US 3576578 A
Description (OCR text may contain errors)
United States Patent Inventor Ernest T. Harper Mountain View, Calif. 687,051
Nov. 30, 1967 Apr. 27, 1971 Appl. No. Filed Patented Assignee Sylvania Electric Products Inc.
OF TWO DISTINCT TRANSMISSION LINES HAVING DIFFERENT CHARACTERISTIC 2,205,874 6/1940 Buschbeck ass/35x 2,241,616 5/1941 Roosenstein 333/35 Re22,374 9/1943 Carter 333/35X 2,406,945 9/1946 Fell 343/863 2,514,344 7/1950 Slaymaker et al. 333/35(UX) FOREIGN PATENTS 1,203,497 1/1960 France 343/791 842,665 6/1952 Germany..... 343/790 866,680 2/1953 Germany 343/791 OTHER REFERENCES Andrew Corp. Bulletin 950, both sides,
Primary Examiner-Herman Karl Saalbach Assistant Examiner-T. Vezeau AttorneysNorman J. OMalley, Elmer J. Nealon and John F.
Lawler ABSTRACT: One half of a dipole antenna is formed by the extension of a center conductor of a coaxial transmission line beyond the point of termination of the outer conductor at the feed point of the dipole. The other half of the dipole is formed by the outer conductor between a broadband cable choke and the feed point. The part of the antenna between the choke and feed point comprises two coaxial lines, one having a characteristic impedance equal to that of the antenna feed line and the other having a characteristic impedance greater than that of the feed line.
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48 O 52 54 5s 58 so 62 FR ouE-cY- MHz l5 r I I FEB so so I00 INVENTOR ERNEST I HARPER m ATTORNEY AGENT FREQUENCY MHZ DIPOLE ANTENNA IN WHICH ONE RADIATING ELEMENT IS FORMED BY OUTER CONDUCTORS OF TWO DISTINCT TRANSMISSION LINES HAVING DIFFERENT CHARACTERISTIC IMPEDANCES BACKGROUND OF THE INVENTION This invention relates to dipole antennas and more particularly to a broadband end-fed coaxial-line-type dipole antenna.
The dipole is inherently a narrowband antenna which characteristically exhibits desirable impedance and gain properties over a relative bandwidth of only a few percent. If an antenna operates over a frequency band from the frequency f to the frequency f and is resonant at the frequency j}, the percentage bandwidth of the antenna is the percentage that the frequency difference f -f is of the frequency f This narrowband characteristic is retained in a coaxial-linetype dipole antenna in which the center conductor forms onehalf of the dipole. One such coaxial dipole antenna, by way of example, includes a quarter-wavelength coaxial sleeve connected to the dipole feed point. This choke joint is inherently a narrowband component and so limits the bandwidth of the dipole. The narrowband characteristic of such an antenna is exemplified by the rapidly increasing voltage standing wave ratio (VSWR) as the frequency is tuned away from the dipole resonance.
A general object of this invention is the provision of a dipole antenna which exhibits high gain and a nominally constant VSWR as seen from the input of the antenna over a broad band of frequencies.
SUMMARY OF THE INVENTION In accordance with this invention, an end-fed coaxial-line dipole antenna comprises an inner conductor as one dipole element and the outer conductors of two coaxial lines having different characteristic impedances as the other dipole element. A coaxial feed line is connected to the coaxial-line end of the dipole through a broadband coaxial cable choke and is one of the coaxial lines comprising the other dipole element.
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic view of an end-fed coaxial-line-type dipole antenna of prior art construction;
FIG. 2 is an end-fed coaxial dipole antenna embodying this invention;
FIG. 3 is a schematic view, partially cut away, of a modified form of this invention;
FIG. 4 is a Smith chart illustrating the operation of this invention; and
FIGS. 5 and 6 are curves showing VSWR characteristics of antennas constructed in accordance with this invention.
DESCRIPTION OF PREFERRED EMBODIMENTS A coaxial dipole antenna typical of prior art construction is shown at 10 in FIG. 1 and comprises a coaxial feed line 11, having an outer conductor 12 and an inner conductor 13, which feeds dipole elements 14 and 13a. Element 13a is an extension of inner conductor 13 from the feed point F at the end of outer conductor 12. Element 14 is a coaxial conductive sleeve that is electrically connected to outer conductor 12 at feed point F and extends back over the feed line for a distance of a quarter wavelength at the center frequency of the antenna. The end 19 of sleeve 14 remote from feed point F is open and thus acts as a choke to block the flow of current from the sleeve to the outer conductor remote fromthe feed point and limit radiation to the dipole elements. Coaxial dipoles of the type shown in FIG. 1 characteristically have a bandwidth of approximately 10 percent.
