WO1997018601A1

WO1997018601A1 - Dual band antenna means

Info

Publication number: WO1997018601A1
Application number: PCT/SE1996/001483
Authority: WO
Inventors: Torsten ÖSTERWALL
Original assignee: Allgon Ab
Priority date: 1995-11-15
Filing date: 1996-11-15
Publication date: 1997-05-22
Also published as: AU7660596A

Abstract

An antenna means enabling operation of a portable radio communication device within two separated frequency bands, for example, at 900 MHz and 1800 MHz. The antenna means comprises a feed portion coupled to circuitry of the portable radio communication device, first and second helical elements wound on a common core, coaxially and longitudinally coextending. The coils are galvanically separated except possibly at one end, they each have one free end and at least one of the coils is coupled directly to circuits of the portable radio communication device.

Description

DUAL BAND ANTENNA MEANS

BACKGROUND OF THE INVENTION

Technical field of the invention

The invention relates generally to a multi-band antenna means having at least two radiating elements coupled, via a feed portion, to circuitry of a radio communication device, wherein the antenna means is capable of performing transmission and reception within each one of at least two, separated frequency bands. Specifically, it relates to a dual band antenna device for a hand-portable telephone, which is capable of both transmitting and receiving on two separated frequency bands.

Description of Related Art

In the past antenna means including helical elements have been used for hand-portable cellular telephones m order to achieve compact dimensions and durability. One feature of a resonant antenna m general is that it is operable within one funda¬ mental frequency band and withm higher frequency bands, but only those having a fixed and predetermined relation to the fundamental frequency band. One problem related to small-size helical antennas m general is their narrow bandwidth.

GB-A 2 206 243 discloses a helical antenna for operating at predetermined upper and lower frequencies having another ratio than that of the natural frequencies of the helical antenna, e.g., the ratio of 2:1 between center frequencies, in order to communicate in two different distress beacon systems. The helical antenna disclosed therein is directed toward solving the problem of providing a helical antenna device capable of operating at two different, selectable frequencies, thus overcoming the fact that a helical antenna is usually restricted to resonating at fixed frequencies being non- harmomcally related to the fundamental frequency of the helical antenna. The proposed solution involves providing a straight conductor inside part of a helical winding and being connected to an upper end thereof. The lower end of the helical winding is connected to transceiver circuitry. By selecting the dimensions of the helical winding and the length and construction of the straight conductor, the two different operating frequencies are to be selected. The disclosed antenna device is exited in different modes and it appears from the disclosed diagram (Fig 2B) as though the upper band is extremely narrow, which would make it difficult for the antenna to operate in a cellular telephone system. Moreover the con¬ struction of this antenna seems complicate manufacturing. In view of the problem to be solved in GB-A 2 206 243, it dis¬ closes the prior art closest to the invention.

US 4 442 438 discloses another helical antenna means with a conductive member extending through a helical structure in order to enable the antenna means to resonate at two different frequencies. The antenna means has a feed port from which the helical structure extends in opposite directions.

EP-A1 0 593 185 discloses a wideband antenna arrangement directed toward providing a coil antenna means which can be used instead of a whip antenna particularly in a digital cellular telephone system proposed for Japan, wherein the center frequencies of the transmission band and the reception band, respectively, are located around 933 MHz with a separation of 130 MHz, giving a ratio of about 1:1,16 between the center frequencies. The main problem to be solved in EP-Al 0 593 185 is to attain a wider operable frequency band than one single helix can provide alone, col. 1, lines 29-30, and lines 45-47. The proposed solution consists of two antenna coils tuned to include different frequencies. The two antenna coils are arranged on a common axis either in line, one inside the other, or interwound. The transmission and reception coils are interconnected at one end and are connected via a duplex filter to transmitter and receiver circuitry, respectively. Arranging two helical elements in close proximity is a very delicate task, and EP-Al 0 593 185 does not seem to provide sufficient instructions on how to arrange and connect the helical elements order to actually obtain a working antenna means having the desired wideband characteristics.

