# Design, Fabrication and Testing of a Multifrequency Microstrip RFID Tag Antenna on Si
## 5 Fabrication and Testing
### Test Setup  
The reflection coefficient and gain of the fabricated RFID antenna were measured with a PicoVNA 106 Vector Network Analyzer (VNA) over the instrument's frequency range of up to 6 GHz. To this end, an SMA connector was attached (as shown in Figure 13b) to the input pads of the RFID antenna via a small piece (10 cm approx.) of 50 ohm RG400 coaxial cable. The SMA input port thus obtained was connected to the VNA port 1 test lead to measure the reflection coefficient \(|S_{11}|\). The transmission coefficient \(|S_{21}|\) was subsequently measured by connecting a wideband dual-ridged TEM horn antenna (Vector Telecom Part No VT10180 DRHA10 SK) to the second VNA port test lead (Figure 14a). The horn antenna was rated, according to specifications, at a maximum VSWR of 2.5 and an average gain of 10 dB over the 1-18 GHz frequency range. Calibration of the VNA was carried out by the standard SOLT method at the test leads, using its accompanying mechanical calibration kit (furnished by the manufacturer). The measurement of \(|S_{21}|\) was repeated for two identical horn antennas, connected to the same VNA test leads (Figure 14b). In all cases, the antennas  
Figure 13: (**a**) RFID tag specimen fabricated on Si wafer. (**b**) RFID tag with attached SMA cable. were placed inside an anechoic enclosure with dimensions of about 150 \(\times\) 90 \(\times\) 60 cm. For mechanical support, expanded polystyrene was used to minimize the multiple scattering effects inside the enclosure. Using the well-known Friis transmission equation (see, e.g., [25] pp. 86-88), and taking into account the antenna input mismatch losses, we obtained the following for the gain \(G_{H}\) of the horn antennas and subsequently for the gain \(G_{R}\) of the RFID antenna:  
\[|S_{21,H}|^{2}=\left(\frac{\lambda}{4\pi R_{2}}\right)^{2}G_{H}^{\prime}\ \Rightarrow\ G_{H}^{\prime}=\left(\frac{4\pi R_{2}}{\lambda}\right)|S_{21,H}| \tag{7}\]  
\[|S_{21,R}|^{2}=\left(\frac{\lambda}{4\pi R_{1}}\right)^{2}G_{H}^{\prime}\ G_{R}^{ \prime}\ \Rightarrow\ G_{R}^{\prime}=\frac{\left(4\pi R_{1}\right)^{2}\left|S_{21,R} \right|^{2}}{\lambda^{2}\ G_{H}^{\prime}} \tag{8}\]  
where  
\[G_{H}^{\prime}=\left(1-|S_{11,H}|^{2}\right)\!G_{H}\ G_{R}^{\prime}=\Big{(}1-| S_{11,R}|^{2}\Big{)}G_{R} \tag{9}\]  
Figure 14: (**a**) Transmission measurement configuration: horn and RFID tag specimen. (**b**) Transmission measurement configuration: two identical horns. and \(|S_{21,H}|\) and \(|S_{21,R}|\) are the S-matrix transmission coefficient values measured for the two configurations of Figure 14a,b. The scattering parameters are defined in the standard manner (see, e.g., [26]):  
\[S_{ij}=\frac{V_{i}^{-}}{V_{j}^{+}}\]  
where \(V_{j}^{+}\) denotes the voltage wave incident on port \(j\), and \(V_{i}^{-}\) denotes the voltage wave reflected from port \(i\) (in the absence of any other incident wave). Obviously, (7) and (8) represent a simplified version of the well-known three-antenna method, using two identical horns. The validity of the simplification was verified by interchanging the two horns transmitting towards the RFID tag antenna and observing closely matched results for the transmission coefficient. Both \(G_{H}^{\prime}\) and \(G_{R}^{\prime}\) incorporate, via (9), the mismatch losses at the test lead ports connecting the horns and the RFID antenna (hence the primed notation). The effect of insertion and mismatch losses along the test leads is compensated by the calibration of the VNA at the test lead ports. Thus, the value of the gain \(G_{R}\) of the RFID antenna is obtained via (9). To compensate for the insertion loss of the small coaxial cable stub attached to the input pads of the RFID antenna, the insertion loss was measured for a cable of the same type and length with SMA connectors at both ends. A value of approximately 0.2 dB was found and added to the results of (9).