realisation of x-band circularly-polarised waveguide-fed microstrip array

Realisation of X-band circularly-polarisedwaveguide-fed microstrip array

W.W. Wu, X.G. Hu, J.J. Huang and N.C. Yuan

A novel design of a circularly-polarised microstrip patch array fed by awaveguide is developed. A special central microstrip patch-to-slottedwaveguide topology helps a simple truncated patch array realisingthe circular polarisation. The simulated and measured return losses,gains, axial ratios, and radiation patterns are presented for this new4 × 4 array at X-band with good results.

Introduction: Microstrip array antennas are widely used in many prac-tical applications in the radar and communication areas [1–3]. Toextend the slotted waveguide-fed microstrip array from a linearlypolarised antenna [1] to a circularly-polarised one, the present workmainly focuses on the hybrid feed network of a novel central microstrippatch-to-slotted waveguide transition and microstrip lines for a circularly-polarised microstrip array at X-band. As a demonstration, a 4 × 4 array hasbeen designed, manufactured and measured. Its results are presented here.

A. Microstrip-to-slotted waveguide transition design: In [1], a relativelysmaller central patch was used to couple microstrip subarrays to theslotted waveguide for a linear waveguide-fed microstrip array. Torealise circular polarisation, two orthogonal polarisation modes withthe same magnitude and 908 phase difference are excited by chamferingcorners of a square patch. The microstrip subarrays of a circularly-polarised array can be composed of several such truncated squarepatches. However, cutting corners of a central square patch over a wave-guide slot cannot help the array to acquire circular polarisation. So, anew central patch-to-slotted waveguide transition is depicted as follows.

Fig. 1a shows the evolution of the special central patch. At the first step,a slot is etched on the square patch. This slot is parallel to the underneathwaveguide slot. Then each section of the patch is truncated. Figs 1b and cshow the two-dimensional (2D) and three-dimensional (3D) views of thetransition. A substrate with thickness of t ¼ 1 mm, dielectric constant of1r ¼ 2.65 and loss tangent of 0.001 at X-band is chosen throughout thesimulations and fabrication. A fabricated 4 × 4 array is shown inFig. 1d. The waveguide slots are at the bottom layer of the microstriparray board. The fabrication process is the same as the linear one in [1].

a

b

c

d

truncated corners

slot

central square patchxz

lslot

wslot

wgap

lpatch

l1

l1

lpatch

l patc

h

dslot

xz

awg

bwg

t

slotted waveguide

slotcentralpatch

microstrip line

xzy

dielectric radome

waveguide with a coax-to-waveguide adaptor

microstrip array

Fig. 1 Configuration of microstrip-to-slotted waveguide transition

a Evolution of central patchb 2D viewc 3D viewd Fabricated array

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The relations between dimension parameters and three indexes, res-onant frequency, coupling, and axial ratio, of this transition model arerespectively investigated. Coupling is the electromagnetic powercoupled to the microstrip lines besides the central patch. Suppose thecross-section dimensions of the waveguide (awg × bwg) are fixed. The res-onant frequency is mainly depend on lslot. The coupling is mainly decidedby lslot and dslot. The axial ratio is more sensitive to the changes of lpatch,wgap, and l1. The optimised dimensions are given in Table 1.

Table 1: Dimensions of transition (mm)

