US 3716867 A
This invention relates to antennas and more particularly to a wire antenna which is multiply-loaded to provide wide band characteristics. By virtue of its novel construction and design, the antenna of this invention will maintain an input impedance value which changes very little over an extended frequency band. Furthermore, single beam omnidirectional radiation patterns can be maintained over a wide band.
Claims available in
Description (OCR text may contain errors)
United States Patent 1 1 Mayes et al.
[ 1 Feb. 13, 1973  WIRE ANTENNA MULTlPLY-LOADED WITH ACTIVE ELEMENT IMPEDANCES  Inventors: Paul E. Mayes, 1508 Waverly Dr., Champaign, . 61820; Andrew J. Poggio, 863 Palomino, Pleasanton, Calif. 94566 22 Filed: Aug.l1, 1970 21 Appl.No.: 62,910
 US. Cl. ..343/70l, 343/747, 343/792.5 [5 1] Int. Cl. ..H0lq 11/04  Field of Search....343/70l, 722, 792.5, 908, 747
 References Cited UNITED STATES PATENTS Hall ..333/80 T 3,277,487 l0/l966 Berry et al .343/792.5 3,098,973 7/1963 Wickersham et al ..343l70l 2,703,363 3/1955 Rines et al ..343/70l Primary Examiner-Eli Lieberman Attorney-Harness, Dickey and Pierce  ABSTRACT This invention relates to antennas and more particularly to a wire antenna which is multiply-loaded to provide wide band characteristics. By virtue of its novel construction and design, the antenna of this invention will maintain an input impedance value which changes very little over an extended frequency band.
Furthermore, single beam omnidirectional radiation patterns can be maintained over a wide band.
10 Claims, 12 Drawing Figures PATENTED 31973 3,716,867
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if 4 77/71/114 zkmwfwrm z/m A l/14v M MIA 16 WIRE ANTENNA MULTIPLY-LOADED WITH ACTIVE ELEMENT IMPEDANCES BACKGROUND OF THE INVENTION The need for antennas possessing wide band characteristics has existed for several years and will become more acute as the complexity of electronic systems continues to increase. This was demonstrated dramatically by the widespread application of log-periodic antennas almost immediately after their invention in the late 1950s. However, numerous applications for wide band antennas remain that are not satisfactorily met by previously-known log-periodic structures. This is particularly true in the area of portable communication systems where it is not possible or perhaps just not convenient to transport a large antenna array. In some such antenna applications, directivity is sacrificed for portability.
Low-directivity or omnidirectional radiation patterns are often obtained by using simple linear radiators such as dipoles and monopoles. The simplest of these are made from thin conductors and, as a consequence, have rather narrow band properties. The center frequency of the operating band usually corresponds either to a self-resonance of the thin-wire antenna or to a tuned resonance obtained by adding loading elements to the basic radiator. An example of a loading element is the familiar top-load consisting of a disc or a number of radial wires. Alternatively, a monopole antenna may be base loaded by placing an inductor at the input terminal. Or some advantage may be obtained by placing the inductor at an intermediate point between feedpoint and tip of a monopole. Usually, these loading techniques result in the antenna being useful across a narrow band which is, at most, only a few per cent of the operating frequency. An exception occurs in the case of loading with resistive elements. This tends to increase the bandwidth at the expense of efficiency and consequently must be avoided in most applications.
SUMMARY OF THE INVENTION This invention provides an antenna which is multiply-loaded by devices, each having a reactance which is negative in sign and increases with increasing operating frequency. An antenna according to this invention may be made up of lengths of conducting radiating elements or sections connected in series by lumped loading elements which are preferably purely reactive to maintain high efficiency. A novel feature of the invention is the use of active loading elements which produce a desirable reactance versus frequency variation.
