United States Patent 3613098

An electrically small cavity antenna especially adapted for flush mounting in an aircraft vertical stabilizer. A cavity is formed in the vertical stabilizer so as to have a pair of radiation windows or apertures on opposite sides of the stabilizer. An energy coupling device is placed substantially in the center of the cavity to couple energy from a feeding network to the cavity. The energy coupling device includes a pair of spaced-apart plates with a tapered portion disposed between the plates. Energy is coupled from the feed network to one of the plates and the narrow end or apex of the tapered portion.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
343/769, 343/789
International Classes:
H01Q1/28; H01Q9/28; (IPC1-7): H01Q1/28
Field of Search:
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US Patent References:
3295137Shortened folded monopole with radiation efficiency increased by ferrite loadingDecember 1966Fenwick et al.
2845624Low drag airplane antennaJuly 1958Burberry
2724052Radio antennasNovember 1955Boyer
2700104Antenna feed systemJanuary 1955Bowman
2644090Recessed slot antennaJune 1953Dorne

Primary Examiner:
Lieberman, Eli
What is claimed is

1. A cavity antenna comprising

2. The invention according to claim 1

3. The invention according to claim 1

4. The invention according to claim 2


This invention relates to a new and improved antenna and, in particular, to a cavity type antenna which is of relatively small physical size.

The fundamental limitations imposed on the performance of a small cavity antenna by its physical size are well known. In general, as a cavity antenna is made smaller, its radiation efficiency and bandwidth become smaller. Such limitations, as the foregoing, become most severe when the maximum physical dimension of the cavity antenna aperture is considerably less than one-half of a free space wavelength at the lowest frequency of interest.

For such small cavity antennas, the bandwidth can be increased by resistive loading. However, such resistive loading decreases the radiation efficiency such that the gain is reduced by the same factor as the bandwidth is increased. That is, the overall gain is the product of the directive gain and the radiation efficiency.

For example, at the very high frequency (VHF) range of communications used by aviation (118 to 136 megahertz, Mhz), a free space half wavelength corresponds to an aperture dimension on the order of 4 feet. It is apparent that, for most aircraft, a cavity having aperture dimensions on the order of 4 feet would seriously disrupt the design of the aircraft for either external or internal (flush) mounting. By employing the aforementioned resistive loading technique, low efficiency cavity antenna dimensions on the order of 20 inches have been flush mounted in relatively large aircraft in the navigation band (108 Mhz.). However, such resistively loaded cavity antennas not only have low efficiency, but are impractical for use in smaller aircraft.


An object of this invention is to provide a new and improved cavity antenna.

Another object is to provide a novel cavity antenna having aperture dimensions considerably less than one-half wavelength without resistive loading but yet having the electrical characteristics of a much larger cavity antenna.

Yet another object is to provide a new and improved energy coupling device.

In brief, a cavity antenna embodying the present invention includes a cavity having at least one aperture and an energy coupling device supportably mounted within the cavity. The coupling device includes an electrically conductive tapered portion having a narrow and a wide end. An electrically conductive plate is supported adjacent the narrow end of the tapered portion. Energy feeding means has separate connections to the plate and to the narrow end of the tapered portion.

In one embodiment of the invention, the tapered portion has the shape of the cone while in another embodiment it has a multisided shape. In either case, another electrically conductive plate is preferably attached to and covers the wider end of the cone or multisided tapered portion.


In the drawings, like reference characters denote like structural elements; and

FIG. 1 is a dimensional view of a flush mounted cavity antenna embodying the invention; and

FIG. 2 is a planned view of an energy coupling device embodying the present invention; and

FIG. 3 is a sectional view taken along the lines of 3--3 of FIG. 2; and

FIG. 4 is a dimensional view of another energy coupling device embodying the invention.


Although the cavity antenna of the present invention can be used in any desired frequency range, it is especially useful for VHF communications. By way of example and completeness of description, the cavity antenna of the invention, herein illustrated, is embodied as a flush mounted aircraft communications antenna. Generally, VHF communications aviation band antennas are required to operate over the frequency range of 118 to 136 Megahertz (Mhz.), to have a voltage standing wave ratio (VSWR) of less than 2:1, to have a nominal gain of zero decibel (db.), to have vertical polarization and to have an essentially omnidirectional radiation pattern in the azimuth plane.

In the flush mounted embodiment shown in FIG. 1, a cavity 10 is formed in a suitable portion of the aircraft frame, such as vertical stabilizer 11, of which only a portion is illustrated. The cavity 10 is formed in a suitable portion of the aircraft frame, such as vertical stabilizer 11, of which only a portion is illustrated. The cavity 10 has a pair of sidewalls 12 and 13, a top wall 14, and a bottom wall 15; each wall being of a suitable electrically conductive material, such as metal, for example, aluminum. Each of the walls may have a geometrical contour, as illustrated, so as to conform to the contour and structural design of the aircraft. The cavity 10 also has a pair of apertures (or windows) disposed on opposite sides of vertical stabilizer 11 and having suitable radomes 16 and 17 attached thereto. The radomes 16 and 17 may be of any suitable electromagnetically transparent material, such as fiberglass, and are attached to stabilizer 11 by any suitable fastening means, such as screws or bolts (not shown).

