SMALL SEMI DIRECTIONAL ANTENNA FOR UWB TERMINAL APPLICATIONS
Raffaele D'Errico, Hassan Ghannoum, Christophe Roblin, Alain Sibille
ENSTA , 32 Bd Victor Paris Cedex 15, France,Email:raffaele.derrico@ensta.fr
ABSTRACT
The design of an ultra wide band (UWB) semidirectional antenna is presented. Both antenna
optimizations by simulation and measurement results
are presented. The final prototype size is 33 mm x 20
mm x 11.5 mm. The achieved input bandwidth is 3.9-15
GHz. The maximum boreside realized gain (BRG) is 7.5
dBi and the maximum Front-to-Back-ratio (FTBR) is 13
dB. The frequency variance of the antenna gain is
exploited in a two-antenna radio link in order to
compensate that of free space attenuation.
1.
INTRODUCTION
The design of UWB antennas should consider the
overall performances of UWB system. For most of
these, the improvement of the link-budget, even by a
few dB, is of practical importance, because of the
constraints imposed by regulation authorities, e.g. FCC
[1] or ECC [2], as regards the transmitted power.
Since regulatory limits are defined in terms of the
effective isotropic radiated power (EIRP) at transmitter
side, one way to enhance system performance is to
introduce antenna gain at the receiver side. Moreover
using a directional transmit antenna helps reduce
emissions in undesired directions. This could be
desirable where a narrower field of view can be
tolerated. Many scenarios in UWB short range
communications indeed are in line of sight (LOS) or
quasi-LOS conditions. However, when the terminal is
mobile, high antenna directionality is not adequate.
Furthermore directional antennas are larger in size than
omni-directional ones, whereas several UWB systems
require small-sized, easy to integrate and particularly
low complexity/cost antennas.
A semi-directional antenna provides a gain of a few dBi
(e.g. 4-6 dBi) with a FTBR of 4-5 dB, and its main lobe
covers a large zone. This should be a good trade-off in
order to improve the link-budget margin, while
preserving the robustness of the link and the small size
of the antenna.
In this paper we improve in terms of size and
performance the design of an UWB semi-directional
antenna, intended for low-cost applications [3].
2.
(DFMM) [5], and it combines a quasi omni-directional
radiator with a dielectric lens which focuses the
radiation. The DFMM antenna size is 33x mm x 20 mm
x1.524 mm, and it is realized in microstrip technology
on a Neltec® substrate (εr = 2.33). This antenna already
presents a slightly directive behaviour in the direction
normal to the monopole plane. The idea is then to
enhance this radiation asymmetry by placing a lens on
the substrate-ground plane side.
ANTENNA DESIGN
The design of the proposed antenna is derived from a
previous work on a dual-fed microstrip monopole
Figure 1. Antenna parameters and final prototype
Such an approach is typical of quasi-optical antenna
design [4], where the lens shape is modelled using
optical rays theory. However a small lens will affect the
radiation differently according to the wavelength, hence
the conventional design method is not appropriate in
UWB because of the large bandwidth. Thus, the method
of moments (MoM) based tool WIPL-D® is used for
simulations, in order to optimize the antenna
performance in terms of radiation and impedance
matching (Z0=50 Ohms).
The adopted lens shape is a half ellipsoid, and its
dimensions have been parameterized as depicted in
Fig.1. In all simulations a coaxial cable connected to the
SMA connector was taken into account, in order to
avoid antenna excitation by superior modes introduced
by the generator implemented in the simulation tool.
Proper de-embedding was carried out in the evaluation
of the impedance matching, but obviously the presence
of this cable will affect radiation results in simulation as
well as in reality, especially at the lowest frequencies
around 4 GHz.
In the presented simulations the lens position is fixed to
L=14 mm, which corresponds to the edge of the ground
plane of the DFMM antenna. As a result the lens is
located on the transition zone between the feeding
circuit and the monopole, where most currents are
concentred.
The semi-axis b=10 mm has been fixed in order to have
a narrower main lobe in the azimuth (X-Y) plane.
Its maximum size is limited by the DFMM width (20
mm).
out that the input bandwidth remains the same, with
some mismatch over the band 8.5- 10.2 GHz. It is
possible to reduce this mismatch by increasing h, as
depicted in Fig.3. The lens height indeed significantly
affects the antenna gain. This is an expected behaviour,
because of the directional effect introduced by the lens.
