US2996713A - Radial waveguide antenna - Google Patents

Description

345 -100 NS? AU 25a Ex PTHS'O xa 299961713 Aug. 15, 1961. I J. M. BOYER 2,995,713

RADIAL WAVEGUIDE ANTENNA l Filed Nov. 5, 1956 (A 2 M'wL. 70p

\ u i l A/M flan E2 10 "/055? INVENTOR.

5&5. BY

United States Patent f 2,996,713 RADIAL WAVEGUIDE ANTENNA Joseph M. Boyer, Redondo Beach, Calif., assignor to Antenna Engineering Laboratories, Torrance, Calif., a

limited partnership Filed Nov. 5, 1956, Ser. No. 620,328 1 Claim. (Cl. 343-745) The'present invention relates to antennas for receiving and transmitting radio frequency energy.

One of the important objects of the present invention is to provide an antenna, useful in the reception or transmission of radio frequency energy which is efiicient for such purposes notwithstanding that its height is relatively small in comparison to a quarter wavelength of the radio frequency energy being received or transmitted.

Another important general object of the present invention is to provide an improved antenna structure capable of being operated in conjunction with any electrically conductive plancwhether such conductive plane be of earth soil, metallic sheet, or defined by other div erse means-and capable of operating at any frequency within the entire electromagnetic spectrum-for the efficient transmission or reception of radio frequency energy.

A specific object of the present invention is to provide an improved antenna structure mounted on the metal roof of an automobile and cooperating therewith for the etficient transmission or reception of radio frequency energy.

-'-' Another specific object of the present invention is to provide an improved antenna, for mobile use, such as on police cars, with the antenna having a very small height in comparison to a quarter wavelength of the energy being transmitted or received and such that the antenna may be completely concealed from view.

Another specific object of the present invention is to provide an improved antenna structure which uniquely utilizes the non-uniform characteristic impedance properties of a radial transmission line" for its operation.

Another specific object of the present invention is to provide an improved antenna structure involving generally uniformly spaced parallel metallic sheets which have adjacent ends interconnected at one end with the energy feedpoint being generally near the center of the sheets and with a condenser interconnecting adjacent ends of the sheets at a location remote from the previously mentioned interconnected ends.

Another specific object of the present invention is to provide an improved antenna structure, especially useful in the transmission and reception of vertically polarized waves.

Another specific object of the present invention is to provide an improved antenna structure, generally in the form of a slot radiator.

Another specific object of the present invention is to provide an antenna characterized by its relatively small height but which has inherently a large amount of loading capabilities so that the amount of external lumped capacity required for proper loading is relatively small.

Another specific object of the present invention is to provide an improved antenna which consists essentially of a radial transmission line.

Another specific object of the present invention is to provide housings for such antenna.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. This invention itself, both as to its 0rganizationand manner of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in connection with the accompanying drawings innrxhich 2,996,713 Fatented Aug. 15, 1961 FIGURE 1 illustrates a portion of an automobile having a novel antenna mounted thereon in accordance with features of the present invention.

FIGURE 2 is a sectional view taken substantially on the line 2-2 of FIGURE 1.

FIGURE 3 is a view similar to FIGURE 1, with however, the top artificial cover removed from the automobile.

FIGURE 4 is a perspective view illustrating the structure shown mounted on the roof of the automobile in FIGURE 3.

FIGURE 5 is generally a transverse sectional view showing the elements of the arrangement illustrated in FIGURE 4 separated, and FIGURE 5 also illustrates the manner in which the antenna element is connected electrically to the metal roof of the automobile as well as to a coaxial cable.

FIGURE 6 is a perspective view illustrating one of the elements shown in FIGURE 5.

FIGURE 7 is a top plan view taken generally as indicated by the arrows 7-7 in FIGURE 5.

FIGURE. 8 is a section through the housing 20.

