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UWB Antenna characteristics
The UWB antenna acts like a filter and is a critical component in UWB radio systems.The basic effect of antennas is that they produce the derivative of the transmitted orreceived pulse waveform (Funk et al., 1995a). This also has the effect of extending theduration of the transmitted and received pulse. This extension of pulse duration
decreases the time resolution of the system. The antenna has a greater impact inUWB than in narrower band systems because of the very large bandwidth of anUWB signal.
In antenna terminology, the frequency range demand must be 6:1 or greater in order
to be ‘ultra wide’ (Taylor, 1995) which means that the upper frequency must be at least
six times greater than the lower frequency of the band. For such very wideband
antennas, issues of linearity, radiation efficiency and impedance match across the band
present difficult problems.
One problem which will arise when a very short time domain (im)pulse (implying
large bandwidth) is used to excite the antenna, is the ringing effect. After the antenna,
the signal is no longer impulse like. Instead the pulse is spread in the time domain.
A typical antenna response is presented in Figure 6.1, where the ringing effect is
modelled using a simple Bessel function.
To avoid ringing, resistive antennas with low Q-values should be used. The resistive
loading will cause the unwanted signal component to die away quickly, leaving a pulse
much closer to the desired shape. The antenna bandwidth can also be increased by
making the Q-value small, since the bandwidth is inversely proportional to Q-value.
However, the low Q-value implies that the efficiency of a resistive antenna is generally
quite poor.
The Q-value for an antenna is given by
where fo, fH, and fL are the centre frequency and the upper and lower 3 dB frequency
values of the antenna respectively.
The frequency domain is also useful for describing the transient response of antennas
because the time and frequency domain are connected by the Fourier transform. The
ability of an antenna to preserve the waveform of the ultra-narrow pulse is investigated in
the time domain. Two of the most important time-domain properties of an antenna are
fidelity and symmetry. The fidelity is defined as the maximum cross-correlation of the
normalized incident voltage and the normalized electric field in the far field region
(Montoya and Smith, 1996). The symmetry is a measure of the symmetry of the waveform
in the far field region (Montoya and Smith, 1996). More theoretical approaches on timedomain
antenna characterization can be found in the literature (Balanis, 1997; Montoya
and Smith, 1996; Shlivinsky et al., 1993; Allen et al., 1993; Lamensdorf and Susman, 1994).
UWB antennas differ from their narrowband counterparts in one basic concept.
Many antennas, especially in the telecommunication applications, are resonant elements
that are tuned to particular centre frequencies and have relatively narrow bandwidths.
In contrast, UWB antenna designs seek much broader bandwidths and require nonresonating
operation.
In the literature, several antenna types have been presented for use in UWB systems.
Applications have used dipoles, log-periodic dipole arrays (LPDA), conical monopole,
spirals, notched, ridged and TEM horns antennas (Taylor, 1995). The main focus in
antenna technology in the existing UWB literature, is for high power radar antennas.
Table 6.1 presents a summary of typical dimensions of different antenna elements
(Taylor, 1995). The remainder of this chapter will explore the practical implementation
aspects of several antenna types.
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