Towards Miniaturisation of UWB Antennas

Towards Miniaturisation of UWB Antennas X. Chen, L. Guo, S. Wang, C. G. Parini Department of Electronic Engineering Queen Mary, University of London, U.K. 29th Nov 27, NPL 1

Overview Introduction Antenna Size and Bandwidth Miniaturised Planar UWB antennas - Monopoles - Half Disc Monopole Antennas - Tapered Slot Antennas Discussion 29th Nov 27, NPL 2

Introduction QMUL Antenna & EM Group The group was found in 1968 Currently: 5 Academic Staff, 8 RAs and 18 PhD students; Funding over this RAE period: > 4M; Current EPSRC funding: >1.39M; Wide links with industry and institutions Research topics - Covering wide areas in antenna engineering and EM applications Facilities: 3 Anechoic Chambers (5 MHz 35 GHz), 2 CATRs (5GHz 35GHz), NSI.9m x.9m NF scanner. 29th Nov 27, NPL 3

Introduction UWB Antennas The quest for small UWB planar antennas Rapid development of UWB wireless communication systems has led to the increasing demand for miniature UWB antennas. Provide flexibility to device designers and easily integrates into space-limited systems such as home-cinema equipment, Portable computers, PC peripherals, PDAs, mobile phones and UWB USB. Main Effects of Size reduction on Antenna Performance: Gain, bandwidth and efficiency 29th Nov 27, NPL 4

Antenna Size and Bandwidth Ground Plane R is the radius of the boundary sphere around an antenna FBW = Δf f 3dB 1 db = Q Higher Q, narrower FBW Lower Q, wider FBW 29th Nov 27, NPL 5

Antenna Size and Bandwidth Minimum attainable radiation Q of a linearly polarized antenna Q = 1 1 ( kr ) kr 3 + Chu s Mclean s Minimum attainable radiation Q of a circularly polarized antenna 1 1 Q = ( + 3 2 ( kr ) Harrington s Mclean s 2 kr ) The lowest values of Q is limited by the size of an antenna! [1] L. J. Chu, Physical Limitations of Omni-directional Antennas, J. Appl. Phys., Vol. 19, pp. 1163-1175, Dec. 1948 [2] R. F. Harrington, Effect of antenna size on gain, bandwidth and efficiency, J. Res. Nut. Bur. Stund., vol. 64-D, pp, 1-12, Jan./Feb. 196. [3] J. S. McLean, A Re-Examination of the Fundamental Limits on the Radiation Q of Electrically Small Antennas, IEEE Trans. Antennas Propagat., Vol. 44, pp. 672-676, May 1996 29th Nov 27, NPL 6

Antenna Size and Bandwidth Both Chu-Harrington and McLean limits indicate that reducing the size of the antenna leads to the decrease of the bandwidth of the antenna. Challenge: Reducing antenna size while retaining UWB property! In practice, a variety of fat element monopoles were found to provide broad impedance bandwidth! 29th Nov 27, NPL 7

Monopoles Classical Broadband Antennas Straight wire monopole Bandwidth: 1% Replacing the wire element with a planar element 29th Nov 27, NPL 8

Monopoles Circular disc monopole-- Vertical type R z L W x h y Geometry of the antenna R=12.5mm, h=.7mm, W=1mm and L=1mm wide bandwidth, satisfactory radiation properties, simple structure, low cost Non-planar 29th Nov 27, NPL 9

Planar Monopoles Optimising the topology of planar antennas to reduce antenna size while retaining UWB. CPW-fed disc was found has a better impedance matching over microstrip-fed disc monopole. FR4 substrate ε r H Z ground plane X h L1 Z y microstrip feed line W W1 R ground plane on the back L z y 5Ω coplanar waveguide g wf W R H substrate εr g h ground plane L Return loss, db -5-1 -15-2 -25-3 -35-4 CPW fed microstrip line fed -45 1 2 3 4 5 6 7 8 9 1 11 12 13 14 15 Frequency, GHz 47mm 41mm with a dielectric constant of 4.7 CPW fed (2.73GHz-15GHz) Microstrip fed (2.78GHz-9.78GHz) 29th Nov 27, NPL 1 1.J. Liang, C Chiau, X. Chen and C.G. Parini, Printed circular disc monopole antenna for ultra wideband applications, Electronic Letters, vol. 4, pp.1246-1248, September, 24.

