Patent Application
20120026049
1. Statement of the Technical Field
The inventive arrangements relate to antennas, and more particularly to antennas having very wide impedance bandwidth and pattern bandwidth.
2. Description of the Related Art
For design purposes, an antenna can be thought of as having two ports. During transmission, the first port (which is commonly referred to as the antenna feed port) allows energy to flow from a transmission line into the antenna. The transmission line will have some characteristic impedance, such as 50Ω. The second port can be envisioned as allowing energy to flow from the antenna into free space. Free space has a characteristic impedance of 120/πΩ.
An antenna is sometimes loosely described in terms of its “bandwidth.” Bandwidth usually refers to the range of operating frequencies over which the antenna is designed to provide some level of satisfactory performance. However, there are actually at least three different performance characteristics that define an antenna's useful operating frequency range. Specifically, antenna performance is appropriately considered with regard to input impedance, gain and radiation pattern, each of which can serve to limit the useful bandwidth or operating frequency range of the antenna. For example, it is usually desirable for antenna input impedance to be maintained within a desired range so that the input VSWR for the antenna is less than about 3:1. The useful bandwidth of an antenna can be limited when the input impedance is outside of such desired range. Likewise, an antenna may not have sufficient gain outside a certain operating frequency range and this factor can limit the useable bandwidth of the antenna. Finally, an antenna can exhibit an undesirable radiation pattern at certain frequencies, and this too can limit the useful bandwidth of the antenna.
Inductors have long been used within antennas to make the antenna appear electrically longer, or stated another way, to make a physically short antenna (<0.5λ) appear electrically resonant. The ideal inductor placement and size is a function of practicality versus functionality. The most efficient implementation of such antennas is an infinitely large inductor at the element end opposed from the feed. This results in a uniform current distribution over the entire length of the antenna element. Conversely, the most practical implementation is generally a small inductor placed near the feed system. This results in a uniform current distribution from the feed to the inductor and then triangular distribution to the element end. Between these limiting cases, the inductor size grows as the inductor is placed further away from the feed point. A trade-off analysis can be performed to determine the most advantageous inductor size and placement to make a short antenna appear electrically resonant in a particular application.
The invention concerns antennas and methods for controlling antennas for producing electromagnetic radiation in a desired direction over a wide range of wavelengths. A current distribution is controlled in one or more conductive radiating elements of an antenna to form, at every wavelength or fractional wavelength within a pattern wavelength range, an antenna radiation pattern having a peak in a direction substantially orthogonal to a length of an elongated conductive radiating element or elements. The pattern wavelength range is exceptionally broad. In the case of a dipole antenna variant, the pattern wavelength range from about ⅓l to at least about 8l, where l is an approximate combined length of a pair of elongated elements forming a dipole antenna. Alternatively, in the case of a monopole antenna variant, the pattern wavelength range can extend from about ⅙l to at least about 4l, where l is an approximate overall length of the monopole antenna.
According to one aspect, the invention includes a dipole or monopole antenna configured for producing electromagnetic radiation in a desired direction over a wide range of wavelengths as described above. The antenna radiation pattern peak has a direction substantially orthogonal to a length of an elongated conductive radiating element of the antenna. This radiation pattern peak is provided in the orthogonal direction over all wavelengths within a pattern wavelength range as defined above.
According to another aspect, the invention includes a radio system. The radio system includes a dipole or monopole antenna excited by a transmitter and configured for producing electromagnetic radiation in a desired direction over a wide range of wavelengths as described above. The antenna is configured for producing an antenna radiation pattern peak in a direction substantially orthogonal to a length of an elongated conductive radiating element of the antenna over all wavelengths within a pattern wavelength range as defined above.
Embodiments will be described with reference to the following drawing figures, in which like numerals represent like items throughout the figures, and in which:
The invention is described with reference to the attached figures. The figures are not drawn to scale and they are provided merely to illustrate the instant invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One having ordinary skill in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operation are not shown in detail to avoid obscuring the invention. The invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the invention.
Antenna performance is evaluated with respect to input impedance, gain and radiation pattern, each of which can serve to limit the useful bandwidth (or useful operating frequency range) of the antenna. Typically, the narrowest bandwidth (when comparing bandwidth limits associated with input impedance, gain and radiation pattern) is the input impedance bandwidth. This bandwidth limit is due to the impedance mismatch as between the transmission line which feeds the antenna and the antenna's input impedance. A matching network is commonly used to increase the input impedance bandwidth which allows energy to get into the antenna. Having resolved that issue with an impedance matching network, the next issues an antenna designer must address are the gain and/or pattern wavelength range.