Radome 17 is shown in FIG. 1 as removed from stabilizer 11 in order to conveniently illustrate the cavity 10 and an energy coupling device 18, which is disposed within cavity 10 when assembled. The arrows in FIG. 1 illustrate the respective locations of coupling device 18 and radome 17 relative to cavity 10 when in assembled form.

The coupling device 18 includes front and backplate portions 19 and 20, respectively, and a tapered or flared portion 21 disposed therebetween. The plates 19 and 20, and the tapered portion 21 may be any suitable electrically conductive material; such as metal, for example, brass, aluminum, copper, and the like. The tapered portion 21, which may suitably have the shape of a cone, has its wide end attached to front plate 19 by any suitable means, for example, a solder or weld joint. The cone 21 is secured by way of a dielectric support 22 to backplate 20 such that the narrow end (apex) of the cone is electrically isolated, as by spacing, from backplate 20 (best seen in FIG. 3). The dielectric support 22 is attached to the backplate 20 and to the cone 21 by any suitable means, such as bonding or hardware. The backplate 20 has been partially broken in FIG. 1 in order to illustrate the support 22.

The coupling device 18 is fed by any suitable feeding device 23, such as the illustrated coaxial cable. The cable 23 extends downwardly through the bottom wall 15 of cavity 10 into the vertical stabilizer 11 and hence to the aircraft fuselage (not shown) where it is connected to a transmitting and/or receiving device (also not shown). The attachment of cable 23 to the coupling device 18 is best seen in FIGS. 2 and 3. The central conductor 24 is attached, as by soldering, to the narrow end of the cone 21 and the outer conductor 25 is attached to the backplate 20. Although the connecting order of the central and outer conductors may be reversed, the illustrated order is preferred for lightening protection. In assembled form, the coupling device 18 is located substantially in the center of cavity 10 and is supported therein by any suitable supporting means, preferably of a low dielectric material. For example, in one cavity antenna embodiment actually constructed, the coupling device 18 has been supported in cavity 10 by means of a low dielectric constant foamlike material. Also in the assembled form, the radome covering 17 is fastened to the vertical stabilizer 11 so as to fit over the cavity aperture and secure the supporting foam and coupling device 18 within the cavity.

For impedance matching purposes, a stub 26 (for example, a strip of metal) is coupled between he backplate 20 of coupling device 18 and the top wall 14 of cavity 10. The stub 26 may be fastened to backplate 20 and the cavity wall 14 by any suitable electrical connecting technique as, for example, a solder joint or a bolted joint. The width and the length of stub 26 are preferably selected for optimum impedance matching of cavity 10 with coupling device 18 and feed cable 23. Although stub 26 may have other locations (e.g., between cone 21 and plate 20), it preferably has the illustrated location for mechanical support as well as lightening protection.

The coupling device 18 serves the function of transferring energy between cavity 10 and the feed cable 23. It has been found that the coupling device 18 permits the apertures of cavity 10 to have physical dimensions substantially smaller than one-half of a free space wavelength at the lowest frequency of interest, but yet provide the electrical characteristics of a much larger cavity. For example, in one cavity antenna design for the VHF aviation communication band, the aperture dimensions were approximately 12 inches. That is, the cavity walls 12, 13, 14 and 15 were each approximately 12 inches in length which corresponds to an aperture dimension on the order of one-tenth of a free space wavelength at the lowest frequency (118 Mhz.) of interest. The azimuth patterns of the constructed cavity antenna (which was vertically polarized) were omnidirectional within 1 db. and were measured every 2 Mhz. from 118 to 136 Mhz. The gains, which were measured by comparing the antenna to a vertically polarized dipole, varied from -2.0 to 1.0 db. with respect to the dipole. The antenna range used to measure the radiation patterns and relative gain was a 100 -foot ground plane range calibrated for measurements in the VHF band. The VSWR was less than 2:1 throughout the frequency band of 118 to 136 Mhz.

The conical shape of the tapered portion 21 is preferred since it can be easily fabricated at low cost. However, other tapered shapes can be employed. For example, a multisided shape, such as the four sided (pyramidal) shape 27 illustrated in FIG. 4 can be used. The plate 19 of the conical and pyramidal coupling device embodiments is desirable to provide symmetry as well as to provide an area which is compatible with the area of the aperture. However, it is to be noted that the cavity antenna will also operate with plate 19 removed.

It should also be noted that cavity 10 could be located in portions of the airframe other than the vertical stabilizer 11 and that the antenna can be designed for frequency bands other than the aviation band. Indeed, the cavity antenna can be embodied in applications other than flush mounted aircraft applications.