In previous simulations, the dielectric constant εr was
chosen with nylon in mind, for which the relative
permittivity varies between 3.2 and 5 depending on the
frequency. The effect of εr for a lens with a=b=h=10
mm is shown in Fig.4. By increasing the lens
permittivity it is possible to confine and concentrate the
waves in order to have a more directional behaviour. At
the same time the impedance matching is improved at
high frequencies.
In all simulations the onset of the matching bandwidth
is close to 4 GHz and close to that of the antenna
without lens [5], which implies that the lens has quite a
small influence on it.
(a)
(a)
(b)
Figure 2. Effect of the lens semi-axis a
a=10, 12,14 ,16 mm; b=h=10 mm εr=4.
Simulated S11 (a) and BRG (b)
As regards the antenna performance evaluation, we
consider the return loss (S11) and the BRG in the
direction normal to the ground plane (X-direction,
Fig.1).
In Fig.2 we show the influence of the semi-axis a, for a
lens of height h=10 mm and a dielectric constant εr = 4.
The antenna is matched over the 4.1-8.4 GHz band with
respect to
S11 < -10 dB, or over the 4.1-15 GHz band
with respect to S11 < -7 dB.
By increasing a, the resonant frequency around 14 GHz
shifts to lowest frequencies, but the input bandwidth
remains unchanged. On the other hand the BRG
decreases at the highest frequencies.
In Fig. 3 the effect of h is shown. The two semi-axis
values are fixed equal to a=b=10 mm, and the lens
dielectric constant is kept equal to εr = 4, while h varies
between 10 mm and 25 mm by step of 5 mm. It turns
(b)
Figure 3. Effect of the lens heights h
h=10, 15 , 20, 25 mm; a=b=10 mm εr=4.
Simulated S11 (a) and BRG (b)
Based on the whole set of simulations, the values of a
and b have been chosen with the purpose of maximizing
the BRG, while the lens height h has been chosen in
order to keep the antenna size reasonably small. The
final adopted lens shape is a half sphere of radius
a=b=h=10 mm, which is also easy to fabricate. This
results in a prototype of the DFFM with Dielectric Lens
(DFMM-DL) of size 33 mm x 20 mm x 11.5 mm. The
lens material is TECAMID 66 GF30, which is
commercially available and composed of Nylon and
30% of glass fiber.
(a)
Figure 5. Measured (solid line) and simulated (marker)
return loss
(b)
Figure 4. Effect of lens permittivity εr on the simulated
S11 (a) and BRG (b) (a=b=h=10 mm)
3.
MESUREMENTS RESULTS
The return loss and the radiation pattern have been
measured in an anechoic chamber with an HP8510C®
vector network analyzer and a calibrated 1-18 GHz Log
Periodic Dipole Array (LPDA) reference antenna. The
radiation characteristics of the DFMM-DL are obtained
after de-convolution of the reference antenna
3.1 Frequency domain results
The measured reflexion coefficient is presented in Fig.5
and compared with the simulated one with εr = 4.
The input bandwidth is 3.9-15 GHz with respect to
S11<-10 dB, with the exception of the band 9-9.8 GHz
where S11<-8 dB, which agrees with simulation quite
well. This mismatch can be tolerated in view of the fact
that the antenna BRG is always positive. Thus the input
bandwidth is improved compared to DFMM antenna,
which is matched over the 4.1-11.5 GHz [5].
In Fig.6 we show the compared BRG and FTBR of the
DFMM-DL and DFMM antennas. The use of the
dielectric lens improves the antenna gain up to 4 dB in
the DFMM input band, except for the frequencies
around 10 GHz, where anyway the FTBR is enhanced.
The maximum BRG is 7.5 dBi at 12.6 GHz, while the
maximum FTBR is 13 dB at 10.6 GHz. In Tab.1 we
show the mean BRG and FTBR computed by averaging
over several bandwidths chosen according to FCC [1] or
to the ECC [2] recommendations. Depending on the
selected sub-band, it is possible to achieve a favourable
mean FTBR or mean BRG, although there appears no
obvious direct relation between both.
