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Title:
ANTENNA
Document Type and Number:
WIPO Patent Application WO/2018/051122
Kind Code:
A1
Abstract:
An antenna comprising: an insulating substrate; an electrode on a first surface of the substrate; a ground plane on a second surface of the substrate, the second surface being on the opposite side of the substrate than the first surface; and a tapered microstrip feed line on the first surface and connected to the electrode, at least a part of the microstrip feed line opposing the ground plane; wherein the electrode is substantially rectangular and has a rectangular slot projecting inwardly from a first edge thereof; the ground plane has a notch projecting inwardly from a first edge thereof, the notch tapering toward the interior of the ground plane; and the antenna has a peak of radiation efficiency in the range of from 400 MHZ to 1200 MHz.

Inventors:
GAO YUE (GB)
ZHANG QIANYUN (GB)
Application Number:
PCT/GB2017/052748
Publication Date:
March 22, 2018
Filing Date:
September 15, 2017
Export Citation:
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Assignee:
UNIV LONDON QUEEN MARY (GB)
International Classes:
H01Q9/40; H01Q1/38; H01Q5/25
Other References:
ZHANG QIANYUN ET AL: "Compact U-shape ultra-wideband antenna with characteristic mode analysis for TV white space communications", 2016 IEEE INTERNATIONAL SYMPOSIUM ON ANTENNAS AND PROPAGATION (APSURSI), IEEE, 26 June 2016 (2016-06-26), pages 17 - 18, XP032983912, DOI: 10.1109/APS.2016.7695717
Attorney, Agent or Firm:
J A KEMP & CO (GB)
Download PDF:
Claims:
CLAIMS

1. An antenna comprising:

an insulating substrate;

an electrode on a first surface of the substrate;

a ground plane on a second surface of the substrate, the second surface being on the opposite side of the substrate than the first surface; and

a tapered microstrip feed line on the first surface and connected to the electrode, at least a part of the microstrip feed line opposing the ground plane; wherein

the electrode is substantially rectangular and has a rectangular slot projecting inwardly from a first edge thereof;

the ground plane has a notch projecting inwardly from a first edge thereof, the notch tapering toward the interior of the ground plane; and

the antenna has a peak of radiation efficiency in the range of from 400 MHZ to 1200 MHz. 2. An antenna according to claim 1 wherein the first edge is a short edge of the electrode.

3. An antenna according to claim 1 or 2 wherein the slot projects inwardly from the middle of the first edge.

4. An antenna according to any one of the preceding claims wherein the width of the slot is in the range of from 1/100 to 1/2, desirably 1/10 to 1/4 of the width of the electrode. 5. An antenna according to any one of the preceding claims wherein the length of the slot is in the range of from 1/2 to 9/10, desirably 2/3 to 4/5 of the length of the electrode.

6. An antenna according to any one of the preceding claims wherein the ground plane does not overlap the electrode and there is a gap between the first edge of the ground plane and the second edge of the electrode. 7. An antenna according to claim 6 wherein the gap has a width in the range of from 0.1 mm to

2 mm, desirably 0.8 mm to 1.2 mm.

8. An antenna according to any one of the preceding claims wherein the notch has curved sides.

9. An antenna according to any one of the preceding claims wherein the length of the notch is in the range of from 1/5 to 4/5, desirably 1/4 to 1/2 of the length of the ground plane. 10. An antenna according to any one of the preceding claims wherein the width of the notch at the edge of the ground plane is in the range of from 1/2 to 4/5 of the width of the ground plane.

11. An antenna according to any one of the preceding claims wherein the thickness of the substrate is in the range of from 1/10 to 1/20 of the width of the electrode.

12. An antenna according to any one of the preceding claims wherein the antenna has a bandwidth of greater than 500 MHz.

13. A transmitter including an antenna according to any one of the preceding claims.

14. A receiver including an antenna according to any one of claims 1 to 12.

15. A data network including a transmitter according to claim 13 and/or a receiver according claim 14.

Description:
ANTENNA

[0001] The present invention relates to antennas, in particular antennas for wavelengths in the ultrahigh frequency (UHF) range.

