Ultra-Wideband Monocone Antenna for UWB Channel Measurements

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					paper n° 1049                                                                                                                                    1




                        Ultra-Wideband Monocone Antenna
                         for UWB Channel Measurements
                 Serge Bories, Christophe Roblin, Member, IEEE, and Alain Sibille, Senior Member, IEEE


                                                                            a ground plane of 100 mm diameter. The monocone structure
   Abstract A new version of well known monocone [1], [2] has               is mechanically held by a radome, made of foam with very
been designed in order to comply with existing (FCC) and                    low permittivity.
future regulations for Ultra-wideband (UWB) communications.
This radiator is also intended to be used for channel
                                                                                                               axis
measurements involving antenna properties close to the real
application. Being both strongly frequency independent and                                      23mm
omni-directional, this antenna has a quite stable main lobe on a
111 % fractional bandwidth. In addition a very low dispersion                                                                      22mm
ensures the respect of the radiated waveform.

   Index Terms— UWB antenna, UWB channel measurements,
linear phase response.

                         I. INTRODUCTION

U    WB communications need stringent information about
     the indoor propagation channel. Those measurements
require specific antennas whose major requirements are a
good matching, omni-directionality and a large and                                                             (a)
frequency-independent main lobe. These three criteria should                                radome
be respected over an ultra-wide band. The design and
performances of a monocone-like antenna extending over a
2.7-9.4 GHz bandwidth are presented.
                                                                                   monocone
                       II. ANTENNA DESIGN
   This antenna is derived from the infinite monocone above a
ground plane [3]. The principle of infinite monocone is to
ensure a theoretical frequency-independence thanks to its
‘scale invariance’ structure. In reality this infinite monocone
                                                                                                                                  ground
can only be truncated upwards and downwards. The lower                                                                            plane
frequency is mainly governed by the radiator length (and to a
lesser extent by the shape of the upper part of the monocone).
                                                                                                               (b)
In the same way, the finite size of the connectorized feeder                Fig. 4 : UWB monocone : (a) simulation with the MoM method (meshed
(here a 50 Ù female sma connector) imposes the upper                        structure in WIPL-D ), (b) experimental prototype
frequency limit of the antenna. The shape of the feeding is
particularly critical [4], and requires that the transition                                          III. PERFORMANCES
between guided and free space propagation is smoothed with
an exponential (in fact elliptical) profile (Fig. 1).                         A. Technical approach
   For UWB channel measurements, we have realized a                            The MoM method tool WIPL-D has been used for (about
22x23 mm height x diameter monocone, made of brass above                    600 unknowns) simulations. The foam radome is not taken
                                                                            into account. The VSWR and the radiation pattern were
  Manuscript received February 7, 2003; revised April 22, 2003. This work   measured in an anechoic chamber with an HP8510C® vector
was supported in part by the European Union under the FP6 project           network analyser and a calibrated 1-18 GHz Log Periodic
ULTRAWAVES.
  The authors are with the Ecole Nationale Supérieure de Techniques         Dipole Array (LPDA) antenna [5]. The following results are
                         ce (e-mail: : bories@ensta.fr).                    de-embedded so that they only show the monocone antenna
                                                                            behaviour.
paper n° 1049                                                                                                                                                  2

  B. Results                                                                  independence of the radiation pattern. The measurements in
   The simulated and measured VSWR are shown in Fig. 2.                       elevation (Fig. 4 (a)) are given for several frequencies (3, 4.5,
The measured input bandwidth is 2.7-9.4 GHz for a VSWR                        6, 7.5, 9 GHz).
less than 2.                                                                     Note that the monopole axis (vertical) is oriented toward
                                                                              theta = 90 deg. The –3 dB beamwidth decreases slowly with
                                                                              frequency from 70° at 3 GHz to 40° at 9 GHz. The direction
                                                                              of the main lobe is quite constant (near 30° above the ground
                                                                              plane) until 7 GHz but the main lobe shifts upwards (45° at 9
                                                                              GHz) with higher frequencies. This is a finite size ground
                                                                              plane effect which is electrically larger as lambda decreases.




Fig. 2 : simulated (blue) and measured (red) VSWR (100 MHz frequency step).


   The gain is weakly frequency dependent : 4.5 dBi (+/- 1
dB) over the whole VSWR bandwidth (Fig. 3). Moreover the
slow increase of the mismatch beyond 9 GHz is balanced by
the low decreasing of the beamwidth so that the gain remains
high until more than 18 GHz. Note that the VSWR and the
gain have no significant “accident” over the frequencies of
interest (3-8 Ghz) but only a slight increase in average (mean
slope of 0.4dB/GHz).                                                                                                      (a)




Fig. 4 : measured (red) and simulated (green) maximum gain.

