READER ANTENNA AND REFLECTOR SIZE

Tests regarding distance range, reader antenna types and reflector sizes (for RCS reference) were executed. First, a test of reading distance was done with two different antennas. The objective was to verify the maximum achievable distance with the measurement setup available at the Laboratory. For this purpose, a 10 cm x 10 cm metallic plate was employed and two antennas were used (shown in Fig. B.1): a quasi-Yagi antenna and a LPDA antenna.

Figure B.1: Implemented antennas for chipless measurements.

(a) quasi-Yagi (b) LPDA

Source: The author.

The results regarding each of the mentioned antennas are shown in Fig. B.2 and B.3. These results showed that larger distance ranges could be achieved with the LPDA antenna. This was expected since its simulated directivity was above 6 dBi, in contrast to approximately 3 dBi of the quasi-Yagi counterpart.

Figure B.2: Measured (a)|S11|(minus clutter) and (b) RCS of loaded ORR with copper tape trace and PET substrate using the quasi-yagi antenna at different distances with 10 cm×10 cm metallic plate.

(a)|S11|

Freq [GHz]

-30 -40 -50 -60 -70

-802.3 2.4 2.5 2.6 2.7 2.8 2.9 3

|S11| [dB]

(b) RCS

RCS [dBsm]

Freq [GHz]

-20

-30

-40

-50

-60

3.2 3 2.8 2.6 2.4

Source: The author.

It is also observed that, when a successful measurement is achieved, the tag information is distinguishable directly from the reflection coeffi- cient (S11) information. Also, as noticed from the results in B.3(b) at 65 cm, slight measurement inaccuracies can turn the RCS results useless.

That is, the result of the measurement showed a good result in S11, but an indistinguishable peak at the RCS. This issue might have been

Figure B.3: Measured (a)|S11|(minus clutter) and (b) RCS of loaded ORR with copper tape trace and Epson paper substrate using the LPDA antenna at different distances with 10 cm×10 cm metallic plate.

(a)|S11|

Freq [GHz]

-50 -55 -60 -65 -70 -75 -80

3 2.9 2.8 2.7 2.6 2.5 2.4 2.3

|S11| [dB] ^50cm_65cm

(b) RCS

Freq [GHz]

-20

-30

-40

-50

-60

3.2 3 2.8 2.6

50cm 65cm

2.4

RCS [dBsm]

Source: The author.

caused by an artifact during the reflector measurement. This is proved by comparing the same measurement with other reflectors as seen next.

Figure B.4 compares the RCS of the tag at 65 cm using reflectors of different sizes as references. Each of the plate sizes are listed in Table.

B.1. Two things can be verified: first, the peak information at such distance remains very prone to error, due to measurement errors and the relative small reflected power from the tag compared to the clutter and metallic plates; second, the absolute value of the RCS of the tag is not the same for each measurement.

Table B.1: Metallic plates dimensions used as RCS references for chipless measurements.

plate ID dimensions [cm×cm]

1 10×10

2 51.8×34.2

3 55.8×46

4 112.4×65.2

For the results calculation shown in Fig. B.4, the theoretical RCS of the metallic plates was used as the reference (RCSplate= 4πA²/λ², whereAis the plate area). To make sure this is a good approximation, the RCS obtained from FDTD simulations were compared for two plates with different areas, as shown in Fig. B.5. The bigger plate presented simulated RCS results very close to the theoretical predictions. The smallest plate showed a very close result to the theoretical calculations within the frequency band of interest. Therefore, these results proved

Figure B.4: Measured RCS of loaded ORR with copper tape trace and Epson paper substrate using the LPDA antenna at a 65 cm distance with different metallic plates.

Freq [GHz]

0 -10 -20 -30 -40 -50

-60 3.2

plate1 plate2 plate3(H) plate3(V) plate4

3 2.8 2.6 2.4

RCS [dBsm]

Source: The author.

that all RCS of the used plates can be approximated to its theoretical formulas.

Figure B.5: RCS of metallic plates, simulated vs theoretical.

(a) plate 1 (b) plate 2

Source: The author.

To find a possible explanation to the results shown in Fig. B.4, another set of measurements were performed. Fig. B.6 shows the mea- sured reflection coefficient (with clutter subtracted) from two plates measured at different distances with two different antennas. It can be observed that in the case of the smaller plate (plate 1), the magnitude of the reflection coefficient changes gradually with distance. On the other hand, the change of the reflected power in the case of the bigger plate is not as expressive as in the first case, for both antennas. This means that the reader does was not able to perceive a difference on the received power.

A possible explanation to this effect is related to the radiation pattern and the beam-width of the maximum lobe of the reader antenna.

It might be the case that when the reflector is considerably big, enough so its main lobe is almost totally reflected, the antenna is not capable of perceive any change on the reflected power, causing a misreading of the RCS. This phenomena is represented in Fig. B.7.

Figure B.6: Measured S11 magnitude of metallic plates vs distance, of (a) plate 1 vs plate 5 with quasi-Yagi antenna, (b) plate 1 vs plate 2 with LPDA antenna.

(a) All sensors

2 2.2 2.4 2.6 2.8 3

-50 -45 -40 -35 -30 -25 -20

Freq [GHz]

|S11| [dB]

Plate 1

2 2.2 2.4 2.6 2.8 3

-50 -45 -40 -35 -30 -25 -20

Freq [GHz]

|S11| [dB]

20 cm 30 cm 40 cm Plate 5

(b) ORR vs Sensor

2 2.2 2.4 2.6 2.8 3

-65 -60 -55 -50 -45 -40 -35

Freq [GHz]

|S11| [dB]

2 2.2 2.4 2.6 2.8 3

-55 -50 -45 -40 -35 -30 -25

Freq [GHz]

|S11| [dB]

40 cm 65 cm Plate 2

Plate 1

Source: The author.

As a conclusion, it can be stated that for small reading distances, big metallic plates may lead to erroneous RCS estimation of the tag.

In contrast, an small plate (such as plate 1) would be accurate enough for the reading distances used in this work. Moreover, its measured RCS is not expected to be much different than the estimated by theory.

For a more exact determination of the appropriate reading distance for a metallic reflector, the beamwidth of the reader antenna should be always considered.

Figure B.7: Beamwidth effect on miscalculation of metallic reflector RCS as a function of distance.

distance Reader

metallic reectors

Source: The author.

C CHARACTERIZATION OF MATERIALS USED IN LOW- COST CHIPLESS SENSORS

C.1 CONDUCTIVE INKS FOR INKJET PRINTED CIRCUITS