US 4 309 707 discloses further wideband antenna arrangements which aims at providing a compact antenna maintaining wideband characteristics, wherem one or several structures, each consisting of two coaxial and longitudinally coextensive helices joined together at one end and having opposite chirality, are arranged close to each other a number of different configurations. This way the structures are coupled m order to provide wide overall bandwidth of the antenna arrangement. The different structures exhibit resonance center frequencies of, e.g., 24.5, 33.9, 43.4, and 51.5 MHz, a ratio of 1:1.19 - 1:1.38 between adjacent center frequencies of the structures. This disclosure is clearly directed toward achieving a continuous wide band.

US 4 772 895 discloses another wideband antenna arrangement wherein a first and a second helical element are arranged concentrically and partly overlapping. The first element is arranged side the second element. One end of each element is free. The other ends of the first and second elements are coupled to a feed portion and a ground portion, respectively, of a coaxial conductor. In order for the first element and second elements to give together a sufficient wide band, e.g., 35 MHz, at, e.g., 155 MHz, the first element has a length of approximately three times that of the second element. This corresponds to resonance center frequencies of, e.g., 115 and 356 MHz, for the first and second elements alone in free space, respectively. Apparently, the described results imply that the second element is connected to ground potential and not to the feed point of the first element.

AU 22 843 discloses various wideband antenna arrangements including helical elements. It teaches several general ways of achieving a continuous wide band using helices, provided that the difference resonant frequencies of the helices is not too large .

SUMMARY OF THE INVENTION

A main object of the invention is to provide a dual band antenna means capable of transmitting and receiving RF signals m each one of two different frequency bands. Specifically the antenna means is intended for a portable radio device capable of both transmitting and receiving two separated frequency bands. This will allow the use of the inventive antenna means to provide operability of a hand-portable telephone in two or more telephone systems, for example, a combination of AMPS (824-894 MHz) and PCS (1850-1990 MHz), or GSM (890-960 MHz) and PCN (1710-1880 MHz), possible also combined with DECT (1880- 1900 MHz) . Another object of the mvention is to provide a dual band antenna means which exhibits high efficiency, radiation lobe pattern without significant "dead angles", and sufficient wide band characteristics each of its operable bands. Moreover, the antenna means of the invention should enable a suitable interface to the radio device, so that no switching means is required in order to operate with more than one frequency band. Yet another object of the invention is to provide a dual band antenna means compact and durable enough for portable radio equipment. Still another object of the invention is to proviαe a dual band antenna means which is suited for manufacturing large quantities.

These objects are attained by a dual band antenna means according to the invention as defined the appended claims. In arranging helical coils coaxially and coextending a compact configuration of the antenna means and operability with separated frequency bands are attained. According to simulation and test results it has been confirmed that an antenna means consisting of two helical coils, tuned to frequencies withm two respective non-overlapp g frequency bands and arranged concentrically, one side the other with a small separation will fulfill the electrical demands for efficiency, lobe pattern, and bandwidth, m spite the fact that they couple capacitively and inductively to each other. This way of arranging the coils does not require any essential enlargement of a housing compared to that which encloses a smgle helical radiating element of an antenna means tuned to the lower frequency band only. In fact, some preferred configurations facilitate the arrangement of a relatively high inner coil serving an upper band approximately twice as high as the lower band. A higher inner coil tends to give a better lobe pattern for its frequency band.

The term feeding portion used m the description and claims should be interpreted as a conductive body at which a coil is fed with RF energy. It may be a part of a thread of a coil, a part of the radio communication device, and/or a body arranged between a coil and the radio communication device.

Although some geometrical configurations of the helical elements disclosure therein may visually appear similar to that of the present invention, EP-Al 0 593 185 is directed towards solving a problem different to the problem herein and provides a solution that operates differently to the present invention from an electrical and an electromagnetic point of view. The inventive antenna means is not directed towards widening an operating band of a helical element, but to enable two (or more) nelical elements that operate close proximity to provide operability with separated frequency bands. In fact, the relative bandwidth of at least one frequency band of the present invention tends generally to be smaller than that of the correspondmg helical element alone.

