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Title:
MODIFIED HELICAL ANTENNA
Document Type and Number:
WIPO Patent Application WO/1998/002936
Kind Code:
A1
Abstract:
A modified helical antenna having increased gain over that of a conventional axial mode helical antenna (200) without increasing the length of the antenna. The modified helical antenna (700) uses a conducting cylinder (702) placed around approximately the first two turn of the helix (716) of the antenna (700). The height of the conducting cylinder (702) should extend from a ground plane (712) to approximately 1.5 helical turns. Closely coupling the conducting cylinder (702) to the helix (716) increases the gain in the main beam (318) as energy in the side lobes (324, 326) decreases. This is true as the inside diameter of the conducting cylinder (702) approaches that of the helix (716) up to a cut-off point. For a particular application, optimization of the inside diameter (710) of the conducting cylinder (702) can be achieved empirically or through numerical analysis.

Inventors:
Nghiem, David (10189 Tilton Street, San Diego, CA, 92126, US)
Stillinger, David C. (2171 Cambridge Avenue, Cardiff, CA, 92007, US)
Joe, Chieu LE. (9405 Gold Coast Drive, A7 San Diego, CA, 92126, US)
Application Number:
PCT/US1997/012399
Publication Date:
January 22, 1998
Filing Date:
July 16, 1997
Export Citation:
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Assignee:
QUALCOMM INCORPORATED (6455 Lusk Boulevard, San Diego, CA, 92121, US)
International Classes:
H01Q11/08; (IPC1-7): H01Q11/08
Domestic Patent References:
WO1993006631A11993-04-01
Foreign References:
EP0635898A11995-01-25
EP0241921A11987-10-21
GB2079063A1982-01-13
US3757345A1973-09-04
Other References:
PATENT ABSTRACTS OF JAPAN vol. 11, no. 46 (E - 479) 12 February 1987 (1987-02-12)
Attorney, Agent or Firm:
Ogrod, Gregory D. (Qualcomm Incorporated, 5775 Morehouse Drive San Diego, CA, 92121-1714, US)
Download PDF:
Claims:
CLAIMS
1. A helical antenna comprising: a central conductor having a plurality of turns wound in the shape of a helix extending from a first end to a second end; and a conducting structure positioned around said central conductor at said first end and coupled to said central conductor.
2. The helical antenna of claim 1, wherein the height of said conducting structure is less than the length of one and onehalf turns of said central conductor.
3. The helical antenna of claim 1, wherein the height of said conducting structure is chosen such that its top edge is roughly equal to a transition point of said central conductor.
4. The helical antenna of claim 1, wherein said conducting structure is a cylinder.
5. The helical antenna of claim 4, wherein a diameter of said conducting structure is equal to or greater than 1.06 times a diameter of said central conductor.
6. The helical antenna of claim 1, wherein said conducting structure comprises material from the group of copper, brass, silver, aluminum, steel, conductive composite or conductive plastic.
7. The helical antenna of claim 1, wherein said conducting structure comprises a conductive material deposited on a support substrate.
8. The helical antenna of claim 1, wherein said conducting structure comprises a wire wound to appear as a solid structure at a first frequency of interest.
9. The helical antenna of claim 8, wherein said conducting structure is manufactured to radiate at a second frequency or interest.
10. The helical antenna of claim 1, further comprising a ground plane.
11. The helical antenna of claim 10, wherein said central conductor is adjusted in height by turning said central conductor about an axis thereby feeding said first end through said ground plane.
12. The helical antenna of claim 10, wherein said conducting structure and said ground plane are formed as a unified body with a conductive material deposited thereon.
13. The helical antenna of claim 12, wherein the unified body comprises a molded plastic material.
14. The helical antenna of claim 1, wherein said central conductor comprises a single conductor wound in a helical fashion.
15. The helical antenna of claim 1, wherein said central conductor comprises a plurality of radiators wound in a helical fashion.
16. The helical antenna of claim 1, wherein the height of the conducting structure relative to the central conductor is adjustable.
17. The helical antenna of claim 16, wherein said height is adjustable during antenna operation.
18. A helical antenna, comprising: radiator means for radiating a circularly polarized electromagnetic wave having a main lobe and one or more side lobes; and directing means electromagnetically coupled to said radiator means, said directing means for redirecting electromagnetic energy from said one or more side lobes into said main lobe.
19. The helical antenna of claim 18, wherein said radiator means is a helical radiator having a decay wave region and a surface wave region, and said directing means is a conducting structure positioned around a base of said helical radiator to decrease the amount of energy radiated by said decay wave region.
20. The helical antenna of claim 19, wherein the height of said conducting structure approximately corresponds to a transition point between said decay wave region and said surface wave region of said radiator means.
21. The helical antenna of claim 18, wherein said directing means is a conductive cylinder.
22. A method for directing energy away from one or more side lobes of a beam pattern of a helical antenna into a main lobe, the helical antenna comprising a central conductor, said method comprising the steps of: positioning a conductive structure around said central conductor; adjusting the height of said conductive structure to a height that is approximately equivalent to a transition point of said central conductor; and adjusting an inner width of said conductive structure to be greater than or equal to 1.06 times the outer diameter of said central conductor.
23. The method for directing energy of claim 22 wherein said helical antenna comprises: a conductive helix having a central conductor with opposite ends, a predetermined outer diameter, a predetermined inner diameter, and a predetermined length; and a ground plane that provides a support surface for said conductive helix and an electrical signal feed point for said conductive helix.
Description:
MODIFIED HELICAL ANTENNA BACKGROUND OF THE INVENTION I. Field of the Invention The present invention relates to communication systems. More particularly, the present invention relates to a modified helical antenna.

