a high efficiency ku-band printed monopulse array

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A High efficiency Ku-band Printed Monopulse Array

D R Jahagirdar* Senior Member IEEE , V G BorkarDirectorate of RF Systems, RCI, DRDO, Hyderabad, INDIA

[email protected]

Abstract

A Ku-band microstrip monopulse array design and performance is discussed in the

following paper, wherein 32% antenna efficiency is achieved at higher end Ku-band

frequency. The printed array is realized on a single layer of low loss dielectric material.

The monopulse comparator, series power divider and array design is described in brief.

The design has been carried out using a combination of moment method in spectral

domain and array theory. The measured results shows a very well defined azimuth and

elevation difference patterns and the results are highly repeatable.

Introduction

Microstrip antennas are increasingly being used for airborne application since inception

because of low profile and light weight [1]. At higher frequencies the ease of fabrication and

repeatable performance also becomes attractive features. The only disadvantage is perhaps

low to moderate power handling capacity [2]. In many radar and communication systems,

monopulse antennas are widely used and there also the microstrip monopulse arrays have

found appreciable use. A Ku-band high gain monopulse microstrip array with integrated

comparator on a single layer is described in the following part of the paper.

Monopulse comparator

The preliminary design starts from the beam-width and gain requirement. An approximatephysical size of the antenna is known from it. An elementary calculation for single ra-

diating patch and progressive phase shift required (separation between patches) gives the

total number elements of the array. A basic configuration of a monopulse comparator for

narrow-band performance is shown in the Fig.1. It is a conventional four hybrid cascade.

Each hybrid is a 90◦ hybrid preceded by an extra line length λm/4. The line widths and

lengths are calculated using full wave moment method formulation. Although the scheme

is narrow-band in perception, the resonant array bandwidth is less or at most comparable to

the comparator’s bandwidth. The isolation between the adjecent ports is expected to be 18-

25 dB while the isolation between distant ports is expected to be 30-50 dB. The monopulse

comparator is fed by four quadrant arrays. In each quadrant, there is a series power divider

feeding each branch.

Quadrant array

Each branch is a series fed patch array [3] as shown in fig 2. Each such array is once again

designed using standard procedure. The procedure is to start from a Taylor’s distribution,

arrive at conductance distribution and then find out widths of the patches to realize the

conductance. Then the revised resonant lengths of each patch are calculated using moment

978-1-4244-4968-2/10/$25.00 ©2010 IEEE



o

C

D−El Lo

BA

D

D−Az Sum

Figure 1: Single layer monopulse

comparator

l p lm

lml p +b+ p

Load Feed

−ve + ve

d

bm =k d sino Q

Q

2 pi

Figure 2: A series fed array branch

method [4] or cavity model [5]. The important dimensions are lengths, widths and a fixed

inter-element spacing λm. For series-fed array, it is difficult to maintain equal spacing since

each patch has a different resonant length. However it does not have any grave effect on

final radiation pattern.

The input impedance can be calculated based on moment method in spectral domain [4].

The dimensions have been arrived at after iterating the design a few times. The height of the

patch dielectric is chosen high in order to make sure that the required bandwidth is realized

and also keeping in mind that smaller widths of feeding line means less damaging effect on

radiation pattern. The calculations should also take into account the dielectric cover effects,

if any. The entire procedure can be iterated for finer calculation. Two types of difficulties are

encountered in the design. One difficulty is to realize the very high and very low values of

radiation resistances. It is relatively easier is case of slots in waveguides. Another difficulty

is in taking into account the inter-element reflections and mutual coupling effects. Once the

two difficulties are worked around, a final optimum design is ready. It can then be realized.

The array layout is shown in a photograph of a sample in Fig.3.

