US3334347A

US3334347A - Shock resistant horn antenna

Info

Publication number: US3334347A
Application number: US358581A
Authority: US
Inventors: Ernest J Wilkinson
Original assignee: Sylvania Electric Products Inc
Current assignee: GTE Sylvania Inc
Priority date: 1964-04-09
Filing date: 1964-04-09
Publication date: 1967-08-01
Anticipated expiration: 1984-08-01

Description

Augl, 1957 E. J. WILKINSON 3,334,347

SHOCK RESISTANT HORN ANTENNA Filed April s, 1964 5 sheets-sheet 1 F/G. l

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ERNEST J. WlLKINSON lay/M 5% Aug. l, 1967 E. J. WILKINSON SHOCK RESISTANT HORN ANTENNA 3 Sheets-Sheet 2 Filed April 9, 1964 WAVELENGTH /WAVELENGTH ATCENTER OF BAND INVENTOR. ERNEST J. WILKINSON Aug. l, 1967 Ef .1. WILKINSON 5 3,334,347

SHOCK RESISTANT HORN ANTENNA A Filed April U, 1964 5 SheetS-Sheet 3 FIG. 7

RELATIVE POWER d b l HG. a

RELATIVE POWER db l I 108 72 36 O 36 72 ANGLE DEGREES IN V EN TOR. v

ATTORNEY United States Patent 3,334,347 SHOCK RESISTANT HORN ANTENNA Ernest J. Wilkinson, Westwood, Mass., assignor to Syl- Vania Electric Products Inc., a corporation of Delaware Filed Apr. 9, 1964, Ser. No. 358,581 Claims. (Cl. 343-719) This invention relates to antennas, and more particularly to ultra high frequency antennas which can function under severe environmental conditions.

. It has become increasingly important for military purposes to provide communication systems which are hardened; that is, which are capable of surviving the extreme environment encountered during a nuclear explosion, and which can function substantially unimpaired after such an explosion. It is a relatively simple matter to protect the electronic equipment itself from the effects of an explosion by locating the equipment in protective underground shelters. It is a more diliicult problem, however, to protect the antenna which must be open to the atmosphere to remain operational. There are three major obstacles that must be overcome in designing a hardened antenna. The most obvious is that the antenna must have the structural strength to withstand the force of an explosion, such as caused by a nuclear bomb. Another is that the antenna must withstand the high temperatures associated with the fireball. Thirdly, the antenna must function in vsurrounding debris, such as dirt, stones, and the like, which may be conductive and which may completely or partially bury the antenna. It is evident that any type of antenna which protrudes above the surface of the ground, even iff it withstands the shock wave and thermal effects of an explosion would be of little use as an antenna since conductive debris which showered the antenna would in all probability short circuit it to ground. It is, therefore, an object of the present invention to provide an eicient, relatively inexpensive, hardened antenna which offers a high degree of protection against shock and thermal effects associated with an explosion, and which, in particular, is not affected by debris accumulation.

Another object of the invention is to provide a iiush mounted antenna electrically equivalent to a stub antenna, this type of antenna having radiating characteristics that have Ibeen found to be particularly suitable for UHF` communication systems.

. A nuclear explosion can cause overpressures outside of the crater region of greater than 1000 p.s.i., and temperatures of 30,000" C. in the vicinity of the fireball. It would be expected that a formidable structure would be required to survive such conditions, and that it would be an even more formidable task to provide an antenna structure that would be operational after su-ch an explosion. In accordance with the present invention, however, a relatively simple structure is utilized to provide both the requisite mechanical strength and electrical operation. Briefly, the antenna comprises a hollow cylindrical pipe flush mounted in the ground which is excited in the TMm mode by means to be described hereinafter, the region `at the bottom of the pipe containing no electromagnetic field. Debris caused, for example, by a ibomb blast, falling into the pipe will collect at the bottom, and, therefore, not interfere with the electri-cal operation of the antenna. Moreover, the antenna structure being below ground, will not receive the brunt of the shock from an explosion. Also, the temperature at the feed point, located about half way down the pipe, will be greatly reduced relative to the temperature at the surface due to the ifact that thermal and nuclear radiation cannot bend around a corner.

