Communications: radio wave antennas – Antennas – High frequency type loops
Reexamination Certificate
1999-03-31
2001-05-29
Phan, Tho (Department: 2821)
Communications: radio wave antennas
Antennas
High frequency type loops
C343S744000, C343S748000, C343S866000
Reexamination Certificate
active
06239760
ABSTRACT:
TECHNICAL ART
The instant invention relates to transmitting and receiving antennas, particularly to helically wound antennas, and to devices which incorporate such antennas as means for coupling electromagnetic energy.
BACKGROUND OF THE INVENTION
The performance of electromagnetic antennas is measured with respect to the distance in a given direction or set of directions over which a given amount radio frequency (RF) power applied to the antenna's input terminals can propagate while having a signal strength above a given threshold. Performance is also measured with respect to the frequency bandwidth over which this can occur. An antenna comprises a collection of radiating elements which convert electrical energy to radiating photons, and the geometry and size of these elements determine the intrinsic radiation pattern of the antenna, representing the distribution of radiated power as a function of angular orientation with respect to the coordinate system in which the antenna is located. The radiation pattern indicates the ability of the antenna to concentrate energy along a given direction or set of directions, and the orientation of the peak of the radiation pattern gives the direction over which the propagation distance in free space will be greatest. The efficiency of the radiation process—i.e. the process of converting electrical energy to radiating photons—is dependent upon the operating frequency and is measured by what is termed here a radiation bandwidth. An antenna also exhibits a frequency dependent complex impedance at its input port or ports which affects the ability of the antenna to absorb power from a given source. This frequency dependency of the input impedance is characterized by the antenna's input impedance bandwidth. The net bandwidth of the antenna is dependent upon both the radiation bandwidth and the impedance bandwidth. An electrical matching network is generally placed between the antenna input port and the feed source to match the impedance of the antenna to that of the power source so as to maximize the amount of real power conducted into and absorbed by the antenna. Some of this absorbed real power is converted to heat due to ohmic losses in the conductive elements comprising the antenna, while the remainder is radiated by the antenna. The impedance bandwidth at the input to the matching network is generally different from that of the antenna. The performance of an antenna is dependent upon the ability of the antenna to absorb electrical energy conducted into the antenna input port, as indicated by the input impedance and impedance bandwidth, and upon the ability of the antenna to convert the conducted electrical energy to radiating photons, as indicated by the radiation pattern and radiation bandwidth. In operation, the orientation of the antenna, and with that the antenna's radiation pattern, relative to that of a given receiving antenna, will affect the maximum propagation distance that can be achieved for a given communications link between the two antennas.
The direction of polarization of an electromagnetic wave is given by the direction of the corresponding electric field component. If the direction of polarization is fixed, the wave is said to be linearly polarized, while if the direction of polarization rotates about the axis of wave propagation, the wave is said to be circularly polarized. The arts pertaining to electromagnetic radiation and propagation generally recognize that electromagnetic waves of a given energy which are linearly polarized in a vertical direction relative to the Earth's surface, i.e. vertically polarized, will propagate farther than corresponding electromagnetic waves of other polarizations. Vertically polarized waves are commonly created with resonant dipoles, or grounded quarter wave monopoles, oriented along a vertical axis. For a dipole, the length of the antenna at resonance—the operating frequency for greatest efficiency—is such that the antenna supports one half of a standing wave. While propagating on or along the antenna structure, the wave is referred as a guided wave, and the guided wavelength is generally about 95% of the free space wavelength for an electric dipole. The length of a resonant quarter-wave monopole will be one quarter of a guided wavelength. The physical size, especially the length, of these resonant dipole and monopole antennas can be a significant disadvantage, especially at low frequencies and for applications requiring a portable, vehicular mounted antenna.
A number of alternative means have been devised for reducing the size, or more particularly the length, of resonant dipole or monopole antennas. When operated at non- resonant frequencies, and particularly at frequencies where the resonant dipole or monopole antenna is electrically short or small, i.e. where the physical length of the antenna is shorter than the corresponding half or quarter guided wavelength, the input impedance becomes complex and likely unmatched to the power source, thereby reducing the amount of power that can be absorbed by the antenna Matching circuits can be used to compensate for this effect and to thereby increase the efficiency of electrically short antennas, and these matching circuits can comprise either passive or active electrical networks. A dipole or monopole antenna can also be constructed with helically wound conductors, whereby the resonance length is governed by the length of the wire and the velocity factor of the helical wave guiding structure, while the antenna length is governed by the overall, and generally significantly shorter, length of the helix. A plurality of electrically short dipole or monopole antennas may also be operated as a phased array to as to concentrate the radiation power in a given direction. The benefits of reduced size in these alternative configurations, however, are generally obtained with the disadvantage of either reduced gain, or increased complexity or cost.
It will be appreciated by one with ordinary skill in the art that a single conductor may comprise a variety of embodiments, including but not limited to a single-element conductor comprising a wire, foil or printed circuit element, each of arbitrary cross section, either solid or hollow; a multi-element conductor comprising a plurality of non-insulated single-element conductors; or a plurality of single-element conductors or multi-element conductors which are insulated from one another; such that a signal applied across two nodes defined at distinct locations along the single conductor is applied across each such conductive element thereof, thereby causing a current to flow in each such element in accordance with Ohm's law. The aforementioned single-element conductor may further comprise a variety of embodiments, including but not limited to a homogeneous or stratified conductive medium, or one or more segments of distinct homogeneous or stratified conductive media conductively joined to one another.
A low profile, i.e. short, vertically polarized antenna would be useful for a number of applications. These applications include portable communications equipment, such as on air, sea and land vessels and vehicles; where the physical length of a protruding antenna could either adversely affect aerodynamic drag, interfere with obstacles, or be overly conspicuous. These applications could also include low frequency land based communications where the height of the antennas is hazardous to aircraft and undesirable to neighboring residents. These tall antennas are also expensive to build and to maintain.
The radiation from an electric dipole or monopole antenna results from the spatial distribution of electric currents associated with the associated standing current waves on the antenna structure. The electric currents oscillate along the linear path of the antenna, and the direction of electric current corresponds to the direction of polarization of the resulting associated radiated wave. Applying the principle of duality of electromagnetic fields, a vertically polarized antenna can also be
Phan Tho
Van Voorhies Kurt L.
VorteKx, Inc.
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