[0002] The TV white space (TVWS) comprises channels that are not used by digital terrestrial television (DTT) or programme making and special events (PMSE) users, and those freed up by the switch-over from analogue to digital TV broadcasting. Idle channels in TVWS could provide services for rural broadband, indoor communications and Machine-to-Machine (M2M) communications.

Owing to low operating frequencies, services working in TVWS benefit from large communication range and good in-building penetration.

[0003] The UHF operating frequency calls for large antennas, which are difficult to fit into modern compact devices. In addition, TVWS may be allocated to services dynamically in different locations and at different times. Thus, radio devices intended to operate in TVWS are required to be able to operate at the entire UHF TV. As a result, antennas designed for devices operating at TVWS have to be either reconfigurable or wideband. Due to the high power consumption of switches of the reconfigurable antennas, wideband antennas have the advantage for mobile terminals. Many research works have explored the miniaturization of wideband antennas operating at UHF TV channels, e.g.: (NPL1) H. D. Chen. "Compact broadband microstrip-line-fed sleeve monopole antenna for DTV application and ground plane effect", Antennas and Wireless Propagation Letters, IEEE, vol. 7, pp. 497-500, 2008.

(NPL2) R. Caso, A. DAlessandro, A. A. Serra, P. Nepa and G. Manara, "A compact dual-band PIFA for DVB-T and WLAN applications." Antennas and Propagation, IEEE Transactions on 60.4, pp. 2084-2087, 2012.

(NPL3) Chi, Yun-Wen, Kin-Lu Wong, and Saou-Wen Su. "Broadband printed dipole antenna with a step-shaped feed gap for DTV signal reception." Antennas and Propagation, IEEE Transactions on 55.11 (2007): 3353-3356.

(NPL4) Huang, Chih-Yu, Bo-Ming Jeng, and Jieh-Sen Kuo. "Grating monopole antenna for DVB-T applications." Antennas and Propagation, IEEE Transactions on 56.6, pp. 1775-1776, 2008.

(NPL5) Huang C Y, Jeng B M, Yang C F. "Wideband monopole antenna for DVB-T applications" [J] . Electronics Letters, 2008, 44(25): 1448-1450.

[0004] In NPL1, sleeve and meandered lines were both used to increase electric lengths of the ground and radiating section respectively. This antenna achieved -10 dB bandwidth from 459 MHz to 891 MHz and had dimension of 174 mm χ 48 mm χ 1.6 mm. A Planar Inverted-F Antenna (PIFA) was combined with inverted L-shape antenna in NPL2, and this antenna covered 470-862 MHz with less than -6 dB return loss within a dimension of 217 mm χ 12 mm χ 8 mm. Through cutting gratings, the volume of the antenna in NPL4 was minimized to 242 mm χ 35 mm χ 0.8 mm. Its voltage standing wave ratio (VSWR) was lower than 2: 1 from 458 MHz to 960 MHz. These antenna are nevertheless large for a compact device.

[0005] There is therefore a need for an improved wideband antenna for the UHF frequency range, in particular one that can achieve high efficiency in a small volume.

[0006] According to an aspect of the invention, there is provided an antenna comprising:

an insulating substrate;

an electrode on a first surface of the substrate;

a ground plane on a second surface of the substrate, the second surface being on the opposite side of the substrate than the first surface; and

a tapered microstrip feed line on the first surface and connected to the electrode, at least a part of the microstrip feed line opposing the ground plane; wherein

the electrode is substantially rectangular and has a rectangular slot projecting inwardly from a first edge thereof;

the ground plane has a notch projecting inwardly from a first edge thereof, the notch tapering toward the interior of the ground plane; and

the antenna has a peak of radiation efficiency in the range of from 400 MHZ to 1200 MHz.

[0007] In an embodiment, the first edge is a short edge of the electrode.

