific to the norms and/or standards of radio frequency identification (RFID), contactless and near-field communication (NFC).

Table 1.1 offers a few examples of jargon applied in different fields of application.

Table 1.1. ISO terminology for the main contactless transmitters and responders

In this book, which deals exclusively with NFC (originally developed by the ECMA in Switzerland, and then taken up again by Sub-Committee SC 06 at the ISO in 2000), we will employ only the official ISO terms: "initiator" and "target". Thus, from this point on, having looked briefly at the principle above, all other terms will be proscribed (or almost).

In the context of NFC applications, this chapter recaps the context, numerous constraints, functional and structural problems relating to the intrinsic content of the NFC protocols and those connected to it, and their direct implications in terms of antennas, which must be dealt with in order to be worthy - in the legal sense (to prevent lawsuits for false advertising) - of the label "Complies with NFC ISO 18092 or 21481 or NFC Forum standard" (in active or passive mode, batteryless or battery-assisted, etc.).

As we will see later on, there are various kinds of such issues.

1.1. Normative constraints

When designing an NFC system and the associated antennas, the technical and protocol constraints needing to be respected are, obviously, the legislational and physical constraints pertaining to the "low layers" 1 and 2 of the open systems interconnection (OSI) model (which are, respectively, the physical and data link/medium access layers), without which the whole setup could never work. As the antenna is part of layer 1 - the physical layer - for all intents and purposes, it is the center of the world.

The forms (appearances, amplitudes, etc.) of NFC signals that must be respected are described in detail in the international standards ISO 18092 NFC IP1 and ISO 21481 NFC IP2 - the lone true standards - which draw extensively on the contactless proximity chip-card standards ISO 14443 A & B (including numerous classes of antennas - 1-6) and on Japanese standard JIS X6319-4 for the patented product FeliCa, and finally those surrounding ISO 15693. In addition to these, there may be proprietary-and/or market-sector-specific standards such as (mainly) NFC Forum, EMVCo and CEN, where the specific operational application characteristics of distances and volumes (in cm3) very frequently involve antennas with different adaptions.

To conclude this introduction, the normative framework of NFC aside, in this book the exchanges taking place between initiators and targets are defined, once and for all, as follows:

1. - "from the initiator to the target" known as uplink;
2. - "from the target to the initiator" known as downlink.

1.1.1. Uplink from initiator to targets

In order to avoid any comprehension problems, note now that, regardless of the intelligence built into the target, it only functions on the basis of "commands" sent by the initiator, which is a TRANSmitter. The initiator also includes a reCEIVER to pick up and interpret communication in the other direction.

Therefore, the initiator is a TRANS. CEIVER; a TRANSCEIVER.

Once again, in order to prevent numerous potential cases of confusion, two potential scenarios are officially defined by the ISO (ISO 19762-3 - Information technology - Automatic identification and data capture (AIDC) techniques - Harmonized vocabulary - Part 3: Radio frequency identification (RFID):

1. - either the energy transmitted by the radio frequency (RF) wave from the initiator is able to provide the necessary power to the target, and in this case the target is "remote-powered" or "batteryless";
2. - or the energy transmitted by the RF wave emitted from the initiator is unable to remotely power the target (which may be the case because of the desired distances of operation, the technologies used, the regulations in force, hostile environments, etc.), and of course, it is important to do something about that. In this case, the target is "battery-assisted", and we are dealing with other types of antennas.

NOTE.- Very frequently - too often, in fact, and incorrectly - remote-powered targets are said to be "passive", while battery-assisted targets are said to be "active". This makes absolutely no sense (see the explanations later on for details).

Nevertheless, it is impossible to put the point across more clearly. Here again, it is important to try to choose the right word.

1.1.2. Downlink from targets to initiator

Independently of the type of power supply to the target (remote-powered or battery-assisted), that device must have an electronic means of communication to perform a downlink from the target to the initiator - also known as the "return link". Downlink can take place in a number of different ways depending on the principles which are used.

It is important not to confuse the power/energy transfer system with the principles of upward and downward communication.

1.1.2.1. "Passive" targets

The adjective "passive" refers to the fact that the downlink - from the target to the initiator - takes place without the use of an RF transmitter.

1.1.2.2. "Active" targets

However, regardless of the way in which the target is powered, if it has a transmitter built in to respond to the initiator, it is said to be "active".

1.1.2.3. Load modulation

In order to achieve downlink, the initiator provides a physical support in the form of a sustained, non-modulated "carrier" frequency, and allows the target to act however it seems fit depending on its own way of working, in order to communicate with the initiator by modulating its electrical characteristics. For this purpose, there are two very similar modulation techniques, and one of which could be considered a "distant relative":

1. - the first, based on a principle of "modulation of impedance (resistance and/or reactance) of the target antenna's load" - known as passive load modulation (PLM) is used by most of the targets available on the market;
2. - the second, which is more recent, is known as active load modulation (ALM), and is still independent of the mode of power supply to the target (whether it is tele-powered or battery-assisted). The target is equipped with a (mini) low-power local transmitter so that just for that period of time, the signals returned to the initiator can be "boosted"., i.e. sent with a newly adapted antenna.

In view of the physical consequences which arise from these two forms of near-field modulation, we typically speak of "retro-modulation" by "magnetic coupling".

In certain systems (such as NFC devices in "peer-to-peer" mode), during the downlink phase in a half-duplex system, the initiator no longer provides a carrier to serve as a support for the return signals. In this particular case, known as active-mode NFC, in order to communicate with the initiator, the target transmits its own wave, calibrated to the same frequency as that of the carrier, and becomes an "active" NFC device.

1.1.2.4. Retro-modulation voltage

Once it has sent interrogation commands, the initiator switches to listen mode, waiting for responses from the target. To this effect, with the exception of P2P mode, the initiator transmits a pure carrier consistently and waits, in the interests of comprehension, for the target to signal its presence and respond by a particular modulation which represents the variation of its load, whether that modulation is passive PLM or active ALM (see the previous sections 1.1.2.2 and 1.1.2.3).

1.1.2.5. Retro-modulation voltage in PLM

In reciprocity of the phenomenon of mutual induction between a secondary party (target) and a primary party...