An exemplary antenna embodiment of the instant invention includes a plurality of lengths of conducting wire and a plurality of twoterminal lumped elements which are predominantly reactive. The frequency dependence of each reactance over at least some portion of the operating band corresponds approximately to a negative inductor, i.e.
where X L is the reactance, m is the radian frequency and [is a constant representing the inductance. The
wire sections are preferably aligned end-to-end with a small gap between the ends of the adjacent wire sections. One end of each section of wire is connected to one terminal of a reactive element and the other end to a terminal of another reactive element with the exception of the innermost terminal of the innermost wire section which becomes an input terminal and the outermost terminal of the outermost wire section which is left unconnected. A balanced antenna is comprised of two of the structures described above which are then fed from a two wire balanced line at the input terminals. Alternately, a single structure may be fed against a ground plane or other ground structure as an unbalanced antenna. The former arrangement corresponds to a dipole; and the latter, a monopole.
The distribution of the active elements along the wire may be somewhat arbitrary. However, two particular distributions are preferred. In one case, the wire sections are all of the same length; the loading is periodic. In the other case, the length of wire sections increases with distance from the feed point. A preferred spacing between loads is log-periodic, i.e., each wire section is shorter than an adjacent one by the same factor which is usually denoted by the Greek letter tau (1). The lengths of wire sections thus form a geometric progression with the shortest at the input end and the longest at the outer end.
The configuration of the electrical network which is used to produce the desired reactance is immaterial insofar as operation of the antenna of this invention is concerned. However, an active network is required and the power supply for each element is preferably selfcontained to avoid coupling between the thin-wire antenna sections and any additional conductors which might be needed to provide for power distribution. One network which will produce the desired reactance variation is a negative-impedance converter (NIC), a four-terminal device which displays at the input terminal pair the negative of the impedance which is connected to the output terminal pair. When terminated in a conventional inductance Lthe input reactance of the NIC will be as given in the formula above.
From the description herein, it will be appreciated that the present invention provides an'easily portable antenna having broad band impedance and pattern characteristics which may be simply constructed from wire and small active two-terminal electrical networks. The active two-terminal electrical networks are preferably distributed along and connected in series with a thin-wire in a log-periodic manner.
The invention will be better understood from the following detailed description thereof taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents a schematic diagram of a balanced antenna constructed in accordance with the principles of the instant invention;
FIG. 2 is a schematic diagram of an unbalanced antenna constructed in accordance with the principles of the instant invention;
FIG. 3 shows a more detailed diagram of a technique for realizing the negative inductive reactance used with antennas of the instant invention;
FIG. 4 shows on a Smith Chart a plot of antenna impedance versus frequency for a typical thin-wire dipole;
FIG. 5 shows a plot of antenna impedance versus frequency which is typical of what may be achieved by an antenna designed according to the principles of the instant invention;
FIGS. 6a, 6b, 6c, 6d, 6e and 6fshow calculated values of current versus location on a typical antenna of this invention for several different frequencies.
FIGS. 7a, 7b, 7c, 7d, 7e, 7f, 7g and 7h show calculated radiation patterns of the antenna having current distributions corresponding to those shown in FIG. 6.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, an exemplary antenna 10 is shown which is designed in accordance with the principles of the instant invention and which is comprised of a plurality of thin-wire sections wherein sections 11a15a comprise one half of the antenna and sections 1 lb-lSb comprise the other half, and a plurality of two-terminal electrical networks 16-l9 (a or b) which are connected in series with the wire sections, l6a-l9a being connected in one half of the antenna and 16b-l9b to the other half. Each terminal of each electrical network 16-19 is connected to one end of a thin wire section. The innermost ends of sections 11a and 1 lb are closely spaced to provide the input terminals of the antenna as illustrated. The outermost ends of sections a and 15b are left unconnected.
In the preferred embodiment of the antenna the lengths of any two adjacent wire sections on each half of the antenna are related by a constant, 1', which is less than one, and the two halves of the antenna are identical to provide for perfect symmetry and balance. However, neither of these requirements is essential to achieve substantially the benefits of the invention and significant departures in construction may be made without materially affecting the antenna performance.
