Durable, lightweight, radar lens antenna

Background of the Invention

The present invention relates to radar antennas, and has particular relation to radar antennas which function as a radome and also faction as lenses to focus the radiation which passes through them.

When a radar is used on an automobile, it must meet the road driving environment by having a radome which will protect the radar from road debris as well as from air. It is desirable for the radome to also perform a focusing function for the antenna. A conventional radome durable enough to provide such service has generally been considered to be unsuitable for even semi-precise focusing.

Scanning an antenna beam rapidly with a lens antenna is usually achieved by electronically switching between multiple feed horns. However, it becomes impractical to do this in most missile sensor applications. Producing an adequate scan field of view results in unacceptably high insertion losses of the series connecting switches. It also results in unacceptably high antenna side lobe levels for far out, off axis beams.

Scanning can be accomplished by moving a durable, lightweight antenna lens back and forth in front of a fixed antenna feed. However, the speed of scanning is inversely proportional to the weight of the lens, and prior art radomes are also heavy. Fortunately, lightness and durability may be traded off with each other. If a moderate weight lens of considerable durability is placed in a protected environment, it can easily be modified to be less durable, but very light. The forward end of a missile, behind a separate radome, is such a protected environment.

Summary of the Invention

The present invention overcomes these deficiencies by modifying a conventional Fresnel lens design. The conventional curved lens surface is replaced with steps approximating it, preferably three steps. The thickness of the stepped lens, at each step, is a half-wavelength or a multiple half-wavelength of the radar operating frequency in the medium of the lens. The half-wavelength or multiple half-wavelength separation of the steps causes reflections from the front and rear surfaces to cancel. This provides a nearly perfect impedance match, thereby minimizing the (undesirable) standing wave between the lens and the feed horn. This thereby further avoids the necessity of using other standing wave reduction methods, such as (a) coating one or both surfaces with an anti-reflection coating, or (b) presenting the curved or stepped side forward to avoid focusing of reflected signals at the antenna feed. Coatings are expensive, and the planar side of the lens (unlike the stepped side) doesn't need to be protected from road debris. The lens and radome can therefore be molded as an integral unit, desirable in the automotive setting.

This lens is also thin enough (and, therefore, light enough) that it can be moved side-to-side very rapidly, preferably by a combination of cams and springs. This allows a very fast scan, albeit over a limited field of view. Gimballing the lens and feed horn together provides a slower scan, but gives an unlimited field of regard. Combining the two gives a fast scan and unlimited regard, desirable in the missile setting.

Brief Description of the Drawings

Figure 1 shows the contour of a Fresnel lens in three embodiments: fill zone <b>10</b>; partial zone <b>12</b>; and step zone <b>14</b>.

Figure 2 shows the details of the step zone lens <b>14</b>.

Figure 3 shows the lens <b>14</b> in operation, refracting radar signals both directly from the feed horn or feed horns <b>46, 48, 50</b> and indirectly from an enclosing housing <b>40</b>.

Figure 4 shows the lens <b>14</b> being moved perpendicular to the beam of radiation through it, thereby changing the beam's direction.

Figure 5 shows gimballing <b>56</b> the lens <b>14</b> and feed horn or feed horns <b>46, 48, 50</b> together.

Detailed Description of the Drawings

In Figure 1, a lens <b>14</b> is made of a material, preferably a plastic which has a suitable index of refraction to electromagnetic radiation of a desired frequency, and which is hard and strong. The plastic sold under the trade name LEXAN has the desired electrical and mechanical properties, and is preferred.

The lens <b>14</b> has a front surface <b>16</b> and a rear surface <b>18</b>, the front surface <b>16</b> being planar. The rear surface <b>18</b> includes a plurality of stepped Fresnel zones <b>20</b>. Each stepped Fresnel zone <b>20</b> consists essentially of a plurality of steps <b>22, 24, 26,</b> preferably three. Each step <b>22, 24, 26</b> is parallel to, and lies a fixed distance from, the front surface <b>16</b>.

Figure 2 shows each stepped Fresnel zone <b>20</b> corresponding to a conceptual, similarly refractive, conventional Fresnel zone <b>28</b>. That is, to determine the location of each step of each stepped Fresnel zone <b>20</b> on the rear surface <b>18</b> of the actual lens <b>14</b>, first determine its distance <b>d</b> from its front surface <b>20</b>. Then look to a conceptual, conventional Fresnel lens <b>10</b>, one which is similarly refractive. It also will be broken up into a like number of zones <b>28</b> on its rear surface <b>30</b>. Find the location of the portion <b>32</b> of the corresponding, conventional, zone <b>28</b> which is the same distance <b>d</b> from its front surface <b>30</b>. This is where the step <b>22, 24, 26</b> should be located.

Figure 3 shows the lens <b>14</b> being driven by a source <b>34</b> of electromagnetic radiation <b>36</b> of the desired frequency. This source is preferably located at a tapered end <b>38</b> of a tapering enclosure <b>40</b> which faces the rear surface <b>18</b> of the lens <b>14</b>. The tapering enclosure <b>40</b> may be conductive or radio-frequency absorptive, as desired.

In Figures 1 and 2, the distance <b>d</b> to each step <b>22, 24, 26</b> from the front surface <b>16</b> is an integral multiple of half the wavelength (λ_ε) at the desired frequency. This allows reflections from the front and rear surfaces <b>16, 18, 22, 24, 26</b> to cancel. This cancellation reduces or eliminates standing waves within the enclosure <b>40</b>.

Figure 4 shows the lens <b>14</b> being moved perpendicular to the beam of radiation through it, thereby changing the beam's direction. The lens <b>14</b>, being so thin, is very light, and may therefore easily be moved in a direction parallel to its front surface <b>16</b> by any suitable means. The preferred moving means is a cam <b>42</b> on one side of the lens <b>14</b> and a spring <b>44</b> on the opposite side. Moving the lens <b>14</b> perpendicular to the radiation <b>36</b> changes the direction of the beam emerging from the lens <b>14</b>, and is much easier than attempting to move the feed horn, or other source <b>34</b> of electromagnetic radiation. This motion allows fine control over the beam's direction. Gross motion may be provided, if desired, by a plurality of feed horns <b>46, 48, 50</b> at the tapered end <b>38</b> of the enclosure <b>40</b>. Since each feed horn <b>46, 48, 50</b> lies at a slightly different position, each produces a beam which emerges from the lens <b>14</b> in a slightly different direction. Gross motion may also be provided by gimballing the entire apparatus.

First and second moving means for moving the lens <b>14</b> in first and second directions parallel to its front surface <b>16</b>, and perpendicular to each other, may be provided. As before, the preferred moving means in both directions is a cam <b>42, 52</b> on one side of the lens <b>14</b> and a spring <b>44, 54</b> on the opposite side.

Figure 5 shows gimballing <b>56</b> the lens <b>14</b> and feed horn or feed horns <b>46, 48, 50</b> together.

Scope of the Invention

While a particular embodiment has been shown in some detail, the true spirit and scope of the present invention are not limited thereto, but are limited only by the appended claims and their equivalents.