Ridged waveguide antenna with concave-shaped sidewalls

A radio frequency antenna comprising a rectangular waveguide having opposing relatively concave-shaped narrow side wall portions extending from a rear wall of the waveguide structure to an aperture of the antenna element. The separation between the sidewalls, at a distance intermediate the rear wall and the aperture, is greater than the width of the aperture. The waveguide includes a ridge-shaped feed structure extending from the rear wall to the aperture. A pair of ground plane conductors having surfaces each with an edge terminating along an aperture edge of a corresponding one of the sidewalls at the periphery provide a pair of aperture edges. Each one of the ground plane conductors extends for a length greater than .lambda./3 where .lambda. is the wavelength at the lowest operating frequency of the antenna. With such arrangement, an antenna element is provided having a relatively constant beam width over the operating bandwidth of the antenna.

BACKGROUND OF THE INVENTION 
This invention relates generally to radio frequency antennas and more 
particularly to radio frequency antennas adapted to operate over 
relatively wide frequency bandwidths. 
As is known in the art, many installations for array antennas impose 
physical constraints on the size of such antennas. For example, in an 
airborne installation each one of the antenna elements in the array 
thereof should have minimum depth, width and thickness. Further, in many 
applications it is necessary that the antenna provide a relatively 
constant beam width over a relatively wide frequency bandwidth. 
SUMMARY OF THE INVENTION 
A radio frequency antenna comprising a rectangular waveguide having 
opposing relatively concave-shaped narrow side wall portions extending 
from a rear wall of the waveguide structure to an aperture of the antenna 
element. The separation between the sidewalls, at a distance intermediate 
the rear wall and the aperture, is greater than the width of the aperture. 
The waveguide includes a ridge-shaped feed structure extending from the 
rear wall to the aperture. A pair of ground plane conductors having 
surfaces each with an edge terminating along an aperture edge of a 
corresponding one of the sidewalls at the periphery provide a pair of 
aperture edges. Each one of the ground plane conductors extends for a 
length greater than .lambda./3 where .lambda. is the wavelength at the 
lowest operating frequency of the antenna. With such arrangement, an 
antenna element is provided having a relatively constant beam width over 
the operating bandwidth of the antenna.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a multibeam radio frequency antenna system 10 
adapted to operate over a relatively wide band of frequencies, here from 
5.0 GHZ to 18.0 GHZ, is shown to include a radio frequency lens 12 having 
a plurality of feedports 14a to 14n disposed along a portion of the 
periphery of such lens 12 and a plurality of, here 8, array ports 16.sub.1 
to 16.sub.8 disposed along an opposing portion of the periphery of such 
lens 12, each one of the plurality of array ports 16.sub.1 to 16.sub.8 
being coupled to a corresponding one of a like plurality of antenna 
elements 22.sub.1 to 22.sub.8, in an array 24, through a corresponding one 
of a plurality of coaxial transmission lines 20.sub.1 to 20.sub.8, as 
shown. The shape of the lens 12, the lengths of the transmission lines 
20.sub.1 to 20.sub.8 and the arrangement of the antenna elements of 
22.sub.1 to 22.sub.8 are such that n collimated beams of radio frequency 
energy are formed in free space by the antenna system 10, each one of such 
n beams having a different direction, as described in U.S. Pat. No. 
3,761,936, issued Sept. 25, 1975, "Multi-Beam Array Antenna", Inventors 
Donald H. Archer, Robert J. Prickett and Curtis P. Hartwig and assigned to 
the same assignee as the present invention. 
Array 24 includes a plurality of, here 8, identically constructed 
conductive members 26.sub.2 to 26.sub.9, an exemplary one of such members 
26.sub.2 to 29.sub.9, here member 26.sub.2 being shown in detail in FIGS. 
2-5, a pair of such members 26.sub.2 to 26.sub.9 forming one of the 8 
identical constructed antenna elements 22.sub.1 to 22.sub.8. Thus, an 
exemplary one of the antenna elements 22.sub.1 to 22.sub.8, here antenna 
element 22.sub.1, includes member 26.sub.1 and 26.sub.2, as shown in FIGS. 
6, 7 and 8. 
Referring now in more detail to members 26.sub.1 to 26.sub.9, each one of 
such members 26.sub.1 to 26.sub.9 is constructed from a block of 
electrically conductive material, here aluminum, here having outer 
dimensions of 1.75 inches (length L) and 2.25 inches (width W). The upper 
surface of such block, as shown more clearly in FIG. 6 for member 26.sub.2 
has machined therein outwardly bow-shaped (or concave) side wall portions 
34, 36 (FIG. 2) and a rear wall portion 38 having a recess notch 40 formed 
therein. The depth of the side wall and rear wall portions is here 0.36 
inches. Also machined into the upper surface 30 of the members 26.sub.1 to 
26.sub.9 is a tapered ridge 42, as shown, here having a width of 0.19 
inches. The tapered ridge 42 has an aperture 44 formed in the upper, flat 
top portion 46 thereof, the flat top portion 46 terminating in a tapered 
portion 48, (FIGS. 5, 6, 7) as shown. The length of the tapered portion 48 
is here 0.9 inches. The distant end of the tapered portion 48 terminates 
at the open end of the waveguide (i.e. at the aperture 49 of the antenna 
element). The depth of the notch 40 formed in the rear wall portion 38 is 
here 0.075 inches, such notch 40 having a length along the rear wall 
portion 38 of, here, 0.588 inches. It is noted that the separation between 
the side wall portions 34, 36 disposed laterally of the tapered portion 48 
increases progressively from the rear wall to a point P approximately 0.8 
inches from the aperture and thus such separation decreases to 0.95 inches 
at the aperture. As will be discussed in more detail hereinafter, 
converging the side wall portions 34, 36 as they extend towards the rear 
wall portion 38 in the region behind the aperture 44 (such aperture being 
the area where the antenna element 22.sub.1 formed by such member 26.sub.1 
together with member 26.sub.2 is fed by the coaxial transmission line 
20.sub.1, (FIG. 1) in a manner to be described) improves the impedance 
matching between the coaxial transmission line 20.sub.1, (here a 50 ohm 
line) and the antenna element 22.sub.1. Member 26.sub.2 also has holes 60 
drilled through it, such being used for bolting the members together with 
bolts and nuts (not shown). 