II. Description of the Related Art Conventional wireless communications systems use antennas to transmit and receive electromagnetic waves through the atmosphere between communication system locations. The signal typically consists of some type of information, such as data, audio, video, voice or other type of information modulated onto a carrier. The carrier may be, for example, a microwave carrier or a radio frequency carrier.

One requirement of wireless communications systems is that the energy in the launched signal be sufficient to enable the receiver to recover the signal accurately. Meeting this requirement is generally hindered by two phenomena: attenuation and interference. These phenomena are commonplace in all communications systems that use antennas to transmit the signals through the atmosphere.

Attenuation is the phenomenon whereby a signal loses energy in such a way as to reduce the strength of the signal in relation to noise in the communication system. Attenuation normally occurs as the signal travels through the atmosphere and may be aggravated by atmospheric sources, such as rain, snow, and water vapor.

Interference is the phenomenon wherein a signal is disturbed and distorted because of interaction with physical objects or electromagnetic energy from other sources. Interference from objects such as trees, buildings and billboards is sometimes referred to as terrestrial object obstruction interference. With terrestrial object obstruction interference, the physical objects obstruct the path of the signal and cause the signal to bounce off the object. The energy in the signal is absorbed into the object and scattered into the atmosphere.

Interference from other signal sources in antenna arrays has sometimes been addressed by using widely spaced walls of conducting material placed between adjacent or nearby antennas to block or absorb radiation. The radiation blocking acts to minimize interference or the "cross-coupling"of signals between antennas. Such walls are shown for

example in Hisamatsu Nakano, Helícal and Spiral Antennas-A Numerical Approach, (1987) chapter 6, page 155. However, such walls do not otherwise improve the operating characteristics of antennas or their respective energy transfer capabilities.

Another mechanism used to overcome both attenuation and interference is to increase the energy of the launched signal. This can be accomplished by increasing the power supplied by the transmitter to the antenna or by increasing the gain of the antenna. However, due to various considerations such as power supply size, battery life, FCC regulations, safety and other like factors, increasing the power of the transmitter is not always a practical alternative.

Therefore, wireless communication system designers frequently turn to the gain of the antenna as a focal point for increasing system performance.

Increasing the antenna length may be an acceptable mechanism for increasing the gain and thereby increasing signal energy in some applications. However, increasing antenna length is not a desirable alternative in applications where the size of the antenna or the communication device is an important factor. With many mobile and portable, and some fixed, applications, there is strong incentive to minimize the size of the antenna. This assists with general aesthetics, minimizes potential for antenna damage, improves mounting characteristics, and aids in minimizing the overall size of some communication devices. In fact, as portable and hand-held applications increase in popularity, the demand for smaller and smaller communication devices increases dramatically.

Other ways in which antenna gain may be increased is by varying the diameter of the antenna, changing the position of the antenna, adding ground planes to antenna assemblies, and reducing the impedance that the antenna sees as it launches or receives a signal. By varying these antenna parameters, antennas can be made to radiate as much energy as is practical into desired directions in order to increase the energy in the signal.

Another mechanism used to increase the gain of the antenna is to vary its shape. For example, an antenna formed in the shape of a helix has behavioral characteristics that are suitable for use in satellite communication systems. For a given number of turns in the helix and for a given length of the antenna, the helical antenna has a standard gain, beam width, and side lobes. A given amount of energy exists in the main lobe and the side lobes.

Conventional helical antennas are made by twisting one or more radiators used to form the antenna into a helical structure. The radiators (or helices or conductors) can be made using wire or strip technology. With strip technology, antenna radiators are etched or deposited onto a thin, flexible substrate. The substrate is then formed, or rolled, into a cylindrical, conical, or other appropriate shape causing the strip radiators to form a helix. Many helical antenna manufacturing techniques are well known.

Helical antennas have good gain and directivity. Moreover, helical antennas exhibit a well understood beam pattern, which consists of a main lobe and at least one side lobe. A given amount of energy is contained in the main lobe and the side lobes. The main lobe also has a predictable beam width.

In addition to characteristics such as favorable gain and directivity, helical antennas are popular for use in satellite communication systems because of an ability to produce and receive circularly-polarized radiation employed in such systems. This allows one antenna to both send and receive signals at the same frequency with minimal interference.

Additionally, because the helical antenna is capable of producing a radiation pattern that is nearly hemispherical, the helical antenna is particularly well suited to certain applications such as mobile satellite communication systems and satellite navigational systems.

While conventional helical antennas have the advantages described above, which make them well suited for various applications in wireless communications systems, there are limitations. One limitation is that the energy in the side lobes of the antenna beam is considered lost energy because useful energy transfer generally takes place in the main lobe. That is, the more energy in the main lobe, and, thus, the less in the side lobes, then the greater the amount of energy useful for communications being radiated from the antenna. Also, energy launched in the side lobes interferes with the energy of the main lobe, with a resulting negative impact on communication signals.