Figure 6. Measured BRG (black) and FTBR (grey), in
DFMM-DL (solid line) and DFMM (dashed line)
Band
3,9-15
(input bandwidth)
3,1-10,6 (FCC)
3,1-4,8 (ECC)
4,8-6 (ECC)
6-8,5 (ECC)
8,5-10,6 (ECC)
10,6-15
(ECC above 10,6)
Mean BRG
(dBi)
Mean FTBR
(dB)
4,49
5,71
3,73
1,53
2,98
4,91
4,32
4,65
5
7,87
4,22
3,65
5,01
7,21
Table 1 Effect of the input bandwidth on mean radiation
characteristics
The frequency-dependence of the radiation pattern of
the DFMM antenna in three different planes is shown in
Fig. 7. The measured realized gain is shown at three
different frequencies in each plane.
the absolute fidelity of the antenna, instead of
computing the maximum magnitude of the normalized
cross-correlation between the transmitted waveform and
the time derivative of the input signal, commonly used
in the literature [6].
(a)
Figure 7. Measured realized gain
3.2 Time domain results
Classically, by means of a frequency domain approach,
it is possible to obtain more or less general information
such as the gain, the main lobe direction and
beamwidth, versus the frequency and phase or group
delay for a given direction (or even for a given solid
angle). In the time domain, it is also possible to access
the waveform of the radiated field as a function of the
angular coordinates.
When an antenna is excited by an incident signal w(t) it
radiates an electric field, whose waveform, e(θ,φ,t) in
the (θ,φ) direction, is a distorted version of the incident
one. Distortion is due to dispersion, i.e. the frequencydependence of the realized gain and the angular
frequency-deviation of the radiation pattern. A common
feature in UWB antenna characterization is a time
domain approach with the purpose to characterize the
distortion introduced by the antenna. This is of practical
importance in pulsed schemes, where distortion may
affect the overall system performance by introducing
inter-symbol interference.
Here the chosen excitation signal w(t) is a gaussian
impulse compliant with the FCC mask. The -10 dB
power bandwidth is 3.2-8.0 GHz (Fig.8).
In Fig.8 we show w(t), properly delayed, and e(θ,φ,t) in
the boreside direction, for the DFMM and DFMM-DL
antennas. Although both antennas distort, they do it in a
visually moderate way.
The normalized correlation between the excitation
signal and the radiated one in certain direction
quantifies the distortion introduced by the antenna in
that direction. This correlation has been used to define
(b)
Figure 8. (a) Spectrum of the excitation signal w(t), (b)
waveform of the excitation signal w(t) (blue), and
waveform of the radiated signal e(t), by DFMM (black
dashed) and DFMM-DL (black solid) in the boreside
direction.
However the antenna distortion may vary significantly
with the direction, thus it must be investigated with
respect to radiation angles. In addition to the absolute
fidelity, it is useful to look at the variation of the
distortion, with the direction which maximizes the
absolute fidelity as reference. Thus a Relative (or
Differential) Fidelity can be defined as (Eq.1), [5]:
DF t (θ , φ ) =
Max
τ
R
R
ee m
em
(θ , φ , τ )
(1)
( 0)
Where Rxy is the (x,y) cross-correlation function,


θ max , ϕ max = ArgMax Max R ew (θ , φ , τ )  ,
and
 τ

em (τ ) = e(θ max , ϕ max ,τ ) .
(
)
The direction maximizing the absolute fidelity turns out
to be the boreside one, which is obviously a favourable
feature [7]. Fig. 9 shows the relative angular fidelity in
the X-Y plane. The fidelity deviation at -1 dB is
respected over an angular range of 55°. Nevertheless the
influence of this variation on the UWB link
performance (e.g. Signal to Noise Ratio or Bit Error
Rate) should be evaluated in order to decide whether it
is acceptable or not.
However at the same time the lens position affects the
antenna gain, as depicted in Fig.11. Increasing L
produces a detrimental effect on the BRG especially at
high frequency. This result can be explained as a
consequence of an angular shift of the main lobe in
elevation, as seen in Fig. 12 which shows the mean
realized gain (MRG) computed over the bandwidth
3.9-15 GHz, for different values of L. Therefore it is
possible to improve the impedance matching, but this
results in an alteration of the radiation pattern.
Figure 9. Angular fidelity deviation of DFMM-DL
antenna in the X-Y plane, in dB
3.3 Effect of the lens position over the ground plane
In the prototype the lens has been placed at L=14 mm
(Fig.1), according to the antenna design (section 2).
