SEMI-DIRECTIONAL SMALL ANTENNA DESIGN FOR UWB MULTIMEDIA TERMINALS Christophe Roblin, Member, IEEE, Alain Sibille, Senior Member, IEEE, and Serge Bories ENSTA, Paris, France –
[email protected]
Abstract—. The design and results of a new UWB printed dipole antenna with a dielectric lens are presented. Index Terms—UWB antennas, small antennas, printed antennas. I.
INTRODUCTION
T
HE design of small size and low cost UWB (Ultra Wide Band) antennas, notably for mass product consumer applications, is nowadays one of the challenges for the “antennists” involved in the UWB communications domain. The targeted applications extend i.e. from LDR (Low Data Rate) systems such as sensor networks, to HDR (High Data Rate) terminals – fixed or nomadic – for Multimedia applications for BAN/PAN (Body / Personal Area Network), Home entertainment or WLAN. For most of these applications (which are, it is worthwhile to recall it, essentially full duplex), omnidirectional antennas are for obvious reasons the first a priori choice. On the other hand, because of the severe constraints compelled by the regulation authorities (FCC i.e.), notably as regards the transmitted power (precisely the EIRP), the link budget is of most concern, even for LOS (Line Of Sight) short range communications. It’s the reason why the improvement of the link budget, even of a few dBs (typically 3 to 6) is of main importance : the transmitted power being restricted, the only way to proceed at the RF HW (Hardware) level is to act at the receiver side, either by reducing the noise figure or by introducing antenna gain (or, of course, both). The latter solution should be in general more cost effective because the UWB VLNA (Very Low Noise Amplifiers) are not cheap. Semi-directional antennas may be a very effective approach in order to mitigate link budget while preserving link robustness. In many scenarios there will be LOS or quasi LOS between the transmitter and the receiver, meaning that most of the channel energy will be contained in the direct paths. Obviously maximizing the antenna gain in the direction of this path will be highly beneficial to the link budget. A further consolidated argument stems from experiments on the obstruction of the direct path by a human at 5 GHz [1], resulting in about 10-15 dB extra attenuation. The
received level can be described by a two-path model diffracted by the edges of the body. Clearly, provided the person intercepted the main path well within the antenna lobe, we can expect a good proportionality of the signal to the antenna gain from such a two path model. However in many of the contemplated scenarios, we cannot force the antenna(s) to have its main lobe be properly aligned on this direct path. Consequently, since gain is associated with directivity and directivity is associated with angular filtering, there is a risk to lose robustness whenever the main path shifts too far from this main lobe. The consequence of that is a tradeoff. Although the antenna may rotate and it is difficult to ensure it will be properly aligned, in many instances it is sensible to think it will be aligned “roughly”, i.e. in the right half space or even in the right quarter of space. Therefore a reasonable choice would be to specify the antenna to be semi-directional, i.e. little gain on its back side, and a main lobe covering about 120° in half power azimuth beam width. A simple design approach is to use a quasi omnidirectional planar radiator as will be described in section II-A, and to add a dielectric lens in order to redirect the major fraction of the radiation on the substrate side of the circuit (§ II-B). Such techniques are well-known for quasi-optical antennas, and present the advantage to start from an existing design, and to be relatively simple in terms of production feasibility in particular through moulding of the lens. II. ANTENNA DESIGN AND RESULTS A. Quasi omni-directional planar dipole design The design is based on several solutions developed in our lab. for the DVB-T (with proper frequency scaling) within the European IST project CONLUENT [2, 3] (during which, notably, we developed cpw-fed elliptical planar dipole UWB antennas) and on the work published in [4]. The adopted geometry is a wide planar dipole fed with a coplanar waveguide (milled over the bottom part of the dipole) : this provide a first size reduction with respect to a thin dipole. The use of a substrate of intermediate permittivity (RO3006® with εr = 6.15 and thickness h = 1.27 mm) offers a further significant size reduction. The "optimized" dimensions obtained after many
simulations with the electromagnetic software WIPL® are (fig. 1) : L = 31 mm, W = 21.64 mm, Lb = 12.1 mm, Lt = 10.4 mm, Ls = 8.5 mm, the size of the central gap between poles being Lg = 0.6 mm and Wg = 5 mm ; for the 50 Ω CPW feeding line, wcpw = 1.2 mm and gcpw = 0.22 mm.