As used herein, the phrase “pattern wavelength range” shall refer to the range of input signal wavelengths over which the antenna will provide an antenna radiation pattern that meets certain predefined performance criteria. For example, in many situations involving dipole and monopole antennas, it is desirable to provide a substantial amount of gain in a direction orthogonal to the length of the antenna. As input signal wavelength decreases (frequency increase), the direction of maximum gain will deviate from this orthogonal direction and a null will begin to form in the orthogonal direction. The antenna will no longer satisfy performance criteria when the direction of maximum antenna gain deviates from the orthogonal by some predetermined amount determined by a designer. The radiation pattern and gain values are controlled by the physical conductor geometry, and more specifically the current distribution on the conductor.
Wideband antennas present special design challenges with regard to pattern wavelength range. In order to more fully understand the problem of pattern wavelength range, it is useful to consider how the pattern of a fixed length dipole antenna will change with variations in frequency. An antenna of fixed physical size or length will appear electrically larger as the frequency is increased. In other words, since a wavelength decreases in size as the frequency increases, a fixed physical length antenna will naturally increase in electrical length as frequency is increased. This occurs because electrical length is measured in terms of wavelengths.
As noted above, the pattern wavelength range is generally defined by the range of wavelengths or frequencies over which the antenna's radiation pattern main beam or lobe extends in a desired direction. The radiation pattern of a dipole begins to distort when the antenna length goes beyond 1.0λ, where λ is the symbol for wavelength. As the length approaches 2λ, a compete cancellation of energy occurs in the direction orthogonal to the length of the antenna. For example, a vertical dipole having a length of 2λ will have a null it its antenna beam in the direction of the horizon. This is highly undesirable for many antenna applications.
Note that the foregoing problem can also be restated by defining the input wavelength in terms of the antenna length. Thus, in the foregoing example for an antenna of length “l” it could be said that the radiation pattern of a dipole begins to distort in undesirable ways when the input signal wavelength λ is less than 1.0l. As the input signal wavelength approaches 0.5l, a compete cancellation of energy is normally expected in the direction orthogonal to the length of the antenna. In the case of a dipole antenna, such energy cancellation will occur for all antenna length l=2nλ, where n is an integer.
In order to understand what causes the above-referenced null in the dipole radiation pattern, it is useful to consider the effect of current distribution along the length of a dipole. When a thin linear dipole's length l is a half wavelength (0.5λ dipole) it will be self-resonant and has a sinusoidal current distribution. This sinusoidal distribution is illustrated by the current distribution shown in
Holding the physical length constant while increasing the frequency by a factor of two results in the electrical length l=1.0λ or λ=1.0l. Under these circumstances, the surface current will still be in the same direction and the radiation pattern beam peak is still orthogonal to the length of the antenna. This situation is illustrated in
However, once the electrical length exceeds 1.0λ (that is, λ<l), a portion of the surface current reverses direction to create two opposing surface currents as shown by the arrows in
According to one aspect of the present invention, inductors are used to modify the current distribution in a dipole antenna to overcome the aforementioned undesirable antenna radiation pattern effects which occur when the antenna has an electrical length of approximately even multiples of λ. Referring now to
In the present invention, the value of inductor 102 in each radiating element is selected to have limited effect when the overall length l of the dipole 100 is less than 0.5λ, where λ is the wavelength of an input signal. However, the inductor values are selected so that, as the wavelength decreases in length (i.e., frequency increases), the inductors 102 control the distribution and magnitude of the electric current in the inner and outer sub-element forming each radiating element of the dipole. When the length l of the dipole is 2.0λ (typically the worst electrical length with equal and opposite currents) the current distribution in each sub-element is controlled so that the radiation pattern from each sub-element, when added with the radiation pattern of each other sub-element, provides a sum radiation pattern that has peak in a direction that is orthogonal to the axis or length of the antenna. In the case of a vertically oriented dipole, this ensures that a beam peak is maintained in a direction toward the horizon. Note that the word “peak” as used herein refers to portions or directions of an antenna radiation pattern where antenna gain is maximized or is substantially at a maximum. For example, in some embodiments the peak can refer to portions of the beam which are within about 3 dB of maximum gain. In other embodiments, the peak can also include portions of the beam which are within about 6 dB of maximum gain.
