Backscatter Radio Links
Passive and semipassive RFID tags do not use a radio transmitter; instead, they use modulation of the reflected power from the tag antenna. Reflection of radio waves from an object has been a subject of active study since the development of radar began in the 1930s, and the use of backscattered radio for communications since Harry Stockman's work (See Chapter 2) in 1949.

A very simple way to understand backscatter modulation is shown schematically in Figure 14: current flowing on a transmitting antenna leads to a voltage induced on a receiving antenna. If the antenna is connected to a load, which presents little impediment to current flow, it seems reasonable that a current will be induced on the receiving antenna. In the figure, the smallest possible load, a short circuit, is illustrated. This induced current is no different from the current on the transmitting antenna that started things out in the first place: it leads to radiation. (A principle of electromagnetic theory almost always valid in the ordinary world, the principle of reciprocity, says that any structure that receives a wave can also transmit a wave. We shall make use of this principle in discussing antennas in greater detail shortly.) The radiated wave can make its way back to the transmitting antenna, induce a voltage, and therefore, produce a signal that can be detected: a backscattered signal. On the other hand, if instead a load that permits little current to flow--that is, a load with a large impedance--is placed between the antenna and ground, it seems reasonable that little or no induced current will result. In Figure 14, we show the largest possible load, an open circuit (no connection at all). Since it is currents on the antenna that lead to radiation, there will be no backscattered signal in this case. Therefore, the signal on the transmitting antenna is sensitive to the load connected to the receiving antenna.

To construct a practical communications link using this scheme, we can attach a transistor as the antenna load (Figure 15). When the transistor gate contact is held at the appropriate potential to turn the transistor on, current travels readily through the channel, similar to a short circuit. When the gate is turned off, the channel becomes substantially nonconductive. Since the current induced on the antenna, and thus, the backscattered wave received at the reader, depend on the load presented to the antenna, this scheme creates a modulated backscattered wave at the reader. Note that the modulating signal presented to the transistor is a baseband signal at a low frequency of a few hundred kHz at most, even though the reflected signal to the reader may be at 915 MHz. The use of the backscatter link means that the modulation switching circuitry in the tag only needs to operate at modest frequencies comparable to the data, not the carrier frequency, resulting in savings of cost and power. (Real RFID tag ICs are not quite this simple and may use a small change in capacitance to modulate the antenna current instead, for reasons we will discuss in Chapter 5.)

Note that in order to implement a backscattered scheme, the reader must transmit a signal. In many radio systems, the transmitter turns off when the receiver is trying to acquire a signal; this scheme is known as half-duplex to distinguish it from the case where the transmitter and receiver may operate simultaneously (known as a full-duplex radio). In a passive RFID system, the transmitter does not turn itself off but instead, transmits CW during the time the receiver is listening for the tag signal. RFID radios use specialized components known as circulators or couplers to allow only reflected signals to get to the receiver, which might otherwise be saturated by the huge transmitted signal. However, in a single-antenna system, the transmitted signal from the reader bounces off its own antenna back into the receiver, and the transmitted wave from the antenna bounces off any nearby objects such as desks, tables, people, coffee cups, metal boxes, and all the other junk that real environments are filled with, in addition to the poor little tag antenna we're trying to see (Figure 16). If two antennas are used (one for transmit and one for receive), there is still typically some signal power that leaks directly from one to the other, as well as the aforesaid spurious reflections from objects in the neighborhood.