GB2243489A - Antenna - Google Patents

Abstract

An axisymmetric antenna for transmitting radio frequency (RF) electromagnetic radiation over a desired radiation pattern, has a dish reflector (2), an aperture (A) for transmitting radiation to provide a first radiation pattern contribution, a feed (6) for providing the RF electromagnetic radiation to the aperture and blockage (4) for blocking radiation from being transmitted through the aperture (A), to provide a second radiation pattern contribution so that the combination of the first and second radiation pattern contributions substantially provides the desired radiation pattern. <IMAGE>

Description

ANTENNAS The present invention relates to antennas, and in particular to axisymmetric antennas.

An antenna, in general, will have a far field radiation pattern characterized by a maximum gain on the boresight of the antenna, and at increasing angles off boresight a series of peaks of gain of maxima less than the boresight gain, and which maxima diminish with increase in offset angle. These gain sidelobes determine, in part, the usefulness of a antenna in a particular application. For example in point to point communications two pairs of antennas, one each of which shares a common station, need sidelobes at critical angles low enough so that the transmissions between the two pairs of antennas do not interfere with each other to too great an extent.

It is not possible to supress side lobes completely so all antennas will produce some side lobe interference, and the critical factor in designing antennas for particular applications may well be sidelobe performance. Antenna design has naturally favoured therefore forms of antenna with intrinsically good, or at least better sidelobe performance. Thus, for example asymmetric or offset dish antennas have found preference to axisymmetric antennas, because the former have better sidelobe performance than the latter. A particular problem with axisymmetric, usually circular, dish antennas is that the necessarily centrally mounted feed, usually feed horn, adds what is termed blockage into the antenna aperture resulting in poorer sidelobe performance.

An example of the effect of a central, circular blockage on the radiation pattern of an axisymmetric (circular) antenna is shown in Figure 1. As will be seen, the first and subsequent odd-numbered sidelobes have increased maxima, while even-numbered sidelobe maxima are reduced, which is symptomatic of central circular blockage. Although sidelobe performance is affected by the feed horn blockage the antenna efficiency can be maintained by keeping the effective blockage area of the feed horn small, and this is the approach antenna design has in the past taken.

The present invention is based upon the realisation that blockage in an antenna aperture can be considered as providing a contribution to the antenna radiation pattern, and that by selecting a form of blockage to provide a radiation pattern contribution complementary to the antenna's aperture radiation pattern, a desired radiation pattern can be provided.

According to a first aspect the invention provides an axisymmetric antenna for transmitting radio frequency (RF) electromagnetic (elm) radiation over a desired radiation pattern, comprising: an aperture for transmitting radiation to provide a first radiation pattern contribution, and a feed for providing the RF electromagnetic radiation to the aperture, characterized in that blockage is provided for blocking radiation from being transmitted through the aperture, to provide a second radiation pattern contribution, so that, in use, the combination of the first and second radiation pattern contributions substantially provides the desired radiation pattern.

Where the antenna has a dish reflector illuminatable with RF EN radiation by the feed and the feed at least partly obscures the aperture from RF EM radiation from the reflector, it is preferable that the said blockage obscures at least part of the feed to the aperture whereby said blockage provides a radiation pattern contribution rather than the obscured part of the feed.

Preferably the axisymmetric antenna has the feed substantially obscured to the aperture by said blockage.

Preferably the aperture is circular and said blockage is rectangular, and it is also preferred that the blockage is symmetric about the diameter of the aperture.

The invention enables the provision of an antenna having a desired radiation which may not be otherwise achievable. Significantly the invention enables axisymmetric antennas to have selective side-lobe performance and permits their possible use in place of offset antennas, allowing the exploitation of the better cross-polar characteristics of the axisymmetric antenna.

In the preferred embodiment mentioned above, particular advantage of the invention is taken by providing blockage to obscure the antenna feed in the aperture so that the selected radiation pattern contribution of the blockage rather than arbitary contribution of the feed has effect.

A prefered embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, wherein Figure 1 is a graph showing the variation of gain relative to boresight against azimuth angle from boresight of a circular antenna having a central, circular, feed-like, blockage.

Figure 2 shows in (a) a front view with hidden detail and in (b) a diammetric cross-sectional side view of an antenna to a preferred embodiment.

Figure 3 shows the effective aperture of the antenna of figure 2.

Figure 4 is an explanatory diagram relating theoretical aperture to the far field in spherical and recti-linear co-ordinate systems.

Figure 5 is a graph showing the variation of gain relative to boresight against azimuth angle from boresight of a circular antenna having a central, rectangular blockage.

A brief physical description of an antenna according to a preferred embodiment of the invention will be given to assist in understanding the subsequent, mathematic description.

Thus, referring to figures 2a and 2b, an antenna 1 has a circular parabolic dish reflector 2 and a circular aperture shroud ring 3. A support beam 4 extends diametrically across the shroud ring 3 and supports a waveguide 5 and feed horn 6 in the aperture A of the antenna A radome 7 is stretched across the shroud ring 3 over the aperture A. The support 4 is substantially rectangular in form seen from the front (Fig. 2a) and provides a substantially rectangular blockage in the aperture A, providing the effective aperture shown in Fig.

3. The blockage may be of conducting material or of EM radiation absorbing material.

A detailed mathematical analysis of the effects of antenna blockage and in particular of preferred embodiment of the invention will now be given.

