Readers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 305

power occurring at the decoupled port 4 . The same applies if power is supplied to
port 3 , in which case the power is divided between ports 1 and 2 .
A directional coupler is described by its coupling loss :

a k =− 20 · ln | P 2 / P 1 | ( 11 . 1 )

and directivity :

a D =− 20 · ln | P 4 / P 2

|

( 11 . 2 )

Directivity is the logarithmic magnitude of the ratio of undesired overcoupled power
P 4 to desired coupled power P 2 .
A directional coupler for a backscatter RFID reader should have the maximum pos-
sible directivity to minimise the decoupled signal of the transmitter arm at port 4 .The
coupling loss, on the other hand, should be low to decouple the maximum possible pro-
portion of the reflected power P 2 from the transponder to the receiver arm at port 4 .
When a reader employing decoupling based upon a directional coupler is commis-
sioned, it is necessary to ensure that the transmitter antenna is well (anechoically) set
up. Power reflected from the antenna due to poor adjustment is decoupled at port 4 as
backwards power. If the directional coupler has a good coupling loss, even a minimal
mismatching of the transmitter antenna (e.g. by environmental influences) is sufficient
to increase the backwards travelling power to the magnitude of the reflected transponder
power. Nevertheless, the use of a directional coupler gives a significant improvement
compared to the level ratios achieved with a direct connection of transmitter output
module and receiver input.

11.2.1.3 Sequential systems - SEQ

In a sequential RFID system the HF field of the reader is only ever transmitted briefly
to supply the transponder with power and/or send commands to the transponder.
The transponder transmits its data to the reader while the reader is not transmitting.
The transmitter and receiver in the reader are thus active sequentially, like a walkie-
talkie, which also transmits and receives alternately. See Figure 11.7.

Quarz Oscillator Modulator Output module Antenna box

Transmission
data

Received
data

Demodulator Amplifier

Figure 11.7 HF interface for a sequential reader system