DE3786787T2 - MONOPULUS ANTENNA WITH IMPROVED SUPPLEMENTAL RADIATION SUPPRESSION. - Google Patents

Description

The present invention relates to an antenna system adapted for radar use and using sum and difference signals to track a target, comprising:

a) a monopulse antenna, which

a1) has a cross-sectional area to emit energy towards a target and to receive return energy;

a2) has an array of excitable elements, which are arranged symmetrically about an azimuth and an elevation axis; where

a3) the cross-sectional area is divided along the elevation axis into a left and a right horizontal strip, along the azimuth axis into an upper and a lower vertical strip, and into four quadrants, each of the strips and quadrants containing a plurality of the excitable elements;

b) circuit means coupled to the excitable elements, the circuit means generating a sum signal and azimuth and elevation difference signals using return energy from the excitable elements, the azimuth difference signal being calculated by omitting the return energy from the excitable elements in the vertical strips, and where the elevation difference signal is calculated omitting the return energy from the excitable elements in the horizontal strips.

The invention further relates to a method for operating an antenna system adapted for radar use, which uses sum and difference signals to track a target, comprising the steps of:

a) transmitting energy towards the target and receiving return energy with a monopulse antenna;

b) dividing a cross-sectional area of the monopulse antenna into left and right horizontal strips along the elevation axis, upper and lower vertical strips along the azimuth axis, and four quadrants, each of the strips and quadrants comprising a plurality of excitable elements arranged symmetrically about the azimuth and elevation axes;

c) coupling the return energy from the excitable elements of the monopulse antenna to circuitry,

d) with the circuit means generating

d1) a sum signal,

d2) an azimuth difference signal, whereby the return energy from the excitable elements is omitted in the vertical strips,

d3) an elevation difference signal, whereby the return energy from the excitable elements is omitted in the horizontal strips.

An antenna and a method of this type are known from the publication "A Multielement High Power Monopulse Feed with Low Sidelobe and High Aperture Efficiency", by Nam San Wong et al., IEEE Transactions on Antennas and Propagation, Volume AP-22, No. 3, May 1974.

More specifically, the invention relates to the optimization of sum and difference patterns in antennas, and in particular to an arrangement for sidelobe suppression in a monopulse antenna that uses sum and difference patterns to track targets.

As is well known, a monopulse antenna can be divided into sections, e.g. using horns or quadrants, with the radar then measuring the displacement of the target by comparing the amplitude and phase of the echo signal for each horn or quadrant.

The RF circuit for a conventional quadrant antenna subtracts the output of the left pair from the output of the right pair to detect any imbalance in the azimuth direction (azimuth difference pattern), and the output of the upper pair from the output of the lower pair to detect any imbalance in the elevation direction (elevation difference pattern). See the Radar Handbook, Merrill Skolnik, Mc Graw Hill, 1970. The outputs of the differencer, i.e. the difference patterns, are zero when the target is on-axis, with the amplitude increasing as the target is displaced from the antenna axis.

A sum signal, typically representing the energy received across the entire aperture, is generated and used as a reference signal for the video input and for gain control.

There are many trade-offs in feed design and radiation patterns because optimal sum and difference signals, low sidelobe levels, polarization diversity, compactness and simplicity cannot all be fully satisfied simultaneously, especially when using a single feed. Historically, a common approach has been to optimize the sum patterns and tolerate the resulting difference pattern signal. However, it is generally recognized that optimizing undesirable features of the difference patterns is important in eliminating significant tracking problems. See Corlin, US 4,525,716; June 15, 1985. For example, strong sidelobes in the difference signals increase the radar's susceptibility to interference from background clutter or other off-axis radiation sources, resulting in tracking error and loss of efficiency.

It is therefore a main object of the present invention to provide an antenna system of the type described above which simultaneously optimizes the sum and difference signals.

