Communications: radio wave antennas – Antennas – Spiral or helical type
Reexamination Certificate
1999-08-12
2001-10-09
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Spiral or helical type
82
Reexamination Certificate
active
06300917
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a dielectrically-loaded antenna for operation at frequencies in excess of 200 MHz, and in particular to an antenna having at least two resonant frequencies within a band of operation.
BACKGROUND OF THE INVENTION
Such an antenna is disclosed in United Kingdom Patent Application No. GB2321785A. This known antenna has a pair of laterally opposed elongate antenna elements which extend between longitudinally spaced-apart positions on a solid dielectric core, the antenna elements being connected at respective first ends to a feed connection and at second ends to a balun sleeve. The antenna elements and sleeve are arranged so as to form at least two conductive paths extending around the core, wherein one of the two paths has an electrical length which is greater than that of the other path at an operating frequency of the antenna. This is achieved using forked antenna elements, wherein each element having a divided portion extending from a position between the top of the dielectric core and the rim of the balun sleeve, the divided portion of at least one of the antenna elements having branches of different electrical lengths. The balun sleeve is split in the sense that longitudinally extending slits are formed as breaks in the conductive material of the sleeve so as to provide isolation between the two sleeve parts, thus defining the two conducting paths. The balun slits are arranged to have an electrical length of about a quarter wavelength (&lgr;/4) in the operating frequency band, the zero impedance point provided by the rim of the sleeve being transformed to a high impedance point between the divided elements, thereby isolating the sleeve parts from one another. As a result of the conductive paths having different electrical lengths, each conductive path resonates at a different frequency and so provides an antenna having a relatively wide bandwidth.
One problem associated with the above antenna is that it is difficult to incorporate slits of sufficient length within the sleeve to provide the quarter wavelength, especially if the sleeve is short. The L-shaped slits disclosed in GB2321785A can be difficult to manufacture and restrict the flow of currents in the sleeve.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a dielectrically-loaded antenna for operation at frequencies in excess of 200 MHz, comprising an electrically insulative core of a solid material having a relative dielectric constant greater than 5, a feed connection, and an antenna element structure disposed on or adjacent the outer surface of the core, the material of the core occupying the major part of the volume defined by the core outer surface, wherein the antenna element structure comprises a pair of laterally opposed groups of elongate elements, each group comprising first and second mutually adjacent elongate elements, which have difference electrical lengths at a frequency within an operating frequency band of the antenna and are coupled together at respective first ends in the region of the feed connection and at respective second ends by a linking conductor extending around the core, the elongate elements of each group thereby defining at least part of an elongate channel which has an electrical length of n&lgr;/2 within the said band, and the major part of which is located between the elements and wherein the first elements of the two groups form part of a first looped conductive path, and the second elements of the two groups form part of a second looped conductive path, such that the said paths have difference respective resonant frequencies within the said band and each extend from the feed connection to the linking conductor, and then back to the feed connection.
Other aspects of the invention, as well as preferred features, are set out in the accompanying claims.
The n&lgr;/2 channel, or slit, makes it possible to provide isolation between conductive loops formed by the antenna elements and linking conductors. Since the major part of this channel is located between the antenna elements, intrusion into other parts of the antenna is reduced. Preferably, the entire channel is located between the antenna elements.
By arranging for the elongate elements and linking conductors to form at least two looped conductive paths with the electrical length of one of the two paths greater than that of the other path at an operating frequency of the antenna, a frequency response with at least two resonant peaks is produced yielding an antenna with relatively wide bandwidth. Indeed, the resonant frequencies can be selected to coincide with the centre frequencies of the transmit and receive bands of a mobile telephone system.
The linking conductor may be formed by a quarter wave balun on the outer surface of the core adjacent the end opposite to the feed connection, this feed connection being provided by a feeder structure extending longitudinally through the core. In one preferred embodiment, the linking conductor is formed by an integral balun sleeve, or trap, each of the conductive paths including the rim of the sleeve. Alternatively, each linking conductor may be formed by a conductive strip extending around the core. The advantage of a balun sleeve is that the antenna may operate in a balanced mode from a single-ended feed coupled to the feeder structure.
In the preferred antenna there are two looped conductive paths extending around the core, each looped path extending from the feed connection, through first or second antenna elements (depending on the operating frequency) of a first group, to the linking conductor, and returning through respective first or second elements of a second group back to the feed connection. The difference in electrical length between the antenna elements in each group, and so between the two looped conductive paths, may be achieved by forming one of the elements in each group of a different width to the other element or elements in the group. In effect, the elements act as waveguides, the wider element propagating signals at a lower velocity than the narrower elements. Alternatively, one of the elements in each group may have a different physical length from the other element or elements in that group.
In the preferred embodiment, the antenna core is generally cylindrical and the feed connection is located on an end-face of the core, each of the elongate elements in each group being coupled together on the end face. The core defines a central axis and the antenna elements are substantially coextensive in the axial direction, each element extending between axially spaced-apart positions on or adjacent the outer surface of the core such that at each of the spaced apart positions, the respective spaced-apart portions of the antenna elements lie substantially in a single plane containing the central axis of the core. In this case, each group of elongate elements comprises first and second antenna elements, the looped conductive paths extending from the feed connection, through first and second antenna elements of a first group of elements to the linking conductor, in the form of the balun sleeve, and returning through the respective first or second antenna elements of a second group of elements to the feed connection. The antenna elements are helical, executing a half-turn around the core. Such a structure yields an antenna radiation pattern having laterally directed nulls perpendicular to the single plane.
The antenna of the preferred embodiment actually has four modes of resonance. This is due to the provision of the balun sleeve, which provides for both single-ended and balanced modes of resonance involving current paths around the balun rim and through the balun respectively. The use of coupled modes in this way is disclosed in our co-pending British Patent Application No. 9813002.4, the contents of which are incorporated herein by references. Accordingly, two modes of resonance are associated with each of the two elements in each group, i.e. one single-ended mode and one balanced mode, the res
Dowsett Mark Roy
Leisten Oliver Paul
Alemu Ephrem
Fulbright & Jaworski
Sarantel Limited
Wong Don
LandOfFree
Antenna does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Antenna, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Antenna will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2558177