A second limitation is that while varying physical parameters of an antenna allows the antenna beam to be customized to a particular application, these are structural variations that may be fixed at the factory where the antenna is made. Thus, the antenna may only be suitable for a particular application contemplated at the time of manufacture. The structural parameters may not be easily adjusted after leaving the factory.

Moreover, adjusting structural parameters can also alter critical characteristics of the antenna such as its frequency response.

While antenna designers are continually attempting to improve the energy transfer of helical antennas by varying the antenna shape, diameter, position, and polarization, and by adding ground planes and lowering impedance, the most significant improvements are obtained by varying the length and number of turns. Optimally, however, it is desirable to have the capability to optimize energy transfer, by increasing antenna gain and directivity, without having to change fixed physical parameters such as the length or number of turns in the helix.

SUMMARY OF THE INVENTION The present invention is a novel and improved modified helical antenna having a conducting structure positioned around the helix to improve the gain of the antenna in a desired direction. The conducting structure is preferably a cylinder as this shape provides uniform behavior when implemented with a helical antenna. The diameter and/or length of the conducting cylinder are determined by the particular application. That is, the dimensions of the conducting cylinder can be modified to tailor the characteristics of the antenna. The conducting cylinder is preferably concentric with the antenna helix, and physically separate from but electrically, or electromagnetically coupled to the conductive helix.

The height of the conducting cylinder is chosen to optimize antenna performance. In one embodiment, the height of the conducting cylinder is adjustable, possibly during antenna operation. The height may be adjusted in order to optimize the antenna performance for a particular operating frequency.

The conducting cylinder can be manufactured from a conductive material including materials such as, for example, copper, brass, steel, silver or aluminum, or other elements or alloys. The cylinder can be made of solid conducive material or a laminate. In laminate form, the conductive material is deposited on a cylindrical substrate or on a flexible substrate which is rolled to form the conducting cylinder.

The conducting cylinder may be manufactured as a solid cylinder or a plurality of conductors formed to appear as a solid cylinder at a frequency of interest. Alternatively, a mesh type material or a material having vertical

extending passages less than a predetermined width or length could be used for some applications.

A ground plane, included in a preferred embodiment of the invention, is comprised of a support surface that supports the conducting cylinder and the conductive helix. The support surface and the conducting cylinder can be formed as a unified body, with a planar surface and cylindrical projection. The unified body may be made from a molded plastic or an acrylic material, for example and coated with a conductive material, or may be made entirely of conductive material.

The conductive helix includes a central conductor having opposite ends. The central conductor may be made of conductive material deposited on a flexible substrate rolled to form the conductive helix at the predetermined diameter. Alternatively, the central conductor can be made of a wire material. The wire material may be aluminum or copper or other conductive material.

The beam pattern of the helical antenna is shaped and energy is directed away from the side lobes by placing the conducting cylinder at one end of the conductive helix around the portion of the conductive helix where the antenna surface current is determined primarily by the decay wave of the antenna.

An advantage of the present invention is that the helical antenna radiation pattern (or beam pattern) can be tuned (or customized) through the use of the conducting cylinder without altering the parameters of the helix such as its length and number of turns. That is, the main lobe can be widened or narrowed, and the side lobes increased or nearly eliminated for a particular application depending on the height and diameter of the conducting cylinder.

For example, for a constant height of the conducting cylinder, the diameter of the conducting cylinder is varied in order to vary the energy in the main beam and the side lobes. Preferably, in this embodiment, the height of the conducting cylinder is chosen to correspond to a point where the surface wave determines the helix surface current density. That is, the height approximately corresponds to a transition point between a decay wave region and a surface wave region of the helix. This can be determined from a helix surface current curve. The diameter is then chosen to obtain the desired beam characteristics.

Alternatively, for a constant diameter of the conducting cylinder, the height of the conducting cylinder is varied to widen or narrow the main

beam and the side lobes. Preferably, in this embodiment, the diameter is chosen to correspond to the point where the axial ratio of the helix is minimized, usually between two and three dB. The height is then chosen to obtain the desired beam characteristics.

In one embodiment, the height of the conducting cylinder is less than 1.5 turns of the helix and the diameter of the conducting cylinder is greater than 1.06% of the helix outer diameter. In this embodiment, the modified helical antenna has modestly increased gain and directivity.

In another embodiment, the height of the conducting cylinder is equal to 1.5 turns of the helix and the inner diameter of the conducting cylinder is equal to 1.06 times the helix outer diameter. Thus, the antenna has optimum gain and directivity.

Optimization of the shape of the beam by varying the conducting cylinder height, diameter, or both, is accomplished either empirically or utilizing numerical analysis.

Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: FIG. 1 illustrates an electromagnetic wave; FIG. 2 illustrates a conventional helical antenna; FIG. 3A is a more detailed representation of the parameters of a conventional helical antenna; FIG. 3B illustrates a conventional axial mode helical antenna power pattern depicted using polar coordinates; FIG. 3C illustrates a conventional axial mode helical antenna power pattern depicted using rectangular coordinates and a logarithmic scale; FIG. 4 is a graphical representation of a current distribution curve for a single helical antenna radiating in the axial mode as determined numerically;

FIG. 5 is a graphical representation of a beam pattern for a conventional single helical antenna radiating in the axial mode as determined using a numeric simulator; FIG. 6A and FIG. 6B are graphical representations of two beam patterns for a conventional single helical antenna radiating in the axial mode as determined empirically; FIG. 7A illustrates a modified helical antenna, according to the present invention; FIG. 7B illustrates a modified helical antenna according to a second embodiment of the present invention; FIG. 7C illustrates a modified helical antenna according to a third embodiment of the present invention; FIG. 8 is a graphical representation of a modified axial mode helical antenna power pattern depicted using polar coordinates as determined using a numeric simulator; and FIG. 9 is a graphical representation of a modified axial mode helical antenna power pattern depicted using rectangular coordinates and a logarithmic scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Overview of the Invention The present invention is directed toward a modified helical antenna that includes a conducting cylinder that is electromagnetically coupled to the antenna helix. The conducting cylinder is used to shape the main beam and the side lobes of the antenna to increase the gain of the antenna without increasing the length of the antenna. The present invention is applicable to a single helix or to an array of helices. The manner in which this is accomplished is described in detail below according to several embodiments.

Antenna Principles Because antennas operate according to the same basic principles of electromagnetic waves, it is useful to provide a description of some of these fundamental principles.

As used herein, a"wave"is an electromagnetic wave. An electromagnetic wave consists of electric and magnetic fields propagating through space. A field is a region where electric or magnetic forces act.

Two physical properties are important in characterizing electromagnetic waves: wavelength and frequency. A wavelength is the minimum distance between any two points on a wave which behave identically. For example, FIG. 1A represents an electromagnetic wave 100.

The electromagnetic wave 100 has a wavelength , and two peaks 102 and 104 The wavelength X is represented by the distance between the two peaks 102 104. The frequency of the wave 100 is the rate at which the wave 100 repeats itself. Frequency is expressed in cycles per second (or Hertz (Hz)). For electromagnetic waves operating in a medium other than free space, the frequency is determined by: ff-v/., where v is the velocity of the wave in the medium.

When an electromagnetic wave is fed into a transmission line, a traveling wave is produced along the transmission line. If the transmission line is short-circuited, the outgoing traveling wave is reflected back into the transmission line, which reflection causes interference. The interference of the reflected wave with the outgoing traveling wave produces a standing wave. When a standing wave is produced in an antenna, the antenna radiates the energy contained in the standing wave.

A traveling wave that is guided along a transmission line which opens out will radiate as a free-space wave. That is, when the wires in a transmission line are opened out, i. e., separated from each other, a distance approaching a wavelength, the traveling wave radiates as a free-space wave.

The guided wave is a plane wave. The guided wave is a plane wave because, among other things, the spacing between the wires in the transmission line is a fraction of the wavelength of the electromagnetic wave. In contrast, the free-space wave is a spherically expanding wave. This is because, among other things, the spacing between the wires in the transmission line approaches, or even exceeds the wavelength of the electromagnetic wave.

A typical antenna operates in the region where the electromagnetic wave undergoes the transition from a guided wave to a free-space wave.

The antenna receives the currents associated with the electromagnetic wave, converts them to electromagnetic fields, and launches the electromagnetic fields into the atmosphere. The currents on the transmission line flow out to the end of the transmission line and the electromagnetic fields associated with the currents continue out, or"radiate"into space.

In order for the antenna to receive the currents from the transmission line effectively, the impedance that the antenna presents to the flow of the currents must be evaluated. Impedance may be characterized as the opposition that an antenna presents to current flow. The impedance of the antenna typically varies as the frequency of the electromagnetic wave varies.

The electric and magnetic fields in a free-space wave traveling outward at a large distance from an antenna convey energy called radiation.

The electric and magnetic fields are vector quantities which are at right angles to each other.

From space, an antenna is characterized by the shape of the radiation pattern. The shape of the radiation pattern (or beam pattern) portrays a region where the radiation is maximum called a main lobe (or main beam), and typically a region where the radiation is less than that contained in the main lobe called a minor lobe. A minor lobe may be a side lobe or a back lobe, depending on the direction in which it is radiating with respect to the main beam. Depending on the particular application, one of the goals of antenna design is to minimize the intensity of the minor lobes. This is because the minor lobes tend to interfere with other transmissions and because energy is lost in the side lobes. Again, in some applications, energy lost in the side lobes may be a significant factor.

The beam pattern also has a beam width. The beam width is the width of the main lobe between the two directions at which the relative radiated power is equal to one half of the value at the peak of the main lobe.

The beam width is typically measured in degrees.

All antennas radiate more strongly in one direction than in other directions. This property is called directivity. Directivity is the ratio of the radiation field strength of the antenna under consideration to the radiation field strength of an (imaginary) lossless, isotropic antenna.

Antennas also exhibit a property called gain. The gain of a conventional antenna is primarily dependent on the size of the antenna.

Typically, the more gain desired, the larger the antenna must be. The gain of an antenna is also related to the directivity of a particular antenna.