According to one aspect of the invention, numerical modeling with optimization is used to select the ideal value of inductor 102 and the ideal length of sub-elements 104, 105 to provide an antenna radiation pattern in which peak gain is directed orthogonal to the dipole length over a wide range of input frequencies. Modeling has shown that for a range of input frequencies over which the dipole 100 has a corresponding electrical length ranging from 0.25λ to 44.0λ, it is possible to limit gain fluctuation orthogonal to the dipole to less than 6 dB. In other embodiments, it has been show that it is possible to limit gain fluctuation orthogonal to the dipole to less than 3 dB. This is a substantial improvement over convention dipole antenna designs such as the one illustrated in
A similar approach as described herein with respect to the dipole antenna in
Numerical modeling with optimization is used to select the ideal value of inductor 108 and the ideal length of sub-elements 111, 110 to provide an antenna radiation pattern in which maximum gain is provided in a direction substantially orthogonal to the length of monopole antenna 106 over an exceptionally wide range of input frequencies. Those skilled in the art will appreciate that with monopole antennas, it is common to have a null in a direction exactly orthogonal to the radiating element due to the effects of limited ground plane size. This is true for monopole antennas even when they are operating within a relatively narrow range of frequencies where l<λ. In contrast, the inventive arrangements can maintain a peak gain in a direction that is a relatively small angle relative to the orthogonal over a much wider range of frequencies as discussed below.
In the present invention, the value of inductor 108 in each radiating element 111, 110 is selected to have limited effect when the overall length l of the monopole 106 is less than 0.25λ, where λ is the wavelength of an input signal. However, the inductor values are selected so that, as the wavelength decreases in length (i.e., frequency increases), the inductor 108 controls the distribution and magnitude of the electric current in the inner and outer sub-element 111, 110. When the length l of the monopole is 1.0λ (typically the worst electrical length for a monopole) the current distribution in each sub-element is controlled so that the radiation pattern from each sub-element 111, 110, when added with the radiation pattern of the other sub-element, provides a sum radiation pattern that has peak in a direction that is substantially orthogonal to the axis or length of the monopole antenna 106. In the case of a vertically oriented monopole, this ensures that a beam peak is maintained in a direction substantially toward the horizon.
A suitable matching network 116 can be provided at the feed point 103 of the antenna 100. Similarly, a suitable matching network 118 can be provided at the feed point 109 of antenna 106. The matching network can be integrated within the feed point 103, 109 or can be provided externally. As is well known in the art, a matching network is used to ensure that the input impedance of the antennas 100 and 106 remains within a predetermined range of values throughout a range of frequencies over which the antenna is designed to operate. For example, it is often desirable for an input impedance of an antenna to be maintained within predetermined limits so that antenna input VSWR does not exceed a predetermined value (such as 3:1) over the entire range of frequencies over which the antenna is designed to operate. Still, those skilled in the art will appreciate that the invention is not limited in this regard.
Impedance matching networks are well known in the art and therefore shall not be discussed here in detail. However, it is sufficient to note that the impedance matching network 116, 118 should be selected to provide an acceptable impedance match to a transmitter (not shown) over a desired operating frequency range of the antenna.
According to some embodiments, the antenna 100 can be excited at feed point 103 by a transmitter 120 with a radio frequency signal having some wavelength within a pattern wavelength range. Likewise, antenna 106 can be excited at feed point 109 by transmitter 122 with a radio frequency signal within some pattern wavelength range.
Referring now to
For purposes of clarity, the inventive arrangements in
The method begins in step 202 with the selection of a desired physical length l of the antenna 100. The physical length can be selected by a designer to satisfy consumer or mission requirements. In step 204, the pattern wavelength range is selected to define the range of input signal wavelengths over which the antenna will provide an antenna radiation pattern that meets certain predefined performance criteria. The pattern wavelength range can be defined in accordance with a maximum wavelength (lower frequency limit) and a minimum wavelength (upper frequency limit).
In step 206, a designer selects as a design goal the maximum acceptable angular deviation that the peak of the antenna radiation pattern will be permitted to deviate from a direction orthogonal to the length of the antenna. For example, the this value can be specified as a maximum acceptable angle that the antenna radiation pattern 3 dB peak bandwidth will be permitted to deviate with respect to an orthogonal direction. Still, the invention is not limited in this regard and the maximum acceptable deviation can be specified in any convenient manner in accordance with the particular numerical modeling program.
In step 208 a designer can choose an initial position p for an inductor along a length of conductive elements. The position p can be selected in any manner compatible with the operation of the numerical modeling program. For example, p can be defined as a distance from the feed point 103, a location defined by a percentage of the overall length of radiating element 101, or by defining lengths of elements 104, 105. Applicants have found that a suitable starting point for the value of p is about 36% of the length of radiating element 101.
