# A Broadband Ultra High Frequency (UHF) Fat-Dipole Antenna for Digital TV Applications
## 2 Antenna Design  
The basic requirements for this design are as follows:  
* Compact dimensions suitable for indoor applications. * Low-cost implementation. * Operating UHF frequency range of 470-700 MHz (current allocated DTV bandwidth). * A 75 \(\Omega\) impedance matching, which is the standard for TV systems. * Single-layer planar structure. * Minimal coupling in 2-element array configuration. The antenna is printed on a low-cost FR-4 substrate, with a 1.6 mm thickness, \(e_{r}=4.3\), and loss tangent of 0.025. The basic idea starts from a fat-dipole, which has a broader bandwidth since they show a lower quality factor (\(Q\)). Given their larger surface area, their current is less concentrated in comparison to thin elements, resulting in a lower inductance per unit length. Figure 2 shows an antenna with the same length but different widths, using a 1.6 mm thick FR-4 dielectric slab. Both antennas are referenced to a 75 \(\Omega\) impedance. Table 1 contains the simulated results for both cases, and the bandwidth is defined as the range where the S11 parameter is less than or equal to \(-10\) dB. It can be seen that the fat element generates a bandwidth response twice as large. The design started from a fat uniform dipole, which was progressively cut in order to further improve its frequency response. The CST Microwave Studio version 2024 optimization yielded the widths and lengths that achieve low reflection losses while maintaining compact dimensions. In addition to the cuts, two parasitic patches were added to enhance design flexibility, similar to the approach reported in [16], where two inverted F-antennas achieved operation in both the LTE 700 MHz band and the 2500-2700 MHz range using parasitic elements. The evolution of antenna design, starting from a fat uniform dipole to the final version, is depicted in Figure 3, and the results in terms of bandwidth are presented in Table 2. The final optimized dimensions and geometry are shown in Figure 4. From the evolution results, it can be seen that the cuts progressively increase the bandwidth, at the expense of shifting upwards its minimum frequency, resulting in a larger element. Therefore, there is a design trade-off between size and bandwidth, which needs to be taken into account for keeping the element with reasonable size. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Band [MHz]** & **Fractional Bandwidth [\%]** \\ \hline Thin & 451–522 & 14.6 \\ Fat & 480–658 & 31.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: S11 comparison of impedance behavior of dipole versions. Figure 2: Simulated S11 comparison of a fat (strip width 70 mm) and thin (strip width 14 mm) dipoles. The antenna can be represented as an RLC circuit, in a first-order approximation. Figure 5 shows the passive network and the respective comparison with the results from the simulated antenna. Their impedances are depicted in both S11 and Smith Chart. Using the Laplace variable \(s\), the input impedance of the passive network \(Z_{in}s\) can be expressed as:  
\[Z_{in}(s)=\frac{R_{1}L_{1}s}{R_{1}C_{1}L_{1}s^{2}+L_{1}s+R_{1}}+L_{2}s+\frac{1}{ C_{2}s} \tag{1}\] \begin{table}
\begin{tabular}{c c c} \hline \hline
**Design** & **Band [MHz]** & **Fractional Bandwidth [\%]** \\ \hline A & 462–583 & 23.0 \\ B & 456–600 & 26.8 \\ C & 458–602 & 27.2 \\ D & 468–645 & 31.7 \\ E & 476–659 & 32.1 \\ F & 483–704 & 37.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: S11 obtained at \(-\)10 dB for the proposed antennas. Figure 4: Antenna and respective dimensions. The light rectangle across the gap is the discrete port position. The respective dimensions are: L1 = 18 mm, L2 = 17 mm, L3 = L4 = 30 mm; L5 = 20 mm; L6 = 10 mm; L7 = 4 mm; L8 = 40 mm; L9 = 15 mm; L10 = 3 mm; W1 = 70 mm; W2 = 60 mm; W3 = 32 mm; W4 = 16 mm; W5 = 10 mm; W6 = 30 mm and W7 = W8 = 5 mm.