Typically, an increase in the directivity of the antenna sharpens the main lobe and increases the gain of the antenna in the direction of the main lobe.

This is because as power is taken away from one portion of the radiation pattern, it is added to the remaining portion of the radiation pattern. This redistribution of power can increase the directivity of the antenna and increase the gain of the antenna in the direction of the main lobe.

The gain of an antenna is measured in decibels. A decibel is expressed as: dB = 10 log (Pl/P2), where Pi is typically the power in the launched electromagnetic wave and P2 is typically the power produced by the incident current.

Another important physical property of an electromagnetic wave is its polarization. Polarization is a quantity describing the orientation of the electric field with respect to the direction of maximum gain. For helical antennas, the diameter, pitch, and number of turns of the helix are factors that can be used to control the polarization state and directivity of helical antennas.

In x, y, z space, the wave, traveling in the x direction, can be described in terms of an x-axis and a y-axis. As the wave travels along the propagation direction, the electric field rotation traces an ellipse as it rotates from its x- axis to its y-axis, to its (-x)-axis to its (-y)-axis, and back around to its x-axis.

The resultant trace is called a polarization ellipse. The ratio of the major axis of rotation to the minor axis of rotation is called the axial ratio.

A wave is said to have linear polarization when the electric field along one axis equals zero. For example, a wave is linearly polarized in the y direction when there is no electric field along the x-axis. In this case the axial ratio is infinity.

A wave is said to have circular polarization when the electric field along the x-axis is equal to the electric field along the y-axis. Ideally, the axial ratio of a circularly-polarized wave is one. That is, an ideal antenna that is circularly polarized has an axial ratio of one. In practice, however, the ideal axial ratio of one is rarely achieved. The true axial ratio of a circularly polarized wave is expressed in dB, where an axial ratio of 0 dB represents an axial ratio of one.

The actual performance of an antenna for launching a circularly polarized wave is referred to as"antenna performance" (AP). The antenna performance of a good antenna generally falls within the range: OdB<AP<3dB An antenna performance of greater than 3 dB is considered poor performance.

The performance of an antenna is commonly described in terms of the above described parameters such as radiation pattern, gain, and

polarization. While the performance of the antenna described herein refers to an antenna as a radiator of energy, the performance applies to an antenna as a receiver of energy as well.

These and other fundamental principles of antennas are well known and are described further in John D. Kraus, Antennas, 17-81 (1988), incorporated herein in its entirety by reference. These principles are described to provide the reader with a basic understanding of antenna theory in general, and should not be construed to limit the scope of the invention.

These and other fundamental principles are also described below with respect to a conventional helical antenna and the modified helical antenna.

Example Environment In a broad sense, the invention can be implemented in any system for which helical antenna technology can be utilized. One example of such an environment is a communication system in which users having fixed, mobile or portable communication devices (such as, for example, telephones) communicate with other parties through a wireless communication link. In this example environment, the communication device utilizes an antenna tuned to the frequency of the communication link.

Another example environment in which the present invention may be implemented is a wireless tracking system in which wireless data terminals are used to exchange information between system users. The information, which can include messages, tracking information, and other information, is transmitted between the wireless data terminals through a communication link. A typical system might include a vehicle message system in which signals are transferred between a central station or hub facility and a vehicle having a communication transceiver.

The present invention is described in terms of these example environments. Description in these terms is provided for convenience only. It is not intended that the invention be limited to application in these example environments. In fact, after reading the following description, it will become apparent to a person skilled in the relevant art how to implement the invention in alternative environments.

Conventional Helical Antennas As described above, an antenna is designed to radiate electromagnetic fields as efficiently and effectively as practical. To ensure efficient and

effective radiation, the specific design of an antenna is dependent upon the particular application. For example, with satellite communication systems, helical antennas enjoy widespread use. Before describing the invention in detail, it is useful to describe the characteristics of some conventional helical antennas.

A conventional helical antenna is an antenna that has one or more radiators that are shaped in a helical fashion. The conventional helical antenna is typically used where circular polarization is desired.

FIG. 2 illustrates one example of a conventional helical antenna 200.

The helical antenna 200 illustrated in FIG. 2 includes a helix 202, a ground plane 204, a feed point 206, end points 208,210, height 209, and a connector 212. Helix 202 is electrically connected to ground plane 204 through feed point 206 using connector 212. Helical antenna 200 has a signal applied to it through feed point 206.

Conventional helical antennas commonly radiate in what is termed the"axial mode."In the axial mode, radiation from the antenna is directed primarily along the axis of the helix. If the circumference of the helix is approximately one wavelength of the incident electromagnetic wave, then the antenna is said to radiate in the axial mode. A typical axial mode helical antenna radiates in the axial mode when 0.75R < C < 1.33k, where C is the circumference of the helix, and X is the wavelength of the incident radiation. A conventional antenna radiating in the axial mode is efficient and has a broad bandwidth. That is, there are considerable operating frequencies at which the antenna radiates with minimal attenuation. Characteristics of helical antennas in general, and helical antennas operating in the axial mode are well known, and described further in John D. Kraus, Antennas, chapter 7 (1988), which is incorporated herein in its entirety by reference, and in Hisamatsu Nakano, Helical and Spiral Antennas-A Numerical Approach, chapter 6 (1987), which is incorporated herein in its entirety by reference.

