SENSOR PROTOTYPE: EXPIRIMENATL RESULTS

The S21 magnitude, phase, and group delay of the device has been measured for different Fig.6. 4 : Measurement set-up for the sensor tag prototype.

(a) (b) Fig.6. 5 : S21 magnitude and phase measured at fundamental frequency for the tag prototype (Fig.6. 2) with a relative humidity variation of 58%- 89% and an ambient temperature of 23°C. a) S21 b) Phase.

Water Prototype

Humidity Temprature Meter

S21 (dB)

RH=

58%

RH=

89%

Frequency (GHz)

ambient temperature conditions inside the plastic box. It should be noted that only the temperature variations inside the box is taken into account. However, the ambient temperature inside the measurement room, which is set to be 23-24°C, was always remains constant. 12 drops of nanowires have been deposited on the C-sections with length l=14.9 mm (shown in Fig.6. 2 (a)); which corresponds to a frequency F(ln)=3.5 GHz; and kept inside the closed plastic box. Variations of S21 magnitude phase and group delay was observed at the fundamental frequency at 3.5 GHz over a bandwidth of 20 MHz. Fig.6. 5 (a) shows the S21 magnitude measured with a temperature inside the box of 23°C (room ambient temperature).

7 dB of magnitude change was observed in this case. Fig.6. 5 (b) shows the change in phase observed in the fundamental frequency for 58%-89% of RH variation.

The group delay was also extracted in this case. It was found that the group delay shows very less variation in the lower fundamental frequency. However, at higher order harmonics a group delay variation of 7.66 ns was observed. Fig.6. 6 (a) and (b) shows the phase and group delay respectively at second harmonics. It is clear that group delay shows a strong variation. Measurement was also conducted for tag prototype without nanowires.

Negligible variation was observed for S21 magnitude and group delay (0.1 dB and 0.01 ns).

Fig.6. 7 shows the S21 magnitude obtained for a humidity variation of 62%-92%.

(a) (b)

Fig.6. 6 : The phase and group delay variation observed at higher order harmonics for a humidity variation of 58%- 89% and an ambient temperature of 23°C. a) Phase, b) group delay.

7.66 ns

Now the ambient temperature inside the box was increased by adding some hot water into it. The physical conditions inside the box were observed. It was found that the temperature increases from 24°C to 29°C. The presence of mist was observed inside the plastic box. The S21 magnitude was always measured using VNA. It was found that Fig.6. 7 : S21 magnitude obtained for the tag prototype without nanowires for a humidity variation of 62%- 92% and an ambient temperature of 23°C. The curves are almost superimposed one on the other.

(a) (b)

Fig.6. 8 : Measured S21 and group delay after increasing the ambient temperature inside the plastic box for a humidity variation of 80%-100% with a temperature range of 24°C- 29°C. a) S21 magnitude, b) group delay.

Frequency (GHz)

S21 (dB)

Frequency (GHz)

S21 (dB)

80%

100%

Frequency (GHz) 100% 80%

4.3 ns

Group Delay (ns)

f=1 GHz f=1 GHz

increasing the temperature improves the S21 magnitude variation. It is due to the known fact that the rise in temperature increases the presence of water molecules inside the box and hence the amount of water molecules captured by the nanowires will also increase. From this, we observe an increase in the S21 magnitude. Fig. 6.8 (a) shows the S21 magnitude observed for a humidity variation of 80%-100% with a temperature variation of 24°C to 29°C. 47dB of magnitude variation was observed over a bandwidth of 1 GHz. Significant variations were observed in the phase and hence group delay also. These variations are much higher than previously observed 7 dB. In addition to this, the range of humidity is significantly reduced here. Fig.6. 8 (a) and (b) shows the variations observed in magnitude and group delay respectively.

If we compare with the existing chipless RFID humidity sensors [15] where a magnitude variation of 5 dB is reported for a bandwidth of 20 MHz, this is the maximum variation reported until now. It is clear from the figure that while humidity is increasing the S21 magnitude and group delay peaks shift to the lower frequency level. It is due to the fact that the nanowires change their permittivity upon water absorption which causes the curves shifting to the lower frequency zone. The repeatability has been tested within an interval of

(a) (b)

Fig.6. 9 : Repeatability test conducted after 4 days. Measured S21 and group delay after increasing the ambient temperature inside the plastic box as 24°C-29°C for an RH variation of 80%-100%. a) S21 magnitude, b) group delay.

