Communications: radio wave antennas – Antennas – Spiral or helical type
Reexamination Certificate
1999-06-15
2004-02-10
Wimer, Michael C. (Department: 2821)
Communications: radio wave antennas
Antennas
Spiral or helical type
C343S702000
Reexamination Certificate
active
06690336
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an antenna for operation in excess of 200 MHz, and to a radio communication unit including the antenna.
BACKGROUND OF THE INVENTION
The antenna requirements of a cellular or cordless telephone handset are primarily that it should be compact and omnidirectional. For a handset operating within the frequency range of 800 MHz to 2 GHz the antenna is typically an extendible rod having a length approximately equivalent to a quarter wavelength when extended, or a helical wire having several turns. The antenna is usually mounted partly within the handset unit and partly projecting from the end of the unit adjacent the earphone. A disadvantage with small antennas such as those designed for personal telephone use is that, in general, they have poor gain over the frequency band in which they are required to operate. It is also known that small resonant antennas generally have a narrower fractional bandwidth than their larger counterparts designed to operate at lower frequencies. Another disadvantage is that, the smaller they are, the greater is their tendency to generate intense near-field electromagnetic radiation, i.e. radiation which is perceived to represent a health hazard when such an antenna is used close to the head for transmission of signals. The measurement of this effect is conducted to produce a parameter usually referred to as the Specific Absorption Rate (SAR).
The latter disadvantage has been addressed to some extent in the Applicant's co-pending British Patent Application No. 2309592, which discloses a twisted loop antenna exhibiting a radiation pattern with an azimuth null when oriented with the axis of the radiating helices upright. With appropriate mounting of the antenna on the housing of a portable telephone, this null can be directed towards the user's head to reduce irradiation in that direction.
It is an object of the invention to provide a small antenna which combines improved bandwidth with good SAR performance.
SUMMARY OF THE INVENTION
According to a first aspect of this invention, an antenna for operation at frequencies in excess of 200 MHz comprises: an insulative core of a solid material having a relative dielectric constant greater than 5′ the outer surface of the core defining a volume the major part of which is occupied by the solid material; a feeder structure comprising a length of transmission line; an electrically conductive structure including a plurality of radiating elements connected to the transmission line at a first position, and a link element separately connecting the radiating elements to the transmission line at a second position, spaced from the first position along the feeder structure, the electrically conductive structure being disposed on or adjacent the outer surface of the core; wherein the core and conductive structure are configured such that the antenna has at least two different modes of resonance which are coupled thereby to define together an operating frequency band for signals fed to or received from the transmission line, the different modes of resonance being associated with different respective radio frequency (r.f.) current patterns in the conductive structure, each pattern including the said radiating elements.
The antenna may be configured such that (a) a first mode of resonance occurs at a first frequency within the said band and is associated with an r.f. current loop including the said antenna elements and beginning and ending at a location at which the antenna elements are connected to the transmission line at the first position, the link element acting as a high impedance blocking element at the first frequency, and such that (b) a second mode of resonance occurs at a second frequency within the said band and is associated with an r.f. current loop running from the location of the connection of the antenna elements to the transmission line at the first position, through the antenna elements and link element in series, to the connection with the feeder structure at the second position.
The antenna may further be configured so that the input reactance component of the load represented by the antenna is substantially zero within the operating frequency band only when the corresponding input resistance component is finite and substantially non-zero. The corresponding Smith chart representation of the load impedance presented by the antenna within the operating band is typically in the form of a looped self-intersecting locus.
In the preferred embodiment, there are two modes of resonance within a single operating band of the antenna, the first resonance being a balanced mode and the second mode of resonance being a single-ended mode. The antenna elements, the link element, in the form of a balun trap, and the transmission line all act as current-carrying elements in both modes of resonance. In this preferred embodiment, the core is cylindrical, having a central axis of symmetry, and the antenna elements are a plurality of axially co-extensive conductors extending between an end of the transmission line and the trap element. These antenna elements are the sole radiating elements, and the antenna has no other elements which act as significant radiating elements in either mode. Effectively, the antenna comprises a unitary structure with a unitary set of conductive elements which act together with both modes to yield two different structural modes of resonance.
It will be appreciated that such an antenna provides an improved operating bandwidth without using a large antenna structure or a plurality of separately fed antenna structures. The frequency responses associated with the respective modes couple together in the frequency domain so as to define the operating bandwidth.
By dimensioning the elements so that the two modes occur within a required band, e.g. the 1710 MHz to 1880 MHz DCS-1800 band for cellular telephones, or the 890 MHz to 965 Mhz European GSM band for cellular telephones, the whole of either of these bands can be accommodated with the bandwidth of the antenna, the two resonant modes coupling such that energy storage associated with one mode of resonance is shared with energy storage in the other mode of resonance, thereby forming a frequency response which is flat-topped or has a non-zero saddle between two resonant peaks. Typically, the modes of resonance are arranged to couple to achieve a combined gain characteristic for the antenna which maintains a response within the 3 dB limits over a fractional bandwidth of at least 3% of the centre frequency of the operating band.
In the preferred antenna in accordance with the invention, the radiating elements comprise a singe pair of elonaate antenna elements interconnected at respective ends by the link element so as to form a path of conductive material around the core with the other ends of the antenna elements constituting a feed connection at a distal end of the transmission line. The antenna elements are co-extensive, each element extending between axially spaced-apart positions on the outer cylindrical surface of the core. The elements may be metallised tracks deposited or bonded onto the core and arranged such that at each of the spaced-apart positions the respective spaced-apart portions of the elements are substantially diametrically opposed. The spaced apart portions all lie substantially in a single plane containing the central axis of the core, and the portions at one of the spaced-apart positions are connected together by the link element to form the loop, the portions at the other of the spaced-apart positions being coupled to feed connections for the loop by cross elements extending generally radially on an end face of the core. The antenna elements are preferably of equal length and are helical, each executing a half-turn around the core between the spaced-apart portions. The feed connections may be connected to a coaxial feeder which forms the transmission line extending through the core on the axis. The other end face of the core is metallised, the resulting conductive layer fo
Agboraw Ebinotambong
Leisten Oliver Paul
Nicolaidis George
Gray Cary Ware & Freidenrich LLP
Symmetricom, Inc.
Wimer Michael C.
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