While the drawings illustrate the antenna associated with an automobile where indeed, the roof of the auto mobile serves as a ground plane, the antenna incorporating features of the present invention has utility elsewhere, for example, on yachts, ships, boats, military vehicles, tanks and indeed also on stationary structures, i.e., at fixed stations. In each instance, a stationary extended area coacts with an antenna element in a new way in achieving the new results described. I

The antenna structure in general comprises the follow ing elements namely: a substantially fiat metallic sheet 10 which in this particular application of the invention is the metal roof 10 of the conventional automobile A; a second metallic sheet I1 uniformly spaced a distance it above the roof 10 with a connection between adjacent. ends of the sheets 10, 11, such connection, in this instance comprising a generally L-shaped tab 11A integrally formed with sheet 11 and connected electrically,

by soldering, brazing, bolting, or the like to the metal? roof 10; a condenser 14 connected between the center conductor 13 of shielded coaxial cable 14 and a point 15 on the sheet 11, such point being substantially in the center of the sheet 11; and a condenser 16 is connected between adjacent points 17, 18 of sheets 10 and 11. The metallic sheath of cable 14 is connected to the sheet 10 at the apertured portion 10A.

These enumerated elements of the antenna are preferably enclosed in insulated structures 20 and 21, which are described later, but such structures are not considered essential to the electrical operation of the antenna.

It is observed that this antenna is a type of slot radiator and it uniquely uses the non-uniform characteristic impedance properties of a "radial transmission line" for its operation.

While the sheet 11 is illustrated as being circular, the same may be square, triangular, elliptical or of any other geometrical shape other than that of a narrow reetangle.

The theory of operation of the antenna may be considered in the light of the following consideration involving the equations applicable to a conventional transmission line in the form of a wire suspended above a ground plane, and also to a "radial transmission line."

In a transmission line comprising a uniform diameter wire of length L suspended at a constant height, Ii, over a large metallic plane body, the characteristic or surge impedance is a constant and is equal to Z =LC, where L is the distributed inductance per unit length and C is the distributed capacity per unit length. When such 0 v t n "ifw s- W 3 line is conductively shorted at one end, and the length L is shorter than one quarter wavelength of an exciting 'alterhating current applied to the line, then such line has a positive .or inductive reactance across itsoutput terminals equal to:

I where X is the reactive magnitude, Z is the uniform characteristic impedance of the transmission line section and o is the electrical degrees of length L. The sign of such reactance for lengths of L less than one quarter wavelength is,positive or of an inductive characteristic. When the open end of the line is terminated by a pure reactance of negative sign, i.e. a condenser having a capacity equal I to iX is connected between the terminals of the open end,

then the line becomes resonant and the maximum voltage appears across the open end of such line section.

As the length of the line L is decreased, the capacity value required to achieve such resonant condition increases rapidly in greater proportion; and at the same time the efiiciency becomes less.

In comparison to transmission lines of the type mentioned above, the "radial transmission line incorporated in the antenna structure produces less losses and thus is more eflicient in energy conversion. These losses are low because of large conductor areas, less concentration of current on thin conductor edges and so forth.

Of much more importance however is that a circular conducting sheet uniformly spaced at a constant height In over a larger metallic sheet, as illustrated, has a non uniform characteristic impedance, i.e. the impedance is different at difierent points spaced radially from the center of the circular sheet. This relationship may be expressed as following:

where T is a constant depending upon the medium, I: is

the spacing, r is the radius of sheet 11, and Z is the characteristic impedance.

Thus it is seen that the characteristic impedance Z is a continuously varying function of r and decreases as r increases. As a practical matter, even with conductors having a finite conductivity, the characteristic impedance at r=0, corresponding to the center of the circle is extremely great.

It is understood that reflection takes place at a point along the transmission line where the characteristics impedance changes. This is used to advantage. The ratio of incident to reflected power is a function of the impedance change Z/Z at a given point of r. Analysis has shown that such discontinuities constitute, in effect, a capacitor or capacitative reactance at that point. Thus. the arrangement inherently has a large amount of self loading, i.e. capacitive reactance so that the amount of external lumped capacity required to be added for proper loading is quite small, even in the case where the radial line is small in terms of the operating wavelength. Further, the multiplicity of non uniform capacitances which may be considered to exist, due to the very nature of the line permit greater efficiencies for artificial loading purposes than lumped condenser means.