partial Eθ gain on boresight varies from 0.9 to 3.3 dB in the same band. The cross component Eϕ level is very low for the whole bandwidth for ϕ = 180 or 0 deg, and, in any case, lower than -20 dB over 3-6 GHz.
L
W
W
z Fig. 4 : gain versus frequency
y Lb
Ls
Lt
Fig. 1 : sketch of the 1st antenna
Note that the choice of the substrate was only dictated for demonstration purpose, considering that many other simulations have demonstrated that cheaper substrate with relative permittivity in the range 4-5 (i.e. FR4) provide as well interesting results. Fig. 5 : measured return loss
The measured input bandwidth is typically [3-10.5] GHz for a VSWR < 2.5 and [3.3-5.2] for a VSWR < 2.
Fig. 2 : simulated return loss
B. Semi-directional planar dipole-lens design The antenna parameters have been “re-optimized” with a dielectric lens (of various shapes) added. Its permittivity should be close to the substrate one (εr = 6.15 was used for both in the simulations). Eventually, the adopted shape is a half prolate ellipsoid (semi axis : a = b = 11 mm, a < c = 25 mm). The obtained characteristics seem adequate with respect to wished performance, with an improved gain of ~5.7 dBi in the main lobe at 5 GHz (3-4 dB gain increase on average).
a
c Fig. 3 : Gain in azimuth (θ = 90 deg – xy plane)
The input bandwidth (VSWR < 1.92) is 2.9 – 10.5 GHz (fig. 2). The FTBR (Front to back ratio) is less than 0.5 dB over the band 3-8 GHz (fig. 3 & 4). The ordinary
Fig. 6 : 3D view of the antenna with lens (εlens = 6.15)
Fig. 7 : simulated return loss (antenna with lens εlens = 6.15)
Fig. 11 : measured return loss (antenna with lens εlens = 2.33)
Fig. 8 : Simulated front and back gains (antenna with lens)
Fig. 12 : measured effective gain in azimuth over [3-8] GHz (Dipole with dielectric lens εlens = 2.33).
5.17 dBi
Fig. 9 : Simulated 3D gain pattern at 5 GHz (antenna with lens)
The following results are concerned with a “variant” of the preceding with a plexyglass lens (εlens = 2.3) instead of the chosen material (Nylon-type), not yet available. Although this results in a significant increase of the lens size, an optimized prototype was fabricated to validate the principle. Fig. 13 : Dipole with lens (εlens = 2.33) ; measured gain versus frequency : effective gain (front : plain blue – back : red circles) & ordinary gain (front : green dashed-dotted).
Fig. 10 : Simulated gain versus frequency (εlens = 2.33).
As can be seen (fig. 11), the antenna with lens offers a gain bandwidth which is closer to [3.5-8] GHz than to the targeted [3-6] GHz. This obvious discrepancy with the simulation, especially over the low frequency part, is due on one hand (but probably slightly) to the fact that the simulation is loss free, and on the other hand to the difficulty of modeling of the feeding part and, in the same time, of the measurement particularly in the case of small antennas. Actually, small antennas without a
“real” screening ground plane are difficult to design, simulate and characterize because, particularly for the lower frequencies, they radiate “as a whole”, the close environment being strongly involved, notably the feeding cable. Nevertheless, it can be observed (fig. 13) that, apart from this “frequency shift”, the achieved gain and front to back ratio (FTBR ~= 6 dB on average) are fully satisfactory. III.