The invention is applicable in a number of versions. The explanation and the table below indicate some possible and some preferred combinations:

1) All coils wound in same direction; 2) At least one coil wound m other directions; 3) All coils coupled to a smgle feed portion; 4) At least two feed portions feeding different coils; 5) No coil being mainly fed mductively/capacitively by another coil; 6) At least one coil being mainly fed through mductive/capacitive interaction with another coil.

3) 4) 5) 6) 1) preferred possible preferred possible

2) possible possible possible possible

3) - not available preferred possible

4) not available - possible not available

In the embodiment of two coils mtercoupled at one end, wound m the same direction, and arranged one side the other, the antenna means has been shown to exhibit specifically good results with regard to efficiency, lobe pattern, and bandwidths as well as to utilization of available space. In that mode the coils interact constructively. Different winding directions seem to give less bandwidth at least one band. Feeding one coil mductively/capacitively from another coil seems to give lower efficiency m at least one band.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows m an exploded view of a first embodiment of an antenna means according to the invention mcludmg a metallic fitting, first and second helically configured elements, a non- conductive core, and a non-conductive housing.

Fig. 2 shows the antenna means of fig. 1 assembled but without the housmg.

Fig. 3 shows in an exploded view of a second embodiment of an antenna means according to the invention including first and second helically configured elements with feed portions, a non- conductive core, and a non-conductive housing with a fastening groove.

Fig. 4 shows the antenna means of fig. 3 assembled but without the housing. Fig. 5 shows in a partly cut-away view another core which is a possible alternative to the cores of figs. 1 - 4. Fig. 6 shows m a diagram the standing wave ratio as a function of the frequency for the antenna means of figs. 1 and 2, wherein the required bandwidths are mdicated by horizontal lines for two separated frequency ranges.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to figs. 1 and 2, a first embodiment of the dual band antenna means according to the invention includes a first coil 1 (helically configured element) and a second coil 2 having a first and a second resonance frequency, respectively, a substantially cylindrical, non-conductive core 3 provided with an external first groove 4 and an external second groove 5 for the first and the second coils, respectively, a non- conductive housmg 6 for housing and protecting the coils and the core, and a conductive fitting 7 (feed portion), to which the coils are coupled and the core fastened, and which connects the antenna means to receiver and transmitter circuits of, e.g., a hand portable cellular telephone (not shown) . The first coil 1 has a diameter larger than that of the second coil 2. The first and second coils are wound in the same direction and the coils and their corresponding grooves m the core are adapted size and configuration m order for the second coil 2 to fit coaxially side the first coil 1. The coils are not in galvanic contact with each other except through the metallic fitting 7, to which they are soldered, pressed, glued, or welded.

Fig. 6 indicates, m a diagram having the frequency along the horizontal axis starting at 500 MHz at the axis intersection increasing by 200 MHz for every marked point on that axis and having the VSWR along the vertical axis starting at 1.0 at the axis intersection increasing by 1.0 for every marked point on that axis, a VSWR characteristics curve m two parts 61 and 63 for the antenna means of figs. 1 and 2 when the first coil 1 and the second coil 2 have winding diameters of 4.8 mm and 7 mm, pitches of 3.5 mm and 6 mm, respectively, coil axial lengths (height) < 24 mm, and equal wire thicknesses of 0.5 mm. The antenna means thereby exhibits operability withm two separated frequency bands at approximately 900 MHz and 1800 MHz - 1900 MHz shown by horizontal lines 62 and 64, respectively. The antenna means exhibiting these characteristics was mounted on a chassis of a typical hand-portable cellular telephone.

