**1. Introduction**

272 Ultra Wideband – Current Status and Future Trends

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Since the US-FCC assigned the ultrawide band (UWB) for unlicensed use in 2002, UWB technology has attracted much attention in both the commercial and academic domains. The characteristics of ultrawide bandwidth from 3.1 to 10.6 GHz and low power emission of - 41.3 dBm/MHz make it a promising candidate for different applications such as high speed communications and radar imaging systems. However, the requirements for impedance matching, constant gain, constant radiation patterns and high radiation efficiency over such a wide bandwidth for the UWB antenna become great challenges to antenna designers.

With the increasing demand for smaller wireless devices, planar antenna, with the advantages of compact size, low profile, low cost, ease of fabrication and ease of integration with RF circuits, appears to be more preferable for UWB applications. Among different planar antennas, monopole antenna has the simplest structure, compact size and omnidirectional radiation pattern and so is one of the best candidates. For a planar monopole antenna to work properly, the ground plane is required to be electrically large enough to approximate an infinite-ground plane and so occupies a large portion of the overall antenna size. Thus to design a compact UWB monopole antenna, the ground plane is usually the one to be minimized. Designing a planar monopole antenna with a small ground plane to cover the UWB is not a difficult task and can be achieved through different techniques [1-7]. The design is usually done using computer simulation. In carrying out the design in simulation, the antenna is fed directly with a signal source without using a feeding cable. However, when the final design is completed and prototyped for measurements, a feeding cable is normally used to connect the antenna to the measurement system. The small ground plane cannot approximate an infinite ground plane well and causes the currents to flow back to the outer surface of the feeding cable, resulting in secondary radiation. This leads to discrepancies between the simulated and measured performances of the antenna and creates uncertainties to the design of the antenna.

© 2012 Liu et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Liu et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

To resolve the problem, a sleeve balun can be placed at the end of the cable to prevent currents from flowing back to the feeding cable [8,9]. A sleeve balun is a metal tube with a length of quarter-wavelength to provide an open circuit for the signal. Although sleeve baluns can be designed to possess good choking characteristics, they are narrowband devices and so are not suitable for UWB antennas. For wideband and high-frequency operation, the feeding cable can be covered with an EMI suppressant material to absorb unwanted EM radiation [10]. By using this method, the shape of the measured and simulated radiation patterns of the antenna will be similar, but the measured efficiency and gain will be lower due to the energy absorbed by the EMI suppressant material. The discrepancies again produce uncertainties to the design of the antenna.

Cable Effects on Measuring Small Planar UWB Monopole Antennas 275

conductor can be made flat to become a planar element and then laid parallel to the ground plane to form a low-profile planar monopole. The planar element can take on different

(a) (b)

**Figure 1.** (a) Center-fed dipole and (b) vertical monopole above infinite ground

**Figure 2.** Planar monopoles using different radiator shapes [13]

**3. Effects of ground plane on small UWB monopoles measurements** 

With the increasing demand for smaller wireless devices, planar monopole antennas with small ground planes have attract much attention. However, in the design of such an antenna, very often, after the antenna performance in terms of gain, efficiency and return

shapes as shown in Figure 2 [13].

In this chapter, the effects of ground-plane size and feeding cable on the measurements of small UWB monopole antennas are investigated. A group of nine planar UWB monopoles with an identical elliptical radiators but different ground-plane sizes are designed using computer simulation where no feeding cable is used. These antennas are also prototyped and measured using the antenna measurement system, Satimo Starlab, where a feeding cable is used [11]. The simulated and measured performances show large discrepancies at low frequencies. To investigate the discrepancies, two different types of feeding cables, a high-frequency coaxial cable and a high-frequency coaxial cable with EMI suppressant tubing, are studied. The simulation models for the two cables are developed and used in computer simulation. With the application of the two cable models, the simulated and measured performances show good agreements. The results show that the feeding cable without EMI suppressant tubing causes many ripples on the 3D-radiation patterns of the antenna.