The foregoing, and other objects, features, and advantages of the present invention, together with a better understanding of its construction and operation, will be more apparent from 'the following detailed description,

3,334,347 Patented Aug. 1, 1967 taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a sectional elevation view of a preferred embodiment of the invention;

FIG. 2 is a fragmentary sectional elevation view showing an alternative means of energizing the present antenna;

FIG. 3 is a sectional elevation view of another embodiment of the invention;

FIG. 4 is a pictorial view of a corner reflector useful to convert the antenna into4 a high gain directional antenna;

FIG. 5 is a pictorial view of a further embodiment of the invention;

FIG. 6 is a plot of the measured VSWR characteristic of the embodiment of FIG. 1;

FIG. 7 is a plot of the measured elevation pattern of the embodiments -of FIGS. 1 and 3; and

FIG. 8 is a plot of the measured elevation pattern of the embodiment of FIG. 4.

Referring` to FIG. l, the illustrated preferred embodiment of the invention comprises a cylindrical conductive pipe 10, typically made of steel, rigidly supported in the ground 12 with its aperture flush with the surface of the ground. The pipe has a middle region 14 of uniform diameter, the top end of which tapers into a region 16 of larger diameter, and the bottom end of which tapers into a region 18 of smaller diameter. The middle region 14 constitutes the antenna and is of a suitable diameter to propagate the specified electromagnetic eld. The enlarged upper region 16 provides impedance matching between the antenna and the atmosphere while the smaller region 18 is of such a Idiameter that this region of the pipe is cut olf at the operating frequency, for reasons to be subsequently explained.

The structural strength required in a particular instance will of course dictate the necessary wall thickness of pipe 10. Calculations have shown that a pipe thickness of approximately four inches should be adequate to withstand the forces and temperatures expected from a nuclear explosion. To further support the antenna, various well known construction techniques can be employed, such as imbedding pipe 10 in concrete. The frequency at which the antenna operates will, of course, govern its dimensions. For an antenna designed to operate at 300 megacycles, for example,

regions

14, 16 and 18 of pipe 10 had inside diameters of 3 feet, 4 feet and 2.5 feet, respectively. The overall length of pipe 10 is 12 feet.

The antenna is energized by a pair of

feeds

20 and 22 mounted at diametrically opposite positions in the lower end of region 14. The feeds each consist of a rectangular cavity formed by conductive wall 24 having an open face llush with the inner wall of pipe 10. Each cavity is excited by a suitably placed probe 26 provided within the cavity, or alternatively by loops or other well known means. Each cavity is filled with a dielectric material 28, such as epoxy-loaded Fiberglas, to protect the feed probes from the effects of a shock wave that may be propagated within pipe 10. Since the antenna feed structure is liush mounted in the walls of pipe 10, shock waves propagated down the pipe pass over the feed without damaging it. Moreover, hot gases present above ground will not significantly affect the antenna structure since only thermal radiation from gases colinear with the antenna axis can enter the pipe to raise the temperature in the region of the feed.

For ease of explanation the operation` of the invention will be described in the transmitting mode, but it is to be understood that it will function equally well in the receiving mode. In operation, energy from a suitable energy source 36 is applied in phase via

equilength cables

30 and 32 and respective probes 26 to corresponding cavities 24 which, in turn, excite a TMm mode electromagnetic field within portion 14 of pipe 10. Since this field configuration is the same as that of a stub antenna, an antenna pattern is produced which is identical to that of a stub antenna.