FIG. 3A provides a more detailed representation of helical antenna 200. Helix 202 includes a diameter 302, a circumference 304, and a pitch angle 306. Helix 202 is represented in terms of an x-axis 308, a y-axis 310, and a z-axis 312. Pitch angle 306 is typically represented by a, where a = arctan S/C = arctan"D, and where S is the distance 330 between the turns in the

helix 202, C is the circumference 304 of helix 202, and D is the diameter 302 of helix 202.

Helical antenna 200 has a current 316 applied to it through feed point 206. The length of one turn in helix 202 (or circumference 304) is expressed in wavelengths (or fractions or multiples thereof). Diameter 302 and circumference 304 of helix 202 refer to the cylinder (not shown) around which helix 202 is wound. The distance between the turns in helix 202 is distance 314.

The cylinder around which helix 202 is wound can be real or imaginary. That is, helix 202 is either supported by or wound around an actual cylinder or cylindrical form, or it can be made of wires or other materials that are sufficiently stiff to be self-supporting such that an actual cylindrical form is not required for support. In one example, the radiators are conductive strips etched or otherwise deposited onto a flexible substrate and the substrate is formed into a cylinder. The substrate can be a known dielectric material on which radiators are formed using known deposition or etching techniques.

FIG. 3B illustrates a conventional axial mode helical antenna power pattern 300 depicted using polar coordinates. The antenna power pattern includes a main lobe 318, a main lobe axis 320, a plurality of minor lobes 324, 326, a half-power beam width 328, and a pitch angle 306. Pitch angle 306 can be determined by considering the helix unwound on a flat surface.

FIG. 3C illustrates a conventional axial mode helical antenna power pattern 300 depicted using rectangular coordinates and a logarithmic (or dB) scale. Point 330 and point 332 mark the half-power beam width for the main lobe.

FIG. 4 is a graphical representation of a current distribution curve 400 for a single radiator radiating in the axial mode as determined numerically.

The vertical of current distribution curve 400 represents the current density at a given location along the length of helical antenna 200. The horizontal axis represents the distance from ground plane 204 in mils.

Current density curve 400 shows that current density decreases as the distance from ground plane 204 increases. Current distribution curve 400 includes a decay wave region 402 and a surface wave region 404. Decay wave region 402 represents the decay wave current of helical antenna 200. The decay wave behaves like an antenna exciter. Surface wave region 404 represents a surface wave current component of helical antenna 200. The surface wave behaves like an antenna director.

As is illustrated in FIG. 4, the surface current on the radiator (or helix or conductor) is initially dictated by the decay wave current component of helical antenna 200. In surface wave region 404, the current in the helix is traveling with a relatively constant amplitude.

Point 406 on current distribution curve 400 represents the point at which the current in the antenna transitions from being attributable primarily to the decay wave to being attributable primarily to the current from the surface wave. The position of transition point 406 varies as the frequency of the incident electromagnetic wave varies. Nonetheless, transition point 406 is usually located at about 1.5 to 2 turns along helix 202 from feed point 206.

A helical antenna that radiates in the axial mode, such as helical antenna 200, typically radiates circularly polarized electromagnetic waves.

Diameter 302, pitch angle 306, and number of turns on the helix in relation to the wavelength of the electromagnetic wave provide control of the polarization state and directivity of helical antenna 200. It is well known that if circumference 304 of helix 202 is approximately equal to the wavelength of the electromagnetic waves in free space, and pitch angle 306 is approximately twelve degrees, then surface current 316 in helical antenna 200 is a traveling wave moving circumferentially and outward along helix 202 from feed point 206. A standing wave exists at end points 208 and 210 and the voltage standing wave ratio is minimum along the center portion of helix 202. The fact that the voltage standing wave ratio is minimum along the center portion of helix 202 indicates that the majority of surface current 316 along the center portion of helix 202 results from a traveling wave.

FIG. 5 is a graphical representation of a beam pattern 500 for a conventional monofilar antenna radiating in the axial mode as determined using a numerical analysis software application. Beam pattern 500 includes two simulated beam patterns 502 and 504 resulting from an electromagnetic wave frequency of 12.75 GHz and an input current density of 13.7484 A/m.

Simulated beam pattern 502 has a half-power beam width of approximately 52°, for example. Simulated beam patterns 502 and 504 approximate the empirically determined beam patterns 600 and 602 of FIG. 6A and FIG. 6B, respectively.

Empirical beam pattern 600 depicts a conventional axial mode helical antenna with a side lobe power level of-9 dB. The half-power beam width of the conventional mode helical antenna is measured at 51.58°. The empirical beam pattern 602 also depicts a conventional axial mode helical

antenna with a side lobe power level of-9 dB. The half-power beam width is measured at 51.48°.

The present invention is sometimes described herein in the context of a monofilar helical antenna. That is, the present invention is described as a modified helical antenna with a single helical radiator. It should be understood, however, that the present invention is not limited to this embodiment and can be implemented with any bi-filar, quadrifilar, or other x-filar helical antenna.