ANTENNA COMPARISONS AND DEMONSTRATION WITH THE ULTRAWAVES PLATFORM
Several tests of antennas performance have been carried out with the UWB communication platform developed in the Ultrawaves IST project [5]. The involved antennas were commercial ones (Skycross and others not presented here) and prototypes designed at ENSTA and ENST Paris (in collaboration). A part of the results is summarized in the following table. The acronyms uses are : • SKY : Skycross (commercial) antenna • SKYG : Skycross antenna with a small home made added ground plane • FPF : F-probe fed planar triangular patch (ENST Paris) • BIC : shaped small bicones [6] (ENSTA) • PDDL : Planar Dipole Dielectric Lens presented antenna (ENSTA)
IV. CONCLUSION The realized antenna gives satisfactory results, in particular as regards the tests performed with an UWB platform in realistic situations. The results concerning the targeted antenna (with a smaller lens of higher permittivity) will be presented at the conference. ACKNOWLEGMENTS This work has been partly funded by the European Union within the framework of the IST ULTRAWAVES project [5]. We address our thanks to all partners and particularly to Bram van der Wal from Philips Consumer Electronics (Eindhoven) for his welcome to his lab. and as well Domenico Porcino for his contribution to fruitful discussions. REFERENCES [1]
[2] [3] [4] [5]
UWB Antennas SKY-FPF
Data Rate Range comment (Mb/s) (m) 30 > 9.5 LOS ; OBST (body) : ~OK @ 3 m SKY-PDDL 30 brief body obst ==> a few err + slight blocking (@ 3m) BIC-PDDL 30 brief body obst. ==> a few err + slight blocking (@ 3m) FPF-PDDL 30 > 9.5 LOS. < 7 m when obstructed by a body (small errors) SKY-SKY 48 6.5 LOS1 Æ episodic blocking & Err ; hand obst. Æ loss of Rx SKYG-SKYG 48 6.8 LOS Æ episodic blocking ; hand obst. Æ No loss of Rx SKY - BIC 48 6 LOS2 Æ episodic blocking & Err ; hand Æ loss of Rx SKYG-FPF 48 > 9.5 LOS SKYG-PDDL3 48 > 8.4 LOS (~OK @ ~9m); hand Æ No loss of Rx FPF-FPF 48 > 9.5 LOS ; Obstructed by a body : ~< 5 m (small errors) FPF - PDDL 48 > 9.5 LOS ; Obstructed by a body : ~< 5 m (small errors) Table 1 : range results in the various antenna configurations and data rates
1
Loss of link rather often Loss of link episodically ; sensitive to proximity body perturbation 3 Planar Dipole with Dielectric Lens (ENSTA) 2
[6]
Jonas Medbo, Jan-Erik Berg and Fredrik Harrysson, Temporal Radio Channel Variations with Stationary Terminal, COST 273, TD(04) 183, Duisburg, 20-22 sept 2004 (also presented at VTC2004-Fall in Los Angeles, September 26-29, 2004) A. Guena, D. Zapparata, A. Sibille and G. Pousset, Mobile diversity reception of DVB-T signals using roof or window antennas, COST 273 TD(04) 014, Athens, Jan. 2004. CONFLUENT (IST-2001-38402) project Deliverable D2.3, Nov. 2003. Y. Kim and D.H. Kwon, “CPW-fed planar ultra wideband antenna having a frequency band notch function”, Electr. Lett. Vol. 40 n°7, 2004. A. Sibille, Ch. Roblin, S. Bories, X. Begaud and A.-C. Lepage, Prototypes and analysis tools for ultra wide band antennas, ULTRAWAVES (IST-2001-35189) project deliverable D6.1, Nov. 2004. H. Ghannoum, S. Bories, Ch. Roblin & A. Sibille, “UltraWideband slightly distorting bicone antenna for UWB channel measurements”, to be presented at IWAT 2005, Singapour, Mars 2005.
