Communications: radio wave antennas – Antennas – Spiral or helical type
Reexamination Certificate
1998-12-03
2001-01-30
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
Spiral or helical type
C343S821000
Reexamination Certificate
active
06181297
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an antenna for operation at frequencies in excess of 200 MHz, and in particular to an antenna which has a three-dimensional antenna element structure.
BACKGROUND OF THE INVENTION
British Patent No. 2258776 discloses an antenna which has a three-dimensional antenna element structure by virtue of having a plurality of helical elements arranged around a common axis. Such an antenna is particularly useful for receiving signals from satellites, for example, in a GPS (global positioning system) receiver arrangement. The antenna is capable of receiving circularly polarised signals from sources which may be directly above the antenna, i.e. on its axis, or at a location a few degrees above a plane perpendicular to the antenna axis and passing through the antenna, or from sources located anywhere in the solid angle between these extremes.
While being intended mainly for reception of circularly polarised signals, such an antenna, due to its three-dimensional structure, is also suitable as an omnidirectional antenna for receiving vertically and horizontally polarised signals.
One of the disadvantages of such an antenna is that in certain applications it is insufficiently robust, and cannot easily be modified to overcome this difficulty without a performance penalty. For this reason, antennas which are to receive signals from the sky in harsh environments, such as on the outside of an aircraft fuselage, are often patch antennas, being simply plates (generally plated metallic square patches) of conductive material mounted flush on an insulated surface which may be part of the aircraft fuselage. However, patch antennas tend to have poor gain at low angles of elevation. Efforts to overcome this disadvantage have included using a plurality of differently oriented patch antennas feeding a single receiver. This technique is expensive, not only due to the numbers of elements required, but also due to the difficulty of combining the received signals.
SUMMARY OF THE INVENTION
According to one aspect of this invention an antenna for operation at a frequency in excess of 200 MHz comprises an electrically insulative antenna core of a material having a relative dielectric constant greater than 5, a three-dimensional antenna element structure disposed on or adjacent the outer surface of the core and defining an interior space, and a feeder structure which is connected to the element structure and passes through the core, the material of the core occupying the major part of the said interior space.
Typically the element structure comprises a plurality of antenna elements defining an envelope centred on a feeder structure which lies on a central longitudinal axis. The core is preferably a cylinder and the antenna elements preferably define a cylindrical envelope which is coaxial with the core. The core may be a cylindrical body which is solid with the exception of a narrow axial passage housing the feeder. Preferably, the volume of the solid material of the core is at least 50 percent of the internal volume of the envelope defined by the elements, with the elements lying on an outer cylindrical surface of the core. The elements may comprise metallic conductor tracks bonded to the core outer surface, for example by deposition or by etching of a previously applied metallic coating.
For reasons of physical and electrical stability, the material of the core may be ceramic, e.g. a microwave ceramic material such as zirconium-titanate-based material, magnesium calcium titanate, barium zirconium tantalate, and barium neodymium titanate, or a combination of these. The preferred relative dielectric constant is upwards of 10 or, indeed, 20, with a figure of 36 being attainable using zirconium-titanate-based material. Such materials have negligible dielectric loss to the extent that the Q of the antenna is governed more by the electrical resistance of the antenna elements than core loss.
A particularly preferred embodiment of the invention has a cylindrical core of solid material with an axial extent at least as great as its outer diameter, and with the diametrical extent of the solid material being at least 50 percent of the outer diameter. Thus, the core may be in the form of a tube having a comparatively narrow axial passage of a diameter at most half the overall diameter of the core. The inner passage may have a conductive lining which forms part of the feeder structure or a screen for the feeder structure, thereby closely defining the radial spacing between the feeder structure and the antenna elements. This helps to achieve good repeatability in manufacture. This preferred embodiment has a plurality of generally helical antenna elements formed as metallic tracks on the outer surface of the core which are generally co-extensive in the axial direction. Each element is connected to the feeder structure at one of its ends and to a ground or virtual ground conductor at its other end, the connections to the feeder structure being made with generally radial conductive elements, and the ground conductor being common to all of the helical elements.
According to another aspect of the invention, an antenna for operation at a frequency in excess of 200 MHz comprises a solid electrically insulative antenna core which has a central longitudinal axis and is made of a material having a relative dielectric constant greater than 5, a feeder structure extending through the core on the central axis, and, disposed on the outer surface of the core, a radiating element structure comprising a plurality of antenna elements which are connected to the feeder structure at one end of the core and extend in the direction of the opposite end of the core to a common grounding conductor. The core preferably has a constant external cross-section in the axial direction, with the antenna elements being conductors plated on the surface of the core. The antenna elements may comprise a plurality of conductor elements extending longitudinally over the portion of the core having a constant external cross-section, and a plurality of radial conductor elements connecting the longitudinally extending elements to the feeder structure at the said one end of the core. The phrase“radiating element structure” is used in the sense understood by those skilled in the art, that is to mean elements which do not necessarily radiate energy as they would when connected to a transmitter, and to mean, therefore, elements which either collect or radiate electromagnetic radiation energy. Accordingly the antenna devices which are the subject of this specification may be used in apparatus which only receives signals, as well as in apparatus which both transmits and receives signals.
In a particularly preferred embodiment of the invention, the antenna includes an integral balun formed by a conductive sleeve extending over part of the length of the core from a connection with the feeder structure at the above-mentioned opposite end of the core. The balun sleeve may thus also form the common grounding conductor for the longitudinally extending conductor elements. In the case of the feeder structure comprising a coaxial line having an inner conductor and an outer screen conductor, the conductive sleeve of the balun is connected at the said opposite end of the core to the feeder structure outer screen conductor.
The preferred embodiment of the antenna, having a core which is a solid cylinder, includes an antenna element structure comprising at least four longitudinally extending elements on the cylindrical outer surface of the core and corresponding radial elements on a distal end face of the core connecting the longitudinally extending elements to the conductors of the feeder structure. Preferably, these longitudinally extending antenna elements are of different lengths. In particular, in the case of an antenna having four longitudinally extending elements, two of the elements are of greater length than the other two by virtue of following meandered paths on the outer surface of the core. In the case of an antenna for circularl
Ho Tan
Symmetricom, Inc.
Wilson Sonsini Goodrich & Rosati
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