In step 210 a value L is chosen for inductor 102. Those skilled in the art will appreciate that the value of the inductor must be chosen so that it has a practical geometry and a self-resonance well beyond the highest frequency at which the antenna is intended to operate. Self-resonance occurs at a frequency determined by the inductor value and the parasitic capacitance in the inductor. It is desirable to avoid inductors having a self-resonance within the intended operating frequency range of the antenna because the inductor in such circumstances will tend to consume all of the energy input to the antenna. In such cases, the absorbed energy will typically cause a rapid temperature rise in the inductor and a substantial reduction in radiated energy. In general, the initial value of the inductor must be chosen to be sufficiently large in value so as to have the desired effect as described herein, but should not be chosen so large that he inductor will enter into self resonance before the upper frequency limit is met.
In step 211, each of the values selected in steps 202, 204, 208 and 210 are provided as an input parameter to a computer based numerical modeling program to create a model of the antenna. A computer based numerical modeling program for the present invention is preferably capable of modeling the electromagnetic fields generated when radio frequency energy over a selected range of wavelengths is applied to an antenna having a physical structure as previously described with respect to
Referring once again to
The evaluation in step 214 can be performed by any suitable means. For example, such evaluation can be performed using computer based statistical analysis to determine whether the antenna satisfies the requirements established in step 206. In some embodiments of the invention, the results of the computer based numerical modeling program can be communicated to a statistical analysis application program for purposes of performing the evaluation in step 214. The statistical analysis application program can be executed on the same computer as the numerical modeling program or a different computer. Alternatively, the numerical modeling program can directly perform such statistical analysis to evaluate the modeling results in step 214.
If, in step 216, a determination is made that the modeled antenna with the current set of parameters satisfies the requirements specified in step 206, then the process can terminate. However, if it is determined in step 216 that a satisfactory set of antenna radiation patterns has not been obtained, then a value defining position p and/or an inductor value L can be incrementally modified in step 218. The process then returns to steps 212 where the modeling, evaluation decision, and adjustments steps are subsequently repeated until acceptable antenna radiation patterns are achieved.
From the foregoing it will be understood that the design process involves determining a length of each sub-element and an inductance value of the first and second inductor based on numerical antenna modeling. In such numerical modeling, the length of each sub-element and the inductance value are iteratively varied to determine optimum values for maintaining the antenna radiation pattern peak in the orthogonal direction throughout the pattern wavelength range. Although not explicitly shown in
The invention has been described in terms of an antenna having a dipole structure as shown in
Using the techniques described herein, two practical designs were developed. In one design a monopole antenna of length 45 inches was developed. Absent the techniques provided herein, the monopole antenna had a useful pattern wavelength range corresponding to frequencies between 30 MHz and 108 MHz. By controlling the current distribution using the techniques described herein, the pattern wavelength range was extended to correspond to a frequency range between 30 MHz and 512 MHz. Significantly, this much broader pattern wavelength range is provided while maintaining a 3 dB peak bandwidth in a direction substantially orthogonal to the length or axis of the antenna throughout the entire pattern wavelength range. More particularly, with the monopole antenna in a vertical orientation, the 3 dB peak bandwidth was maintained within 10 degrees of the horizon throughout the entire pattern wavelength range. In this design, a 250 nH inductor was positioned at a location p that was approximately 13 inches from the feed. In the absence of controlling the current distribution using the techniques described herein, a deep null in the antenna radiation pattern for this antenna would be present in the orthogonal direction at approximately 270 MHz. This null is eliminated by using the techniques described herein.
In a second design, a monopole antenna of length 13 inches was developed. Absent the techniques provided herein, the monopole antenna had a useful pattern wavelength range corresponding to frequencies between 30 MHz and 512 MHz. By controlling the current distribution using the techniques described herein, the pattern wavelength range was extended to correspond to a frequency range between 30 MHz and 870 MHz. Significantly, this much broader pattern wavelength range is provided while maintaining a 3 dB peak bandwidth in a direction substantially orthogonal to the length or axis of the antenna throughout the pattern wavelength range of the antenna. More particularly, with the monopole antenna in a vertical orientation, the 3 dB peak bandwidth was maintained within 10 degrees of the horizon throughout the entire pattern wavelength range. In this design, a 47 nH inductor was positioned at a location p that was approximately 9 inches from the feed. In the absence of controlling the current distribution using the techniques described herein, a deep null in the antenna radiation pattern for this antenna would be present in the orthogonal direction at 760 MHz. This null is eliminated by using the techniques described herein.
