# Design of Two Compact Wideband Monopoles Through Loading with Linear Approximated Lumped Components
## 2 Antenna Design
### Design of Colinear Loaded Monopoles  
As shown in Figure 3, two monopoles are designed in a colinear structure with a parallel lumped RLC circuit loaded dispersedly on the vertical copper tube of the antenna. First of all, the lower monopole is designed to cover a 30-750 MHz band. The length of the lower monopole is \(L_{1}\) = 175 cm. And a cross blade with a length \(L_{3}\) = 15 cm is arranged at the top working as a hat load for obtaining a good match at the lower side of the concerned band. Seven parallel lumped RLC circuits are loaded on the lower monopole for obtaining acceptable gain in all directions across the concerned band while lowering the VSWR to some extent. Since it is hard to obtain a transmission line transformer working for the 30-750 MHz band, an attenuator is attached to the port of the lower monopole for lowering the VSWR further to an acceptable extent. All element values and the position of these parallel lumped RLC circuit loads are determined by optimization later.  
Figure 3: Parallel lumped RLC loaded monopoles arranged in colinear structure.  
The upper monopole is designed to cover the 800-3000 MHz band and its length is \(L_{2}=15\,\mathrm{cm}\). Since \(L_{2}\geq L_{3}\) is satisfied, the cross blade also works as an elevated ground for this upper monopole. Three parallel lumped RLC circuits are loaded on the lower monopole for obtaining acceptable gain in all directions across the concerned band while lowering the VSWR to some extent. This upper monopole is fed by a coaxial line traveling through the inside of the tube of the lower monopole, as depicted in Figure 4. In this case, the outer surface of the outer conductor of the coaxial feeding line and the internal surface of the tube would form another coaxial waveguide. To remove this waveguide effect, the coaxial feeding line is attached to the internal surface of the tube so that the outer surface of the outer conductor of the coaxial feeding line and the tube are an equipotential body now. Moreover, at the load position, the coaxial feeding line is wound so that the outer surface of the outer conductor of the coaxial feeding line could be designed as a coil inductor. Combined with the patch resistor and the patch capacitor, there is a completely parallel lumped RLC circuit load unit arranged at the load position, as shown in Figure 4.  
As the outer surface of the outer conductor of the coaxial feeding line is designed to be identical to the radiator of the lower monopole, special consideration should be given to the port of the lower monopole. As depicted in Figure 5, some ferrite beads are arranged on the coaxial feeding line as a choke to feed the current to the lower monopole effectively, since the outer surface of the outer conductor of the coaxial feeding line is commonly connected to the ground at the connector of the upper monopole. Moreover, as shown in Figure 5, combined with two other patch resistors, a \(\pi\)-type circuit with attenuation characteristics is designed for the attenuator of the lower monopole, as depicted in Figure 3. By adjusting this two resistors' values carefully, it is easy to obtain the required attenuation characteristics. Then, this attenuator can be connected to a 50 ohm connector for the lower monopole by a 50 ohm microstrip transmission line, as shown in Figure 3.  
Figure 4: The coaxial line for feeding the upper monopole travels through the inside of the tube of the lower monopole. The figure shows the practical load design for the lower monopole.