Modified Helical Antenna FIG. 7A is a diagram illustrating a modified helical antenna 700 built and operating according to one embodiment of the invention. According to the embodiment illustrated, modified helical antenna 700 is comprised of a helix 716 partially surrounded by a conducting structure 702 along a predetermined length near the base. According to the embodiment illustrated in FIG. 7A, antenna 700 is a monofilar antenna (i. e., a single- radiator helix). The embodiment illustrated also includes a ground plane 712 and an electrical connector 714.

In a preferred embodiment, helix 716 is cylindrical in shape. The invention is described in terms of this cylindrical embodiment and, as such, conducting structure 702 is referred to as a"conducting cylinder 702."As is the case with conventional helical antennas, helix 716 can be conical or some other appropriate shape as well. After reading the description, it will become apparent to a person skilled in the relevant art that in the preferred as well as the alternative embodiments, the conducting structure 702 may be of cylindrical or other appropriate shape.

Helix 716 is described in terms of a helix diameter 704 and a helix length 706. Conducting cylinder 702 is described in terms of a cylinder height 708, and a cylinder diameter 710.

According to one embodiment of the invention, modified helical antenna 700 is comprised of a conventional helical antenna 700 in combination with a conducting structure 702. Conducting structure 702 is coupled to ground plane 712 and closely coupled to helix 716. By closely coupling conducting cylinder 702 to helix 716, the beam of the antenna can be reshaped, or even customized for a particular application. By placing conducting structure 702 at the base of helix 716, the side lobes are reduced and the main lobe is narrowed and its gain increased (i. e., the antenna has

more directivity). The transfer of power from the side lobes to the main lobe increases the gain of the antenna in the preferred direction.

Conducting structure 702 is generally cylindrical in shape, and can be considered a tuning element, and can also be referred to as a choke, or as an energy direction or re-direction element. By adjusting a height 708 and an inner dimension (width) or diameter 710 of conducting structure or cylinder 702, beam characteristics may be tailored to suit a particular purpose. In this manner, beam characteristics can be tuned (or customized) using the conducting cylinder without having to alter the helix. By adjusting height 708 and/or diameter 710 of conducting cylinder 702, the main lobe can be widened or narrowed, and the side lobes increased or nearly eliminated for a particular application. The affect on the beam shape can be determined either empirically or utilizing numerical analysis.

Inner width or diameter 710 of conducting cylinder 702 is preferably determined by the input impedance and axial ratio for which the antenna is being designed. Alternatively, for a constant height 706 of conducting cylinder 702 roughly corresponding to transition point 406, diameter 710 of the conducting cylinder is varied to widen or narrow the main beam and the side lobes.

Conducting structure 702 can be manufactured from electrically conductive materials such as copper, silver, aluminum, brass, steel, etc. The conductive material also may be soldered or otherwise fixed to ground plane 712. Conducting structure 702 may also utilize conductive (e. g., impregnated) plastics or a carbon composite type material, or conductive material deposited on a flexible substrate rolled to form the cylindrical shape. The plastic or carbon composite material can be bonded in place using known adhesive compounds or conductive epoxies.

Conducting structure 702 is generally cylindrical in shape with an elliptical or circular cross-section, but can be manufactured with slightly rectangular, square, or other cross-sections as desired which support appropriate electrical currents to redirect signal energy, as discussed below.

The conducting structure or cylinder also need not be a solid material, although this is preferred for well behaved current circulation, but could be formed as a conductive mesh, or have occasional passages extending horizontally along its circumference. While a small vertical gap may be possible, the impact on current circulation and energy transfer is not believed to be appropriate for efficient coupling to the main lobe, as discussed below. In any configuration, it is believed that openings or

passages should be maintained with dimensions smaller than about one quarter of the wavelength of interest in order to prevent undesirable energy loss or coupling.

In order to make the height of conductive structure or cylinder 702 adjustable it can be mounted using one of several know techniques. For example, a cylindrical extension projecting upward from the ground plan can have a threaded surface which mates with a matching threaded interior surface on conductive cylinder 702. Alternatively, or in addition, a set screw can be used to interact with such a the projection to secure the conductive cylinder in place at a desired height.

In a preferred embodiment, the conductive cylinder is made by forming plastic into a cylindrical shape using injection molding techniques.

A conductive material is then deposited on the surface of the plastic cylinder. This is illustrated in the side view of FIG. 7B, where a modified helical antenna 720 has a conductive cylinder or choke 722 surrounding a lower portion of the base of helix 716. The ground plane 724 and inner portion 726 of conductive cylinder 722, are formed from a plastic material, such as by molding. This material, or structure, is then coated with a conductive material 728. The use of plastics and injection molding techniques makes the structure light, cost effective, and easy to implement.

The height may be adjusted using known etching techniques. With injection molding techniques a unified ground plane and conductive cylinder structure can be implemented.

Alternatively, the conducting cylinder may be manufactured from a material such as copper, brass, silver, steel, aluminum, conductive plastics, conductive composites or other conductive material, and then mounted on ground plane 712. In yet another embodiment, conductive material is deposited on a flexible substrate and the substrate rolled to form the conducting cylinder.

