# A Broadband Ultra High Frequency (UHF) Fat-Dipole Antenna for Digital TV Applications
## 2 Antenna Design
Figure 3: Evolution of the simulated S11 of the elements starting from the thick dipole (**A**), (**B**) one-section enlargement, (**C**) one-section cut, (**D**) second diagonal cut, (**E**) third cut and final design (**F**), with parasitic patch and slot close to the feeding point. A particular concern regarding this dipole-like antenna is the balun. Simulations were conducted using a so-called discrete port, whose terminals provide an ideal balanced excitation between its two extreme points. However, this representation is idealized and may not accurately reflect real-world implementation. To investigate the actual excitation, two solutions were employed: one with an F-type connector directly attached to the antenna, with the shield and inner conductor soldered to each of the dipole elements, and another using an infinite balun, where the coaxial shield is soldered to one of the elements and the inner conductor contacts the other dipole element. Due to the antenna's two central slots, the infinite balun was positioned off-center, offset from the geometric midpoint. The presented simulations used the off-centered discrete port to closely match the prototype configuration. Field and surface current plots provide valuable insights into how each antenna component affects overall performance. These visualizations reveal the spatial distribution of electromagnetic effects, complementing the global performance data obtained from insertion loss curves. Figure 6 displays the computed surface currents at the lowest, middle and highest frequencies. The off-centered feed position creates an asymmetric current pattern, with the largest intensity concentrated in the central area. This observation validates the strategic placement of slots and parasitic patches in this high-sensitivity region. At 710 MHz (upper frequency), the current distribution exhibits limited propagation towards the dipole extremities due to the shorter wavelength. The farfield pattern for three different frequencies of a single element is shown in Figure 7. The plots exhibit characteristics consistent with those expected from a standard dipole, despite the presence of cuts and parasitic elements. The omni-directional nature of the radiation pattern across the XY plane is particularly relevant for broadcast applications, where signals can arrive from multiple directions and the indoor placement of antennas is often random, depending on each user's specific situation. The observed asymmetry is caused by the off-axis placement of the feed, due to the presence of slots. Figure 5: Passive network representation of the simulated antenna, along with the comparison results in both magnitude in dB and Smith Chart, blue line: simulated antenna and orange line: circuit representation. For MIMO applications, where both vertical and horizontal deployments are utilized for antenna arrays, it is essential to achieve good isolation between co-polarized and cross-polarized components to prevent signal absorption from orthogonal polarizations. In indoor applications, where the wireless channel rarely contains a line-of-sight (LOS) path, received signals arrive from multiple directions. This necessitates an isolation analysis, defined as the difference in dB between the gains for the desired co-pol polarization and the orthogonal cross-pol, for each point across the entire 3D space. Figure 8 presents a 2D projection of the 3D isolation patterns at two frequencies, obtained through full-wave simulations. The isolation performance degrades at the upper frequency limit, particularly along the \(z\)-axis (\(\theta=0^{\circ}\) and \(180^{\circ}\)). Figure 6: Simulated normalized absolute value of the surface current for three different frequencies, in dB. The triangle at the horizontal center is the port excitation symbol. Figure 7: Simulated gain in dB for three different frequencies. The antenna is oriented along the \(z\)-axis.