# A Broadband Ultra High Frequency (UHF) Fat-Dipole Antenna for Digital TV Applications
## 3 Results
The first observed out-of-band resonance in the measurements with the balun occurred at approximately 420 MHz, and it was likely caused by the balun itself; therefore, it has a non-radiating character. Table 3 summarizes the results in terms of fractional bandwidth (considering the \(-10\) dB threshold). It is clear that the direct connection, despite its simplicity and mechanical ruggedness, imposes a significant constraint on the coupling of currents to and from the antenna dipole, resulting in a much narrower bandwidth and unstable operation, given the observed common-mode currents flowing in the cable. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Band [MHz]** & **Fractional Bandwidth [\%]** \\ \hline Simulation & 483–712 & 38 \\ Direct connection & 513–562 & 9 \\ Infinite balun & 394–747 & 62 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Bandwidth comparison. Figure 10: Comparison of the different excitation methods and the simulation. The upper horizontal level of the rectangle is at the \(-10\) dB threshold for the return loss. In detail, the infinite balun’s actual implementation. Figure 9: Comparison of the measurement, with direct connection and the simulations. F-solver stands for FEM and T-solver for the FIT. The antenna element gain (in dB) at the broadside direction was simulated and measured using a 75 \(\Omega\) input impedance network analyzer in a laboratory environment, as illustrated in Figure 11. Two identical antennas were positioned at a known distance to compute the individual gain from the \(S_{12}\) parameter using the Friis free-space propagation formula. Three-dimensional electromagnetic simulations predicted a uniform gain across the frequency range, as the virtual model employed Perfect Matched Layers (PMLs) to absorb reflections at the simulation boundaries. In contrast, experimental measurements revealed significant gain oscillations, primarily attributed to the cluttered indoor environment that introduced numerous reflections and multipath effects. A notable gain reduction was observed from 625 MHz to the upper limit of the measured frequency band. This phenomenon can be attributed to two primary factors: (1) increased reflection and refraction effects as the wavelength becomes shorter, and (2) potential energy absorption by the implemented balun. Although the balun showed no apparent mismatches in the S11 reflection coefficient (Figure 10), it may not have been effectively radiating the entire input energy.To mitigate these issues, implementing a 1:1 75 \(\Omega\) balun could potentially improve the antenna's performance, albeit at increased cost and system complexity. Since the antennas were intended for deployment in a MIMO operation, a 2-element array was investigated. Minimizing the mutual coupling between the elements was essential to reduce the correlation and thereby enhance diversity gain. The first test examined the influence of the relative positions of the antennas, considering both collinear and orthogonal deployments. Both elements were positioned 51 cm apart, and as shown in Figure 12, the orthogonal arrangement was favored due to a lower level of coupling, except for a small band near 600 MHz and at the lower end of the band. The distances are defined as depicted in Figure 12, and this choice is justified when compared to the conventional center-to-center length, as the primary interest here is the overall array dimension. In other words, the parameters \(d_{o}\) and \(d_{c}\) (representing orthogonal and collinear distances, respectively) were always set to be equal, to ensure that the MIMO antennas are of similar size. It is important to note, however, that to maintain this condition, the orthogonal antennas were electrically closer than their collinear counterparts. Figure 11: Environment where the gain was measured, and the respective results are shown with the simulation curve. Distance between elements was set to 1 m.  
Focusing solely on the orthogonal case, Figure 13 presents the coupling results for different \(d_{o}\) distances.