The diameter of region 1S being dimensioned beyond cutoff at the operating frequency, no energy propagates into this region, and it is clear that foreign matter that may be located withinpthis region will not affect antenna performance. Thus, debris from a bomb explosion, or other cause, falling into pipe will accumulate in the bottom of the pipe where no field exists and will not in any way detract from the performance of the antenna. The length of region 18 can be suitably chosen to accommodate the expected amount of debris. Debris accumulating around the lip of the pipe aperture has been found to have a negligible effect on antenna patterns or impedance, since the principal fields about the antenna are those within the aperture itself.

Although the invention is especially attractive as a hardened antenna, it is by no means limited to such use but can also be employed in more conventional applications either as a tracking antenna or as a monopulse feed for a parabolic refiector or lens. It will be recalled that the antenna radiates in the TMm mode when feed probes 26 are energized in phase. If, however, the probes are energized in opposite phase, the TEU mode is excited, with the resulting antenna pattern being normal to the aperture. It will be appreciated that this broadside pattern has maximum intensity on the antenna axis, while the TMm mode produces a pattern having a minimum intensity or null on the antenna axis. These patterns will be recognized as the familiar sum and difference patterns of a monopulse tracking antenna. The present invention, therefore, has the capability of providing monopulse information simply by switching the excitation from inphase to opposite-phase. One means by which this switching can be accomplished is shown in FIG. 2, wherein probes 26 are connected via

cables

30 and 32 to a hybrid junction 40 havinga sum port 42 and a difference port 44. When sum port 42 is energized by a signal from a signal source, the probes are fed in phase and a difference pattern results; conversely when difference port 44 is energized, the probes are fed 180 out of phase causing a surn pattern to be produced. The antenna operating in this manner is particularly useful as a monopulse feed for a parabolic refiector since it is hollow and would, therefore, not interfere with optical alignment of the refiector. It can also be used directly mounted in the ground as shown in FIG. l, for developing rough elevation tracking information in hardened or other applications.

Another embodiment of the invention is illustrated in FIG. 3, and consists of a conductive cylindrical pipe 50 of uniform cross-section, except for an aperture portion 52 of enlarged diameter which functions as an irnpedance transformer, as in the embodiment of FIG. 1. Diametrically disposed, at a position approximately midway of the length of pipe 50', are an upper pair of energizing

feeds

54 and 56, and a lower pair of feeds 58 and 60, separated from the upper pair by a distance equal to a quarter wavelength at the center frequency of operation. Each feed comprises a dielectric loaded cavity energized by a probe suitably placed within the cavity as described hereinabove. Upper feeds 54 and 56 are energized by probes 62, while lower feeds 58 and 60 are energized by probes 64. Probes 62 are connected by equal length cables 66 and 68 to an energy source 70 via a T-junction 72. Probes 64 are connected by

equal length cables

74 and 76 through T-junction 72 to energy source 70. The length of arm 78 of T-junction 72 is one quarter wave longer than arm 80. A signal applied via arm 78 to probes 64 will, therefore, lag by 90 the signal applied to probes 62. Since the upper and lower feeds are displaced from each other by a quarter wavelength, as measured in the guide, it is evident that the fields produced by each pair of feeds will combine in phase in the region adjacent the upper -feeds and above, while these fields will cancel in the region below the lower feeds. The region below the lower set of feeds, therefore, contains no electromagnetic field and debris falling into this region does not affect the antenna performance.

This embodiment is more frequency sensitive than the embodiment depicted in FIG. 1, since the field cancellation depends upon precise relative phasing of the energizing feeds and by the accuracy of the quarter wave spacing between the two sets of feeds. In practice, about 99% cancellation of the field in the region below feeds 58 and 60 has been attained. This embodiment, however, has some mechanical advantage over that of FIG. 1 in that the walls of the pipe are straight throughout its length, except for the impedance matching section; thus, there are no protuberances that would receive the force of a shock wave that may propagate down the pipe.

