US RE40129 E1
A wideband multi-mode antenna having low VSWR operating characteristics. The antenna is has a shape similar to a helical antenna, but is formed from a right-triangularly shaped piece of conductive material. The result is a rolled planar antenna having a height and diameter predetermined to provide optimum VSWR for a given frequency range.
1. A wideband multi-mode antenna, comprising:
an antenna element made from a single right triangularly shaped sheet of conductive material, the material having a height and a base dimension;
wherein the conductive material has a rolled shape, such that the antenna has the height of the conductive material, a number of turns having spacing between them, and a base diameter, and a pointed tip .
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18. A diopole type antenna, comprising:
two antenna elements, each made from a single right triangularly shaped sheet of conductive material, having a height and a base dimension;
wherein the conductive material has a rolled shape, such that the antenna has the height of the conductive material, a number of turns having spacing between them, and a base diameter, and a pointed tip ;
wherein the antenna elements are connected to form a dipole.
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21. A method of manufacturing an antenna, comprising the steps of:
forming a right-triangularly shaped sheet of conductive material, having a height and a base dimension; and
rolling the material along the height dimension, to form the antenna such that the antenna has the height of the conductive material, a number of turns having spacing between them, and a base diameter, and a pointed tip .
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34. A wideband multi-mode antenna, comprising:
a substantially triangular sheet of conductive material, rolled such that the material has one or more turns;
wherein the antenna has a height along the axis of the turns and a diameter determined by the outside surface of the turns; and
wherein the turns have spacing between them.
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51. A method of manufacturing an antenna, comprising the steps of:
rolling a sheet of generally triangular material, thereby forming a rolled shape having a height along the axis of the rolled shape, a diameter around the outer surface of the rolled shape, and one or more turns having spacing between them.
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This invention relates to antennas, and more particularly to an antenna based on a tapered helix configuration and having low VSWR over a wide bandwidth, and multi-mode operation.
Most antennas are capable of efficient operation over only a limited range of frequencies. For efficient energy radiation or reception, conventional antenna designs calls for antenna dimensions that are on the same order as the operating wavelength (λ). Efficient antenna operation requires not only efficient radiation or reception, the antenna must also be matched to the specific source or load for maximum energy transfer.
Antenna match quality is determined by the voltage standing wave ratio (VSWR) of the antenna at each frequency of interest. A perfect match requires a one-to-one ratio at all frequencies.
For broadband applications, there are several types of antennas that can be designed to provide a low VSWR over wide frequency ranges. Some of these antennas are inherently directional such as the conical spiral and log periodic. Others are omni-directional such as the bicone and tapered blade antennas.
Although there are a number of antenna configurations that can provide a low VSWR over a wide band, most have some limitations that make them unacceptable for many applications. For example, a log periodic antenna can easily be designed to provide low VSWR over several frequency octaves. But, the log periodic phase center, the effective radiating point of the antenna, varies with frequency and a log periodic is physically quite large compared to the wavelength of its lowest operating frequency. A bicone antenna or its monopole is capable of providing low VSWR over a wide bandwidth, but occupies a large volume compared to narrow band antennas having the same low end operating frequency. Generally, wideband antennas are difficult to design for low VSWR over more than an octave frequency range and are generally much larger than a wavelength at their lowest usable frequency.
Antenna, whether broadband or not, are not required to have low VSWR; many conventional antenna designs typically exhibit a high VSWR (>3:1). However, a high VSWR adversely affects efficiency, unless some form of compensatory matching network is used. But matching networks create new problems—most matching networks are not broadband and they tend to decrease the power available for transmission. For high power transmitters, matching networks must often be designed with electro-mechanical tuning elements. Such designs are costly and make automation of the matching function must slower than is possible with lower-power solid state tuning elements. In general, impedance matching to achieve a low VSWR is relatively easy for narrow band antennas (<10% of center frequency), but more difficult for wideband antennas (>20% of center frequency).
One aspect of the invention is a wideband multi-mode antenna. In its simplest form, the antenna is made from a single right triangularly shaped sheet of conductive material, having a height and a base dimension. The planar material is rolled, such that the antenna has the height of the planar material, a number of turns having spacing between them, a base diameter, and a pointed tip.