With reference to figs. 3 and 4 a second embodiment of the dual band antenna means according to the invention includes a first coil 21 and a second coil 22 having at one end, respectively, for coupling the antenna means to receiver and transmitter circuits of, e.g., a hand portable cellular telephone (not shown) , feed portions extending parallel to the axis of the coils and having a first and a second resonance frequency, respectively, a substantially cylindrical, non-conductive core 23 provided with an external first groove 24 and an external second groove 25 for the first and the second coil, respectively, and a non-conductive housing 26 provided with a third groove 27 intended for fastening the antenna means to a chassis of the cellular telephone. The first coil 21 has a diameter larger than that of the second coil 22. The first and second coils are wound in opposite directions and the coils and their corresponding grooves in the core are adapted in size and configuration in order for the second coil 22 to fit coaxially inside the first coil 21. The coils are not in galvanic contact with each other except, possibly, through the transceiver circuits. The antenna means according to figs. 3 and 4 exhibit two possible and independent electrical features different from those of figs. 1 and 2, namely that its coils are wound in opposite directions and that its coils may easily be connected separately to different transceiver circuits.

As an alternative, in the embodiments of figs. 1-4, it is possible to essentially feed one of the coils 1, 2 mductively/capacitively from the other coil 2, 1, which is fed mainly via the feeding portion.

Fig. 5 shows, with a partial section 46, an alternative con¬ figuration of the core which is to carry the coils. The depicted core 43 is hollow and substantially circular cylin¬ drical. It is provided with an external groove 44 and an internal groove 45 intended for receiving the first and second coils (not shown) , respectively.

Although the invention is described by means of the above examples, naturally, many variations are possible within the scope of the invention. Such variations are, for example, variation of the helical diameter of one or both coils along the length thereof, using more than two helical coils in order to attain operability within more than two separated frequency bands and/or to increase the bandwidth of at least one of the separated non-overlapping frequency bands. It would also be possible to use additional elements in this antenna means for exclusively transmitting or receiving within a frequency band covered by the antenna. Moreover, the coupling of the coils to the feeding portion(s) could be achieved by capacitor means coupling RF energy.

Claims

1. An antenna means comprising: at least one feed portion coupled to circuitry of a hand- portable radio communication device, a helically configured conductive first element havmg a first axial length, first and second ends, as well as a first pitch and a first diameter, a conductive second element having a second axial length and third and fourth ends, said first and second elements being arranged substantially coaxially and, at least partially, axially coextending with each other, in order for said antenna means to perform transmission and reception withm each one of a first frequency band and a second frequency band, which are separated from each other, and whose center frequencies have the ratio of approximately 1:2 c h a r a c t e r i z e d b y said second and fourth ends being free ends, - said second element bemg helically configured and having a second pitch and a second diameter.

2. An antenna device accordmg to claim 1, c h a r a c t e ¬ r i z e d b y - said first element essentially effecting transmission and reception withm said first frequency band, said second element essentially effecting transmission and reception withm said second frequency band separated from said first frequency band.

3. An antenna device accordmg to claim 1 or 2, c h a r a c ¬ t e r i z e d b y said first end and said third end bemg mter-coupled at said at least one feed portion.

4. An antenna device according to any one of claims 1-3, c h a r a c t e r i z e d b y said second diameter bemg smaller than said first dia¬ meter, - said second element being arranged mside said first element.

5. An antenna device according to claim 1 or 4, c h a r a c ¬ t e r i z e d b y - one of the first and second elements being fed mainly through mductive/capacitive coupling to the other one, which m turn is coupled to said at least one feed portion

6. An antenna device according to any one of claims 1-4, c h a r a c t e r i z e d b y said second diameter being substantially equal to said first diameter, said second pitch being substantially equal to said first pitch.

7. An antenna device according to any one of claims 1-6, c h a r a c t e r i z e d b y reception and transmission of RF signals according to a first telecommunication standard being performed with said first frequency band, reception and transmission of radio signals according to a second telecommunication standard, other than said first telecommunication standard, bemg performed withm said second frequency band.

8. An antenna device according to any one of claims 1-4, c h a r a c t e r i z e d b y said first and second elements being wound in the same direction.

9. An antenna device according to any one of claims 1-3, c h a r a c t e r i z e d b y said first and second elements being wound in opposite directions.

10. An antenna device according to claim 1, c h a r a c t e ¬ r i z e d b y at least one further helically configured conductive element having a resonance peak at a frequency different from both said first and second frequencies.