Using a radial line antenna, as previously described, with the diameter of the sheet 11 being 0.04 wavelength of the wavelength of the current applied thereto via cable 14, the circumferential aperture formed between the edge of sheet 11 and the metal surface forms an efiicient radio antenna of extremely small height, h. A normal working height for "it" has been found to be 0.002 wavelength at the operating frequency.

' The term eflicient as used above has reference to a condition wherein there is not more than a difference of two decibels between the field strength, from the antenna, as described herein, when compared with the field strength developed by a quarter wavelength rod an enna operating over the same ground plane with the same applied power, the field intensities in each instance being measured one mile distant from the antennas.

Also, tests have shown that when the diameter of the sheet 11 approaches 0.08 wavelength the field sttcngth radiated therefrom is substantially identical with that radiated from a full quarter wavelength rod antenna. The antenna as described herein, produces a vertically polarized signal.

The condensers 12 and 16 may be fixed condensers but are preferably adjustable condensers. In general, condenser 16 is adjusted so that the "radial line" defined by the adjacent surfaces of sheet 10, 11 is resonant; and condenser 12 is adjusted so that the input impedance as seen" at the input terminals represented by the coaxial cable 14 in FIG. 5 is a pure resistance having a magnitude comparable to the characteristic impedance of commercially available cable connected to such input terminals, the condenser 12 serves essentially as a means for obtaining a satisfactory impedance transformation between the coaxial cable impedance and that of the feedpoint on sheet 11, i.e. the fecdpoint of the antenna.

For esthetic, security, or other purposes, the antenna may be confined in an insulating housing 20 and/or 21.

The housing 20 as shown in FIGURES 4 and 8 is essentially a flanged cylinder having a fastening flange 20A and an upper inturned peripheral portion 208 for engaging and confining the peripheral edge 7 of the circular sheet 11.

In order to preserve the streamlining of the automobile, a false top 21 of insulating material covering the antenna is mounted on top of the automobile. This housing 21 is a streamlined structure which blends in with the streamlining of the automobile. The housing 21 is of fibreglass molded to conform to the outline of the vehicle and is faired in. Such top 21 is retained by conventional means such as self tapping screws with weatherstripping between the housing 21 and vehicle roof 10.

As shown, the height of the housing 20 is approximately 1% inches and the housing 20 is approximately two feet in diameter at its upper portion.

The housing 21 is spaced a maximum distance of approximately l /i inches above the vehicle roof to accommodate the housing 20.

Either housing 20 or 21 may be filled with foam material 25 to provide mechanical rigidity.

Preferably, the condensers 12 and 16 are air condensers, i.e. have air as the dielectric medium.

It is observed, from the above, that the height of the antenna, i.e. the quantity h is approximately l n of one quarter wavelength of the operating frequency.

As mentioned above, the sheet It may have other shapes than a circular shape. The thickness of the metallic sheet 11 is immaterial so long as it has good conductivity. Its thickness is usually dictated by mechanical stability. Also, as previously indicated. the sheet 11 should have a large extended area defined by a geometricnl configuration in which one surface dimension should not greatly exceed the other surface dimension. The

longest and shortest surface dimensions should fall within a ratio not greater than approximately a ratio of l2 to 1. Further, while, as described above, the center conductor of the coaxial cable is connected via condenser 12 to the geometrical center of the sheet 11, the connecting point on the sheet 11 may deviate from the geometrical center. However, such deviation should not exceed a distance greater than approximately .OOl of a wavelength of the operating frequency.

It is also noted from the above that the radial transmission line is operated in its lowest or zero mode.

While the particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications 5 may be made without departing from this invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

An antenna of small height comprising a large conducting plane, a thin conducting sheet mounted at a substantially uniform height over said plane, an air capacitor, said sheet being conductively shorted to said plane at a first region and being reactively connected to said plane by said air capacitor at a second region remote from the first ;region, a coaxial cable having a metal sheath connected to said plane and having a conductor, a second air capacitor, said conductor being connected through said second air capacitor to said sheet at substantially its geometrical center, said second capacitor serving to provide an impedance transformation between said conductor and said center.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Flush Mounted Antenna for Mobile Application," by

Rhodes, Electronics, March 1949, pages 115 to 117.

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