A High Efficiency Ku-Band Printed Monopulse Array
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Transcript of A High Efficiency Ku-Band Printed Monopulse Array
8/13/2019 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
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
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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
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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.
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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.