Frequency (GHz)

S21 (dB)

80%

100%

4.3 ns

Group Delay (ns)

80%

100%

Frequency (GHz)

four days. It was observed that the S21 magnitude and group delay shows the same kind of behavior as in the previous case as shown in Fig.6.9 (a) and (b) respectively. In this case also, 47 dB of variation is observed for S21 magnitude over a bandwidth of 1 GHz. The group delay also shows significant variation (4.3 ns). Thus it is clear from the study that the temperature of water has a significant impact. When there is more water molecules in the water, nanowires will capture more water molecules and this will increase the rate of variation of magnitude phase or group delay.

Measurement was also done for the prototype without nanowires also. The same ambient atmosphere was re-created with an ambient temperature varies between 25-30°C. It was observed that the prototype gives 6 dB of variation in the fundamental frequency as shown in Fig.6.10. When the density of water molecules deposited on the prototype increases, it can cause a permittivity change and that may be the reason for the variation in magnitude.

This measurement can be considered as the reference measurement and this variation is low compared to the variation produced by the presence of nanowires, which was 47 dB. This proves the role of nanowires as a good catalyst. Fig.6. 10 shows the S21 magnitude variation for the prototype without nanowires which is considered to be the reference measurement. In the all above explained cases, for better comparison, same replicas of prototypes have been used.

Fig.6. 10 : Measured S21 for prototype without nanowires after increasing the ambient temperature (25°C-30°C) inside the plastic box with an RH variation of 80%-100%.

Frequency (GHz)

S21 (dB) 80%

100%

It has been found that when temperature is increasing, the S21 magnitude, phase and hence group delay exhibits more variation. This is due to the fact that when the temperature is increasing; there will be more water molecules in the medium so that the nanowires can capture more number of water molecules than previously which results in a significant variation. Thus, for a given temperature, the change in humidity induces variation in S parameters. However, for different temperatures, they show a small percentage of error. This can be rectified by expressing the rate of change of variation in magnitude or group delay as a function of absolute humidity. Absolute humidity (AH) is related to the absolute amount of water molecules in air. Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum for that air pressure and temperature. It means that, for two different temperatures for a given RH, the density of water molecules in air will be different. More the temperature is, more will be the density of water molecules are (the storing capacity of water molecules in air for high temperature will be high). When we take two different temperature variations, it is always better to express the S parameter as a function of AH. Thus, AH can be expressed as,

, where M_e and M_a is the molecular mass of the water (18 g/mols) and air (29g/mols) respectively and

,

, where P is the standard atmospheric pressure (101325 Pa) and Pe is the saturation pressure

of water depending in temperature and can be expressed as .

Thus we see that AH can be expressed as a function of temperature expressed in Kelvin.

Fig.6. 11 (a) and (b) shows the rate of change of S21 magnitude in dB for different temperatures as a function of relative and absolute humidity respectively. Curve fitting has

(a) (b)

Fig.6. 11 : Rate of change of S21 magnitude in dB for two different temperatures as a function of a) relative humidity, b) absolute humidity. The cross curves represents the curves obtained using curve fitting method with the expression AH or RH=a*S21dB+b, where a & b are the constants obtained from curve fitting method.

Fig.6. 12 : Percentage of error calculated for the two different temperatures (Fig.6.11) for relative and absoulte humidity .

T=23.3°C-23.6°C T=24°C-24.3°C T=23.3°C-23.6°C

T=24°C-24.3°C

been done in order to find the analytical expression between S21 in dB and relative humidity or absolute humidity. It has been found that S21 is linearly proportional to relative humidity and absolute humidity as per the following equation,

RH or AH=a*(S21)dB+b; where a and b are constants which have been calculated using curve fitting in MATLAB. This study has been carried out for a very small range of humidity (10%) and hence it results in a linear variation. However, for large range of relative humidity, the variation was found to be inversely proportional as we will see in the next section.

Fig. 6.12 shows the percentage of error calculated as a function of S21 for relative humidity and absolute humidity. The percentage of error is less for AH. However, we cannot end up with such a conclusion since the two temperatures are too close to each other. A good way to clearly obtain an idea is to repeat the measurements for different temperatures whose values are far from each other. In order to obtain this, the measurement time can be increased and hence a value of RH more than 90% can be obtained. This measurement can be compared with another measurement which has a higher value of temperature. In short, from the obtained results we can conclude that while considering two different temperatures, it is always better to express the S parameters as a function of AH. More precisely, we are not able to say whether the observed variation is due to variation in AH.

However, it is clear that the measurement is a function of temperature and therefore not only be related to the change in RH. However, if we know the temperature, and if the sensor is calibrated, it is possible to use it to measure RH.The next section explains the backscattering sensor measurement conducted for the chipless tag shown in Fig.6. 2 (b).