RESULTS AND DISCUSSIONS

The backscattered response of the tag was measured using the horn antenna which is connected to the two ports of the PNA. Variations of magnitude (we can call it as S21), and phase of the reflected signal from the tag and hence group delay were observed over certain band of frequencies. However, near the fundamental frequency at 3.7 GHz, strong Fig.6. 13 : Measurement set-up for sensor tag application.

Humidity-Temperature Meter

Tag Dual polarized

horn antenna

VNA

variations were found.

Fig.6.14 shows the S21 magnitude obtained for a humidity variation of 60.2%-88% for an ambient temperature of 23°C. The S21 magnitude shows a clear variation near the fundamental frequency relative to the humidity change. A variation of 30 dB was observed. It is clear from the figure that, while increasing the humidity variation, the S21 is shifting to the lower frequency region. This can be explained by the fact that the nanowires changes permittivity while absorbing water.

The changes are inversely proportional as shown in Fig.6. 15 (a). Using curve fitting method, the analytical expression between RH and the S21 magnitude in the fundamental frequency for a temperature of 23°C has been found as,

RH= a/S21(dB) + b

where a=2.918e+3, b=145 are constants which are calculated using the curve fitting method. From the figure we can see that this equation is valid for range of humidity between 65%-86%. Again, we observed that when S21 is increasing, RH is decreasing which corresponds to the already stated expression, RH=a*(S21_dB)+b with a<0 (for 10% RH range).

The expression obtained for a high range of RH is not linear, instead is inversely proportional.

Fig.6. 14 : S21 magnitude of the sensor tag (as shown in Fig.6. 2(b)) for a relative humidity variation of 60.2%-88% for an ambient temperature of 23°C.

Frequency (GHz) 40 MHz RH=88%

RH=60.2%

S21 (dB)

Fig.6. 15 (b) depicts group delay variation with respect to a change in relative humidity. As shown in the figure, a group delay variation of 13 ns was observed in the positive real axis. A negative group delay of 10 ns was observed after reaching certain humidity range which can be attributed to the presence of absorption in an already highly dispersive structure. In this limited frequency band, the group velocity no longer represents the velocity of a signal or of energy transport and ceases to have a clear physical meaning [16]. However, it is still possible to retrieve this information which is directly related to the humidity. The group delay peak is minimum for a minimum value of the relative humidity of 62.5%. The delay peak increases with respect to humidity and reaches the maximum value.

This is due to the known fact that, a high permittivity causes the signal traveling through a medium to be delayed (with a small group velocity) which in turn increases the group delay.

This high permittivity may be due to the presence nanowires which increases permittivity upon humidity absorption. In order to interpret the variations in terms of change in permittivity or loss, a simulation study was conducted. A thin dielectric material was added on the top of the C-section as nanowires, with a thickness of 10 m as shown in Fig.6. 16. The

(a) (b)

Fig.6. 15 : S21 magnitude and group delay variation during the backscattering measurement of the chipless sensor tag. a) S21 and relative humidity variation as a function of time. The green curve represented using diamonds is in accordance with the relation RH= a/S21 (dB) + b ; where a=2.918e+3, b=145. b) Group delay variation of the sensor tag near fundamental frequency for a relative humidity variation of 60.2%-88%.

Frequency (GHz) 40 MHz

12.3 ns RH=88

%

Group Delay (ns) 10 ns RH=60.2

% RH=85.8%

S21 (dB)

impact of change in permittivity r and losses tan on the group delay values were observed.

Each time, these two values were varied by taking the aspect that nanowires change their permittivity and losses upon humidity absorption. However, this simulation cannot take into account of the thickness of the nanowires and the concentration of nanowires in the deposition zone. This simulation was conducted just to give an idea about the evolution of group delay as function of permittivity and losses.

As shown in Fig.6. 17, same kind of behavior as in the case of measurement was observed. In the simulation the permittivity has been varied from 1 to12 and loss tangent ranges between 0.01-0.4. It was found that group delay tends to its negative value while the loss tangent is increasing. It proves the already stated fact that negative group delay arises with highly dispersive medium followed by absorption. It is also clear that a change in permittivity will shift the group delay curve. Since it is not possible to take into account of the exact geometry of nanowires, the magnitude of the group delay varies from that of measurement. This is a simple model to explain the accumulation of water molecules in the tag surface and the change in permittivity of the nanowires upon water molecules absorption.

However, this model study was enough to explain the behavior observed in the measurement.

Fig.6. 16 : The simulated sensor tag model in CST in order to explain the observed evolution of group delay as a function of permittivity and loss tangent.

nanowires copper

Now coming to the measurement, measurement was carried out without nanowires also. No significant variation was observed for S21 magnitude, phase, and hence group delay (1.44 dB on the S21 magnitude and 0.7 ns on the group delay). Fig.6. 18 (a) and (b) shows the variation of magnitude and group delay respectively for an RH variation of 58%-88%. This measurement was conducted for the same tag shown in Fig.6. 2 (b) with l=14.9 before depositing the nanowires.

Fig.6. 17 : Evolution of group delay as a function of permittivity and loss tangent in simulation study (see Fig.6. 16).

(a) (b)

Fig.6. 18 : Results obtained for measurement for tag without nanowires for an RH variation of 58%-88% for an ambient temperature of 23°C. a) S21, b) group delay.

Group Delay (ns)

Frequency (GHz)

S21 (dB)