Radome

Abstract

A one-half-wavelength radome is made of a corrugated plate of ordinary dielectric material. The corrugated plate is made such that the pitch of corrugation is smaller than the wavelength at the operating frequency and the height of corrugation is effectively equal to one-half the wavelength at the operating frequency. A radome of this construction exhibits good electrical properties of small reflection and low dielectric loss and high mechanical strength.

Description

BACKGROUND OF THE INVENTION

This invention relates to a radome and more particularly to a radome for the superhigh frequency band.

There are many kinds of radomes in the prior art, such as thin plate radomes, one-half-wavelength radomes, etc. A thin plate radome is made of a thin dielectric plate which is thin in comparison with the wavelength at the operating frequency, thus being simple to construct. It has been, therefore, used frequently for the VHF band or the lower frequency bands. Where the wavelength is shorter, however, it is necessary to make its thickness very small in order to ensure the requisite electric properties. Therefore, this type of radome is no longer practical because of the difficulty with respect to mechanical strength.

A one-half-wavelength radome is made of a dielectric plate with the thickness equivalent to one-half the wavelength at the operating frequency. This eliminates the reflection of the wave by the radome. In actuality, however, especially where the frequency is high, there is a residual reflection due to the dielectric loss produced in the interior of the dielectric material. Thus, the desired properties may not be obtained. Moreover, the reflection increases as the dielectric constant of the dielectric material increases. It is, therefore, necessary to select dielectric material having low dielectric constant and low dielectric loss as the dielectric material for a one-half-wavelength radome.

SUMMARY OF THE INVENTION

It is a primary object of this invention to overcome the disadvantages found in the prior art.

It is another object of this invention to provide a radome usable for high frequency band, particularly for the SHF band.

Another object of this invention is to provide a one-half-wavelength radome having good electrical properties of small reflection and low dielectric loss.

Still another object of this invention is to provide a one-half-wavelength radome made of ordinary dielectric material, without the use of a dielectric material having an especially low dielectric constant and low dielectric loss, thus having good electrical properties of small reflection and low loss.

The radome according to this invention is composed of a corrugated plate of dielectric material. The corrugated plate is constructed such that the pitch of corrugation is smaller than the wavelength at the operating frequency and the height of corrugation is effectively equal to one-half of the wavelength at the operating frequency. Such a corrugated plate has an effective low dielectric constant, which is smaller than the dielectric constant of a flat plate, and it may be substituted for a flat dielectric plate with an equivalent dielectric constant. Moreover, the corrugated plate shows low dielectric loss because of the existence of air between the crests of corrugations.

Accordingly, it is possible to obtain a radome having good electrical properties of small reflection and low dielectric loss, using an ordinary dielectric material. Furthermore, the radome has good mechanical properties of high mechanical strength and high flexibility because of the corrugated structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of the radome embodying this invention.

FIGS. 2 and 3 are the drawings for illustrating the operation of the radome according to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a corrugated plate 1 is made by corrugating a dielectric plate of FRP (fiber-reinforced plastic) or the like. The pitch P of the corrugations of the corrugated plate 1 is made smaller than the wavelength at the operating frequency and the height t of the corrugation is made effectively to be one-half the wavelength at the operating frequency. The form of the corrugations of the corrugated plate 1 may be as desired, such as a triangular wave, sine wave, continuation of arcs, etc.

Now the operation of the radome according to this invention will be explained with reference to FIGS. 2 and 3. SInce the pitch P of the corrugations of the corrugated plate 1 is smaller than the wavelength at the operating frequency, if the cross section of the corrugated plate 1 is observed along any desired line A-A' parallel to the plane of the corrugated plate 1 as shown in FIG. 1, it is noted that there are two pieces of dielectric layers 2 having a thickness .DELTA. t in every pitch P of the corrugations as shown in FIG. 2. Even though the distance between these two pieces of dielectric layers 2 varies depending on the position of the cross section line A-A', the distance between them is constant in every cycle of the pitch. Consequently, the equivalent dielectric constant of the corrugated plate 1 is equal to the equivalent dielectric constant of a model which is composed of two pieces of dielectric layers 2 having a thickness .DELTA. t in every pitch P as shown in FIG. 3.

When the incident direction of the wave is parallel to the normal line of the plane of the corrugated plate 1 and the direction of the electric field thereof is perpendicular to the plane of the paper, the equivalent dielectric constant of this model is:

.epsilon..sub.e = 1 + ( .epsilon..sub..gamma. - 1) 2 .DELTA. t/P (1)

when the direction of the electric field is parallel to the plane of the paper, the formula is: ##SPC1##

Suppose that P = 10 mm and .DELTA. t = 1 mm in the case the wavelength is 25 mm and that the dielectric material is FRP and its dielectric constant .epsilon..sub..gamma. is 4. Then, from the formula (1)

.epsilon..sub.e = 1.6 . (3)

Also, from the formula (2)

.epsilon..sub.e = 1.177 . (4)

In both cases, the dielectric constant is considerably lower than a flat plate of the same material.

Comparing the formulas (3) and (4), the formula (4) shows a smaller dielectric constant. Therefore, since it is possible to select the relationship between the corrugated plate 1 and the direction of polarization of the wave, it is advantageous to select such a relationship wherein the direction of the electric field is parallel to the plane of the paper.

It is apparent from the formulas (1) and (2) that the corrugated plate 1 shown in FIG. 1, which is made of dielectric material having dielectric constant .epsilon..sub..gamma., may be substituted by a uniform dielectric plate having dielectric constant .epsilon..sub.e given by the formula (1) or (2).

In consequence, if the thickness t of the corrugated plate 1 shown in FIG. 1 is effectively to be one-half a wavelength then using formulas (1) and (2)

t = .lambda./2.sqroot..epsilon..sub.e

where .lambda. is the wavelength of the operating frequency. Thus, it is possible to decrease the effective dielectric constant as can be seen from the figures of the formulas (3) and (4).

Moreover, since there are only two pieces of dielectric layer in every pitch P of the corrugations of the corrugated plate 1 and the remainder of the space is filled with air, the dielectric loss, as well as the dielectric constant, is also decreased.

As described above, the radome according to this invention shows good electrical properties of a low dielectric constant and loss, though an ordinary dielectric material is used instead of special dielectric materials having especially low dielectric constants and losses.

Furthermore, the radome of this invention has superior mechanical strength in the direction of the ridges of the corrugations of the corrugated plate. It is therefore advantageous to design the radome to bear the load in that direction. Also, it has flexibility with respect to bending deformation in the direction normal to the ridges of the corrugations, so that it is easy to make cylindrical or other shaped radomes with the corrugated plate.

Claims

What is claimed is:

1. A radome comprising a corrugated plate of dielectric material wherein the pitch of corrugation is smaller than the wavelength at the operating frequency, the height of corrugation is one-half of a wavelength at the operating frequency, and the section thickness t of said corrugated plate is equal to:

.lambda./2.sqroot..epsilon..sub.e ,

where .epsilon..sub.e = equivalent dielectric constant of said corrugated plate as determined from the dielectric constant and thickness of the plate material, and .lambda. = the wavelength at the operating frequency.