awg ¼ 22 bwg ¼ 9 lslot ¼ 10 wslot ¼ 1.5

dslot ¼ 8 lpatch ¼ 7.6 wgap ¼ 1.5 l1 ¼ 1.8

a

b

d

0

–5

–10

–15

–20

–25

–30

–35

–40

simulated result of conventional CP array

simulated result of new CP array

tested result of new CP array

9 10 11frequency, GHz

S11

,dB

12

181716151413121110

9876543210

10.0 10.2 10.4 10.6 10.8 11.0frequency, GHz

simulated gain of conventional CP array

simulated AR of conventional CP array

simulated gain of new CP array

simulated AR of new CP array

measured gain of new CP array

measured AR of new CP array


tested result of new CP array at 10.4 GHz













gain

,dB

patte

rn,d

Bpa

ttern

,dB

AR

,dB

–10

–20

–30

–40

–50

0

–60–150 –100 –50 0 50 100 150

–10

–20

–30

–40

–50

0

–60

angle, deg

angle, deg–150 –100 –50 0 50 100 150

c





Fig. 2 Simulated and measured results

a Simulated and measured S11

b Simulated and measured gains and axial ratiosc Simulated and measured E-plane patternsd Simulated and measured H-plane patterns

At the central resonant frequency 10.6 GHz, the gain of the centralpatch is 2.1 dB and the axial ratio is 2.5 dB. The unloaded microstriplines get 30% coupling.

B. Array design: The square microstrip patch of the subarray has thesame dimension as the central patch, lpatch ¼ 7.6 mm. The distancebetween two neighbouring subarrays is nearly half of the waveguide’s

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wavelength d ≃ lg/2 and should be adjusted till these two subarrays arein phase. Based on (3) in [1], another dimension parameter dslot shouldbe readjusted to let each microstrip subarray have the same coupling.Finally, the optimised distance is d ¼ 18.4 mm and the offset of thewaveguide slot is dslot ¼ 8.5 mm.

Simulated and measured results: A fabricated 4 × 4 array has beendesigned, manufactured and measured. The results are shown inFig. 2. The simulated results of a 4 × 4 truncated corner patch arrayfed only by a conventional microstrip line network are also given forcomparison.

The impedance bandwidth of the new circularly-polarised array is700 MHz (6.8%). The gain is higher than 16.5 dB in the frequencyband. The radiation aperture of the array is 75 × 70 mm. So the apertureefficiency is 61%. At 10.6 GHz, S11 is equal to 237 dB, the axial ratioof the array is close to 0dB. When using microstrip lines to feed the 4 ×4 truncated patch array, it is difficult to achieve circular polarisation,shown in Fig. 2b. The measured half power beam width of theE-plane is HPBWE ¼ 248 and of the H-plane HPBWH ¼ 208, shownin Figs 2c and d.

Conclusion: Using a special square patch to couple the electromagneticwave from the waveguide to a normal truncated patch array realises thecircular polarisation of the array. In the frequency band, the realised gainof the array is around 17dB with an aperture efficiency of 61%. A

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700MHz (6.8%) impedance bandwidth and circular-polarisation band-width of 400MHz (3.8%) are achieved. The presented design can beextended to form bigger arrays with only some adjustments.

# The Institution of Engineering and Technology 201212 June 2012doi: 10.1049/el.2012.1988One or more of the Figures in this Letter are available in colour online.

W.W. Wu, X.G. Hu, J.J. Huang and N.C. Yuan (Department ofElectronic Science and Engineering, National University of DefenceTechnology, Changsha, Hunan, People’s Republic of China)

E-mail: [email protected]

References

1 Wu, W.W., Yin, J.X., and Yuan, N.C.: ‘Design of an efficient X-bandwaveguide-fed microstrip patch array’, IEEE Trans. Antennas Propag.,2007, 55, (7), pp. 1933–1939

2 Shahabadi, M., Busuioc, D., Borji, A., and Safavi-Naeini, S.: ‘Low-cost,high-efficiency quasi-planar array of waveguide-fed circularly polarizedmicrotrip antennas’, IEEE Trans. Antennas Propag., 2005, 53, (6),pp. 2036–2043

3 Borji, A., Busuioc, D., and Safavi-Naeini, S.: ‘Efficient, low-costintegrated waveguide-fed planar antenna array for Ku-bandapplications’, IEEE Antennas Wirel. Propag. Lett., 2009, 8, pp. 336–339

CS LETTERS 30th August 2012 Vol. 48 No. 18

realisation of x-band circularly-polarised waveguide-fed microstrip array

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