Polarization properties of reflector antennas used as radio telescopes
Bibliographic record
Abstract
The distribution of cross polarization across the main beam and near sidelobes of a reflector antenna is calculated. Results are expressed in terms relevant to imaging in radio astronomy, using Stokes parameters, as plots of instrumental polarization Q/I, U/I, and V/I, showing conversion of total intensity of a signal which is unpolarized into apparent linear and circular polarization. The calculations use GRASP8, software that is based on physical optics and the physical theory of diffraction. For purposes of calculation, the symmetrical paraboloidal reflector (diameter ∼40 wavelengths) is fed at the prime focus with a linearly polarized signal. Computed radiation patterns at a number of feed orientations are averaged to establish the antenna response to an unpolarized radio astronomy signal. The results of the computations are consistent with measurements of instrumental polarization of the Dominion Radio Astrophysical Observatory Synthesis Telescope at 1420 MHz made using unpolarized radio sources. For this telescope, the dominant source of instrumental polarization across the field is the cross polarization of the feed. The next most significant effect is scattering by the feed struts; both three‐strut and four‐strut configurations are examined. Struts affect performance in linear polarization but also introduce some instrumental circular polarization. The contribution to instrumental polarization from the reflector itself is comparatively small. Roughness of the reflector surface has relatively little effect in the main beam in Q and U but introduces V and also randomizes the polarization of the sidelobes. In all cases considered, the computations show that the first and subsequent sidelobes are highly polarized, with levels of instrumental polarization up to 50%.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.003 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".