An embodiment of this invention is shown at 20 in FIG. 2 and comprises a coaxial feed line 21,- having an outer conductor 22 and an inner conductor 23, connected to dipole elements 24 and 25 by a ferrite cable choke 26. Choke 26 does not per se constitute part of this invention and is described more particularly in an article entitled Charting The Bandwidth Of Isolating r-f Chokes," pages 1 l2 and 1 l3, Electronics, June 13, 1966 (McGraw-Hill Publishing Co.). Briefly, this cable choke consists of several turns 27 of the coaxial transmission line 21 wound on a solenoidal core 28 of ferrite material. This choke is designed with an inductance that resonates with the self-capacitance between the turns 27 at or near the resonant frequency of the dipole antenna. Therefore, a resonant impedance is produced at the center frequency of operation of the antenna. However, the cable choke continues to provide a very large impedance at frequencies away from the center frequency by virtue of the fact that the inductance to capacitance ratio of the choke is very large. The ferrite material from which core 28 is made is judiciously selected to minimize magnetic losses in the material over the operating frequency band of the antenna. Within these limitations, cable chokes are designed to provide large impedances over as high as octave bandwidths or greater.
Dipole element 24 is comprised of parts or sections 24a and 24b of coaxial line that are coupled by connector 29. Part 24a is the end portion of coaxial line 21 that extends above the choke 26 and has a length l,,. Connector 29 may be oneof the conventional mating connectors for connecting two coaxial cables. Dipole element 25 is an extension of the center conductor of coaxial line 24b.
An important feature of this invention is the dual impedance characteristic of the coaxial lines forming dipole element 24. More specifically, the characteristic impedance of a section 24a of element 24 adjacent to the choke 26 is equal to the characteristic impedance of feed line 21. The characteristic impedance of section 24b constituting the remainder of dipole element 24 is substantially higher than that of section 240. By way of example, the impedance of feed line 21 and of section 24a typically is 50 ohms, and the impedance of section 24b is to ohms. A general rule that may be followed in calculating the characteristic impedance (Z,,) of section 24b is: (I) Z, is approximately twice that of the feed line 21 for bandwidths in the order of 1.25:1; (2) Z, is approximately 2% times that of the feed line 21 for bandwidths in the order of 1.5:1 to 1.75:1; and (3) Z, is approximately 3 times that of feed line 21 for bandwidths in the order of 2: lwhere the bandwidth ratio is f /f zfi lf for an antenna operating between the frequencies f and f,,,,,,,.
Dipole elements 20 and 24 are each physically a quarterwavelength long as is required for the operation of a dipole antenna. Since line 24b is a length of coaxial transmission line as well as a quarter-wavelength impedance transformer, the dielectric constant of the insulation thereof causes its physical and electrical lengths to be different.
Line section 24b is electrically a quarter-wavelength long at the resonant frequency of the dipole. The physical length 1 of line section 24b is therefore representable as where h )\/2,)\ is the wavelength at the resonant frequency of the antenna, h/2 is the physical length of the lower half of the antenna from the cable choke to feed point F and v, is the phase velocity of signals in line 24b in percent of the speed of light. The phase velocity v,, is representable as where c is the speed of light, and and e are the permeability and permittivity relative to free space, respectively, of the dielectric filling line 24b. Since V is a ratio of two velocities, it is dimensionless.
A modified form of this invention is illustrated in FIG. 3 wherein coaxial dipole antenna 20 is supported in a tube 31 made of low-loss dielectric material such as fiberglas. Since antennas 20 and 20' are similar, like elements are designated by primed reference characters in FIG. 3. Antenna 20, however, includes a torroidal ferrite cable choke 26 comprising several turns 27' of coaxial line 21' wound on a torroidal core 28' of ferrite material. Discs 3235 inclusive, fit snugly over the associated dipole elements for supporting the latter in the center of tube 31. Similarly, disc 36 fits snugly over torroidal cable choke 26' for the same reason. The ends of the tube are sealed by discs 32 and 37. The discs are also made of a dielectric material having low-loss properties and may be bonded or otherwise secured to the tube 31 to provide a rigid structured assembly. Connector 38 provides for electrical connection between feed line 21 and equipment such as a transmitter or receiver (not shown).
