UWB Antenna TOH ppt

Stable boresight radiation patterns are achieved across the entire operating frequency band, by suppressing the high order mode resonances.. However, when the height of the patch antenna

GHz (48%) for a reflection of coefficient of 10 dB, and an

average gain of 7.7 dBi Stable boresight radiation patterns are achieved

across the entire operating frequency band, by suppressing the high order

mode resonances This design exhibits good mechanical tolerance and

man-ufacturability.

Index Terms—Broadband antennas, microstrip patch antennas,

sus-pended plate antennas, ultrawideband (UWB) antennas.

I INTRODUCTION

A low profile and embeddable unidirectional antenna is required for

certain ultrawideband (UWB) [1] communication, imaging,

localiza-tion, and radar applications The lower and upper UWB spectrums are

3.1–4.8 GHz (43%) and 6.0–10.6 GHz (55%), respectively The

ex-isting broadband directional antennas, such as the Vivaldi [2],

log-pe-riodic, cavity-backed, waveguide, horn, and dish antennas, cover the

entire 3.1–10.6 GHz band (109%) However, they are electrically large,

and have a high profile in the direction of wave propagation Omni- and

bi-directional antennas, such as the planar monopoles [3]–[5], disc cone

[6], and slot antennas [7], have a low gain and back radiation pattern,

therefore they are not suitable for sectorial or unidirectional

commu-nication Also, it is a challenge to maintain a stable radiation pattern

across the whole frequency band, since the radiation aperture is

fre-quency dependent

To lower the Q-value for increasing the impedance bandwidth of

the patch antenna, the dielectric substrate is replaced by air, the patch

is usually suspended at a height of 0:06o, whereo is the free

space wavelength However, when the height of the patch antenna

is increased, the high inductance [8] from a long feeding probe

makes it difficult to achieve impedance matching To broaden the

impedance bandwidth, notches and slots have been employed, such

as the E-shaped [9], U-slot patches [10], and planar electric dipole

with shorted patches [11], to compensate the large input inductance

Furthermore, other broadband feeding techniques such as the aperture

coupled feed [12], L-probe feed [13], center slot feed [14], suspended

probe feed [15], and folded feed [16] have been used Although a

broadband impedance matching is achieved, the radiation patterns

of such patch antennas at the higher frequency are degraded by the

occurrence of high order modes Consequently, the co-polarization

Manuscript received January 15, 2008; revised January 15, 2008 First

pub-lished May 02, 2009; current version pubpub-lished July 09, 2009.

The authors are with the RF & Optical Department, Institute for Infocomm

Research, Singapore 138632, Singapore (e-mail: wktoh@ieee.org).

Color versions of one or more of the figures in this communication are

avail-able online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TAP.2009.2021968

II ANTENNASTRUCTURE

The schematic diagram of a single-element planar antenna is shown

in Fig 1 It consists of a radiator and a feeding structure The radiator measures 222 30 mm, and is positioned at a height of H1= 13 mm The n-shape microstrip feeding line is excited by a 50

nector, while the other end is connected to the radiator through a ver-tical strip The width of the feeding line is 5 mm wide (89

at a height ofH2= 2:5 mm, using Styrofoam material with a relative permittivity of"r 1 The antenna is designed to operate in the lower UWB band of 3.1 to 4.8 GHz At 3.0 GHz, the free space wavelength

o= 100 mm The radiator structure measures 0:22 2 0:3 2 0:13o, and the ground plane is1 2 1o The copper plates are 0.5 mm thick The 7.5 mm wide (70 1 feeding line is optimized to improve the impedance matching A gradual 50-70-89

tion provides a broadband impedance matching between the feeding line and the patch

III RESULTS

All the simulations in this communication were conducted using

a full-wave simulator IE3D The antenna was subsequently pro-totyped for experimental verification The reflection coefficient measurement was taken using a HP8510C vector network analyzer

A broad impedance bandwidth covering from 3.1–5.1 GHz (48%) for

jS11j < 010 dB is shown in Fig 2 The simulated resonances are in good agreement with those of the measurement

The measured radiation patterns at 3.0 , 4.0, and 5.0 GHz are shown

in Figs 3(a)–(c) The maximum H-plane cross-polarization levels grad-ually increase with increasing frequency, and peaked at = 045.