To give the antenna a directional pattern, a suitable refiector, such as the corner reector illustrated in FIG. 4 may be used in conjunction with the present antenna. As shown in FIG. 4, the corner reflector 80 consists of a conductive plate 84 supported by a block of concrete 86 or other material to provide structural strength capable of withstanding an explosive blast. It will be remembered that the present antenna is electrically equivalent to a stub antenna, as indicated by the field configuration illustrated in the aperture 82; therefore, any type of refiector usable with a stub antenna can be similarly employed with the present antenna.

The present invention can also be embodied in a rectangular duct, illustrated in FIG. 5, which is excited in the T M11 mode to produce a bi-directional pattern in the azimuth plane. This embodiment consists of a rectangular conductive duct 90, which can be mounted vertically in the ground in the same manner as described hereinbefore, and which is energized by an upper pair of

feed structures

92 and 94 disposed on opposite side walls of duct 90, and a lower pair of

feed structures

96 and 98 also disposed on respective side walls and vertically disposed one quarter wavelength from

feeds

92 and 94. Each feed structure is similar to that described hereinabove, and consists of a rectangular cavity open to the inside of duct 90; however, in this instance each cavity is fed by a plurality of probes 100 uniformly spaced within the cavity to provide the requisite energization to produce the TMll radiation mode. This ernbodiment has higher gain than that of the circular pipe embodiments since a bi-directional azimuth pattern is produced, rather than the omnidirectional azimuth pattern of the circular pipe antenna. A unidirectional antenna pattern can be provided by placing a vertical planar reflector parallel to one side wall.

The performance of antennas constructed in accordance with the invention is illustrated in FIGS. 6, 7 and 8. FIG. 6 shows the measured VSWR plot of an antenna of the type shown in FIG. 1. The solid curve is the VSWR of the antenna operating without debris, while the dotted curve 112 is the VSWR when the antenna aperture is surrounded by debris. It can be seen that a VSWR of 2.4 or less is maintained over a twenty percent frequency band, and that debris around the aperture has a minimal effect on impedance. No measurable change in impedance could be detected due to debris located in the bottom of the antenna pipe. It is evident, then, that antenna performance is not degraded by the presence of debris around the aperture or in the antenna pipe.

FIG. 7 depicts the measured elevation pattern of an antenna of the type shown in FIGS. 1 or 3. The diameter of the aperture of the measured antenna was approximately three-quarters of a wavelength. The antenna pattern is identical for either embodiment since the pattern is determined by the field configuration within the aperture regardless of the energizing means by which this field configuration is produced. It will be noted that this antenna pattern closely resembles that of a stub antenna, as mentioned hereinabove.

The directional elevation pattern of an antenna of the type depicted in FIG. 4 is shown in FIG. 8, and is seen to be essentially one-half of the omnidirectional pattern of FIG. 7 but having higher gain.

From the foregoing, it is evident that a simple, broadband antenna has been provided which can withstand the extreme environmental conditions caused by a nuclear explosion and which can function unimpaired in the presence of debris.

While there has been shown what are now thought to be preferred embodiments of the present invention, various alternatives will occur to those versed in the art without departing from the true spirit and scope of the invention. Accordingly, it is not intended to limit the invention by what has been particularly shown and described, except as indicated in the appended claims.

What is claimed is:

1. An antenna capable of operating under severe environmental conditions comprising, a conductive cylindrical pipe of uniform inside diameter and having an impedance `matching aperture section on one end thereof, a first pair of energizing feeds diametrically disposed in the wall of said pipe, a second pair of energizing feeds diametrically disposed in the wall of said pipe and axially displaced from said first pair of feeds by a distance equal to a quarter guide wavelength, means for applying energy to said first pair of feeds, and means for applying energy to said second pair of feeds, said last mentioned energy being in phase quadrature with respect to said energy applied to said first pair of feeds.

2. The antenna according to claim 1, wherein each energizing feed comprises a dielectric lled rectangular cavity having an aperture end disposed iiush with the inside Wall of said cylindrical pipe.