Radiation Patterns CPW fed The H-plane pattern of the CPW-Fed disc monopole antenna is omni-directional at 3 GHz but distorted at 8.8 GHz 33 3 33 3 3 6 3 6 27-4 -3-2 -1 9 27-4 -3-2 -1 9 24 12 24 12 21 18 15 21 18 15 At 3 GHz At 8.8 GHz The simulated (red line) and measured (blue line) H-plane radiation patterns of CPW-Fed disc monopole antenna 29th Nov 27, NPL 11

Measurement of Monopoles Measurement set-up 29th Nov 27, NPL 12

Mechanism of UWB monopoles How exactly a printed UWB monopole operates across the entire bandwidth? Does a planar UWB monopole break Chu- Harrington s limit? 29th Nov 27, NPL 13

Mechanism of UWB monopoles The disc supports multiple resonant modes Return Loss, db Simulated return loss curves -5-1 -15-2 -25-3 2R λ 1 /4-35 2 4 6 8 1 12 14 2R 2R λ 2 /2 Frequency, GHz 2R 3λ 3 /4 2R λ 4 First resonant freq: 2R λ 1 /4 Second resonant freq: 2R λ 2 /2 Third resonant freq: 2R 3λ 3 /4 Forth resonant freq: 2R λ 4 29th Nov 27, NPL 14

Mechanism of UWB monopoles Current distribution At 3GHz At 5.6GHz At 8.6GHz At 12.8GHz 3 3 35 Magnitude of H-field 25 2 15 1 5 Magnitude of H-field 25 2 15 1 5 Magnitude of H-field 3 25 2 15 1 5 5 1 15 2 25 3 35 Distance, mm 5 1 15 2 25 3 35 Distance, mm 5 1 15 2 25 3 35 Distance, mm H-field distributions along the edge of the half disc 29th Nov 27, NPL 15

Mechanism of UWB monopoles Hybrid mode: standing wave + travelling wave Good feeding structure for traveling wave at high freq. Standing wave Travelling wave f1 f2 f3 f4 Frequency 29th Nov 27, NPL 16

Slot Monopoles Elliptical / circular slot antennas CPW-fed: -5-1 Circular slot Elliptical slot Return loss, db -15-2 -25-3 -35-4 Microstrip line fed: -45 2 3 4 5 6 7 8 9 1 11 12 Frequency, GHz Measured return loss Return loss, db -5-1 -15-2 -25-3 -35 Circular slot Elliptical slot At 3.1GHz At 1GHz E-Plane pattern -4 2 3 4 5 6 7 8 9 1 11 12 Frequency, GHz Measured return loss H-Plane pattern 29th Nov 27, NPL 17

Half Monopoles Halve the disc antennas to achieve a 4% reduction in size. The miniaturisation improves the impedance bandwidth but reduces the gain and degrades the polarization purity in the upper frequency range. Original size: 17 17 mm with a LTCC dielectric constant of 7.8 Reduced size: 1 17 mm with a LTCC dielectric constant of 7.8 iwat 7, M. Sun, Y.P. Zhang, NTU, Singapore Magnitude of S11 (db) -5-1 -15-2 -25-3 -35-4 -45 Miniaturized Un-miniaturized 2 3 4 5 6 7 8 9 1 11 12 Frequency (GHz) Gain (dbi) 6 2-2 -6-1 -14-18 -22 Miniaturized Un-miniaturized 2 3 4 5 6 7 8 9 1 11 12 Frequency (GHz) Measured return losses Measured gain 29th Nov 27, NPL 18