Referring now to
Note that in
Referring now to
As would normally be expected where l is 1.0λ, the antenna radiation pattern in
Referring now to
As would normally be expected where l is 2.0λ, the antenna radiation pattern in
Referring now to
As would normally be expected where l is 3.0λ, the antenna radiation pattern in
Based on the plots in
From the foregoing, it will be appreciated by those skilled in the art that a method has been provided for producing with a dipole antenna electromagnetic radiation in a desired direction over a wide range of wavelengths. The method consists of exciting with a radio frequency signal first and second elongated conductive radiating elements of a dipole antenna having a combined overall length l to produce an oscillating time varying electric current within the first and second elongated conductive radiating elements.
The method further includes selectively varying the radio frequency signal to have any wavelength λ within a pattern wavelength range of the antenna extending from at least about 0.5l to at least about 2.0l. In other embodiments, the method includes selectively varying the radio frequency to have any wavelength within a pattern wavelength range of the antenna extending from about ⅓l to about 4l. The method also includes selectively controlling a current distribution along a length of the first and second elongated conductive radiating elements to form at any wavelength selected within the pattern wavelength range, an antenna radiation pattern having a peak in a direction substantially orthogonal to an axis aligned with a length of the elongated conductive radiating elements.
The method also involves selectively controlling the current distribution along the length of the first and second conductive radiating elements to limit variations in gain orthogonal to the axis of the antenna. In some embodiments, these gain variations have been limited to a value less than about 6 dB. In other embodiments, the current distribution is controlled to limit variations in gain orthogonal to the axis to a value less than about 3 dB over the entire pattern wavelength range of the antenna. Optimal values of p and L can be selected using the numerical modeling techniques described herein for limiting such gain variations.
Notably, in each of the embodiments of the invention described herein, the current distribution in the radiating element has been described herein as being controlled using an inductor component conductively connected between a plurality of sub-elements which collectively define the elongated conductive radiating element. However, the invention is not limited in this regard and a combination of inductors and/or capacitors arranged in a network can also be used in place of the single inductor as described herein. Moreover, resistors can also be used in place of the inductors described herein, although such arrangements will have a lower overall efficiency as compared to inductor based designs.
Those skilled in the art will appreciate that once the antenna design has been optimized to provide a suitable antenna radiation pattern over the entire pattern wavelength range, the method can continue with the step of impedance matching an input of the dipole antenna to a transmitter that is intended to provide the radio frequency signal over an entire range of the pattern wavelength range.
Further, in the case of a monopole antenna a similar methodology can be used to extend the pattern wavelength range of an antenna over a wide range of wavelengths. In such cases the method involves exciting with a radio frequency signal, a monopole antenna including an elongated conductive radiating element having an overall length l to produce an oscillating time varying electric current within the elongated conductive radiating element. The method further includes selectively varying the radio frequency signal to have any wavelength λ within a pattern wavelength range of the antenna extending from at least about 0.25l to at least about 1.0l. In other embodiments, the radio frequency signal can be varied to have any wavelength λ within a pattern wavelength range of the antenna extending from at least about ⅙l to about 2l. As with the dipole embodiment, this is accomplished by selectively controlling a current distribution along a length of the elongated conductive radiating element to form at any wavelength selected within the pattern wavelength range, an antenna radiation pattern peak in a direction substantially orthogonal to an axis aligned with a length of the elongated conductive radiating element.
As with the dipole arrangement, the current distribution along the length of the monopole conductive radiating element can be controlled so as to limit variations in gain orthogonal to the antenna length to a value less than about 6 dB. However, in other embodiments, current distribution along the length of the conductive radiating element can be controlled to limit variations in gain orthogonal to the axis to a value less than about 3 dB.
The inventive arrangements and methods described herein are not limited to dipole antennas. Instead, these techniques can also be used in disc cone antennas and a variety of other basic antenna designs to increase the upper frequency boundary of the pattern wavelength range. In the disc-cone case, an inductor would be placed along the length of each of the disc-cone antenna elements using techniques similar to those described above relative to the dipole embodiment. Further, it should be appreciated that the dipole and monopole antenna elements described herein can be combined with other antenna elements. For example, a dipole element as described herein can be used as a driven element and may be combined with parasitic elements such as reflector elements and/or director elements. In other embodiments, the antennas described herein can be used without limitation to form antenna arrays or other more complex antenna designs.
Applicants present certain theoretical aspects above that are believed to be accurate and that appear to explain observations made regarding embodiments of the invention. However, embodiments of the invention may be practiced without the theoretical aspects presented. Moreover, the theoretical aspects are presented with the understanding that Applicants do not seek to be bound by the theory presented.
Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.
Also, while various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. For example, the various embodiments of the invention are not limited with regard to any particular type of antenna described herein. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