Slight variations of L have been experimentally tested
on the prototype and are shown here. As depicted in
Fig.10, by moving the lens into the Z-direction we can
have a beneficial effect on the impedance matching. In
particular, for L = 19 mm and L=21.5 mm the antenna
respects S11<-10 dB over the all 3.9-15 GHz band.
(a)
Figure 10. Effect of the lens position on S11 (measured);
(b)
Figure 12. Effect of lens position on MRG (measured)
4.
Figure 11. Effect of the lens position on BRG
(measured)
Antenna performance in a LOS radio link
Let us consider a point to point LOS radio link with two
DFMM-DL antennas. The frequency dependence of the
antenna BRG (Fig.6) can be exploited in order to
compensate that of free space attenuation, when two
such antennas are used in the radio link. As depicted in
Fig.13, the transmitted/received power ratio of the
overall system is almost constant in the LOS direction
over the 3.4-9.5 GHz band (maximum variation of 2.5
dB). Such a feature is rarely obtained (see the two
isotropic ideal antennas case in Fig.13), and is very
interesting in that all sub-bands under a constant EIRP
spectral density will exhibit identical performance [7].
the frequency dependence of the gain has been exploited
in a point to point LOS radio link, achieving a
frequency flat two antenna system, which has some
virtues given current UWB regulations.
6.
Acknowledgments
The authors would like to thank G. Poncelet for his help
with the fabrication of the antenna prototype.
This work was in part supported by the European
Commission under IST integrated project PULSERS
(FP6).
Figure 13. LOS transmitted/received power
Figure 14. 2 DFMM-DL antennas system in LOS
However this is the most favourable case, while the
antenna orientation may change. We consider a
symmetrical rotation α from the boreside direction,
identical for the two antennas. The antenna rotation is
along the Z-axis, i.e. the azimuth plane (Fig.14).
In Fig.15 we show the influence of this rotation.
Performances are degraded, but even with a shift α=25°,
corresponding to a “global” variation of 50° from the
LOS boreside direction, the path loss remains roughly
frequency flat (maximum variation 2.5 dB) over the 3.48.8 GHz.
Figure 15. 2 Transmitted/received power in a 2
DFMM-DL system for different orientations in the
azimuth plane
5.
Conclusions
A design for a small UWB semi directional antenna has
been proposed. A gain improvement has been obtained
by the use of a dielectric lens with a quasi
omnidirectional antenna (maximum gain 7.5 dBi). The
lens design has been optimized in order to achieve a
trade off between the input bandwidth (3.9-15 GHz), the
antenna gain and the size. Time domain results
demonstrate a quasi non-distorting behaviour. Finally
References
1. Federal Communications Commission (FCC),
Revision of Part 15 ofthe Commission’s Rules
Regarding Ultra-Wideband Transmission Systems,
Notice of Proposed Rules Making (NPRM), ET-Docket
98-153
2. Electronic Communications Committee (ECC),
Decision of 24 March 2006 on the harmonised
conditions for devices using Ultra-Wideband (UWB)
technology
in
bands
below
10.6
GHz
(ECC/DEC/(06)04)
3. Roblin Ch., Sibille A., Bories S., Semi-directional
small antenna design for UWB Multimedia Terminals,
ANTEM 2005 Proc. p.142-143, St. Malo, 2005
4. Filipovic D. F., Gearhart S. S., and Rebeiz G. M.,
Double slot antennas on extended hemispherical and
elliptical silicon dielectric lenses, IEEE Trans.
Microwave Theory Tech., vol. 41, pp. 1738–1749, Oct.
1991
5. Ghannoum H., Bories S., D’Errico R., Small-size
UWB planar antenna and its behaviour in
WBAN/WPAN applications, IEE Seminar on Ultra
Wideband Systems, Technologies and Applications,
London, Apr. 20 2006.
6. Allen O. E, Hill D. A., Ondrejka A. R., Time-Domain
Antenna Characterization, IEEE TRANSACTIONS ON
ELECTROMAGNETIC COMPATIBILITY, VOL. 35,
NO. 3, AUGUST 1993
7. Sibille A., Bories S., D’Errico R., Roblin Ch., UWB
antenna
performance
evaluation
from
the
communication system point of view, ISWCS ’06,
Valencia, 5-8 Sept. 2006.