By way of example, dipole antennas embodying this invention which have been constructed and successfully operated have the following dimensions and characteristics:
h, inches.... 106 84 11, inches 53 42 12, inches.. 36 32 13, inches 17 10 Characteristic impedances:
Cable 21 (and part 24a), ohms O 50 Part 24b, ohms 95 125 Choke (Solenoidal) Outside diameter, inch 0.75 1. 25 Inside diameter, inch 0. 5 1.0 Thickness, inch. 0.75 0. 75 Number of turns 12 5 Frequency range, rnHZ 48-62 50-100 VSWR (average) 3.0: 4.011
1 Material=Q,-3 ferrite, manufactured by Indiana General.
The dual impedance feature of dipole element 24 results in a substantially constant VSWR over the operating frequency band of the antenna. This occurs because section 24b is a quarter-wave transformer and because cable choke 26 is effective over a broad band of frequencies to prevent currents from flowing on the outer conductor 22 of the feed line. This operation is illustrated graphically on the Smith chart shown in FIG. 4 for the dipole A referenced above.
Consider an example where the VSWR of the antenna is to be maintained nominally constant and equal to 3:1 over the frequency band from 48 MHz. to 62 MHz. The center 41 of the Smith chart represents a perfect match between the antenna impedance of feed line 21 to which the antenna impedance is normalized. Circle 42 is a plot of normalized antenna impedance for a constant VSWR of 3: 1. Curve 43 is a plot of the antenna impedance, normalized to the characteristic impedance (50 ohms) of feed line 21, over the frequency band. Curve 43 illustrates the operation of antenna 20 if the dipole is fed directly by coaxial line 21, i.e., if section 24b were an extension of feed line 21. Reference to curve 43 reveals that the VSWR of this antenna is substantially greater than 3:1 at the band edges, 48 MHz. and 62 MHz. (The VSWR is proportional to the linear distance between point 41 and a point of interest.) More particularly, the VSWR is approximately :1 at 48 MHz. This same information is illustrated in a slightly different manner by curve 43 in H6. 5.
Curve 45 in FIG. 4 shows the antenna impedance, again measured at feed point f but referenced to the characteristic impedance (95 ohms) of section 24b through which the antenna is actually fed. Curve 46 illustrates the antenna impedance shown in curve 45 after it has been transformed through the quarter-wavelength high-impedance section 24b of the antenna to connector 29. Curve 46 is obtained by rotating curve 45 in the clockwise direction. The amount that each point on curve 45 is rotated corresponds to the electrical length of section 24b at the frequency associated with that point. Only the point 47 (corresponding to the resonant frequency of the dipole) on curve 45 is rotated exactly a quarter-wavelength at the center of the band.
Since the antenna impedance represented by curve 46 is referenced to the characteristic impedance ohms) of line 2412, whereas the dipole is actually fed by a 50 ohm coaxial line 21, the antenna impedance (curve 46) must be referenced to 50 ohms. Curve 48 illustrates the antenna impedance at the connector 29 normalized to the characteristic impedance of feed line 21. This same information is shown as curve 48 in FIG. 5. Reference to curve 48 reveals that the VSWR remains at the nominally constant valve of 3:1 over the operating frequency band. Comparison of curves 43 and 48 (FIG. 5) shows that the resultant VSWR of an antenna employing the dual impedance feature of this invention is lower and more nearly constant over a considerably broader band of frequencies than that of a similar antenna wherein the characteristic impedance of the lower half of the antenna is constant and equal to the impedance of the feed line.
Curves 49 and 50, see FIG. 6, illustrate the operation of dipole B and are similar to curves 43 and 48', respectively.
Changes, modifications and improvements may be made to the above-described preferred embodiment of the invention without departing from the spirit of the invention. The scope of the claims define the advance the invention makes in the art.
1 claim: 1. A broadband end-fed dipole antenna having two dipole elements, comprising:
first and second coaxial transmission lines electrically connected together, each of said lines having an inner conductor and an outer conductor, the outer conductor of said first line and at least part of the outer conductor of said second line constituting one of said dipole elements,
the other of said dipole elements comprising an extension of the inner conductor of said first line,
feed line means for energizing said dipole elements, and a broadband choke operatively coupling said feed line means with said second line,
the characteristic impedance of the first line being greater than that of the second line and the characteristic impedance of the second line being equal to that of the feed line means.
2. The antenna according to claim 1 in which said feed line means and said second line comprise different portions of the same coaxial cable, said choke comprising a magnetically permeable core with a predetermined length of said coaxial cable wound therearound and having an inductance resonant with the self-capacitance between cable windings at the center operating frequency of the antenna, having a large inductance-to-capacitance ratio, and providing a large impedance at the center frequency and frequencies spaced therefrom.
3. The antenna according to claim 5 wherein said first line has an electrical length substantially equal to a quarter wavelength at the resonant frequency of said antenna, said first line having a physical length satisfying the relationship where v, is the phase velocity of signals in said first line and his the wavelength at the resonant frequency of said dipole.