At 5.0 GHz, the increase in cross-polarization levels is due to the oc-currences of high order mode resonances Nonetheless, the radiation patterns remain stable, and there is no squinting from 3.0–5.0 GHz Fig 4 shows the maximum co-polarizations gain profile, which is also directed at the boresight, and the maximum cross-polarization on the E- and H-planes An average gain of 7.7 dBi is achieved with61.5 dBi

of gain fluctuation As the H-plane cross-polarization levels increase, the co-polarization gain decreases Fig 5 shows the 3-dB beamwidth plot for both the E- and H-planes, varying from 40to 65and 70to

90respectively

Fig 6 shows the simulated boresight gain and reflection coefficient characteristics for the antenna, when the ground plane is reduced from infinity to 402 40 mm The measured gain profile on a 100

2 100 mm ground plane agrees well with that of the simulation, comparing Fig 4 and Fig 6 Both readings show a peak gain of 9.4 dBi at 3.75 GHz, and a decreasing gain of 6.5 dBi at 5.0 GHz, with less than 0.4 dBi of differences The impedance matching remains unchanged, when the ground plane is varied from infinity 0018-926X/$25.00 © 2009 IEEE

Fig 1 Schematic diagram antenna.

peak gain varies from 8.0 dBi for an infinitely large ground plane,

provides the most stable gain performance

IV ANALYSIS

The lower and higher resonant frequencies of this antenna are

mainly determined by two components, (a) the length of the radiator

Ltop, and (b) the height H1 of the vertical strip The resonant

frequencies are estimated by

f 2(L3 2 108 top+ nH1)

fl; n = 1

Fig 2 Measured and simulated reflection coefficient jS j.

where f is in GHz, Ltop and H1 are in mm Fig 7(a) and (b) depict the instantaneous current distribution and vector plot of a cycle, at 3.4 and 4.6 GHz, respectively The two current minima

Fig 3 Measured radiation patterns at (a) 3.0 GHz, (b) 4.0 GHz, and

(c) 5.0 GHz.

of the current distribution on the patch depict the half-wavelength

resonant frequencies At the lower resonance, the vertical strip is

considered as part of the radiating element The dash-lines depict the

half-wavelength resonant Therefore, the effective radiating length is

43 mm, andf1 3:488 GHz The difference between the estimated

and measured lower resonating frequencies is 2.5% At the higher

resonance, the radiation on the vertical strip is substantially weaker

than that of at the lower resonance Therefore, the effective radiating

length is 30 mm, and fh 5:0 GHz The difference between the

estimated and measured higher resonating frequencies is 5.2% It is

validated that (1) could be used as design guidelines to estimate the

resonating frequencies for the proposed antenna These guidelines

are not applicable when the height of the patch is raised from

fh are far apart from each other; where the coupling between the

n-shaped microstrip line and patch is reduced

A parametric study of this antenna, by varying the lengthLbottom

and heightH2of the n-shaped feeding line, and heightH1of the patch

Fig 4 Measured maximum gain on the E- and H-planes.

Fig 5 3-dB beamwidth of the radiation patterns on the H- and E-planes.

Fig 6 Simulated reflection coefficient and boresight gain performances for various ground plane dimensions.

were conducted The results are shown in Figs 8 and 9 WhenH1is in-creased from 13–20 mm (53.8%), the impedance bandwidth is reduced

to 3.1–4.4 GHz (17.3%) The boresight gain is reduced, and begins to squint after 4.3 GHz This is due to the reduced coupling between the n-shaped microstrip line and patch WhenLbottomis extended from 26–30 mm (15.3%), there are no changes to the gain performance, no squinting, and the impedance matching remains atjS11j < 010 dB

Fig 8 Maximum gain, boresight gain, and reflection coefficient plots for

con-figurations A (original), B (extended patch height), and C (extended n-shaped

feed line), using simulator.

Fig 9 Maximum gain, boresight gain, and reflection coefficient plots for

con-figurations D (H = 2:5 mm), E (H = 2:0 mm), and F (H = 3:0 mm),

using simulator with infinite ground plane.

Fig 9 shows the impedance bandwidth and gain profile when height

H2 = 2:5 6 0:5 mm (620%), using infinite ground plane It can be

seen that there are minimal changes to the impedance matching and

gain profile Therefore this design has a good mechanical tolerance due

to the low Q value of the antenna Furthermore, from Fig 7(b), it is

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