The publication by Nam San Wong et al. mentioned in the introduction describes an approach to optimize the sum and difference signals by sidelobe suppression. As a result of a mathematical model, the signals received by 32 elements are multiplied by excitation coefficients, ie weighted. The excitation coefficients are complicated fractional expressions and different for the calculation of the sum, the azimuth difference and the elevation difference.

The document US-3,711,858 also deals with a monopulse radar antenna which uses sidelobe suppression. In this antenna, waveguide sections are provided in the four quadrants, with adjacent ends of the waveguides in the quadrants being staggered in such a way that certain waveguides located in one quadrant extend into the adjacent quadrant and vice versa. This reduces the lobe of the phase transition from one quadrant to the other.

The arrangement of antenna elements according to a specific pattern, although for a different purpose (namely for switching a monopulse antenna between wide-lobe detection mode and narrow-lobe tracking mode), is already known from US 3,965,475.

DE-A-27 36 497 also deals with sidelobe suppression. However, this publication primarily aims to generate a combined difference signal.

According to the present invention, an antenna system of the type mentioned at the outset is characterized in that

c) the circuit means

c1) provide the sum signal as the sum of the return energy from all excitable elements,

c2) provide the azimuth difference signal as the difference between

c2.1) the sum of the return energy from the excitable elements in the two left quadrants and the left horizontal strip, and

c2.2) the sum of the return energy from the excitable elements in the two right quadrants and the right horizontal strip,

c3) provide the elevation difference signal as the difference between

c3.1) the sum of the return energy from the excitable elements in the two upper quadrants and the upper vertical strip, and

c3.2) the sum of the return energy from the excitable elements in the lower two of the quadrants and the lower vertical strips.

In addition, the procedure mentioned at the beginning is characterized by the following steps:

e) generating the sum signal as the sum of the return energy from all excitable elements,

f) Generating the azimuth difference signal as the difference between

f1) the sum of the return energy from the excitable elements in the two left quadrants and the left horizontal strip and

f2) the sum of the return energy from the excitable elements in the two right quadrants and the right horizontal strip,

g) Generating the elevation difference signal as the difference between

g1) the sum of the return energy from the excitable elements in the two upper quadrants and the upper vertical strip and

g2) the sum of the return energy from the excitable elements in the two lower quadrants and the lower vertical strip.

The present invention is therefore characterized in that the sum signal is calculated as the sum of the return energy of all the excitable elements; that the azimuth difference signal is calculated as the difference between the sum of the return energy of the excitable elements in the two left quadrants and the left horizontal strip (i.e. a left segment) and the sum of the return energy of the excitable elements in the two right quadrants and the right horizontal strip (i.e. a right segment); and that the elevation difference signal is calculated as the difference between the sum of the return energy of the excitable elements in the two upper quadrants and the upper vertical strip (i.e. an upper segment) and the sum of the return energy of the excitable elements in the two lower quadrants and the lower vertical strip (i.e. a lower segment).

The effect of the above measures is that the sum and difference signals are optimized simultaneously.

In the drawing:

Fig. 1 is a conventional five-horn antenna for providing sum and difference patterns;

Fig. 2a is an embodiment of the invention showing an aperture having an array of elements divided into quadrants and stripes;

Fig. 2b is a sum and difference network for providing desired sum and difference signals for the embodiment of Fig. 2a;

Fig. 3a is an alternative embodiment (not subject to the present invention) showing an aperture having an array of elements divided into quadrants and an optionally excluded central region;

Fig. 3b is a sum and difference network for providing desired sum and difference signals for the embodiment of Fig. 3a;

Fig. 4a is a comparison of the elevation difference pattern signals for the embodiment of Fig. 2a, before and after optionally excluding elements along the elevation axis when generating the signals, and

Fig. 4b is a comparison of the azimuth difference pattern signals for the embodiment of Fig. 2a, before and after elements along the azimuth axis were selectively excluded in generating the pattern signals.