Half Monopoles In QMUL, The CPW-fed disc monopole was cut into half, the reduced antenna size is 16.33 4 mm with a dielectric constant of 3. The return loss is below -1 db within 1.94-11.59GHz frequency band. y 16.33mm x 5 Miniaturized Un-miniaturized 3 33 3 6 3 33 3 6 4mm 12.5mm B ) (d L oss rn R etu -5-1 -15-2 -25-3 -35 27 9-4 -3-2 -1 24 12 21 15 18 At 2.45GHz 27 24 21 18-4 -3-2 -1 15 12 At 5.64GHz 9 E-Plane pattern -4-45 2 4 6 8 1 12 Frequency (GHz) 3 33 3 6 3 33 3 6 Measured return losses half disc- blue line Full disc red line 27 24 9-4 -3-2 -1 12 27 24-4 -3-2 -1 12 9 H-Plane pattern 21 15 21 15 18 18 A similar design was proposed by B. Sanz-Izquierdo and J.C. Batchelor, University of Kent, LAPC 27 37 x 2mm 34.25 x 12.5mm 29th Nov 27, NPL 19

Half Monopoles Mechanism of the miniaturized antenna L L1 No radiating radiating 2.46 GHz 3.1 GHz Firstly, the disc monopole antenna is simply chopped half Current distributions L3 L3 L3 The relationships between L2, L3 and the first resonances L2 L2 L2 Further verification by introducing a ground plane on the other side with different length L2 First Resonance f1 L2=4mm, L3=31.3mm 2.72 GHz L2=7mm, L3=28.3mm 2.76 GHz L2=1mm, L3=25.3mm 2.84 GHz L=25 mm (full disc) 3.1 GHz -5-1 -15 wavelength λat f1 11.3mm 18.7mm 15.6mm 99.7mm Return Loss (db) -2-25 -3 L3/λ.28.26.24.25-35 -4-45 L2=1mm, L3=25.3mm L2=7mm, L3=28.3mm L2=4mm, L3=31.3mm -5 2 4 6 8 1 12 Frequency (GHz) 29th Nov 27, NPL 2

Tapered Slot Antennas As an travelling wave antenna, TSA has demonstrated multi-octave bandwidth. A typical TSA should have a aperture width of slot W λ /2 and the length of the TSA L from 2 to 12λ. It is not suitable for application when low-profile is essential. L W Return loss, db -5-1 -15-2 -25-3 Measured Simulated 7.6mm 6mm -35 2 3 4 5 6 7 8 9 1 11 12 Frequency, GHz Measured: 3.3GHz 1.8GHz Simulated: 3.GHz 1.5GHz 29th Nov 27, NPL 21

Miniature Tapered Slot Array Antenna The miniature tapered slot antenna in an array environment can obtain a bandwidth of 4-16GHz with wide scan angle. The element is only.12λin width and.3λin depth at 4GHz. The wideband impedance matching is achieved essentially due to the constructive effect of the mutual coupling among elements. 23mm 9mm E- and H-plane Scanned Active Reflection Coefficient (db) of the TSA array as the function of scan angle Ө and frequency. The bandwidth achieved is approaching 4:1 from 4-16GHz 29th Nov 27, NPL 22

Conclusions Planar monopoles are capable of supporting multiple resonance modes, overlapping of the evenly spaced modes leads to UWB characteristic Chu-Harington limit no longer hold in multiple resonances! A remarkable more than 5% size reduction can be achieved for printed disc monopole antenna for UWB systems by simply halving the original antenna and tuning other parameters The small tapered slot antenna can achieve wideband performance in an array environment due to strong mutual coupling between elements. 29th Nov 27, NPL 23

Q&A Thank You 29th Nov 27, NPL 24