[0008] In an embodiment, the slot projects inwardly from the middle of the first edge.

[0009] In an embodiment, the width of the slot is in the range of from 1/100 to 1/2, desirably 1/10 to

1/4 of the width of the electrode.

[0010] In an embodiment, the length of the slot is in the range of from 1/2 to 9/10, desirably 2/3 to 4/5 of the length of the electrode.

[0011] In an embodiment, the ground plane does not overlap the electrode and there is a gap between the first edge of the ground plane and the second edge of the electrode.

[0012] In an embodiment, the gap has a width in the range of from 0.1 mm to 2 mm, desirably 0.8 mm to 1.2 mm.

[0013] In an embodiment, the notch has curved sides.

[0014] In an embodiment, the length of the notch is in the range of from 1/5 to 4/5, desirably 1/4 to 1/2 of the length of the ground plane.

[0015] In an embodiment, the width of the notch at the edge of the ground plane is in the range of from 1/2 to 4/5 of the width of the ground plane.

[0016] In an embodiment, the thickness of the substrate is in the range of from 1/10 to 1/20 of the width of the electrode.

[0017] In an embodiment, the antenna has a bandwidth of greater than 500 MHz.

[0018] According to an aspect of the invention, there is provided a transmitter including an antenna as described above. [0019] According to an aspect of the invention, there is provided a receiver including an antenna as described above.

[0020] According to an aspect of the invention, there is provided a data network including a transmitter and/or a receiver as described above.

[0021] The present invention can therefore achieve a compact and low profile UHF UWB antenna for TVWS applications. The radiating body is optimised. The radiating body and ground are printed on a substrate which helps to reduce size of the antenna and makes manufacture cheap. The present invention is particularly applicable to wireless network provision.

[0022] Embodiments of the invention will be described below with reference to the accompanying drawings, in which:

[0023] Figure 1 is a plan view of an antenna according to an embodiment of the invention;

[0024] Figure 2 is an end view of the antenna of Figure 1 ;

[0025] Figure 3 is a side view of the antenna of Figure 1 ;

[0026] Figure 4 depicts a first mode of the antenna of Figure 1 ;

[0027] Figure 5 depicts a second mode of the antenna of Figure 1;

[0028] Figure 6 depicts a third mode of the antenna of Figure 1 ;

[0029] Figure 7 depicts a fourth mode of the antenna of Figure 1 ;

[0030] Figure 8 depicts a fifth mode of the antenna of Figure 1 ;

[0031] Figure 9 is a graph of characteristic angle vs frequency for the first to fifth modes of the antenna of Figure 1;

[0032] Figure 10 is a graph of voltage standing wave ratio vs frequency for the first to fifth modes of the antenna of Figure 1 ;

[0033] Figure 11 is a graph of voltage standing wave ratio vs frequency for different values of the width of the gap between electrode and ground plane of the antenna of Figure 1 ;

[0034] Figure 12 is a graph of voltage standing wave ratio vs frequency for different values of the length of the notch of the antenna of Figure 1 ;

[0035] Figure 13 is a graph of measured and simulated voltage standing wave ratio vs frequency of the antenna of Figure 1 ;

[0036] Figure 14 depicts simulated and measured E-plane radiation patterns of the antenna of Figure 1 at 474 MHz;

[0037] Figure 15 depicts simulated and measured E-plane radiation patterns of the antenna of Figure 1 at 630 MHz;

[0038] Figure 16 depicts simulated and measured E-plane radiation patterns of the antenna of Figure 1 at 786 MHz;

[0039] Figure 17 depicts simulated and measured H-plane radiation patterns of the antenna of Figure 1 at 474 MHz;

[0040] Figure 18 depicts simulated and measured H-plane radiation patterns of the antenna of Figure 1 at 630 MHz; [0041] Figure 19 depicts simulated and measured H-plane radiation patterns of the antenna of Figure 1 at 786 MHz; and

[0042] Figure 20 is a graph of measured gain and radiation efficiency of the antenna of Figure 1 over UHF TV spectrum.

