## 3 Design of the Dual-Band Coupled-Fed Dipole Antenna  
Figure 3 shows the proposed dual-band dipole antenna with an electromagnetically coupled-fed structure. The antenna comprises internal and external dipoles with lengths \(H_{out}\) and \(H_{in}\) respectively, of which the values independently determine each resonance frequency. The internal dipole is fed directly by the circuit board with the balun, and the external and internal dipoles are electromagnetically coupled to achieve the additional T-DMB band matching characteristics. This coupled-fed mechanism, which has been employed in various broadband microstrip patch antennas [12; 13; 14], is first applied to the design of dual-band dipole antennas for the VHF frequency band. The coupled-fed mechanism used for the dipole element can achieve the dual-band matching characteristics, which can improve the illuminator of opportunity for PCL systems. The radius of the internal and external dipole radiators is denoted by \(R_{in}\) and \(R_{out}\), respectively. The separation gap between the internal and external dipoles is \(S_{d1}\). The optimal design parameters of the proposed antenna were derived using the FEKO electromagnetic simulator [16] in conjunction with a genetic algorithm [17], and they are listed in Table 1. Photographs of the proposed coupled-fed dipole antenna and the circuit board with the embedded balun are presented in Figure 3b. The two ports of the internal dipole inside the external dipole are firmly attached to the housing by screw-type connectors.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline \(H_{out}\) & 460 mm \\ \hline \(H_{in}\) & 670 mm \\ \hline \(R_{out}\) & 62.5 mm \\ \hline \(R_{in}\) & 15 mm \\ \hline \(S_{d1}\) & 47.5 mm \\ \hline \(S_{d2}\) & 1.6 mm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Design parameters of the proposed antenna.  
Figure 2: Frequency modulation (FM) and terrain digital multimedia broadcasting (T-DMB) broadcast stations with potential coverage area in the Seoul-Gyunggi urban area.  
The fabricated antenna was measured at an outdoor test site to investigate the radiation characteristics. Figure 4a shows the antenna under testing. Figure 4b is the outdoor test site, where the test antenna is on the left and the reference antenna is on the right. The distance between the test and reference antennas is 10 m.  
Figure 5 shows the VSWR of the proposed coupled-fed dipole antenna and the conventional thin dipole, where the solid and dashed lines represent the measured and simulated results, respectively. In the VSWR simulation, the balun is also included to achieve more accurate results. The FM and T-DMB regions are displayed in gray. There is an additional resonance by the external radiator in the second gray T-DMB region, as shown in Figure 5b. The proposed antenna shows good matching characteristics in the FM and T-DMB bands, ranging from 88 to 217 MHz (VSWR \(<\) 2.5).  
Figure 3: Geometry of the proposed antenna; (**a**) designed structure; (**b**) photographs of the fabricated antenna.  
Figure 6 shows the simulated bore-sight gains compared to the measured data. The results of the proposed antenna are 2 dBi and 2.9 dBi at 100 MHz and 200 MHz, while the results of the conventional thin dipole antenna are 0.8 dBi and \(-7\) dBi. The 3-dB gain bandwidth ranges from 83 to 250 MHz.  
Figure 4: Outdoor test site: (**a**) antenna under testing; (**b**) outdoor test configuration.  
Figure 5: Impedance matching characteristics of the proposed antenna: (**a**) conventional thin dipole; (**b**) proposed antenna.  
Figure 6: Bore-sight gains: (**a**) conventional thin dipole; (**b**) proposed antenna.