Communications: radio wave antennas – Antennas – Spiral or helical type
Utility Patent
1999-01-13
2001-01-02
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
Spiral or helical type
C343S802000
Utility Patent
active
06169523
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to helical antennae used for radiation and reception of electromagnetic energy. More specifically the invention relates to electronically tunable helical antennae, specifically electronically tunable helical antennae with built-in chokes. Specifically, the invention relates to an electronically tunable helical antenna system consisting of a collinear array of independently controlled and fed antennae and fixtures.
BACKGROUND OF THE INVENTION
Modern communications systems deploy a multitude of radios operating independently. Often each radio has a dedicated antenna. In some cases it is desirable to connect two or more antenna elements to one radio. It would be desirable to be able to collinearly mount these independent antennae and their control and feed lines on one support, in a manner that allows their independent operation. In so doing, each of the antennas can be controlled by separate independent radios and/or any two or more antennas may be electrically connected to a single radio. Furthermore, it would be desirable to provide the means to include the electrical control of electrical fixtures mounted on top of the collinear array, such as a warning light.
U.S. Pat. No. 4,924,238 (the '238 patent) to the present inventor discloses an electronically tunable helical antenna that radiates broadside, and that has been demonstrated to operate over very wide frequency bands, i.e. is tunable over as much as a 15:1 frequency band. In the '238 patent, the helical turns of the radiating portion of the antenna are formed of tubular material which may be in the form of a single length of tubing or may comprise a number of parallel coaxial cables with their outer conductors in electrical contact. The antenna is tuned by a series of oppositely poled pairs of diodes that are connected, at spaced points, to the radiating coils of the antenna. When the diodes are biased to be conductive, a section of the radiating helix is short-circuited, reducing the antenna electrical length and increasing its tuned frequency. Bias voltages to control the diodes are provided by leads inside the radiating turns of the helix. Each lead for a pair of diodes emerges at a point electrically balanced between the two spaced points that are connected to the associated diodes. The electrical balance results in no r-f current flows on the bias leads. This type of antenna may be a monopole helical antenna or a dipole antenna with two oppositely disposed arms. The helix diameter typically runs between 0.001 and 0.05 wavelengths. Its length is typically less than 0.05 wavelengths at the lowest operating frequency in the case of the monopole and twice that for the dipole case. The dimension of a typical tunable helical monopole with a minimum operating frequency of 30 MHZ is less than 16 inches in height and less than 4 inches in diameter.
Many existing collinear helical antenna array systems require a large physical space in which to operate, for example, to provide a large enough helix diameter to ensure decoupling of the antenna from a conductive center supporting post, and to provide adequate length for the desired operating wavelength. U.S. Pat. No. 2,293,786 to Krause requires an axial helical physical length preferably in the range between 1.5 and 5 operating wavelengths and a helix diameter and pitch such that the circumference of each turn preferably equals an integral number of operating wavelengths, i.e. diameter>0.3 wavelengths. At an operating frequency of 30 MHZ, this corresponds to a diameter greater than 10 feet and a length of about 48 feet, impractical dimensions in most cases for collinear arraying of elements.
It is known, to form a compact array of independent center-fed collinear dipoles, that feed lines running parallel to the dipole element are required. In order to avoid scatter from these feed lines, which may cause azimuth pattern distortion, it is desirable to run the feed line through the center of the radiating elements. The difficulty is in achieving this without coupling from the dipoles and causing rf currents along the feed lines that limits antenna performance. This is accomplished in the Krause design by proper adjustment of the helix length, helix diameter, and diameter of the centrally located feed line package. The approach allows almost all the energy traveling along the helix length to be radiated from the helical coil before reaching the end of the coil, thereby leaving little to be coupled to the center located feed lines. This is possible because of the greater than 1.5 wavelength long elements used.
It would thus be desirable to be able to array in a collinear fashion the much smaller electronically tunable helix antenna that is a fraction of a wavelength long, that can be tuned and operated over a much wider band than the fixed tuned helical element described by Krause.
SUMMARY OF THE INVENTION
The present invention can provide a compact array of tunable helical antennae, and other devices, having feed and control lines running parallel to the dipole element wherein there is a centrally located conductive tube internal to the array of devices and radiating helices and through which the feed and control lines run, such that multiple antennae and devices may be collinearly mounted along one central conductive tube enabling, for example, multiple antennae with one antenna per radio or multiple antennae per radio.
A first embodiment of the invention, (shown in FIG.
1
), includes a collinearly mountable electronically tunable helical monopole antenna system with a helical antenna coil having an open center space, a base, and a centrally located conductive tube disposed axially through the center space of the helical antenna coil and containing feed and control lines for antennae and devices collinearly mounted above the helical monopole. There is an automatic choke means that decouples the centrally located conductive tube from the helical radiator. This is accomplished by shorting the center conductive tube to the helical antenna coil at the base of the helical antenna coil near where the feed is connected to the helical antenna coil. The feed line feeds the antenna beyond where the antenna coil is shorted to the center conductive tube. Both the helical antenna coil and the center conductive tube are connected to a metallic ground plane. The combination of the helix radiator coil and the center conductive tube is effectively a coaxial (coax) transmission line, with the outer conductor/shield being the helix and the center conductive tube being the coax center conductor.
The collinearly mountable antenna system also has at least one selectable means for shorting sections of the helical antenna coil such that the helical antenna coil is electrically shortened by the at least one selectable means for shorting, rendering the helical antenna system tunable by altering the electrical length of the helical antenna. When the helical antenna is operational, it is tuned to be resonant at the desired operating frequency. The tuning is accomplished preferably by appropriately biasing at least one PIN diode switch (which is the preferable selectable means for shorting) located along each helix surface. Resonance occurs when the helix is electrically one quarter (¼) the desired wavelength long, or any odd multiple of ¼ wavelength (such as ¾ wavelength). As the electrical length of the radiator is tuned, the electrical length of the choke, i.e. the coaxial line consisting of the center conductive tube and helical coil is also changed and thereby tunes the automatic choke to the operating frequency of the helical radiator.
A center fed dipole version of the present invention can be implemented by introducing the mirror image of the described helical monopole and center conducting tube. Multiple monopoles, dipoles or combinations of monopoles, dipoles and/or other types of antennae or devices such as lights, may all be mounted along one center conductive tube which houses feed and control lines. All an
Devine, Millimet & Branch P.A.
Ho Tan
Kohler Kristin
Remus Paul C.
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