[0043] In the Figures, like parts are indicated by like references.

[0044] An embodiment of the present invention provides a novel compact antenna designed for white space devices. The present invention incorporates a modified rectangular plate having a slot in its middle thereby providing a radiating structure having wider bandwidth. An embodiment of the invention can realize 87.5% bandwidth from 474 MHz to 1212 MHz within a dimension of 0.36λι χ 0.06λι (where λι is the wavelength of the minimum operating frequency 474 MHz), and ultrawideband behaviour is achieved by exciting multiple modes able to radiate efficiently. It radiates

omnidirectionally on the magnetic field plane and realizes 1.68 dB gain and 86.4% radiation efficiency in average over UHF TV spectrum. Integrating the proposed antenna onto the client of a TVWS testbed, both uplink and downlink signals had high SINRs even if the client was separated over 120 m from the transmitter. The present invention thus provides an advantageous antenna for devices working in TVWS owing to its compactness, low profile, UWB property and reasonable gain.

[0045] An antenna 10 according to an embodiment of the invention comprises a modified rectangular electrode 11 printed on an insulating board 12 backed by a ground plane 13 as shown in Figures 1 to 3. Board 12 may be formed of a glass-reinforced epoxy laminate sheet such as FR4, and has length L, width W and thickness H. The electrode has length 11 and width W. The electrode 11 has a slot 15 projecting inwardly from a first edge 1 la thereof. Slot 15 is centrally located in the electrode and rectangular, with width d and length 13.

[0046] Electrode 11 is fed through a tapered microstrip line 14 which connects to the middle of a second edge 1 lb of the electrode, opposite to the first edge 1 la. Microstrip line has a width wm at its distal end and a width wf where it meets the electrode 11 and over most of its length, wm > wf. The length of the wider part of the microstrip is 14 and the length of the taper connecting the wider part to the narrower part of the microstrip is 15. At least a part of microstrip line 14 lies over (i.e. opposes) ground plane 13 so as to form a transmission line in combination with ground plane 13.

[0047] Ground plane 13 is also generally rectangular, with width W and length l_ground, but has a notch 16 in a first edge 13a thereof. The first edge of the ground plane is nearly aligned with the second edge of the electrode, with a gap g therebetween. Notch 16 is centrally located in first edge 13a of ground plane 13 and extends towards the centre thereof. Notch 16 is tapered, narrowing toward the centre of ground plane 13, with curved sides 16a, 16b. The width of the notch 16 at its interior end is w3. The width of the notch at the exterior end is W - 2-w4, where w4 is the width, on each side, of the remaining part of first edge 13a of the ground plane 13. The length of the notch is l_ground - l_rect, where l_rect is the length of the rectangular, un-notched part of the ground plane. [0048] An antenna structure is capable of supporting multiple orthogonal modal currents J„ which can be analysed using computational electromagnetics software such as FEKO provided by Altair Engineering, Inc..

[0049] Eigenvalues (λ η ) are introduced to predict radiation ability of each mode and are defined in equation (1) where X and R are reactance and resistance operators respectively. Eigenvalues can be described by characteristic angle (angle n ) acquired from (2). Resonance occurs when angle„ is 180°, and in other cases energy will be stored in the structure. The present inventor has determined that modes 1 and 3 have resonances at around c/2L and c/L (where C is the speed of light in vacuum) and their characteristic angles pass through 180° softly and they have potential to contribute to wideband performance.

X Q n ) = A n R(J n ) (1) angle n = 180° - tan _1 (A n ) (2)

[0050] Focusing on modes having vertical currents (J and JO, the antenna of the present invention has more flat gradients near resonances than a conventional rectangular plate, which is helpful to achieve wider bandwidth .

[0051] The characteristic modes of a specific embodiment of the invention were analysed with a 50 Ω-impedance excitation being taking into account. After being excited, the resultant current on a structure can be decomposed into a linear combination of modal currents based on (3), where a„ is the weight contributed by each mode and can be calculated from modal coefficient according to (4) and (5). In (5) E 1 is the incident electrical field.