                                                                                                                         (b)
  The cross-polarization maximum level is always 20 dB
below the co-polarization one.                                                Fig. 4 : measured gain : (a) in elevation, (b) in azimuth in the main lobe (θ = 45
                                                                              deg). 3GHz (blue), 4.5GHz (cyan), 6GHz (green), 7.5GHz (orange), 9GHz
                                                                              (yellow).
  Another major requirement for both channel measurement
and UWB communication applications, is the frequency-                           The measured radiation patterns in azimuth (Fig. 4b) show
                                                                              an omni-directional behaviour (offset of, at worst, +/-1 dB for
paper n° 1049                                                                                                                                     3

the lowest frequencies (2-3 GHz)).                                  explain the global increase of the standard deviation of the
                                                                    group delay. Whereas for the monocone, the active zone is
                                                                    growing with frequency but it is always centred on the
  In the UWB systems based on short-pulse modulations               feeding
(Impulse Radio), the question of the distortion induced by the
antennas has to be addressed. For this purpose, the phase of
the radiated far field was also measured (Fig.4). It is
essentially linear in the main lobe.                                                                è                             è




                                                                               (ns)




Fig. 4 : simulated phase response vs frequency for :
theta= 30° (blue), theta=60° (black), theta=0° (red)                                                                                     (°)

   To quantify and interpret efficiently the phase linearity
(with frequency) with respect to the angular coordinates call        Fig. 5 : measured standard deviation of group delay in nanoseconds vs elevation
                                                                    è in degrees for the UWB monocone (black) and the LPDA (blue)
for the definition of global, simple and efficient parameters.
As an example and for sake of simplicity, the standard              zone.
deviation of the group delay in a given direction :
            1 f2                                                       To conclude, the “ gd criterion” seems to indicate that the
            ∆f ∫ f1 g
σ gd =             (τ − τ g )2df
                                                                    behaviour of the UWB monocone in the pulsed transmit mode
will be used here. Nevertheless this criterion needs a lot of       is mainly dispersionless in the main lobe.
points on the frequency axis (70 frequency points here, over
3-10 GHz).                                                                                      IV. FABRICATION
                                                                       Fabrication of this kind of monopole is a tricky issue
   Thus gd( ) is used here to evaluate the dispersion as a          because the thinnest part (0.9 mm diameter) is both the most
function of the radiated direction. It is computed from the         fragile and the feeding area. The monocone has been
measurements over the 3-10 GHz VSWR bandwidth (Fig. 5)              machined from solid cylinder of brass with a turning machine
for the presented monocone (black). The dispersion is low           whose minimum step (axial and radial) was 0.01mm. Ground
( gd = 1ns) around the main lobe (è=30°). Note that the             plane was cut in a 4mm depth plate in order to shape the
dispersion increases sharply off the main lobe (typically           feeding area and to position accurately the connector backside
beyond                                      the ground plane).      thanks to a sink footprint. The foam radome is also machined
This value of a dispersionless beamwidth corresponds roughly        to support mechanically the monocone so that the antenna
to the gain beamwidth in the middle of the 3-10GHz                  can be manipulated risk-free during a measurement
bandwidth.                                                          campaign.
   As a matter of comparison, this parameter is also computed
(Fig. 5) in the case of a measured LPDA antenna [5]. The              To demonstrate the low sensitivity of this ‘frequency
minimum value of ógd is 1.5 ns (i.e. 50% higher than the            independent’ antenna structure, the optimum height of the
monocone). The LPDAs are known to be dispersive because             feeding rod (h = 7.2mm) is shifted by different values (Äh =
of their phase center shift with frequency. [6]. The shift of the   0.3, 1.3, 2.3 mm) which modifies the aperture profile (Fig. 6).
active zone with frequency along the antenna structure can          This sensitivity experiment shows that the measured VSWR
paper n° 1049                                                                            4

tolerates fabrication or connectorization errors of less than 1.3
mm for the 2-10 GHz bandwidth.

dB

       -5


     -7.5


     -10


 -12.5


     -15


 -17.5


     -20




            0    0.2    0.4    0.6     0.8   1     1.2   1.4    1.6    1.8           2
                                                                                    10
                                                                             x 10

Fig. 6 : Measured VSWR vs frequency for different values of feeding rod height
   = 0 (black), 0..3 (blue),1.3 (green), 2.3 mm (magenta).



                                     V. CONCLUSION
  Experimental results have demonstrated that the proposed
monocone is able to achieve the main requirements needed to
perform UWB channel measurements. Future work will
address the reduction and shaping of the ground plane, which
will result in pulling down the direction of the main lobe
towards the horizontal ground plane. To increase mechanical
robustness and/or to avoid using a foam radome, future
prototypes will be lighter, by scooping out the radiator. More
details on the technical approach and results will be given
during the workshop.


                                     ACKNOWLEDGMENT
  The authors thank G. Poncelet for help with the fabrication
and measurements of prototypes.

                                       REFERENCES
[1]         S. A. Schelkunoff, Electromagnetic Waves, Van Nostrand, Princeton, N.J.
            1943 ch. XI.
[2]         C.H. Papas and R. W. King, “Input impedance of wide-angle conical
            antennas fed by a coaxial line”, Proc. IRE, vol. 37, pp.1269-1271, 1949

[3]         S. N. Samaddar, E. L. Mokole, “Biconical antennas with unequal cone
                    IEEE Trans. Antennas Propagat., vol.46, No 2 Feb. 1998,
            pp.181-193

[4]         E. Zollinger , “Extremely wideband antennas” in COST Action 231 -
            Digital mobile radio towards future generation systems - Final report, ch.
            3, pp.80-82.

[5]         [The measuring antenna is an ESLP 9145 from SCHARZBECK MESS
            www.schwarzbeck.de
[6]         C. A. Balanis , “Antenna theory, analyse and design” 2nd edition , Ed.
            Wiley. ch. XI p.556