Considering first in general terms a centrally blocked antenna the radiated field of a blocked aperture can be considered as the superposition of the field of an unblocked aperture with that of an antiphase aperture corresponding to the blocked area. Usually the blockage will be due to a feed or subreflector and so the blocked area will be far smaller than the unblocked area. The small blockage aperture produces a radiated field with a broad main lobe much lower in level than the main lobe of the unblocked aperture and in antiphase to it. Because the phase of the unblocked aperture radiated field reverses between consecutive lobes of the pattern, the blockage results in the first sidelobe of the pattern being raised, the second lowered, the third raised, and so on.The difference in radiation pattern of an axisymmetric antenna with and without a central circular blockage can be seen in Figure 1.

In the preferred embodiment a circular aperture is blocked with a rectangular area and the aperture radiates a field which is equal to the unblocked diffraction pattern minus the blockage diffraction pattern.

The radiation in a plane perpendicular to an aperture of a given width is equivalent in form to that which would result from a line source of the same length as the aperture has width with a suitable current distribution in it located near or in the aperture and parallel to the plane in which the radiation is measured. A planar circular aperture for example may have a general aperture field distribution, more accurately referred to as an equivalent current distribution of the form 1a (x,y). A linear line source, parallel to the horizontal x-axis, may be located nearby, with a general current distribution of the form I1(x).

The far field is found by vector summation of the contributions of each very small element within the aperture at a given point in the far field, using an expression of the form

exp [j k sin. e (x cos + + y sin )) dA where ris the distance from the antenna to the far field point, and 8, 0 refer to a conventional spherical coordinate system, as shown in Figure 4.Because we are only concerned with the far field in the horizontal plane, = = 0, and the expression may be reduced to

The summation, or integration, may be separated into summations in the x and y dimensions,

which reduces to a single summation in the x dimension once the summation in the y dimension has been evaluated,

where I'(x) is in general a complex quantity to allow a for the fact that the phase of the current may not be uniform.

Setting the current in the line source, at a point x=xl, equal to the current in the x dimension summation so that I(xl) = Ia(xl) radiation in the plane containing the x dimension, the horizontal plane, may arise indistinguishably from one current distribution or the other.

For a straight line source of uniform phase the radiated field is given by the Fourier Transform relationship:

where Ex(p) = electric field distribution along the line source, p = 2irx a u = a.sine a = length of line source, and F(u) gives the far field pattern amplitude.

In the case of uniform line source illumination Ex(p) = 1 and equation 1 simplifies to give:

which, when integrated, gives F(u) = sin(iru) #u For a circular aperture the radiation pattern is given by the Fourier Transform relationship: F(u) = 2 # Ex(p).J0(pu)p dp (2) 2 o where p= sr is the normalised radius, a u = 2a.sine a = radius of circular aperture, and Jg is the zero order Bessel function of the first kind. Setting Ex(p) = 1 for a uniform illumination and integrating gives: F(u) = 2 Jl(ru) .

#u For a circle of area A and a rectangle of area B we arrive at expressions for the circular aperture of Fl(u) = A.2 Jl(#u) XTU and for the rectangle aperture of F2(u) = B.sin(ru) #u Hence, the resultant field of the blocked uniformly illuminated aperture is given by F(u) = Fl(U)-F2(U) = A.2 Jl(ru) - B.sin (irk) nu su The horizontal radiation pattern of a uniformly illuminated circular aperture with and without a central circular blockage shown in Figure 1, where the alternate raising and lowering of sidelobes is apparent, can be compared with the same unblocked aperture pattern with and without horizontal rectangular blockage of equivalent height to the circular blockage shown in Figure 5. It is apparent that the sidelobe levels have been effected, and in fact all sidelobes are reduced in level by a small amount.

The invention can be applied to apertures other than that described in the prefered embodiment, for example planar, cylindrical or spherical apertures. The blockage may be formed by EM radiation absorbing material on the reflector dish of an antenna or elsewhere in the antenna, or by modifying the radiating field with a dielectric material. Blockage may be designed into an antenna, e.g.

in an array antenna.

Claims (9)

1. An axisymmetric antenna for transmitting radio frequency (RF) electromagnetic radiation over a desired radiation pattern, comprising an aperture (A) for transmitting RF electromagnetic (EM) radiation to provide a first radiation pattern contribution, and a feed (5,6) for providing the RF electromagnetic radiation to the aperture (A), characterized in that blockage (4) is provided in the aperture (A) for blocking radiation from being transmitted through the aperture (A) to provide a second radiation pattern contribution, so that, in use, the combination of the first and second radiation pattern contributions substantially provides the desired radiation pattern.

2. An axisymmetric antenna as claimed in claim 1 having a dish reflector (2) illuminatable by the feed with RF EM radiation and the feed at least partly obscuring the aperture from RF EM radiation from the reflector, wherein said blockage (4) obscures at least part of the feed (5,6) of the aperture (A) whereby said blockage provides a radiation pattern contribution rather than the obscured part of the feed.

3. An axisymmetric antenna as claimed in claim 2 wherein the feed (5,6) is substantially obscured to the aperture (A) by said blockage (4).

4. An axisymmetric antenna as claimed in any preceding claim where the aperture (A) is circular and said blockage (4) is rectangular.

5. An axisymmetric antenna as claimed in any preceding claim wherein said blockage (4) is symmetric about the diameter of the aperture (A).

6. An axisymetric antenna as claimed in any preceding claim wherein the said blockage (4) supports the feed (5,6).

7. A telecommunication system having an antenna as claimed in any preceding claim.

8. An antenna characterized by the provision of selected blockage whereby a tailored aperture field distribution is provided.

9. An antenna substantially as herein described and with reference to the accompanying drawings.