Shaped Hologram Illumination
Shaped hologram illumination using a dual reflector feed system (DRFS) simplifies the hologram manufacturing and improves the hologram operation at the horizontal polarisation. The shaped illumination provides amplitude taper to the hologram aperture field without the need for narrowing the slots in the hologram pattern. The narrowing of the slots does not provide adequate tapering of the aperture field at the horizontal polarisation and thus the quiet-zone field quality is poor at this polarisation. In addition, the etching of these narrow slots is complicated.
Ray tracing based synthesis procedure is based on the following approximations: rays are assumed infinitely small, reflector surfaces are assumed locally planar, power propagates in flux tubes formed by adjacent rays, hologram illumination is assumed rotationally symmetric. Design tools were implemented as MATLAB -software.
The DRFS consists of two shaped hyperboloid-type reflectors that change the Gaussian radiation pattern of the primary feed into the shape hologram illumination.
Figure 1. Photograph of the 310-GHz dual reflector feed system.
Simulations of dual reflector feed system beam
Dual reflector feed system beam was simulated with GRASP8W. Physical Optics (PO) and Physical Theory of Diffraction (PTD) were used to computed the incident field in the hologram aperture.
Figure 2. Simulated DRFS beam amplitude [dB] at 310 GHz.
Figure 3. Simulated DRFS beam phase deviation from ideal spherical wave at 310 GHz.
Measured dual reflector feed system beam
Dual reflector feed system beam was measured at 310 GHz using a planar scanner
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Figure 3. Measured and simulated amplitude and phase in the horizontal cross- cut of the beam.
650-GHz Dual Reflector Feed System
For designing the DRFS for the frequency of the 650 GHz, the existing in-house dual shaped reflector synthesis software was upgraded and a new DRFS was designed. The operation of the DRFS was verified with planar near-field measurements at 650 GHz. A photograph of the DRFS is shown in Fig. 4.
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Figure 4. The 650-GHz dual reflector feed system.
Contact person: Juha Ala-Laurinaho