Dielectric-loaded antenna

Details Dielectric-loaded antenna Dielectric-loaded antenna

1997-07-10

2001-02-06

Wong, Don (Department: 2821)

Communications: radio wave antennas

Antennas

Spiral or helical type

C343S821000, C343S859000

Reexamination Certificate

active

06184845

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to dielectric-loaded antenna for operation at frequencies in excess of 200 MHz, and having a three-dimensional antenna element structure on or adjacent the surface of an elongate dielectric core which is formed of a solid material having a relative dielectric constant greater than 5.
BACKGROUND OF THE INVENTION
An antenna as described above is known from published UK Patent Application No. GB 2292638A which discloses a quadrifilar antenna having an antenna element structure with four helical antenna elements formed as metallic conductor tracks on the cylindrical outer surface of a cylindrical ceramic core. The core has an axial passage with an inner metallic lining and the passage houses an axial feeder conductor, the inner conductor and the lining forming a coaxial feeder structure for connecting a feed line to the helical antenna elements via radial conductors formed on the end of the core opposite the feed line. The other ends of the antenna elements are connected to a common virtual ground conductor in the form of a plated sleeve surrounding a proximal end portion of the core and connected to the outer conductor of the coaxial feeder formed by the lining of the axial passage. The sleeve, in conjunction with the feeder structure forms a trap, isolating the helical elements from ground, yet providing conductive paths around its rim interconnecting the helical elements. This antenna is intended primarily as an omnidirectional antenna for receiving circularly polarised signals from sources which may be directly above the antenna, i.e. on its axis, or at smaller angles of elevation down to a few degrees above a plane perpendicular to the axis. It follows that this antenna is particularly suitable for receiving signals from global positioning system (GPS) satellites. Since the antenna is also capable of receiving vertically or horizontally polarised signals, it may be used in other radiocommunication apparatus such as handheld cordless or mobile telephones.
A dielectric-loaded antenna which is particularly suited to portable telephone use is a bifilar helical loop antenna in which two diametrically opposed half turn helical elements form, in conjunction with a conductive sleeve as described above, a twisted loop yielding a radiation pattern which is omnidirectional with the exception of two opposing nulls centred on an axis perpendicular to the plane formed by the four ends of the two helical elements. This antenna is disclosed in our co-pending U.S. patent application Ser. No. 08/664,104 the contents of which form part of the disclosure of the present application by reference. When this loop antenna is appropriately mounted in a mobile telephone handset, the presence of the nulls reduces the level of radiation directed into the user's head during signal transmission. While the antenna gain is superior to many prior mobile telephone handset antennas, it is significantly less than the maximum value above and below a central resonant frequency. It is an object of this invention to provide an antenna of relatively wide bandwidth or capable of operating in two frequency bands.
SUMMARY OF THE INVENTION
According to a first aspect of this invention, there is provided a dielectric-loaded loop antenna for operation at frequencies above 200 MHz comprising an elongate dielectric core formed of a solid material having a relative dielectric constant greater than 5 and, on or adjacent the surface of the core, a three-dimensional antenna element structure including at least a pair of laterally opposed elongate antenna elements which extend between longitudinally spaced-apart positions on the core, and linking conductors extending around the core to interconnect the said elements of the pair, the elongate elements having respective first ends coupled to a feed connection and second ends coupled to the linking conductors, wherein the said elongate elements and the linking conductors together form at least two looped conductive paths each extending from the feed connection to a location spaced lengthwise of the core from the feed connection, then around the core, and back to the feed connection, the electrical length of one of the two paths being greater than that of the other path at an operating frequency of the antenna. Since the looped conductive paths have different electrical lengths, their resonant frequencies are different and can be selected so as to coincide, for example, with the centre frequencies of the transmit and receive bands of a mobile telephone system.
The linking conductors may be formed by a quarter wave balun on the outer surface of the core adjacent the end opposite to the feed connection, the latter being provided by a feeder structure extending longitudinally through the core. In one preferred embodiment, the linking conductors are formed by mutually isolated parts of a balun sleeve so that each of the two looped conductive paths includes the rim of a respective sleeve part. The sleeve parts are isolated from each other by longitudinally extending slits in the conductive material forming the sleeve, the electrical length of each slit from a respective short-circuited end to the relevant sleeve rim being at least approximately equal to a quarter wavelength at the operating frequency so that isolation between the two sleeve parts is provided at their junctions with the elongate antenna elements.
Alternatively, each linking conductor may be formed by a conductive strip extending around a respective side of the core from one elongate antenna element to another. In another alternative, one linking conductor may be formed in this way, and the other may be formed by the rim of a quarter wave balun sleeve, with or without the slits described above. The advantage of incorporating a balun sleeve is that the antenna may then operate in a balanced mode from a single-ended feed coupled to the feeder structure.
Advantageously, the antenna element structure has a single pair of laterally opposed elongate antenna elements each of which is forked so as to have a divided portion which extends from a location between the first and second ends of the element as far as a respective one of the linking conductors. The difference in electrical length between the two looped conductive paths may be achieved by forming one or both of the divided portions as branches of different electrical lengths. Each branch may then be connected to respective linking conductors extending around opposite sides of the core which, at least in the region of the elongate elements are isolated from each other. It will be appreciated that the difference in path lengths may be achieved not only by making the branches of different lengths, but by forming the linking conductors differently on opposite sides of the core.
Particularly satisfactory operation can be achieved by arranging for the electrical length of each branch to be approximately 90° (or (2n+1)&lgr;/4 where n=0, 1, 2 . . . ) at the resonant frequency of its respective conductive path, &lgr; being the corresponding wavelength. The linking conductors represent a location of low impedance at the operating frequency, and each 90° length acts as a current-to-voltage transformer so that the impedance at the fork of each forked element is relatively high. Accordingly, at the resonant frequency of one of the conductive paths, excitation occurs in that path simultaneously with isolation from the other path or paths. It follows that two or more distinct resonances can be achieved at different frequencies due to the fact that each branch loads the conductive path of the other only minimally when the other is at resonance. In effect, two or more mutually isolated low impedance paths are formed around the core.
In the preferred antenna in accordance with the invention, the advantageous low impedance connection point for the antenna elements at their junction with the linking conductor or conductors is provided by annular linking conductors in the form of a cylindrical split conductive sleeve which operates

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