Referring again to Fig. 1, there is shown a conventional five horn antenna for providing sum and difference signals. As shown in Fig. 1, five horn antennas A, B, C, D, E are arranged such that antenna A is the left antenna; B is the upper antenna; C is the right antenna; D is the lower antenna; and E is the antenna which fills the central space around which antennas A, B, C and D are arranged. An elevation difference signal is obtained by subtracting the return energy from antenna D from the return energy from antenna B, and an azimuth difference signal is provided by subtracting the return energy from antenna C from the return energy from antenna A. A sum signal is provided by the return energy from antenna E alone. These as well as other forms of antenna feeds have been used in radar tracking systems, but have suffered from the problem of achieving high sum gain while maintaining small sidelobes in the difference patterns. Similar problems are encountered where the antenna consists of a single aperture containing an array of transmitting elements, and where the difference patterns are also generated using one half of the aperture minus the opposite half of the aperture.

Referring now to Fig. 2a, an embodiment of the invention is shown. In Fig. 2a, the antenna 10 is shown having a circular aperture 12 and an array of transmitting and receiving elements 20. The antenna is a broadband antenna designed to operate, for example, in a projectile. The aperture is divided into substantially equal and symmetrical quadrants 14, 15, 16 and 17. The quadrants 14 and 15 define the upper elevation hemisphere for the aperture 12 while the quadrants 16 and 17 define the lower elevation hemisphere for the aperture 12. More specifically, quadrant 14 defines the upper left quadrant, quadrant 15 the upper right quadrant, quadrant 16 the lower right quadrant, and quadrant 17 the lower left quadrant.

Also shown in Fig. 2a is a horizontal strip of elements 24 along the elevation axis and a vertical strip of elements 26 along the azimuth axis. The strip 24 comprises a strip A which contains elements which can be taken in equal parts from the quadrants 14 and 17. The strip 24 also comprises a strip I which contains elements which can be taken substantially equally from quadrants 15 and 16. Strip 26 comprises a strip H which contains elements which can be taken substantially equally from quadrants 14 and 15 and a strip J which contains elements which can be taken substantially equally from quadrants 16 and 17. As further shown in Fig. 2a, quadrants A, B, C and D indicate the remainder of quadrants 14, 15, 16 and 17 in Fig. 2a after the respective elements for strips 24 and 26 have been taken.

In operation of this embodiment, the strips 24 and 26 are selectively excluded from the generation of the difference pattern signals, which, as further explained below, leads to a reduction in the side lobes of the azimuth and elevation difference patterns.

Referring to Fig. 2b, a diagram of the sum and difference network is shown which connects the return signals from the quadrants and strips of Fig. 2b to achieve low difference pattern sidelobes.

The sum pattern to be used for the antenna of Fig. 2a and provided by the network of Fig. 2b is (A + B + C + D) + (H + I + J + K); the azimuth difference pattern is (A + D + K) - (B + C + I); and the elevation difference pattern is (A + B + H) - (C + D + J).

To achieve the required combination of returns to obtain the desired sum and difference patterns mentioned above , first the return from each quadrant and strip is selectively coupled to that of another quadrant or strip in parallel hybrids 41, 42, 43 and 44. The hybrids are commercially available standard sum and difference hybrids, i.e. magic sum and difference T's commonly used in comparator circuits. The coupling coefficient of each hybrid will vary depending on the aperture design and will be chosen to provide as close to an ideal sum distribution pattern as possible. As in Fig. 2b, the returns from strips K and I are fed into hybrid 41. The returns from strips H and J are respectively fed into hybrid 42. The returns from quadrants A and D are fed into hybrid 43. The returns from quadrants B and C are fed into hybrid 44. K and I are combined in hybrid 41 to produce (K + I), and at hybrid 41 the difference is taken to produce (K - I). The same process is repeated for H and J in hybrid 42 to produce (H + J) and (H - J); at hybrid 43 to produce (A + D) and (A - D); and at hybrid 44 to produce (B + C) and (B - C).

With further reference to Fig. 2b, the outputs of hybrids 41, 42, 43 and 44 are selectively added and subtracted to achieve further desired combinations of quadrants A, B, C, D and strips H, I, J and K. The (K + I) output of hybrid 41 and the (H + J) output of hybrid 42 are combined in phase in hybrid 41 to produce the output of hybrid 51 (H + J + K + I). The (B + C) output on hybrid 44 is subtracted from the (A + D) output of hybrid 43 at hybrid 52 to provide (A + D) - (B + C) at the output of hybrid 52, and is then combined in phase with (B + C) to provide (A + D + B + C). The (A - D) output of hybrid 43 is likewise combined with the (B - C) output of hybrid 44 to provide (A + B) - (C + D) at the output of hybrid 53, and is subtracted at hybrid 53 to provide (A + C) - (B + D) at the output of hybrid 53, which is not used and is therefore shorted.

In order to form the sum signal (A + D + B + C) + (H + I + J + K), the azimuth difference signal (A + D + K) - (B + C + I), and the elevation difference signal (A + B + H) - (C + D + J), the outputs of the hybrids 51, 52 and 53 are further optionally combined.

To provide the sum signal, the output of hybrid 51 (H + J) + (K + I) is combined with the (A + B) + (C + D) output of hybrid 52 at hybrid 61 to form (A + B + C + D) + (H + I + J + K).

To produce the azimuth difference signal, the (K - I) output of hybrid 41 is combined with the (A + D) - (B + C) output of hybrid 52 at hybrid 62 to provide (A + D + K) - (B + C + I) at the output of hybrid 62.

To form the elevation difference signal, the (H - J) output of hybrid 42 is combined with the (A + B) - (C + D) output of hybrid 43 at hybrid 63 to provide (A + B + H) - (C + D + J) at the output of hybrid 63.

In Figures 4a and 4b, comparisons of measured data are shown for the original difference signals using the entire ("original") aperture return signal from Figure 2a compared with the difference signals with the horizontal and vertical stripes optionally excluded using the return from Figure 2b. The difference signals are for all practical purposes identical on either side of the line of sight and the discussion below applies to the sidelobe patterns both to the right and left of the line of sight.

Referring now to Fig. 4a, the elevation sum and difference signals for the original arrangement are shown (left figure) and the elevation difference signal with the horizontal stripes I and K removed (right figure). From Fig. 4a, it can be seen that the original elevation difference pattern has a near noise of about -15 dB in a side lobe at about 20º. Compare this with the right figure in Fig. 4a, which shows the elevation difference pattern with the horizontal stripe removed. Here, the near noise in the side lobes decreases rapidly to almost -25 dB at 30º, forming deep nulls.

Even more dramatic results are shown in Fig. 4b. Shown are the original sum and difference signals of the azimuth (left figure) and the azimuth difference signal with excluded stripes H and J (right figure). The original azimuth difference pattern shows near-end noise in sidelobes of -15 dB at approximately 25º. The azimuth difference pattern with excluded vertical stripes is noticeably different. The near-end noise in the sidelobes is -27 dB at 25º and deep nulls are formed.

In both cases, the sidelobes have been suppressed by 10 dB or more. This is highly significant for the case of close interference in sidelobes, which is critical for the concerns of clutter and interference from other transmitters.

Fig. 3a shows an embodiment (not subject of the present invention) in which a central region of the elements is selectively excluded in generating the difference patterns. Fig. 3b shows a sum and difference network for providing the desired sum and difference signals. The circuit of Fig. 3b has the advantage of using only five hybrids, which is of great benefit for applications where space is very important (i.e. missile radar systems etc.). Data for the embodiment shown in Figs. 3a and 3b are comparable to that for the embodiment shown in Figs. 2a and 2b.