FIGQZ shows what is substantially one-half of the balanced antenna of FIG. 1 together with a ground plane 20. This unbalanced version of the antenna is fed by a coaxial cable 30 with outer conductor connected to the ground plane and inner conductor connected to thefree end of wire section 1 la.
FIG. 3 shows how a negative-impedance converter 31- can be used together with an inductor 32 across the output terminals to achieve reactance with frequency dependence of inductive reactance but opposite in sign which is suitable foruse as the two-terminal networks 16l9 and, to this end, is provided with a pair of terminals 34 for series connection with a thin wire or the equivalent. Since negativev impedance converters are known to the art, a detailed description thereof will not be presented here.
.FIG. 4 shows how the input impedance of a typical thin-wire dipole changes appreciably with frequency when operated over an extended frequency band. The
dots on the impedance locus correspond to various.
values of L/Awhere L is the half-length of the dipole and A is the wavelength. The Smith Chart plot displays the multiple resonances of the dipole which are caused by reflections of the current wave from the open ends The Smith Chart plot of FIG. 5 illustrates how the particular reactive loading of the instant invention can reduce the variation in impedance. Evidence of low and high impedance resonances are no longer seen. For 0.5 L/A 7.0 the impedance locus is confined to 'a small area on the chart.
The reason for the reduced influence of resonances is demonstrated in FIGS. 6a-6f. It is noted there that standing wave current distribution which is characteristic of the thin-wire dipole is in evidence only in FIG. 6a which corresponds to L/)\=0.5. As frequency increases the current decreases due to the 8c reactive loading so that only small currents remain at the end of the dipole (Z/L 1.0). Hence, there is almost no current wave to be reflected from the open end. For Ll).
2.0, 4, 5.2 and 6.2 as shown in FIGS. 60, d, e, andf, the current decays to negligible values at points more and more distant from the dipole end. Note that the figures show the current which would be present on a monopole; however, the same current would occur on one-half of a dipole and its mirror image would be present on the other half so that the information given here can be readily interpreted as dipole data as well as monopole data.
FIGS. 7a-h show one quadrant of the radiation pattern in any plane through the axis of a typical antenna of the instant invention. The patterns are shown for values of L/)\ from 0.3 to 7 corresponding to a frequency band of more than 20 to 1. Only the upper half of one side of the beam is shown, the lower half and the other side of the beam follow as mirror images by virtue of symmetry of the antenna. Note that maximum radiation occurs along the horizontal axis for all L/)\ 5.0. Evidence of a beam scanning effect is seen for LI). 5 5.0. This phenomenon is caused by the periodic loading which was used on this particular antenna, with log periodic loading, the beam scanning can be eliminated.
A more detailed presentation of the foregoing analysis appears in Technical Report AFAL-TR-69-180 by Andrew J. Poggio and Paul E. Mayes entitled Numerical Solution of Integral Equations of Dipole and Slot Antennas Including Active and Passive Loading published by Air Force Avionics Laboratory and Air Force Systems Command, Wright-Patterson'Air Force Base, Ohio, which is available from the Library of Congress, the teachings of which are incorporated in this application by reference thereto. As will be apparent in view of the above report, the present invention is useful for a variety of forms of antennas including cylindrical dipole antennas and planar slot antennas.