In yet another alternative, the conducting cylinder may be manufactured using wire wound in a cylindrical shape. In this case the pitch angle is very small in order to achieve the shorting property for the helix at the wavelength of interest necessary to mimic the behavior of a conductive cylinder. This is illustrated in the side view of FIG. 7C, where a modified helical antenna 730 has a conductive coil or choke 732 surrounding a lower portion of the base of helix 716. Conductive coil 732 has an appropriately small pitch angle to interact with the energy from the side lobes for helix 716 in the same manner as a solid conductive cylinder.

Alternatively, conducting cylinder 702 may also be made, in some applications, to fit another (longer) wavelength and act as a second conductive helix, such as when connected to a connector 734. The result is a dual band antenna assembly with a greatly improved inner antenna.

The beam pattern of helical antenna 700 is shaped and energy is directed away from the side lobes by placing conducting cylinder 702 at one end of helix 716 generally around or along the portion of helix 716 where the helical antenna 700 surface current is determined primarily by the decay wave of the helical antenna 700.

In one embodiment, height 708 of conducting cylinder 702 is optimal when the top edge of conducting cylinder 702 is at roughly the same location as transition point 406 of the antenna is on helix 716. In this embodiment, conducting cylinder 702 is placed around the portion of helix 710 where the decay wave contributes to the radiation pattern. Typically, at this height, the cylinder will have a diameter that is near six percent of the wavelength of the incident electromagnetic wave. In being so placed, conducting cylinder 702 acts as a choke and minimizes the amount of energy radiated into space from the decay wave. In essence, conducting cylinder 702 compensates for the fact that the decay wave behaves as an exciter that parasitically directs energy away from the main lobe. Conducting cylinder 702 redirects that energy into the main lobe.

In another embodiment height 708 of conducting cylinder 702 is lower than transition point 406. In this embodiment, modified helical antenna 700 has a wider main beam.

Because transition point 406 varies as the frequency of the incident electromagnetic wave varies, the optimal height of conducting cylinder 702 also varies with the frequency of the incident electromagnetic wave. To allow adjustment in the field, either or both the length 706 of the helix 716 and the height of conducting cylinder 702 may be made adjustable. In one embodiment, length 706 of helix 716 can be adjusted by turning helix 716 about an axis and feeding the lower portion through a hole in ground plane 712 (i. e., by effectively screwing helix 716 into or out of ground plane 712).

Additionally, the length 706 of the helix 716 also may be made adjustable in a dynamic sense.

In an alternate embodiment, height 708 of conducting cylinder 702 remains constant, preferably at transition point 406, and diameter 710 of conducting cylinder 702 is increased or decreased. Varying diameter 710 of conducting cylinder 702 may alter the axial ratio of the antenna. For

example, for a constant height 708 of conducting cylinder 702, diameter 710 of conducting cylinder 702 may be decreased in order to alter the axial ratio of the antenna. As the diameter 710 narrows, the axial ratio increases, and the polarization becomes more elliptical. In order to maintain circular polarization, however, the axial ratio should remain between two and three dB, or better.

In still another embodiment, the goal is to reduce the side lobes, but not necessarily to narrow the beam width. In this embodiment, diameter 710 of conducting cylinder 702 is larger than an optimal diameter. The greater diameter 710 is, the less effect conducting cylinder 702 has on the axial ratio. In other words, the axial ratio remains constant at between two or three dB, or better.

FIG. 8 is a graphical representation of a beam pattern 800 for a conventional single helix (or conductor or radiator) 700 radiating in the axial mode as determined using a numerical analysis software application. Beam pattern 800 includes two simulated beam patterns 802 and 804 resulting from an electromagnetic wave frequency of 12.75 GHz and an input current density of 13. 7484 A/m. Both simulated beam patterns 802 and 804 have half-power beam widths of much less than 52°, which is the beam width of conventional helical antenna 700. Moreover, the side lobes of both beam patterns 802 and 804 demonstrate a simulated value of less than-15 dB.

Simulated beam patterns 802 and 804 approximate the below-described empirically determined beam patterns 900 and 950 of FIG. 9.

Empirical beam pattern 900 depicts modified helical antenna 700 of the present invention with a side lobe power level of-14 dB. The half- power beam width of the conventional mode helical antenna is measured at 48.19°. This represents a measured increase in antenna power of about 1 dB with a theoretical improvement of about 1.5 dB over conventional helical antenna 700.

The dimensions of conducting cylinder 702 can be readily determined by a person skilled in the art utilizing either numerical analysis or empirical analysis. Unless otherwise specified, the dimensions of the modified helical antenna are measured in mils, in which one mil equals one thousandth of an inch. In a preferred embodiment, results are verified utilizing both numerical and empirical analyses.

In a preferred embodiment, diameter 704 of helix 710 is 260 mil, length 706 of helix 710 is 1.5 inches, diameter 710 of conducting cylinder 702 is 380 mil, height 708 of conducting cylinder 702 is 400 mil, the area of

ground plane 712 is 4 square inches (or two inches by two inches), and electrical connector 212 is an SMA connector. Moreover, in the preferred embodiment, the frequency of the electromagnetic wave is within one of the frequency bands currently used for domestic fixed-satellite commercial earth stations, such as Ku band. The Ku band uses the 11.7-12.2 GHz region down link and the 14.0-14.5 GHz region up-link. In alternative embodiments, each of these parameters can be altered to suit the application.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What we claim as the invention is: