Array Feed of Reflector

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© 2009 Sonnet Software, Inc. (315)453-3096 [email protected] 1 Offset Parabolic Reflector Simulation Using CST Studio Suite Microwave Studio ® at Sonnet Software, Inc. Dr. James R Willhite Using a Far Field Pattern as a Source for Simulation of a Larger Model Parabolic reflector Patch array feed 54 λ

description

antenna

Transcript of Array Feed of Reflector

Page 1: Array Feed of Reflector

© 2009 Sonnet Software, Inc. (315)453-3096 [email protected]

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Offset Parabolic Reflector

Simulation Using CST Studio Suite Microwave Studio® at

Sonnet Software, Inc.Dr. James R Willhite

Using a Far Field Pattern as a Source for Simulation of a Larger Model

Parabolic reflector

Patch array feed

54 λ

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12m Offset Parabolic ReflectorAn offset parabolic reflector was built in Microwave Studio with an OD of 12m. This was done by using a CST supplied macro which draws analytical curves. A parabola was drawn and rotated to build a surface. This was then intersected with a 12m OD cylinder to make an offset parabolic surface.

The focal point of the dish is at (0, 0, 7.2m) in the coordinate system shown here and the axis of the parabola is in the z-direction.

We studied this reflector when it is illuminated by a circularly polarized patch array operating at 1.35 GHz. At this frequency the dish is 54λ across.

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Patch ArrayAs a feed structure for the parabolic dish, a simple 7 element array of square patches was designed. A finite, circular ground disk was used behind the patches.

The T-solver (time domain) in MWS was used for this initial study because of its speed.

The patches were sized so that the array radiated most efficiently at 1.35 GHz. An un-tapered simultaneous excitation of the patch was used to generate a far field pattern. The port pairs on each patch were phased to give RCP radiation.

The radius of the ground disk for the array was 232.5 mm, much smaller than that of the reflector.

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Polar Cut Through Array Feed Pattern

The radiation pattern from the 7 element array was RCP with a peak beam of 15.5 dBi. The sidelobes were down 27.7 dB.

The far field pattern was generated with an origin at the center of the coordinates used to build the array. It could have been shifted to the phase center if needed.

Since a finite ground was used on the array, there are LCP backlobes to the feed pattern.

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Array Radiation as Excitation of Dish

The far field pattern from the patch array was saved as an excitation file. The parabolic reflector was then built in MWS and simulated with the I-solver using the array far field pattern as the excitation. The I-solver is best for models which are many λ in extent.

The origin for the field pattern was placed at the focal point of the dish and the pattern was directed to the center of the projected circle of the dish.

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Array Radiation as Excitation of Dish

The simulation required 10G of RAM and 8 hours on a dual socket, dual core computer. The array field is shown here on an 80dB scale, clearly showing sidelobes in the feed.

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Currents on Reflector with CP Feed

The array feed generates currents on the reflector in a somewhat circular pattern. This pattern is effected by the parabolic shape of the reflector.

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Reflector LCP Beam

The main beam from the array feed is RCP. The beam from the parabolic reflector is LCP and has high peak directivity.

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Directivity on Phi=90º Cut

On this cut through the directivity pattern for the dish and the source pattern, you can see the main beam of the dish peaked at θ=0 and the feed pattern directed back at the dish with a peak at θ=-140. Near the main beams the patterns are strongly circularly polarized but away from the peaks the polarization changes rotation.

RCP feed beam

LCP reflector beam

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15-Patch Array A quick study was done to increase the efficiency of the reflector. We wanted to:

1) fill the reflector as much as possible,

2)drop the illumination at the edges below -20dB reducing diffraction, and

3)reduce the spillover around the dish.

We moved from the original 7 patch array with no amplitude weighting to a 15 patch array with weighting.

The radius of the ground disk on the array increased to 367mm, still much less than the reflector.

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Feed Beam Shaping

dish spillover

This figure shows the radiation pattern from the feed array on a phi=90 cut, normalized to the peak value. The reflector subtends 39.8 if the feed beam is centered. The amplitudes on the feed array elements are cos2 (2*d/c*) with d the distance from the array center, c the array cell size, and a variable. Weighting with =3.5 seemed to match our requirements.

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Parabolic Reflector & Feed StructureThe feed array was position at the focal point as before, with a 39.8ºrotation off the z-axis.

To increase the fidelity of the simulation, a supporting system was added. This is a bipod of thin metal strips. Electrical continuity between the ground disk of the array and the reflector was now made through this supporting structure.

The simulation of the reflector with the I-solver now includes all the structure shown here.

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Weighted Feed Radiation

The far field pattern from the T-solver simulation of the array was located as shown here. The feed pattern includes the effects of the array “illumination” of the ground disk. Now the scattering from the supporting bipod and the blockage of the reflector beam by the disk and support are included in the I-solver simulation.

support for feed

parabolic reflector

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Currents on Reflector with Weighted Feed Beam

The tapering of the array excitations has dropped the current at the edges of the reflector.

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Radiation Patterns

Using a weighted feed beam has dropped the forward portion of the beam from the reflector outside the main beam by approximately 10dB. The diffraction from the edges was reduced by 5dB.

Backlobe of feed

diffraction from edges of reflector

original feed

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Summary• The T-solver and I-solver in Microwave Studio

can be used together to solve physically large antenna models.

• A far field radiation pattern from a smaller feed structure can be obtained with the T-solver.

• The far field pattern can be used as the source for a simulation of a larger reflector or other structure using the I-solver.

• Design studies are possible even with models as large as the 54λ OD reflector studied here. These can include the effects of support structures.