Test results and performance

Frequency (Ku-band) f o ± 25MHz

Gain 33 dBi

Return loss 23 dB min

Isolation ∆Az − ∆El 35 dB min

Sidelobe level (H-plane) 18 dB first (30 dB avg)

(E-plane) 18 dB first (28 dB avg)

Beamwidth (both planes) 3.0 ± 0.1◦

Null depth 28 dB (Both-planes)

Null shift with freq. 0.05◦ H-plane

0.05◦ E-plane

Cross-over 4-dB (H-plane)

5-dB (E-plane)

Cross-talk 28-dB

Cross-pol 32-dB



Figure 3: The autocad sketch of array

The electrical performance is very good. The return loss is very good at all the ports (better

than 23 dB). The isolation between the co-located ports is also very good (better than 24 dB)

while the isolation between Az and El difference ports is as high as 35 dB. The radiation

pattern performance is also very good. The sum pattern sidelobe levels in E-plane are better

than 20 dB while the sidelobe levels are better than 18 dB in H-plane. The branch current

distribution is in H-plane and since branch current distribution is proportional to impedance

ratio, it is very difficult to achieve desired branch current distribution. That is the reason

why the H-plane patterns look bulged and the first sidelobe level is poor in H-plane. The

subsequent sidelobes are below 30 dB average. In case the array size is small, this problem

won’t be there and better side lobe level can be achieved.

The difference patterns also shows a good symmetry and the null depths are better than 28

dB equal in both planes. The cross-talk between the Az and El plane is better than 28 dB

within 3-dB beamwidth. The cross-polarization performance is also excellent, better than

32 dB. These type of antennas exhibit a fractional bandwidth, the antenna has exhibited

about 0.5% bandwidth. Mainly the gain decrease rapidly with frequency. The 1 dB gain

bandwidth is 25 MHz while overall performance is very good over 50 MHz bandwidth. The

overall efficiency was 32% and gain is 33 dBi. Considering the losses in microstrip feed

and comparator, it is a very good figure. The measured patterns of monopulse array are

shown in Fig.4 and Fig.5.

Conclusion

Microstrip antennas have a lot of applications in radar systems because of its low profile

and lightweight features. The printed monopulse arrays are very good alternative in case

transmit power is low or moderately high. A single layer Ku-band printed monopulse array

design and performance analysis is presented in this paper. The design is on a single layer

wherein the monopulse comparator is realized in the space between four quadrants. The

electrical performance is excellant. The above monopulse antenna is particularly suited for

airborne applications.



Az D

S

−10 100 5 15 2−5−15

0

−10

−20

−30

−40

Figure 4: Azimuth plane patterns

D

S

−15

0

−10

−20

5 10 150−5−10

−40

−30

El

Figure 5: Elevation plane patterns

Acknowledgements

The author would like to thank Dr. A. Ghosh for masurement support. The author would

like to acknowledge the encouragement of R. Das, Technology Director, RF Systems and

S.K.Ray Director, RCI, Hyderabad.

References

[1] R. E. Munson, “Conformal microstrip antennas and microstrip phased arrays,” IEEE

Trans. Antennas and Propagation, vol. AP-22, pp. 74–78, Jan. 1974.

[2] D. M. Pozar and D. Schaubert, Microstrip Antennas. IEEE Press, New Jersy, 1995.

[3] F. C. Bevan B. Jones and A. Seeto, “The synthesis of shaped patterns with series-fed mi-

crostrip patch arrays,” IEEE Trans. Antennas and Propagation, vol. AP-30, pp. 1206–1212, Nov. 1982.

[4] D. M. Pozar, “Input impedance and mutual coupling of rectangular microstrip anten-

nas,” IEEE Trans. Antennas and Propagation, vol. AP-34, pp. 1191–1196, Nov. 1982.

[5] Y. T. Lo, D. Solomon, and W. F. Richards, “Theory and expriment on microstrip anten-

nas,” IEEE Trans. Antennas and Propagation, vol. AP-27, pp. 137–145, Mar. 1979.

a high efficiency ku-band printed monopulse array

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