A pair of ground plane surfaces 70, 72 of the member 26.sub.2, (FIG. 6) 
have edges 73, 75 terminating along the aperture 49 and the sidewalls 34, 
36. Thus, the ground plane surfaces 70, 72 are adapted to allow 
flush-surface mounting of this antenna within an airborne vehicle (not 
shown). 
Referring now to the bottom surface 28 of member 26.sub.1 (shown more 
clearly in FIGS. 3-6) such surface 28 also has a tapered ridge 85 formed 
thereon; here, however, the flat portion 83 of the ridge 85 has a turret 
shaped conductive post 86 (here shown) press fit therein by a pin-shaped 
end 87 as shown in FIG. 9. Post 86 has a hole 88 drilled therein as shown 
for receiving the center conductor 102 of a coaxial connector 100 (FIGS. 7 
and 8) in a manner to be described in detail in connection with FIG. 9. It 
is noted from FIGS. 3 and 5 that the tapered ridges 48, 85 formed on the 
upper and lower surfaces of member 26.sub.1 are in alignment or 
registration with each other. Further, it is evident from FIGS. 2-6 that 
the post 86 of member 26.sub.1 fits into the aperture 44 of member 
26.sub.2 as shown in FIGS. 7, 8 and 9. 
When members 26.sub.1, 26.sub.2 are affixed together, the lower surface 28 
of member 26.sub.1, and the upper surface 30 of member 26.sub.2 form 
opposing upper and lower wide surfaces of a hollow rectangular, open ended 
waveguide structure and side wall portions 34, 36 and rear wall portion 38 
form narrow side and rear walls of such open ended, rectangular waveguide. 
More particuarly, the affixed members 26.sub.1, 26.sub.2 formed a tapered 
ridge rectangular waveguide antenna element 22.sub.1. Surfaces 105, 106 of 
member 26.sub.1 contact surfaces 109, 110 of member 26.sub.2 respectively 
as shown in FIG. 6 so that the flat portions 46, 83 of the ridges 42, 85 
are separated a distance "d" (FIG. 9), and the side walls of the 
waveguide, i.e. surfaces 28, 30 are separated a distance "b". The 
distances "b" and "d" are designed so that the waveguide propagates in the 
TE.sub.10 mode. Here, "d" is 0.045 inches and "b" is 0.325 inches. The 
tapered ridge waveguide antenna elements 22.sub.1 to 22.sub.8 are fed by 
the coaxial transmission line 20.sub.1 to 20.sub.8 through coaxial 
connectors 100 (FIGS. 7, 8) having a center conductor 102 (FIG. 9) passing 
through hole 104 (FIGS. 7, 9) and the end of such center conductor 102 
press fit to post 86 to provide electrical and mechanical contact to post 
86. The outer conductor 105 is electrically and mechanically connected to 
the member 26.sub.2 through screws (not shown). The inner conductor 102 is 
separated from the walls of the hole 104 by a dielectric sleeve 103 as 
shown. A ferrite ring 107 is disposed around the inner conductor 102 
between the dielectric 103 and the post 86, as shown in FIG. 9 to provide 
impedance matching between the coaxial connector 100 and the post 86. 
Radio frequency energy fed to the antenna element 26.sub.1 via connector 
100 thus launches radio frequency energy into cavity 108 (FIG. 8). The 
gradually curved contours of the side walls to the relatively narrow 
aperture permits the electric field to continue to propagate towards the 
reduced aperture 49. The ground plane conductors' surfaces define the 
beamwidth of the height-plane and provide approximately constant beamwidth 
in the width dimension W as a function of frequency. There, the length A 
(FIG. 2) of each of the ground plane conductors 70, 75 is 0.36 inches. The 
dimension A is constrained and that in a particular array it would not 
exceed .lambda./2 at the upper frequency. It could, however, be changed 
for non-array applications. In any event, the length A should be greater 
than .lambda..sub.L /3 where .lambda..sub.L is the wavelength at the 
lowest operating frequency to provide a substantially constant beamwidth 
over the operating band of frequencies. The launched energy then travels 
towards the open end or aperture 49 of the cavity in the TE.sub.10 mode 
having an electric field vector extending between the wide surfaces of the 
waveguide as shown by arrow E in FIGS. 8 and 9. 
Having described a preferred embodiment of the invention, it is now evident 
that other embodiments incorporating these condepts may be used. It is 
felt, therefore, that the invention should not be restricted to the 
disclosed embodiment but rather should be limited only by the spirit and 
scope of the appended claims.