# Design of Two Compact Wideband Monopoles Through Loading with Linear Approximated Lumped Components
## 1 Introduction  
Monopole antennas, with their omnidirectional characteristics, are widely used in many wireless communication systems, such as broadcasting [1, 2], vehicle communication [3, 4, 5], indoor local communication [6, 7], and electromagnetic sensors [8, 9, 10]. With the development of multifunctionality on a platform, more and more wireless communication systems are assembled on a single platform [11, 12]. This leads to many antennas working for different systems being placed nearby, which can result in unwanted electromagnetic interference problems [13, 14]. Even if each system is well designed with satisfactory isolation, the use of too many antennas increases the complexity on a platform. Therefore, an omnidirectional antenna covering all system bands is promising, as it means that only one antenna needs to be considered on the platform. In addition, as the demand for information capacity grows, the bandwidth of wireless communication systems is designed to be wider over the years [15, 16, 17, 18]. Wideband or multiband antennas are always a hot research area as they have the potential to meet the wideband requirements of wireless communication systems. As a basic concept, the ordinary quarter-wavelength monopole is usually equivalent to a quarter-wavelength open-circuit lossless transmission line. It is a resonant structure, and the current on it conforms to a standing wave distribution [19]. The quarter-wavelength monopole has satisfactory gain in all directions and a good voltage standing wave ratio (VSWR) only in a narrow bandwidth. An increase in frequency causes a reverse current, which results in gain degradation in all directions according to the superposition theorem [20]. On the other hand, when the frequency decreases, the monopole becomes electrically small, and its equivalent circuit is gradually turned toward an open circuit [21]. It is hard to feed an electrically small antenna with a 50 ohm port. Therefore, there is anatural bandwidth limitation of the ordinary quarter-wavelength monopole, and bandwidth expansion of the monopole has always been a popular research topic in various monopole applications. To expand the bandwidth of the monopole, one popular method is to add a hat at the free end of the monopole, which is equivalent to adding a capacitor between the open circuit and ground. A displacement current can exist strongly in the low-impedance path of the equivalent capacitor, and a traveling wave distributes on the monopole, which could result in a wide bandwidth characteristic. With printed circuit board (PCB) technology, the hat load of the monopole can be designed in various shapes and is compact in microwave applications [22; 23]. Some novel structures with special functions can also be embedded in the monopole with a hat. Ref. [24] presented a printed monopole antenna with a modified stepped impedance resonator (SIR) in the hat load to achieve dual-band operation. The size of the monopole can be reduced by introducing an embedded resonant structure [25]. To expand the bandwidth further, other techniques such as a sleeve structure can be employed at the bottom of the monopole [26]. Since physical size is not a primary concern in microwave applications, it is quite flexible to design antennas in various structures using PCB technology. In VHF/UHF applications, the size of the monopole antenna increases considerably, especially on the lower side of the band. The classical wire monopole is always a popular candidate compared with other structures and is widely used in omnidirectional applications. Moreover, as the outline of the wire monopole is small, the wind resistance of the antenna can be effectively reduced in outdoor applications [27]. To expand the bandwidth of the wire monopole while maintaining its outline, loading with lumped elements is feasible as long as a suitable loading strategy is employed. Loading the monopole with only resistance is the simplest way to try to expand the bandwidth of the antenna.