**1. Introduction**

Ultra-wide band systems transmit and receive ultra-short electromagnetic pulses having limited effective radiated power. The system performance is determined primarily by the characteristics of the radiators that have to conform with stringent frequency and time domain requirements in the entire operating band [1]. These requirements are namely, a non-dispersive phase centre; constant radiation and impedance over the frequency range with no excitation of higher order modes [2]. Planar monopole antennas with wide operating bands are most often well matched multi-resonant structures. In [3], Ma et al. have described the time domain performance of a printed dipole antenna employing a tapered slot feed. It is reported that the received pulses are distorted and broadened to more than 1 ns in spite of the antenna being well matched over the entire band. The higher order modes generated tends to shift the antenna phase centers with frequency and can lead to

broadening of the radiated pulse. The same authors in [4] reports a much lesser pulse distortion for a tapered slot antenna operating in the 3.1–10.6 GHz UWB band. The present chapter intends to outline the design, simulation and measurement steps followed to characterize UWB antennas in the time domain.

*U* !

*DOI: http://dx.doi.org/10.5772/intechopen.94546*

*u* !

!

antenna transfer function is always band limited.

*rad*ð Þ *<sup>ω</sup>*,*r*, *<sup>θ</sup>*, *<sup>φ</sup>* ffiffiffiffiffi *z*0 p ¼ *h*

*rad*ð Þ *t*,*r*, *θ*, *φ* and the applied voltage pulse *u*

*r*

*h* !

*<sup>∂</sup> <sup>t</sup>* � *<sup>r</sup> c* � � <sup>⊗</sup>

*h* !

!

*Tx*ð Þ¼ *t*, *θ*, *φ*

*Rx*ð Þ *t*, *θ*, *φ* . As a result, an ideal antenna (with *h*

!

*Tx*ð Þ¼ *<sup>ω</sup>*, *<sup>θ</sup>*, *<sup>φ</sup> <sup>j</sup><sup>ω</sup>*

The time domain relation between the transmitted electric field pulse

1 2*πc*

impulse response *h*

the received pulse.

**2.2 Transient radiation**

and,

antenna is,

*e* !

where,

response *h*

**111**

*rad*ð Þ *<sup>t</sup>*,*r*, *<sup>θ</sup>*, *<sup>φ</sup>* ffiffiffiffiffi *z*0 <sup>p</sup> <sup>¼</sup> <sup>1</sup>

transmit impulse response *h*

!

input voltage pulse.

*e* !

the applied voltage pulse is,

*E* ! *Rx*ð Þ *<sup>ω</sup>*,*r*, *<sup>θ</sup>*, *<sup>φ</sup>* ffiffiffiffi *zc* p ¼ *h*

*Time Domain Performance Evaluation of UWB Antennas*

In the time domain, the corresponding relation is,

*Rx*ð Þ *<sup>t</sup>*,*r*, *<sup>θ</sup>*, *<sup>φ</sup>* ffiffiffiffi *zc* p ¼ *h*

and azimuth angles, the receive antenna transfer function *h*

!

!

*Rx*ð Þ *ω*, *θ*, *φ*

*Rx*ð Þ *t*, *θ*, *φ* ⊗

*zc* and *z*<sup>0</sup> are the antenna port and free–space characteristic impedance respectively, while ⊗ indicates convolution operation. If ð Þ *θ*, *φ* represents the elevation

An ideal receiving antenna should receive a voltage pulse of the same shape as the one incident on it from any direction. This means that it should have a Dirac– delta impulse response which is also be independent of the angle of arrival. In other words, the antenna transfer function should have a uniform amplitude and a linear phase response (or constant group delay). However, in practice, the receiving

The frequency domain relation between the transmitted electric field pulse and

2*πc h* !

*∂ ∂t h* !

1 2*πc*

The convolution with the delta function in Eq. (5) represents the time delay attributed to the finite speed of light, denoted by *c*. Eq. (6) indicates that the

radiate an electric field pulse which would be a first–order time derivative of the

*∂ ∂t h* !

!

*Rx*ð Þ *t*, *θ*, *φ* � � <sup>⊗</sup>

*Tx*ð Þ *t*, *θ*, *φ* is a time derivative of the receive impulse

!

*e*�*jωr=<sup>c</sup> r*

*Tx*ð Þ *ω*, *θ*, *φ*

*E* !

> *e* !

*Rx*ð Þ *t*, *θ*, *φ* are considered as a function of the angle of arrival of

*rad*ð Þ *<sup>ω</sup>*,*r*, *<sup>θ</sup>*, *<sup>φ</sup>* ffiffiffiffiffi *z*0

*rad*ð Þ *<sup>t</sup>*,*r*, *<sup>θ</sup>*, *<sup>φ</sup>* ffiffiffiffiffi *z*0

!

*UTx*ð Þ *<sup>ω</sup>*,*r*, *<sup>θ</sup>*, *<sup>φ</sup>* ffiffiffiffi *zc*

*Tx*ð Þ *t*,*r*, *θ*, *φ* at the transmitting

*Rx*ð Þ *ω*, *θ*, *φ* (4)

*uTx*ð Þ *<sup>t</sup>*,*r*, *<sup>θ</sup>*, *<sup>φ</sup>* ffiffiffiffi *zc*

*Rx*ð Þ *t*, *θ*, *φ* (6)

*Rx*ð Þ¼ *t*, *θ*, *σ δ*ð Þ*t* ) will

p (5)

p (3)

p (1)

p (2)

*Rx*ð Þ *ω*, *θ*, *φ* and its

Two monopole antennas with similar design topology is considered for the present analysis: the SQMA with a square radiator and RMA with a rectangular radiator. In both the antennas, the ground plane is optimally designed for a wide impedance bandwidth. The general practice to realize ultra wide-bandwidth in square/rectangular radiating elements is to modify the ground–radiator interface [5–10]. In SQMA, the inherent resonances of a square patch is matched over the wide band by incorporating cuts in the ground plane at specific locations [11]. The designs proposed in [11, 12] have been contemplated to design the RMA. In RMA, the PCB footprint is greatly reduced and radiation patterns in the 3.1–10.6 UWB is stable without much radiation squinting which is otherise observed in wideband antennas at higher frequencies primarily due to the current distribution on the antenna ground plane [13]. Incase of RMA, this aspect is taken into consideration in the design ensuring broadside radiation all through the band of operation. This is further confirmed from the spatio-temporal transfer characteristics and the transient response analysis, making it suitable for UWB applications.
