# Conductive Ink Printed Fabric Antenna with Aperture Feeding Technique
## 3 Antenna Design
### Measurement  
Figures 3-6 show the simulated and measured results of the two printing techniques with a summary comparison shown in Table 1. The measurements were performed with both a vector network analyzer and in an anechoic chamber to verify the electrical performance of the antennas. The radiation efficiency is a measured ratio of the total radiated power of the antenna in a 3D sphere with the input power at a given frequency point. (center frequency). \begin{table}
\begin{tabular}{l c c c c c c c} \hline  & **Conductive** & **Bulk Conductivity** & **Antenna Size** (**Length** \(\times\) & **Bandwidth** & **Meaned** & **Rad. Eff.** & **FBR** (dB)** \\  & **Ink** & **(MS/m)** & **Width)** & **(GHz)** & **(dBi)** & **(dBi)** & **(\(\Omega_{\text{eff}}\))** & **(dB)** \\ \hline Screen-printed & \begin{tabular}{c} Electrodag \\ 479 SS \\ \end{tabular} & 1.97 & 66 \(\times\) 66 \(\times\) 1.24 & 2.37–2.44 & 6.77 & 59.72 & 7.08 \\ \hline Direct/inkjet & \
\begin{tabular}{c} Materpincs \\ SCAG-003 \\ \end{tabular} & 0.04 & 66 \(\times\) 66 \(\times\) 1.70 & 2.39–2.46 & 4.66 & 44.31 & 10.76 \\ \hline Copper-sheet &
\begin{tabular}{c} Copper sheet \\ (60 MHz) \\ \end{tabular} & 58.7 & 65 \(\times\) 65 \(\times\) 1.06 & 2.36–2.42 & 7.57 & 83.79 & 9.61 \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of direct/inkjet printing vs. screen-printing of fabric patch antenna. Figure 3: \(511\) curve of screen-printed antenna. From Figure 3, the screen-printed antenna obtained a simulated bandwidth of 2.37 to 2.43 GHz (60 MHz) and a measured range of 2.37 to 2.44 GHz (70 MHz) at \(-10\) dB \(\rm S11\). Although the simulated and measured reflection coefficients (\(S11\)) results were similar, the radiation patterns recorded differing gains of 4.17 dBi and 6.77 dBi at 2.4 GHz for simulated and measured, respectively. This can be attributed to incorrect simulation values for the bulk conductivity of the base fabric (conductive PET). The radiation pattern in Figure 5 nonetheless shows conformal patterns in both the simulated and measured results with cross-polarization values lower than \(-20\) dB. Figure 4: \(S11\) curve of direct ink-printed antenna. Figure 5: Normalized radiation patterns of screen-printed antenna at 2.4 GHz. Figure 6: Normalized radiation patterns of direct ink-printed antenna at 2.4 GHz. Figure 4 shows the reflection coefficient of the direct printed method with a visible shift in the simulated bandwidth compared to the measured. Simulated \(S11\) at \(-10\) dB is observed from 2.38 to 2.42 GHz (40 MHz bandwidth) and for measured \(S11\) from 2.39 to 2.46 GHz (70 MHz). In addition to incorrect bulk conductivity approximations, the difference could be attributed to fabrication errors with the epoxy application, SMA pin misalignment, and poor curing of epoxy resin. The radiation pattern shown in Figure 6 recorded a 3.4 dBi simulated gain and a 4.66 dBi measured gain at 2.4 GHz. The measured front-to-back ratio (FBR) of the direct printed antenna indicates large back radiation which can be attributed to the uneven nature of the printed ground plane resulting in an increased surface current and also fringing effects from the thick 0.35 mm conductive ink layer. The comparison in Table 1 shows a better performance of the screen-printed fabric antenna over the direct printed. Although the poorly measured gain of 4.66 dBi can be attributed to the low bulk conductivity, the printing technology could not permit the use of a denser conductive ink without clogging the nozzle of the printer. The table also shows a similarly designed probe-fed patch antenna with 0.05 mm thin copper film on a 1 mm felt substrate. This was designed as an ideal reference for the two printing methods. As can be seen, with a bulk conductivity of 58.7 MS/m, the copper-based variant obtained a radiation efficiency of 83.79%, and a measured gain of 7.57 dBi but with a narrower bandwidth.