3. An antenna capable of operation under severe environmental conditions comprising, a conductive duct of uniform cross section and having an impedance matching aperture section on one end thereof, a first pair of energizing feeds respectively disposed in opposite side walls of said duct, a second pair of energizing feeds respectively disposed in said side walls and axially displaced from said rst pair of feeds by a distance equal to a quarter guide wavelength, means for applying energy to said first pair of feeds, and means for applying energy to said second pair of feeds, said last-mentioned energy being in phase quadrature with respect to said energy applied to said first pair of feeds.

4. An antenna capable of operation under severe environmental conditions comprising, a c-onductive cylindrical pipe having a central portion of a diameter which can sustain a TMm electromagnetic field, an impedance matching aperture region on one end of said pipe of a diameter larger than said central portion, a taper section connecting said aperture region and one end of said central portion, a region of smaller diameter than said central portion on the other end of said pipe dimensioned to be incapable of sustaining said TMm field, a taper section connecting said region of smaller diameter and the other end of said central portion, first and second dielectric filled feed cavities diametrically disposed in the wall of said central portion, each having an aperture end iiush with the inside wall of said central portion, first and second feed probes disposed Within respective cavities and operative to couple energy thereto or therefrom, and means for coupling energy to or from said feed probes.

5. An antenna capable of operating under severe environmental conditions comprising, a conductive cylindrical pipe having a central portion of a diameter which can sustain a specified electromagnetic field, an impedance matching aperture region on one end of said pipe of a diameter larger than said central portion, a taper section connecting said aperture region and one end of said central portion, a region of smaller diameter than said central portion on the other end of said pipe dimensioned to be incapable of sustaining said specified field, a taper section connecting said region of smaller diameter and the other end of said central portion, first and second feed cavities diametrically disposed in the wall of said central portion, each having an aperture end ush with the inside wall of said central portion, and means for coupling energy to or from said cavities.

References Cited UNITED STATES PATENTS 2,343,531 3/ 1944 Buchholz 343-786 2,369,808 2/ 1945 Southworth 343-719 X 2,398,096 4/1946 Katzin 343-786 X 2,587,055 2/1952 Marshall 33.3-10 X 2,748,350 5/1956 Miller 333-10 2,809,371 10/1957 Carter et al. 343-786 3,267,475 8/1966 Howard 343-786 X ELI LIEBERMAN, Primary Examiner.

CHESTER L. JUSTUS, Examiner.

R. E. BERGER, Assistant Examiner.

Claims

5. AN ANTENNA CAPABLE OF OPERATING UNDER SEVERE ENVIRONMENTAL CONDITIONS COMPRISING, A CONDUCTIVE CYLINDRICAL PIPE HAVING A CENTRAL PORTION OF A DIAMETER WHICH CAN SUSTAIN A SPECIFIED ELECTROMAGNETIC FIELD, AN IMPEDANCE MATCHING APERTURE REGION ON ONE END OF SAID PIPE OF A DIAMETER LARGER THAN SAID CENTRAL PORTION, A TAPER SECTION CONNECTING SAID APERTURE REGION AND ONE END OF SAID CENTRAL PORTION, A REGION OF SMALLER DIAMETER THAN SAID CENTRAL PORTION ON THE OTHER END OF SAID PIPE DIMENSIONED TO BE INCAPABLE OF SUSTAINING SAID SPECIFIED FIELD, A TAPER SECTION CONNECTING SAID REGION OF SMALLER DIAMETER AND THE OTHER END OF SAID CENTRAL PORTION, FIRST AND SECOND FEED CAVITIES DIAMETRICALLY DISPOSED IN THE WALL OF SAID CENTRAL PORTION, EACH HAVING AN APERTURE END FLUSH WITH THE INSIDE WALL OF SAID CENTRAL PORTION, AND MEANS FOR COUPLING ENERGY TO OR FROM SAID CAVITIES.