# Programmable Organic Chipless RFID Tags Inkjet Printed on Paper Substrates
## 4 Experimental Tag Validation and Results  
In this section, three different 20-bit tags with all bits set to '1' were fabricated with one, two, and three printed ink layers in order to compare the effect of the resonator ink conductivity on the measured envelope signal. The tags consisted of a linear chain of resonant elements, identical to the one of the reader line. In addition, two trigger bits set to '1' were added at the beginning and at the end of the tag chain in order to know the start and end of the reader system to acquire the data signal. The tags were fabricated by inkjet printing using PEDOT:PSS on ordinary paper substrates (DIN A4). The measured dielectric constant and loss tangent of the paper were \(\varepsilon_{\text{r}}=3.11\) and tan\(\delta=0.036\) and the measurements were carried out by means of the resonant cavity _Agilent 85072A_.  
The measured envelope functions of the fabricated tags are depicted in Figure 5. As can be appreciated, the functional resonators (providing the logic state '1') are detected by means of dips in the envelope functions, and tag reading provides the corresponding ID code. However, as the number of printed layers decreases (representing a reduction in the conductivity), the modulation index in the envelope function decreases, and this jeopardizes the detection of the ID code by means of post-processing stages. For example, in the tag printed with one ink layer, it is difficult to obtain the ID code, especially when the tag is read face down. On the contrary, by printing two or three layers, good modulation index for both sides of the tag are obtained, and it suffices to detect the tag code on both faces. Note that the different signal offset levels were obtained as a function of the tag-reading orientation. When the tags are read face up, the envelope signal offsets are in the same level (near to 1,5V). However, when the tags are read face down, they are in a lower level and different for the three tags. This difference is, in part, due to the ink conductivity and the air gap between the tag and the reader [57].  
Another example of a 20-bit tag envelope signal is depicted in Figure 6. In this case, only two layers were printed, and it was enough to detect the ID code clearly. Note that when a bit is set to '0', a very small variation of the envelope signal is observed because of the absence of a certain resonant element at their predefined position (the chain resonator is detuned). With these results, the functionality of the proposed tags, implemented by means of organic inks and printing two layers on ordinary paper substrates, was demonstrated.  
Figure 4: Van der Pauw measurements as a function of one, two, and three PEDOT:PSS ink layers.  
The small difference between the face up and face down measured envelope functions allows the addition of a cellulose layer on top of the printed resonators in order to hide them. This approach improves the security of the printed tags because after printing, the resonant elements are buried (sandwiched) in the paper substrate.  
Figure 5: Measured envelope functions of the fabricated tags as a function of one two and three ink layers printed.  
Figure 6: Measured envelope functions of the fabricated tag printed with two ink layers.