**2. UWB monopole antennas**

Dipole antenna, often called dipole, is one of the simplest but most widely used types of antennas. The typical structure of a dipole consists of two thin-wire conductors normally having equal length *L* as shown in Figure 1(a) where it is assumed *L*=λ/2 with λ being the wavelength at the resonant frequency. At resonance, the currents form the standing waves on both conductors, as shown in Figure 1(a), giving rise to electromagnetic (EM) radiations. Monopole antenna, often called monopole, has half the size of a dipole. An ideal monopole normally consists of a single thin-wire conductor perpendicularly mounted on an infinite ground plane as shown in Figure 1(b). At resonance, the current forms a standing wave on the conductor which radiates EM fields. The EM fields incident on the infinite ground plane are reflected as if they were radiated from the monopole image having the same current distribution as that of the lower conductor of the dipole in Figure 1(a). Thus a monopole can be viewed as the corresponding double-length center-fed linear dipole [12]. A monopole radiates energy into only the upper half space. So for a given input power, a monopole has the radiated power and hence the gain twice as much as the corresponding dipole.

A thin-wire monopole has a simple structure, but a very narrow bandwidth, making it unsuitable for UWB applications. To broaden the impedance bandwidth, the thin-wire 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 shapes as shown in Figure 2 [13].

274 Ultra Wideband – Current Status and Future Trends

antenna.

**2. UWB monopole antennas** 

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

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

Dipole antenna, often called dipole, is one of the simplest but most widely used types of antennas. The typical structure of a dipole consists of two thin-wire conductors normally

wavelength at the resonant frequency. At resonance, the currents form the standing waves on both conductors, as shown in Figure 1(a), giving rise to electromagnetic (EM) radiations. Monopole antenna, often called monopole, has half the size of a dipole. An ideal monopole normally consists of a single thin-wire conductor perpendicularly mounted on an infinite ground plane as shown in Figure 1(b). At resonance, the current forms a standing wave on the conductor which radiates EM fields. The EM fields incident on the infinite ground plane are reflected as if they were radiated from the monopole image having the same current distribution as that of the lower conductor of the dipole in Figure 1(a). Thus a monopole can be viewed as the corresponding double-length center-fed linear dipole [12]. A monopole radiates energy into only the upper half space. So for a given input power, a monopole has

the radiated power and hence the gain twice as much as the corresponding dipole.

A thin-wire monopole has a simple structure, but a very narrow bandwidth, making it unsuitable for UWB applications. To broaden the impedance bandwidth, the thin-wire

λ/2 with λ

being the

having equal length *L* as shown in Figure 1(a) where it is assumed *L*=

discrepancies again produce uncertainties to the design of the antenna.

**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 loss has been optimized using computer simulation, the measured performance of the prototyped antenna does not agree with the simulated performance. Large discrepancies usually occur at lower frequencies. This creates uncertainties and doubts in the design of the antenna. As will be shown in the following sections, the discrepancies at low frequencies are mainly caused by the feeding cable used to connect the antenna to the measurement system.

Cable Effects on Measuring Small Planar UWB Monopole Antennas 277

The simulated 3D-radiation patterns for the two cases are shown in Figures 4(c) and (d). It can be seen that both radiation patterns have peak-radiation at elevation from the horizontal ground plane, typical for monopoles with finite ground planes. For the antenna with the large ground, peak-radiation is stronger and at a smaller elevation angle than those for the antenna with the small ground plane. Peak-radiation in the lower hemisphere of the radiation pattern is much weaker for the antenna with the large ground than for the antenna with the small ground plane. For the antenna with the small ground plane, Figure 4(c) shows that ripples occur in both the upper and lower hemispheres of the pattern. However, for the antenna with the large ground plane, Figures 4(d) shows no ripple in the upper hemisphere of the pattern, but many ripples with much smaller magnitudes in the lower

**Figure 3.** Illustration of two possible ways affecting measured results of antenna: (a) reflections of EM

(a) (b)

Ripples on a 3D-radiation pattern are the results of EM fields with different phases adding together constructively and destructively in different spatial directions. To study the causes of these ripples on the 3D-radiation patterns of Figures 4(c) and (d), computer simulation is carried out on the same antenna with the same large and small ground planes but without using the feeding cable. Results at resonant frequency are shown in Figure 5. For the antenna with the small ground plane, Figure 5(a) shows that the ripples disappear. Thus the ripples in Figure 4(c) are mainly caused by the feeding cable. In fact, this agrees with Figure 4(a) which shows that, at the resonant frequency, a standing wave is developed on the feeding cable which gives out EM radiation. The EM fields are added together constructively and destructively in different spatial directions, producing ripples in the 3Dradiation pattern in Figure 4(c). For the antenna with the large ground plane and without using the feeding cable, Figure 5(b) shows that the 3D-radiation pattern is about the same as that in Figure 4(d) using the feeding cable. This agrees with Figure 4(b) which shows no standing wave developed on the feeding cable and so no radiation from the cable. Thus the feeding cable has no effect on the radiation pattern. However, Figure 5(b) shows that the

fields from antenna and (b) currents flowing back to feeding cable

hemisphere.
