# Compact Ultra-Wideband Series-Feed Microstrip Antenna Arrays for IoT Communications
## 4 Discussion  
The main results taken from the previous section are compiled in Table 3, for the three developed microstrip arrays with log-periodic structure with five, seven, and nine elements. Regarding the total efficiency parameter, provided at three different frequencies, it is possible to conclude that all arrays are properly designed, using materials with good behavior at high frequencies, since it is possible to observe always high efficiency values, well above the 80%. Likewise, the gain is presented at the same three frequencies. It is possible to observe that the developed antennas have gains mostly in the 10-12 dBi range, which are values considerably higher than those of the typical known elementary antennas; therefore, allowing us to combat propagation issues typical in these frequency bands, improving communications.  
Finally, the results regarding bandwidth are shown, as absolute values and also as a fractional value using the fc of 28 GHz considered in the design. The results reveal that with the increase of the number of elements in the array, the bandwidth obtained also increases, reaching almost 44% of bandwidth for the nine-elements prototype. Both antennas, according to [33] are considered ultra-wideband structures.  
Table 4 compiles a set of characteristics from a group of selected state-of-the-art papers, which better represent different approaches proposed to enable wider bandwidths using antenna arrays with series configurations. Although it is not possible to make a linear comparison, as they are all distinct and composed of different number of elements, it is possible to find out that the nine-element array proposed in this paper is clearly the solution with the greatest bandwidth, keeping a compact structure.  
Figure 13: Variation of the Gain and Efficiency of the 9-element array over the frequency.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \begin{tabular}{c} **Bandwidth** \\ **(GHz)** \\ \end{tabular} & \begin{tabular}{c} **Fractional** \\ **BW BW/fc** \\ \end{tabular} & **Gain (27 GHz)** & **Gain (28 GHz)** & **Gain (29 GHz)** & \begin{tabular}{c} **Efficiency** \\ **(27 GHz)** \\ \end{tabular} & \
\begin{tabular}{c} **Efficiency** \\ **(28 GHz)** \\ \end{tabular} &
\begin{tabular}{c} **Efficiency** \\ **(29 GHz)** \\ \end{tabular} \\ \hline Array 5 & 6.91 & 24.7\% & 11.0 dBi & 10.6 dBi & 10.1 dBi & 88.8\% & 87.7\% & 87.0\% \\ Array 7 & 8.30 & 29.6\% & 10.3 dBi & 10.7 dBi & 9.40 dBi & 83.3\  
Lastly, it is also important comparing the results in terms of bandwidth of the proposed arrays with typical series-fed patch antenna arrays. In [34] a 4 \(\times\) 4 array of microstrip patches operating at 28 GHz is presented, composed of four-element patch series-fed arrays which have an approximate bandwidth of 1.5 GHz, corresponding to 5.35%. In both solutions proposed in this article using log-periodic architecture, it is possible to substantially exceed these values, allowing to achieve bandwidths of 25%, 30% and 44%, respectively, evidencing the advantage of this approach when compared to conventional techniques.