Microstrip Antenna for Polarimetric Gbd

Microstrip Antenna for Polarimetric Gbd SAR Johan Granholm, ... This paper outlines the design and the measured performance of a 224 ... elevation bea...

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MicrostripAntenna for Polarimetric G b d SAR Johan Granholm, Kim Wwlders', Mikael Dich, and E.Lintz Christensen Electromagnetics Institute, Technical University of Denmark, Building 348 DK-2800 Lyngby, Denmark

Abstract This paper outlines the design and the measured performance of a 224-element dual-linearly polarized microstrip array antenna with low cross-polarization. The array is currently being flown on the Danish high-resolution polarimetric C-band synthetic aperture radar (SAR) [1,2].

Design The antenna design is based on the work presented in [31. The antenna size is 1.3 x 0.31 x 0.15 m (LxHxDl, and consists of 4 identical panels (fig. 1).Each panel contains 56 microstrip patches, organized as 7 identical linear sub-arrays, each with 8 probe-fed patches.

Fig. 1. Antenna, front view. The patch configuration is shown in fig. 2. Solder joint,

.

/Patch

2

.

L

*

L = 21 2 m m 'Feed network

D=EOmm

Fig. 2. Patch configuration, cross section and top view. The patch is a square with side lengths L = 21.2 mm. It is fed using one probe, offset D = 5 mm from the edge, for each polarization. It is etched on a 0.381 mm Rogers RTiduroid 5870 substrate. The patch substrate is mounted on a 2.0 mm Rohacell 31 H F low permittivity = 1.05 62 5 GHz) substrate, which again is mounted on a 3 mm silver-plated aluminum ground plane. On the other

0-7803-2009-3/941$4.00 &2 1994 IEEE.

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side of the aluminum ground plane the patch feed network, made on 0.787 mm RTlduroid 5870, is mounted. The antenna feed-network h a s three levels: The patches in each 8-element sub-array are fed through a network with two 50 R input ports, one for the H- and one for the V-polarization (the patch feed network). The sub-arrays within a panel are fed through two identical (€IN) 7-way power splitters (the sub-array feed network), which implement the elevation beam shaping. The four panels are fed through two 4-way power splitters (the panel feed network). The microstrip circuit, which implements the patch feed network in the 8-element linear sub-array is shown in fig. 3 (the location of the patches is also outlined). It is designed to give equal amplitude and phase excitation of all patches. The V-port network is a straightforward resonant design. In the H-port network, the 8 patches can be considered as organized in four pairs. The patches in a pair have opposite location of the feed probe, and are fed 180" out of phase to obtain the same effective patch excitation. This pair-wise antiphase feed technique has significant impact on the array performance. First, the transmission between the H- and V- ports of the feed network is practically eliminated (see fig. 6, SHV),which means, that cross-polar radiation due to leakage between the antenna H-and V- ports i s significantly reduced. Second, for the H-polarization, the cross-polar radiation due to the patch feed probe is reduced, because the probes in a patch pair are fed with opposite phases, and therefore cancel each other to some degree. This significantly improves the cross-polarization suppression in the elevation plane for the H-polarization.

Fig. 3. Patch feed network. The sub-array feed network is a 7 way power divider implemented in microstrip (fig. 4a). I t has been designed to provide the unequal amplitude and phase excitations required t o establish the elevation beam shaping, which approximates the desired modified cosec2-pattern. The H- and Vsub-array feed networks are mounted perpendicular to the patch feed networks of a panel. The connections between the patch feed networks and subarray feed networks use 90" microstrip Eplane transitions (fig. 4b). The return loss for these transitions is better than -30dB.

Fig. 4. Sub-array feed network. a) Layout, b) Cross section

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The panel feed network consists of two high-power stripline 4 way dividers, which are connected to the panels with coax cables. In order to improve the SAR-system azimuth-ambiguity performance, the power dividers apply a small taper to the 4 panels.

Performance Below we have summarized the measured performance of the antenna : Operating frequency Peak directivity Loss Cross-polarization suppression H N tracking, magnitude WV tracking, phase

: 5.3 GHz k 50 MHz : 28 dBi : < 1.4 dB : 30 dB (within the 3 dB limits of the main lobe)

-

:c t 0.3dB :
The radiation patterns (directivity) in the azimuth (cp = 0")and elevation (cp = 90") planes for both polarizations are shown in fig. 5. All radiation pattern measurements shown in this paper were performed a t the TUD-ESA spherical near-field test facility [41. The radiation pattems shown are the H- and V-polarized field components according to Ludwig 3 [51.The notation used is that E w is the V-polarized field component from an antenna fed a t the H-port. dBi

90

dBi

dBi

-U)

-io

o

io

a0

30 e

90

H-port. 0 = 90

v-port, 9 I O

-20

-io

o

io

a,

31

e

dBi

Fig. 5. Azimuth (cp=O) and elevation ( ( ~ ~ 9radiation 0) patterns (5.3GHz).

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Fig. 6 shows the measured input reflection coefficients for the H- and V- ports, and the transmission between the ports.

, dB

0 -10

-20 -30 -40 -50

5

5.1 5.2 5.3 5.4 5. &GB:

Fig. 6. Input reflection coeffcients (SHH and

Sw)and transmission (Sm).

During SAR system operation the antenna is mounted in an unpressurized pod, in which the temperature a t the nominal altitude of 41000 ft, is -25'. The radar signal is a 2 kW peak, 60 W average alternating H- and V-polarized pulse train. No performance degradation compared to operation at laboratory conditions has been detected.

-

1 Madsen,

-

S.N.,Christensen, E.L., Skou, N. and Dall, J. :

"The Danish SAR System: Design and Initial Tests", IEEE R a n s . Geoscience and Remote Sensing, Vol. GE-29, May 1991, pp. 417-426. 2 Skou, N. : "The Danish Polarimetric SAR Svstem". Second Intemational Workshop onRadar'Polarimetry, Nantes, France, Sept. 1992, pp. 525-533. 3 Woelders, K and Granholm, J. : "Design and Performance of a Dual-Linearly Polarized C-band Microstrip Array Antenna", Second International Workshop on Radar Polarimetry, Nantes, France, Sept. 1992, pp. 369-378.

3.E.and Jensen, F. : "Spherical Near-Field Scanning a t The Technical University of Denmark, IEEE Trans. Antennas and Propagation, Vol. Ap-36, June 1988,pp. 734-739. 5 Ludwig, A.C. : "The Definition of Cross Polarization", IEEE Trans. Antennas and Propagation. Vol. AP-21, January 1973, pp. 116-119. 4 Hansen.

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