Utilizing the information'provided above, consider the design of a dipole antenna which operates in the 10-55 MHz band (wavelengths from 30 to 5.5 meters). From FIG.'5, it can be seen that the impedance locus is compact for a dipole halflength/wavelength, LIA between 0.5 and 7.0. However, the radiation pattern shown in FIG. 7 indicates that the pattern remains more stable for L/)\ less than 5.0. The 5.5 to 1.0 band can be taken between L/)\ 0.5 and 2.75 while keeping overall dipole length relatively small. FIGS. 5 and 7 indicate that periodic spacing of the active elements will provide excellent impedance and pattern properties. If needed, wider bandwidths can be obtained by spacing the loads in a log-periodic fashion, as previously indicated. Parameters for a near-optimum design can be obtained from the aforementioned report, Numerical Solution of Integral Equations of Dipole and Slot Antennas Including Active and Passive Loading. For example, as indicated in the aforementioned report, excellent performance can be obtained with loads periodically spaced along each dipole arm. The desired sign and frequency dependence of the reactive elements is obtained by terminating a negative impedance converter with an ordinary inductor as shown in FIG. 3. The following table, derived from the information provided in the aforementioned report, gives the relative location of each load on the dipole arm the value of reactance at L/)\ 1.0 (f=Ml-lz and the value of inductance which is used to terminate the negative impedance converter in order to achieve the desired reactance;
As seen above, the reactive values are tapered with smaller values near the feed point and larger values near the ends of the dipoles. This particular design is offered for illustrative purposes only and should not be interpreted as restricting the invention to any particular method for spacing the loading elements or achieving the negative reactance which is proportional to frequency or the values of reactance used. In this regard, other designs can be derived from the information set forth above.
It will now be appreciated that the present invention provides an improved antenna which can be made small in size so as to be conveniently portable yet which possesses a broad band impedance and pattern characteristic. Therefore, it is exceptionally suitable for use for emergency portable communication systems as well as police communication systems and the like where the capability of tuning to frequencies over a relatively large band is highly desirable. Accordingly, this invention is believed to constitute an important advancement in the communications art generally, and the portable communication equipment field specifically.
While it will be apparent that the teachings herein are well calculated to teach one skilled in the art the method of making the preferred embodiments of this invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or meaning of the subjoined claims.
What is claimed is:
1. An antenna structure for operation over a predetermined frequency band comprising:
a plurality of radiating sections, each being of a predetermined length and arranged to generally form said antenna structure, said antenna structure having at least one end; and
a plurality of active electrical devices with respective ones of said plurality of active electrical devices connected intermediate adjacent ones of said sections in series with said adjacent ones of said sections, each of said plurality of active electrical devices having a reactance which is negative in sign and increases with increasing frequency over at least a portion of said frequency band so as to reduce resonances on said antenna structure by reducing the current wave toward said end of said antenna structure to thereby reduce the current wave reflecting from said end.
2. An antenna structure according to claim 1 wherein said radiating sections are each a thin conducting wire.
3. An antenna structure according to claim 2 wherein said wire sections are aligned end-to-end along a straight line, and at least one of said wire sections being adapted to be connected to a source of radio frequency energy to form a monopole antenna. 4. An antenna structure according to claim 2 wherein said" wire sections are aligned end-to-end along a straight line, and at least two of said wire sections being adapted to be connected to a source of radio frequency energy to form a dipole antenna. 5. An antenna structure according to claim 1 wherein said electrical means are evenly spaced with respect to said antenna structure. 6. An antenna structure according to claim wherein said means are spaced in a log-periodic distribution with respect to said antenna structure. 7. An antenna structure according to claim 1 wherein said active electrical device includes a negative-impedance converter. 8. An antenna structure according to claim 7 wherein said radiating sections are each a thin conducting wire.
9. An antenna structure for operation over a predetermined frequency band comprising:
a plurality of radiating sections arranged in logperiodic fashion; and an active electrical device connected intermediate adjacent ones of said sections in series with said adjacent ones of said sections, each of said active electrical devices having a reactance which is negative in sign and increases with increasing frequency over at least a portion of said frequency band.
10. An antenna structure for operation over a predetermined frequency band comprising:
a plurality of radiating sections arranged in periodic fashion; and an active electrical device connected intermediate adjacent ones of said sections in series with said adjacent ones of said sections,'each of said active frequency over at least a portion of said frequency electrical devices having a reactance which is hand. negative in sign and increases with increasing I s