An advantage of the invention is that the antenna is compact, low cost, and easily manufactured. It provides a low VSWR that can be easily matched over a wide range of frequencies. With these features, the antenna has ready application to many existing systems as well as to new systems now under development for the wireless market. The potential for small size and wide bandwidth make the antenna especially useful for mobile communications and multi-mode radios.
Overview of Structure and Operating Characteristics
As illustrated in
Variations of antenna 10 may be constructed with triangularly shaped material, where the hypotenuse is curved (concave or convex) rather than straight. Mesh or grid material, having fine mesh or grid spacing has been shown to have equal performance especially when weight or wind loading are factors.
As further explained below, antenna 10 is electrically small. Its largest dimension (its height) is less than a wavelength at the lowest useful frequency (the frequency at which the VSWR rapidly improves). Dimension Y is typically about 0.2 to 0.24 λ at the lowest useful frequency.
Dimension D is typically about 0.02 λ at that frequency. Although in cross section, antenna 10 is not a true circle and tapers from base to tip, its diameter, D, is taken at the base and is approximated by twice the radius from the center to the outermost point on the antenna.
In operation, antenna 10 may be configured as a monopole and mounted above a conductive ground plane. However, antenna 10 may also be used as elements of other configurations, such as dipole antennas or antenna arrays.
For performance evaluation purposes, antenna 10 may also be compared to a “fat” monopole, or a flat planar surface equivalent to an unrolled monopole. These configurations represent examples of rolled and unrolled limiting configurations of antenna 10. For example, a fat monopole approximates antenna 10 as the spacing between turns decreases to zero and the number of turns increases for a given base dimension.
Antenna Modeling for VSWR Characteristics
For computer-aided modeling of antenna 10, a suitable method is to use approximations to the antenna current distribution. This may be accomplished by using finite element methods, which break the planar surface into a large number of mathematically definable segments that approximate the surface. There are a number of methods for generating and solving a large number of segments.
Among the more popular computer-aided modeling methods is the method of moments (MoM). One example of a specific modeling tools using the MoM is GNEC-4, which is a Microsoft Windows compatible version of the basic NEC (Navel Electric Code) software. Analysis of Wire Antennas and Scatters (AWAS) is another example of MoM antenna simulation software.
Current distribution simulations demonstrate how the current distribution for each configuration changes as a function of frequency. For example, antenna 10, as compared to a “fat” monopole or unrolled version, has a better current distribution across a wider range of frequencies.
A feature of all configurations of antenna 10, such as those of
In addition to VSWR simulations, antenna pattern simulations provide insight into the expected radiation pattern of antenna 10.
Simulated and Experimental VSWR Data
An attempt was made to establish a rough numerical correlation by simplifying the data. Thus,
The fact that it is possible to build antennas with such good correlation to simulation implies that it is relatively insensitive to small variation in physical dimensions (<1%). This is remarkable, because for a typical conical spiral antenna, a change of only 0.01 λ in the separation of spiral arms makes an essential difference in the observed RF current distribution.
Although VSWR performance of antenna 10, in its various configurations, is insensitive to small irregularities, there appear to be three critical physical parameters. These are the height to base ratio, the feed point height above the ground plane, and the turns spacing. Of these three parameters, only feed height above the ground plane may be difficult to control in field situations. The other two parameters are primarily a function of the antenna's fabrication accuracy. Turns spacing, as well as the number of turns, are functions of the unrolled surface horizontal dimension. Referring again to
The following are design guidelines for monopole antennas. The guidelines are applicable to various configurations of antenna 10, such as the examples of
For a monopole antenna having minimum size, the following are design parameters for providing low VSWR over an octave bandwidth:
The design guidelines set out above have been especially developed for a single octave antenna. However, with appropriate testing, wider bandwidths may be achieved. Notably, for certain values of K as the ratio of height to base decreases, the VSWR bandwidth widens to more than two octaves for VSWR less than about 3:1 with a 50 ohm reference.
With respect to feed points, different feed points along the spiral base change not only the VSWR bandwidth of the first resonance to second resonance of a monopole, but also the second resonance and higher resonance regions. With VSWR control techniques, such as shaping the hypotenuse of the planar surface, it may be possible to design a very broadband antenna or antenna with an useful frequency stop band (high VSWR region). Use of tunable feed points may yield a broadband antenna with a built-in tunable stop band filter. This could serve the interference mitigation purpose in the frequency domain that null-steering serves in the spiral domain.
