WO2005078862A1

WO2005078862A1 - Multi-band antenna using parasitic element

Info

Publication number: WO2005078862A1
Application number: PCT/GB2005/000392
Authority: WO
Inventors: Dedimuni Rusiru Vinodaka Leelaratne
Original assignee: Harada Industry Co., Ltd.
Priority date: 2004-02-06
Filing date: 2005-02-04
Publication date: 2005-08-25
Also published as: JP2007520964A; JP4586028B2; GB2410837A; GB2410837B; GB0402689D0; DE112005000302T5

Abstract

A multi-band antenna includes an elongated first antenna element and one or more elongated further antenna elements extending in parallel spaced relationship with the first antenna element. A dielectric extends between the first antenna element and the one or more further antenna elements for maintaining physical spacing between the antenna elements. The antenna has a helical first antenna element and a second antenna element that includes a linear section that extends parallel to the longitudinal axis of the first element. In one form of the antenna, the second element includes a helical section extending coaxially around the first element and in serial connection with the linear section. In another form, the linear section of the second element comprises the whole of the second element, and the second and third elements extend parallel to the longitudinal axis of the first element on respective opposite sides of the first element. The antenna elements may be formed of either wire or metallic tape. An advantage of the antenna is that resonant frequency bands can be more closely spaced than previously; a correspondingly greater number of resonant frequency bands is then possible.

Description

MULTI-BAND ANTENNA USING PARASITIC ELEMENT

This invention relates to antennas, and more particularly relates to a multi-band antenna which includes at least one parasitic element extending parallel to a main antenna element and providing capacitive coupling to increase the number of bands in multi-band operation. Recent years have witnessed a dramatic growth in radio usage for automotive applications. This has raised a need for the conventional amplitude-modulated/frequency-modulated (AM/ FM) vehicular antenna (often in the form of a mast antenna) to be operational at other frequencies, such as the Digital Radio Broadcasting (DAB) and radio telephony (GSM) frequencies. DAB consists of two frequency bands, known as DAB Band III (~200MHz) and DAB L Band (~1450MHz) . The use of a ^λchoke' in the form of a tightly-wound coil has enabled multi-band operation at much higher frequencies, such as GSM and DAB L Band. However, it has not proven possible to produce an antenna that can operate reliably when the central frequencies of the upper and lower frequency bands to be covered are in a ratio of less than approximately 2.5:1, for instance, an antenna able to reliably cover both FM (~100MHz) and DAB Band III (~200MHz) . The antennas described in the preferred embodiments are directed to overcome this limitation. The subject invention is a multi-band antenna having a feed end connectable to electrical circuitry for transmitting and/or receiving signals, the antenna including a helical first antenna element and a second antenna element that includes a linear section that extends parallel to the longitudinal axis of the first antenna element. Interaction between the antenna elements provides operative frequency bands that are additional to bands resulting from use of the first antenna element alone. In a first form of the antenna, the second antenna element may also include a helical section that extends coaxially around the first antenna element and is in serial connection with the linear section. The linear section may extend from the helical section in a direction away from that end of the helical section that is most proximate the feed end of the antenna. Alternatively, the linear section may extend from the helical section in a direction away from that end of the helical section that is most distant from the feed end of the antenna. Preferably, the helical section surrounds a longitudinally-central portion of the first antenna element. Preferably, the second antenna element extends for substantially the whole length of the first antenna element. Preferably, the antenna includes dielectric means extending between the first and second antenna elements for maintaining physical spacing between those antenna elements . In a second form of the antenna, the antenna also comprises a linear third antenna element. In this form, the linear section of the second antenna element comprises the whole of the second antenna element . The , second and third antenna elements each extends parallel to the longitudinal axis of the first antenna element. Interaction between the first, second and third antenna elements provides operative frequency bands that are additional to bands resulting from use of the first antenna element alone. The second antenna element may extend on an opposite side of the first antenna element from the third antenna element, and preferably, they extend at positions directly opposite each other on the first antenna element . The second form of the antenna may also include a fourth antenna element that is helical and surrounds the first antenna element at a position either longitudinally-outward or longitudinally-inward on the first antenna element from the second and third antenna elements . In the second form of the antenna the second and third antenna elements may have substantially the same length. The antenna may include dielectric means extending on the outside of the first antenna element for maintaining physical spacing between the first antenna element and the other antenna elements . In either the first or second forms of the antenna, the antenna may include an elongated dielectric support, with the first antenna element being wound around the dielectric support. In those antennas with the dielectric means, the dielectric means may include a dielectric sheath that extends to cover both the dielectric support and the first antenna element wound around the dielectric support. The elongated dielectric support may be a fibreglass rod. One or more of the antenna elements may be formed of wire, and in such case, the wire of one or more of the antenna element may have a dielectric coating. The dielectric coating may be enamel . One or more of the antenna elements may be formed of metallic tape. The antenna may include a metal sprung section that extends between the feed end of the antenna and one end of the first antenna element and allows flexure. The metal sprung section may be a helical metal spring with a shorting strip extending in parallel for reducing the inductive loading effect of the spring. The antenna may include an outer dielectric sheath, and in such case, the sheath may be configured so as to reduce wind noise of the antenna. The first antenna element and one or more of the other antenna elements may be relatively positioned such that four resonant frequency bands exist, the four frequency bands having respective center frequencies in the approximate ratio of 1:2:3:4. The antenna may be a vehicular mast antenna. Such mast antenna may include means for anchoring the mast antenna to a roof of a vehicle such that the antenna extends at an angle of approximately 60 degrees to the horizontal. The physical spacing between the first antenna element and the one or more further antenna elements may be at least 0.02mm. Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: - Figure 1 is a side view of a conventional form of antenna; Figure 2 is a side view of a first embodiment of an antenna of the subject invention, the view omitting an outer dielectric sheath fitted over the outside of the antenna; Figure 3 is a graph comparing return loss as a function of frequency for the antennas of Figures 1 and 2 ; Figures 4(a) and 4(b) are equivalent circuit diagrams for first and second wire elements that are, respectively: (i) directly-connected at their one end, and (ii) capacitively- coupled rather than directly-connected; Figure 5 is a graph illustrating a comparison of return loss as a function of frequency for directly-connected and capacitively-coupled first and second wire elements, as respectively shown in Figures 4(a) and 4(b); Figure 6 is a graph illustrating, for two bandwidths, optimum bandwidth as a function of the wire thickness of the second wire element; Figure 7 is a graph illustrating return loss as a function of frequency for an antenna of similar type but different design to that shown in Figure 2 , the graph covering the same frequency range as shown in the left half of Figure 3 ; Figure 8 illustrates typical radiation patterns that were measured using the antenna of Figure 2 , when mounted on the rear roof of a vehicle at an angle of 60 degrees to the horizontal; Figure 9 illustrates a side profile of a vehicle, and the position of an antenna on the rear roof used to obtain the patterns illustrated in Figure 8; Figure 10 is a side view of a second embodiment of the antenna of the subject invention, the view omitting an outer dielectric sheath fitted over the outside of the antenna; Figure 11 is a graph illustrating received power (in dB) as a function of frequency, over a lower frequency range of 55 to 75MHz, for the antenna of Figure 10; Figure 12 is a graph illustrating received power (in dB as a function of frequency, over a higher frequency range of 110 to 170MHz, for the antenna of Figure 10; and, Figure 13 is a side view of a third embodiment of the antenna of the subject invention, the view omitting an outer dielectric sheath fitted over the outside of the antenna. The following paragraphs describe the invention in terms of preferred embodiments, and the embodiments relate to a vehicular mast antenna such as that shown on the roof of the vehicle in Figure 9. The antenna of the subject invention is intended, however, to have broader application and to cover any antenna having a structure covered by the appended claims . Current AM/FM mast-antenna technology uses a composite structure as presented in Figure 1. This consists of a bottom joint 20 that screws into an amplifier base (housing electrical circuitry) , and a reinforced helical spring (shorting strip) 22 providing flexibility and having a top joint 24. A fibreglass rod 26 is connected to the upper end of the joint 24. A helical member 28 is formed by metallic tape that is wound around the rod 26 in a helical configuration. Alternatively, the helical member might be formed by wire that is wound around the rod 26 to form a wire helix. It is also possible to use wire of sufficient thickness that the wire helix is self-supporting, in which case fibreglass rod 26 may not be needed. Lengths of FM masts vary typically from 20cm to 80cm. The 80cm masts use a monopole antenna design, i.e. a straight conductor, such as a metal rod. A reduction in height to typically 20cm to 50cm is obtained by using a helical antenna, where the conductor is wound in a helical shape. A reduction in mast height results in reduced FM gain. This can be compensated for by adding amplification through electrical circuitry typically housed in a moulded base at a bottom end of the mast antenna. The mast can usually be unscrewed from the base . Figure 2 illustrates a first preferred embodiment of the mast antenna of the invention. The antenna of Figure 2 differs from that of Figure 1 by the addition of a conductive linear member (or 'parasitic element') 30 that extends in parallel spaced relationship with the helical member 28. In the Figure 2 embodiment, the helical member 28 is formed of wire that is coated with enamel, the enamel coating providing dielectric insulation for the wire. The linear member 30, which may be wire or metallic tape, is then placed along the surface of the enamel-coated wire of the helical member 28. An outer insulating cover (not shown) is then formed by placing, for instance, a heat-shrinkable envelope over the structure; after being shrunk, the heat-shrinkable envelope retains the linear member in position. It is also possible to use metallic tape to form the helical member 28. In such case, it will be required to place a dielectric sheath, for instance a heat-shrinkable envelope, around the outer surface of the rod 26 and the metallic tape wound thereon; this fixes the tape position on the rod. Then, the linear member 30 (wire or metallic tape) is placed on the outer surface of the dielectric sheath, an outer dielectric sheath is then applied, with the linear member 30 effectively being retained in position by being 'sandwiched' between the two dielectric