Vn = Jn - E i dS = a n (l + j n ) (4)

_ § J n -E l dS _ V n

n ~ 1 + η 1 + η >

[0052] The input impedance of an antenna can be derived from the current and voltage at the feeding port noted by 7j n and V in . In (6) and (7), ]ζ and E[ n are current density and electrical field at the feeding port respectively.

n =∑ CtnJn (6)

[0053] Since the total input impedance is not a linear sum of modal impedances, modal admittance Yin is calculated instead in (8). Finally, voltage standing wave ratio (VSWR) of each mode is obtained from (9).

VSWR n = (9)

[0054] The characteristic modes become more complex when the ground plane, feeding strip and substrate are taken into account, and five significant modes are depicted in Figures 4 to 8. In the first mode (Figure 4), currents on the radiating body and feeding strip have same direction, while in mode 2 (Figure 5) a current null appears and moves from the bottom of the radiating body to the feeding strip with increase of frequency. Both the third (Figure 6) and fourth (Figure 7) modes have a null on the radiating body and another on the feeding strip. However their currents on the ground are different. Two nulls emerge on the radiating body in the fifth mode (Figure 8) and its feeding strip also sees one current null.

[0055] Modal characteristic angles and VSWRs of the five modes are plotted in Figures 9 and 10. Curves of each mode do not extend over the whole observed spectrum because on a specific structure a mode can just exist within a segment of frequencies. As shown in Figure 9, except for mode 1 whose characteristic angle passes through 180° steeply at 365 MHz, all of the other modes present wideband potential. As shown in Figure 10, antenna performance over 600 to 950 MHz is decided mainly by mode 2. Modes 3 and 4 are dominant within 950 to 1 150 MHz and the highest resonance is contributed by mode 5. However, VSWRs of all the five modes between 450 MHz and 600 MHz are inferior, but the composite VSWR is lower than 2. Observing weighted currents ( n J n ) of all modes at frequencies within this region, it is found that the imaginary part of weighted current of the dominant mode, which is mode 1 for frequencies lower than 460 MHz and mode 2 within 460 to 600 MHz, is far from 0. However, multiple non-radiating modes, like those having intense current on the ground or on the surface of the substrate, contribute to obtain an imaginary part of the cumulative current close to 0 and as a result good total impedance matching and low overall VSWR.

[0056] In an embodiment of the invention, the thickness H of insulating board 12 is 0.8 mm. The width wm of the feeding strip at its distal end (at the bottom of the antenna) is desirably 1.6 mm to match with the 50Ω SMA connector. The width wf of the rest of the feeding strip is desirably 1.1 mm to match with impedance of the radiating body. Values of other significant parameters to obtain good performance are discussed below. The performance of the antenna is not found to be particularly sensitive to the dimensions of the slot. In particular, the width of the slot can be in the range of from 1/100 to 1/2, desirably 1/10 to 1/4 of the width of the electrode and provide good bandwidth. The length of the slot can be in the range of from 1/2 to 9/10, desirably 2/3 to 4/5 of the length of the electrode and provide good performance.

[0057] i). Gap between the radiating body and ground

Current is concentrated near the gap between the radiating body and the ground. As Figure 1 1 indicates, when the width g of the gap increases, the VSWR < 2 bandwidth starts from slightly lower frequency, but the antenna becomes multiband instead of ultra-wideband as expected, and its VSWR increases at high TVWS channels, which can be improved by reducing width of the gap. However, a narrow gap will result in shrinking of the bandwidth and unsatisfying performance at low frequencies of UHF TV spectrum. After careful trials, a 1 mm- wide gap was selected to make the antenna cover lower frequency TVWS channels and realize widest bandwidth. A larger gap width can be selected to provide improved performance a higher frequencies. A lower gap width can be selected to provide improved performance at lower frequencies. Values of g may be in the range of from 0.1 mm to 2 mm, desirably 0.8 mm to 1.2 mm.

[0058] ii). Size of the notch

The notch on the ground can improve impedance matching, and it is further optimized to have tapered edges to extend electric length. The ground can be regarded as a combination of a rectangular part and two truncated quarter ellipses. Keeping the total length of the ground constant, portions of the rectangular part and truncated quarter ellipses decide the depth and width of the notch and affect impedance matching. As Figure 12 illustrates, increasing the length of the rectangular part, l_rect, the rectangular part takes a higher proportion of the ground plane, and consequently impedances at low frequencies of UHF TV spectrum are better matched and hence lead to lower VSWR. But VSWRs get worse at high frequencies of UHF TV spectrum due to poorer matching. Increasing the proportion of the ground plane taken by the truncated ¼ ellipses, has the opposite effect. In an embodiment l_rect is selected to be 58 mm. Thus, the length of the notch (l_ground - l_rect) is about 1/3 of the total length of the ground plane. In an embodiment, the length of the notch is in the range of from 1/5 to 4/5, desirably 1/4 to 1/2 of the length of the ground plane.

[0059] Table 1. Dimensions of the Antenna.

[0060] A prototype with the optimized parameters shown in Table 1 was fabricated and measured. Its simulated VSWR < 2 bandwidth was from 474 MHz to 1260 MHz and its measured bandwidth was 474 to 1212 MHz, agreeing well as shown in Figure 13.

[0061] To observe radiation patterns of the proposed antenna, three significant frequencies 474 MHz, 630 MHz and 786 MHz, which are central frequencies of the first, middle and last TVWS channels, were analysed. Radiation patterns of the antenna on both electrical field and magnetic field planes at these frequencies are shown in Figures 14 to 16 and 17 to 19 respectively. Over the whole UHF TV spectrum the proposed antenna has omnidirectional radiation patterns on the H-plane, and two radiation nulls appear on the E-plane. The radiation patterns at 474 MHz, 630 MHz and 786 MHz are almost the same. Small discrepancies between measurements and simulations could be caused by effects introduced by cables and unavoidable scatterings in the EMC screened anechoic chamber.

[0062] Realized gain and radiation efficiency of the proposed antenna over UHF TV spectrum were also measured to evaluate its performance at the maximum radiation direction and all areas surrounding it respectively. As shown in Figure 20, both gain and radiation efficiency are uniform over the UHF TV spectrum and the average gain reaches 1.68 dB. The radiation efficiency is between 76.2% and 92.4%, and hence most power delivered to the antenna can be radiated.

[0063] The antenna was tested in a Carlson Wireless TVWS system. A client equipped with the proposed antenna was placed 127.5 m away from the base station. Between the transmitter and receiver there were two blocking buildings, trees and a road.

[0064] The base station transmitting power was 23 dBm and gain of its antenna was 11 dBi and there was 1 dBi loss on the cable connecting the base station and antenna. Due to the complicated communication environment, signals arriving at the client were non-directed, and the communication quality can be ensured by the high radiation efficiency of the proposed antenna. Signal-to- interference-plus-noise ratio (SINRs) of both uplink (UL) and downlink (DL) with the client antenna pointing in different directions were consistent and SINRs of both DL and UL do not vary much with the change of client antenna direction. Connecting a computer to the client, the Internet could be accessed through TVWS signals. An online application 'SPEEDTEST' (http://www.speedtest.net/) was used to monitor network speeds and the averaged DL and UL speeds are 9.5 Mbps and 3.6 Mbps respectively. The test results not only prove the good performance of the proposed antenna in a practical scenario but also its pointing direction independence and reasonable gain.

[0065] Having described exemplary embodiments of the invention, it will be appreciated that modifications and variations of the described embodiments can be made. It will be appreciated that the antenna of the present invention can be used for both transmission and reception. The antenna of the present invention can be used for digital and analogue signals using all types of modulation. The invention is not to be limited by the foregoing description but only by the appended claims.