Specific Design Parameters for 225 to 400 MHz Antenna
Unrolled planar material used to form the antenna was cut as a right triangle with the height of the vertical axis Y determined by the low frequency f of the desired bandwidth. Specifically, the vertical axis length Y is determined by Y=0.86(c/4f).For an f of 225 MHz the value for Y was 0.287 meters.
The base dimension X of the right triangle is determined by X=Y/K, where K can range from 1.3 to 1.7. Although less than a 3:1 VSWR octave bandwidth was desired, a value of K=1.6 was selected for mechanical design reasons.
Antenna 120 is mounted on a metal plate 126, which provides a ground plane. The ground plane to antenna spacing S ranges between S=Y/80 to Y/30. A good compromise value for S in a 50 Ohm system would be: S=Y/50. For example, a spacing S could be set at 0.0036 meters for an overall height H of the monopole above the ground plane of 0.293 meters.
The conductive material spacing was held equal as the material was rolled to produce an antenna with maximum (at the base) spiral diameter dimension of D=of H/10. In other words, the height to diameter ratio is 10:1. The number of turns is set by the H dimension and the value selected for K.
The feed point for maximum low VSWR bandwidth (one octave above the first resonance) is the innermost point of the base spiral.
The thin copper material selected for the antenna may be protected or supported by some form of mechanical support to produce a physically rugged antenna. In the example of
For a lower frequency range, a different construction technique could be more appropriate. For lower frequencies requiring larger antenna elements it would be possible to form the antenna surface from metal mesh supported by a rigid rod along the inside edge. Antennas of this type should be suitable for fixed locations. Wind loading could also be minimized by use of metal mesh.
For higher frequencies, the size of the antenna should be small enough that thin material (<0.2 mm thick) should provide enough rigidity so that foam potting would not be necessary.
As indicated in
Moreover, antenna 120 would not normally require a matching network. Essentially, it incorporates a matching network into the antenna configuration. The additional inductance and capacitance of the configuration is distributed in a manner that changes the current distribution on the antenna as a function of frequency. The change in current flow is complex and significantly different than current on a simple linear monopole of the same size. This change in distribution appears to provide additional VSWR bandwidth beyond what would be possible by simple impedance transformation matching networks applied to a conventional monopole.
In general, the above observations are true of the various configurations of antenna 10.
A fundamental difference between antenna 10 and conventional planar helical antennas is that the surface of antenna 10 has both a linear surface component along the antenna axis and a helical component around the antenna axis. This allows both linear and helical current to flow on the same structure. Planar and tapered surface helical antennas have only helical surfaces (no linear current flow), so current only follows a helical path.
Antenna 10 does not act like a large number of monopoles connected in parallel. It does provide multiple sheet current paths for signals applied at one point at its base. One dominant path is the straight path produced by the quarter wave resonance due to the vertical edge of the inside turn. Another dominant path is the resonant path produced by the outside spiral edge of the helix. When the feed point at the base is moved along the bottom spiral, an optimum VSWR point can be reached for an antenna having a given number of turns.
In simulations, there are similarities between the impedance of the “fat” monopole and unrolled versions of antenna 10 in monopole form. Specifically, except for much lower values of real and imaginary impedance both have the same form although the imaginary impedance of the unrolled version is more inductive (positive reactance). In contrast, for antenna 10 the real and imaginary impedance curves have an additional resonance. This additional resonance is apparently responsible for the partial cancellation of the normal monopole capacitive reactance while decreasing the value of the real impedance thus producing a wide VSWR bandwidth.
More Complex Configurations
Antenna arrays using monopoles or dipoles (yagi, planar array, corner reflector, etc.) as their fundamental elements may benefit by replacing conventional dipole/monopole elements with elements having the configuration of antenna 10.
As a fundamental antenna element, such as a loop or dipole, antenna 10 may be configured to form arrays. The resulting array provides a wider bandwidth than arrays comprised of conventional elements. When so mounted, VSWR may be easily referenced to a 50 ohm impedance system.
Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.