sheaths. For optimum performance, no direct electrical connection is made between the helical member 28 (primary element) and the linear member 30 (parasitic element) ; instead, they interact only through capacitive-coupling. Figure 3 illustrates that the two frequency bands associated with the antenna having the single element 28 of Figure 1 has become four frequency bands after the parasitic element, i.e. linear member 30, has been added (as shown in Figure 2) . The single-element conventional mast resonates at frequency bands around 70MHz and 210MHz. The resonance at the frequency band of 70MHz (40) is known as the 'fundamental' or first-order mode of the mast, and the resonance at the frequency band of 210MHz (42) is the third- order mode. Such conventional masts only provide operation at the odd modes. The introduction of the parasitic radiating element generates additional modes and not only allows for operation at the frequency bands of 50MHz (44) and 190MHz (48) , similar to the first and third modes of the conventional mast, but also at frequency bands of 125MHz (46) and 260MHz (50) . The length of the helical member 28 and the linear member 30 are adjusted for tuning to the required frequency bands. Figures 4(a) and 4(b) respectively illustrate the equivalent circuits for direct coupling and capacitive-coupling of the helical member 28 and the linear member 30. The direct connection (in which the members 28 and 30 are physically connected together at their one end) requires a longer production process than the capacitively-coupled connection (where they are not physically connected) , resulting in greater time and cost for producing the former compared to the latter. And the direct connection also appears to result in a smaller (less favourable) bandwidth than for the capacitively-coupled connection. As an illustration, the typical effect on band- width and performance for the two types of connection are illustrated in Figure 5. The -3dB bandwidth is increased from 1.5% for a direct connection to 4% for the capacitively-coupled connection. The increase in bandwidth may be explained by the use of equivalent circuits, as shown in Figures 4(a) and 4(b). For the direct-connected circuit in Figure 4(a) , the 'non-resonant' arm presents a residual reactive load to the second arm at resonance. This results in a net increase in the 'Q' of the resonance, in other words, a narrower bandwidth than for a single resonant arm, i.e. a single element. If the second arm is capacitively-coupled, as shown in Figure 4 (b) , then the coupling capacitor, Cl, is in series with the residual reactive load of the non-resonant arm. This has the effect of reducing the net reactive load on the resonant arm, giving an increased bandwidth. The thickness of the metallic tape or wire used for the linear member 30 allows a degree of control over the four resonant frequency bands shown in Figure 3. Experiments were conducted using the two lowest frequency bands 44 and 46, which henceforth will be respectively referred to as the lower and upper resonant frequency bands. The ratio of the two frequencies can be altered by altering the thickness of the linear member 30. However, the element thickness is best tuned for the optimum radiation efficiency; making the wires thinner or thicker than the thickness for optimum radiation efficiency results in reduced radiation efficiency. When wire was used as the linear member 30, an optimum wire gauge was found from experimentation. The impedance bandwidth of the design was optimized by the technique illustrated in Figure 6, in which -3dB %bandwidth was measured for various thicknesses of wire used for the linear member (parasitic element) 30; measurements were made for two frequency bands. It can be seen than an optimum value for -3dB %bandwidth occurred when the wire used for linear member 30 had a thickness (diameter) in the range of 0.5mm to 0.7mm. The spacing between the helical member 28 and the linear member 30 was found to alter the capacitive-coupling and the loading on the helical member 28. As the spacing was increased, an increase in the lower and upper resonant frequency bands, i.e. 44 and 46 in Figure 3, was noticed. The thickness of enamel coating on wire that forms the helical member 28, or the minimum thickness of the heat-shrinking envelope covering metallic tape that forms helical member 28, is such that the spacing is less than 1/10 of the wavelength of a signal at the central frequency of the highest resonant frequency band of the antenna. The antenna of the invention can be readily packaged for both mechanical and weather-proofing requirements. However, the effect of any heat-shrinkable or over-moulding material used for such purposes has to be taken into account. Over- moulding material provides additional loading on the mast due to the effective dielectric constant of the over-moulding material being higher than the dielectric constant of free space . The proposed design is primarily designed for operation at an upper-to-lower frequency ratio of approximately 2:1 (note the frequencies of the frequency bands 46 and 44 in Figure 3), but is not limited to such ratio. As mentioned above, the design is not application-limited, and can be used for other purposes, such as mobile telephony and in-home radio receivers. Figure 7 illustrates another result when return loss was measured as a function of frequency for a mast antenna having the construction shown in Figure 2. Two frequency bands

(corresponding to the frequency bands 44 and 46 in Figure 3) were chosen that were not application-specific. The increase in bandwidth for the upper band was obtained by a combination of a higher-order mode of the helical member 28 and the fundamental mode of the linear member 30. Figure 8 presents typical radiation patterns measured using the first embodiment of the mast antenna when mounted at 60 degrees off the horizontal on the rear roof of the vehicle 54 shown in profile in Figure 9; the mast antenna is identified as 56. Performance was found to be related to the location and angle of installation of the mast antenna on the vehicle. The parasitic element is not limited to the linear member 30 of Figure 2. Figure 10 illustrates a second embodiment which includes the helical member 28, but in which the linear member 30 has been replaced by a parasitic member 58 that includes a linear section 60 connected at one end to a helical section 62. As with the first embodiment, dielectric material extends between the helical member 30 and both sections of the parasitic member 58. Experimentation has shown that if the antenna of the invention requires a parasitic linear element so long that it exceeds the length of the helical member 28, one solution is to form a parasitic element that includes the helical section 62 as a loading coil. The presence of the helical section 62 does result in a slight obstruction, and hence a reduction in gain of the lower resonant frequency band (a band corresponding to band 44 of Figure 3) . The degree of reduction is dependent on the extent of the obstruction. The bandwidth of the upper resonant frequency band (a band corresponding to band 46 in Figure 3) is controllable by the position of the helical section 62 relative to the helical member 28. A significant increase in bandwidth is obtained when the helical section 62 is positioned towards the bottom end of the helical member 28. Figures 11 and 12 illustrate the effect of the design and positioning of the parasitic member 58 on the signal level and the bandwidth of received power, for the lower and upper resonant frequency bands, respectively. The wire-with-coil-at-centre embodiment is the one illustrated in Figure 10. The wire-with-coil-at-bottom embodiment is not illustrated, but involves the helical section 62 being positioned to surround the bottom end of the helical member 28, with the linear section 60 extending upward from helical section 62, i.e. turning the parasitic member 58 of Figure 7 upside down. Figure 13 illustrates a third embodiment, in which multiple parasitic members 70, 72 and 74 are used. The first member 70 corresponds to the linear member 30 of the first embodiment. The second member 72 is a second linear member extending on an opposite side of helical member 28 from linear member 70. The third member 74 is a second helical member, positioned at an upper end of the mast antenna and not connected to either linear member 70 or second linear member 72. The introduction of multiple parasitic members results in a combined effect, and for manufacturing and aesthetic reasons this may be necessary. As shown in Figure 13, the parasitic members are not required to be identically formed, i.e. they need not be all linear members. In the arrangement shown in Figure 13, the linear members 70 and 72 are used in the lower region of the helical member 28 to ensure that the most efficient area of helical member 28 is not obstructed, and the second helical member 74 is positioned near the top of the mast antenna where it ensures maximum loading. A parasitic member is not limited in shape, and could be loosely wound around the helical member 28. The wind-noise reduction feature used on standard masts can be metalised to construct a wound parasitic member. The location of the parasitic member on the helical member 28 can also be varied. By varying the location, the frequency ratio between the upper and lower frequency bands can be altered. As discussed thus far, the parasitic antenna element, i.e. linear member 30 in Figure 2, has been on the outside of the helical member 28. However, it could be positioned inside of, and dielectrically-spaced from, the helical member 28. Such inside configuration, however, has been shown to result in a poorer radiation efficiency than the outside configuration. However, in certain situations an inside configuration might have to be used, for instance with 'slimline' masts, and in such cases the trade-off in radiation efficiency may be acceptable. Although the foregoing description has referred to the primary element as being helical, i.e. helical member 28, experimentation has shown that a linear member might instead be used as the primary element, although less effectively. The parasitic members described in the first to third embodiments would be usable with such linear primary element. While the present invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation, and that changes may be made to the invention without departing from its scope as defined by the appended claims . Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features. The text of the abstract filed herewith is repeated here as part of the specification. A multi-band antenna includes an elongated first antenna element and one or more elongated further antenna elements extending in parallel spaced relationship with the first antenna element. A dielectric extends between the first antenna element and the one or more further antenna elements for maintaining physical spacing between the antenna elements. The antenna has a helical first antenna element and a second antenna element that includes a linear section that extends parallel to the longitudinal axis of the first element. In one form of the antenna, the second element includes a helical section extending coaxially around the first element and in serial connection with the linear section. In another form, the linear section of the second element comprises the whole of the second element, and the second and third elements extend parallel to the longitudinal axis of the first element on respective opposite sides of the first element. The antenna elements may be formed of either wire or metallic tape. An advantage of ^'the antenna is that resonant frequency bands can be more closely spaced than previously; a correspondingly greater number of resonant frequency bands is then possible.

Claims

1. A multi-band antenna having a feed end connectable to electrical circuitry for transmitting and/or receiving signals, the antenna comprising: a helical first antenna element; a second antenna element that includes a linear section that extends parallel to the longitudinal axis of the first antenna element; wherein interaction between the antenna elements provides operative frequency bands that are additional to bands resulting from use of the first antenna element alone.

2. The antenna of claim 1, wherein the second antenna element also includes a helical section that extends coaxially with the first antenna element and is in serial connection with the linear section.

3. The antenna of claim 1 or 2, wherein the linear section extends from the helical section in a direction away from that end of the helical section that is most proximate the feed end of the antenna.

4. The antenna of claim 1 or 2, wherein the linear section extends from the helical section in a direction away from that end of the helical section that is most distant from the feed end of the antenna.

5. The antenna of claim 3 or 4, wherein the helical section is adjacent a longitudinally-central portion of the first antenna element.

6. The antenna of any preceding claim, wherein the second antenna element extends for substantially the whole length of the first antenna element .

7. The antenna of any preceding claim, comprising dielectric means extending between the first and second antenna elements for maintaining physical spacing between those antenna elements .

8. The antenna of claim 1, also comprising: a linear third antenna element; wherein: the linear section of the second antenna element comprises the whole of the second antenna element; the second and third antenna elements each extend parallel to the longitudinal axis of the first antenna element; and, interaction between the first, second and third antenna elements provides operative frequency bands that are additional to bands resulting from use of the first antenna element alone .

9. The antenna of claim 8 , wherein the second and third antenna elements are disposed externally of the first antenna element .

10. The antenna of claim 8 or 9, wherein the second antenna element extends on an opposite side of the first antenna element from the third antenna element, and preferably directly opposite thereto.

11. The antenna of any of claims 8 to 10, comprising a fourth antenna element that is helical and surrounds the first antenna element .

12. The antenna of claim 11, wherein the fourth antenna element is longitudinally-spaced on the first antenna element from the second and third antenna elements.

13. The antenna of any of claims 8 to 12 , wherein the second and third antenna elements have substantially the same lengt .

14. The antenna of any of claims 8 to 13 , comprising dielectric means extending on the outside of the first antenna element for maintaining physical spacing between the first antenna element and the other antenna elements .

15. The antenna of any of claims 1 to 6 or 8 to 13, comprising an elongated dielectric support, wherein the first antenna element is wound around the dielectric support .

16. The antenna of claim 7 or 14, comprising an elongated dielectric support, wherein the first antenna element is wound around the dielectric support, and wherein the dielectric means comprises a dielectric sheath that extends to cover both the dielectric support and the first antenna element wound around the dielectric support .

17. The antenna of any of claims 15 or 16, wherein the elongated dielectric support is a fibreglass rod.

18. The antenna of any preceding claim, wherein one or more of the antenna elements are formed of wire.

19. The antenna of claim 18, wherein the wire of one or more of the antenna element has a dielectric coating.

20. The antenna of claim 19, wherein the dielectric coating is enamel.

21. The antenna of any of claims 1 to 17, wherein one or more of the antenna elements are formed of metallic tape.

22. The antenna of any preceding claim, comprising a metal sprung section that extends between the feed end of the antenna and one end of the first antenna element and allows flexure .

23. The antenna of claim 22, wherein the metal sprung section is a helical metal spring with a shorting strip extending in parallel for reducing the inductive loading effect of the spring.

24. The antenna of any preceding claim, comprising an outer dielectric sheath.

25. The antenna of claim 24, wherein an outside face of the outer dielectric sheath is configured so as to reduce wind noise of the antenna.

26. The antenna of any preceding claim, wherein the first antenna element and one or more of the other antenna elements are relatively positioned such that four resonant frequency bands exist, the four frequency bands having respective center frequencies in the approximate ratio of 1:2:3:4.

27. The antenna of any preceding claim, wherein the antenna is a vehicular mast antenna.

28. The antenna of claim 27, comprising means for anchoring the mast antenna to a roof of a vehicle such that the antenna extends at an angle of approximately 60 degrees to the horizontal .

29. The antenna of any preceding claim, wherein the physical spacing between the first antenna element and the one or more further antenna elements is at least 0.02mm.