It can thus be seen that an embodiment of the invention can reduce the sidelobes for the azimuth difference pattern by selectively removing a vertical strip of elements along the azimuth axis, and can reduce the sidelobes for the elevation difference pattern by selectively removing a horizontal strip of elements along the elevation axis.

It can also be seen that excluding other predetermined aperture cross-sectional patterns can allow the signals to be further optimized, i.e. allowing other combinations to reduce the sidelobes in the difference patterns while minimizing circuit complexity and maintaining the sum signal quality.

Claims (3)

1. Antenna system adapted for radar use and using sum and difference signals to track a target, comprising: a) a monopulse antenna (10) which a1) has a cross-sectional area to emit energy towards a target and to receive return energy; a2) has an array of excitable elements (20) which are arranged symmetrically about an azimuth and an elevation axis; wherein a3) the cross-sectional area is divided along the elevation axis into a left and a right horizontal strip (K,I), along the azimuth axis into an upper and a lower vertical strip (H,J), and into four quadrants (A,B,C,D), wherein each of the strips (K,I;H,J) and quadrants (A,B,C, D) contains several of the excitable elements (20); b) circuit means coupled to the excitable elements (20), the circuit means generating a sum signal and azimuth and elevation difference signals using return energy from the excitable elements (20), the azimuth difference signal being fed into the vertical strips (H,J) while omitting the return energy from the excitable elements (20). and wherein the elevation difference signal is calculated by omitting the return energy from the excitable elements (20) in the horizontal strips (K,I); characterized in that c) the circuit means c1) provide the sum signal as the sum of the return energy from all excitable elements (20), c2) provide the azimuth difference signal as the difference between c2.1) the sum of the return energy from the excitable elements (20) in the two left quadrants (A,D) and the left horizontal strip (K), and c2.2) the sum of the return energy from the excitable elements (20) in the two right-hand quadrants (B,C) and the right horizontal strip (I), c3) provide the elevation difference signal as the difference between c3.1) the sum of the return energy from the excitable elements (20) in the two upper quadrants (A,B) and the upper vertical strip (H), and c3.2) the sum of the return energy from the excitable elements (20) in the lower two of the quadrants (D,C) and the lower vertical strips (J). 2. Antenna system according to claim 1, characterized in that the circuit means comprise a hybrid network (41-44; 51-53; 61-63). 3. A method of operating an antenna system adapted for radar use that uses sum and difference signals to track a target, comprising the steps of: a) transmitting energy towards the target and receiving return energy with a monopulse antenna (10); b) dividing a cross-sectional area of the monopulse antenna (10) into a left and a right horizontal strip (K,I) along the elevation axis, an upper and a lower vertical strip along the azimuth axis, and into four quadrants (A,B,C,D), each of the strips (K,I;H,J) and quadrants (A,B,C,D) comprising a plurality of excitable elements (20) arranged symmetrically around the azimuth and elevation axes; c) coupling the return energy from the excitable elements (20) of the monopulse antenna (10) to circuit means d) with the circuit means generating d1) a sum signal, d2) an azimuth difference signal, the return energy from the excitable elements (20) being omitted in the vertical strips (H,J), d3) an elevation difference signal, wherein the return energy from the excitable elements (20) is emitted into the horizontal strips (K,I), characterized by the steps: e) Generating the sum signal as the sum of the return energy from all excitable elements (20) f) Generating the azimuth difference signal as the difference between f1) the sum of the return energy from the excitable elements (20) in the two left quadrants (A,D) and the left horizontal strip (K) and f2) the sum of the return energy from the excitable elements (20) in the two right quadrants (B,C) and the right horizontal strip (I), g) Generating the elevation difference signal as the difference between g1) the sum of the return energy from the excitable elements (20) in the two upper quadrants (A,B) and the upper vertical strip (H) and g2) the sum of the return energy from the excitable elements (20) in the two lower quadrants (D,C) and the lower vertical strip (J).