Choice of Cassegrain Geometry

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PH0101 Unit 2 Lecture 4 Wave guide Basic features

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Presentation transcript:

Choice of Cassegrain Geometry SUMMARY Background The problem of long wavelengths The possibility of a larger subreflector The possibility of an ultrawideband feed P.Napier EVLA Feed/FE PDR, 12 Feb02

Requirements Impacting Feed System Design Phase I : provide secondary focus feeds for 1- 50 GHz Phase II : provide prime focus feeds for 0.3 – 1 GHz Keep all bands working during transition period Budget constraints P.Napier EVLA Feed/FE PDR, 12 Feb02

Long Wavelength Problems Subreflector subtends 18 deg. Feed aperture is 7 wavelengths. Feed is large at L Band. Possible solutions: (a) Use a larger subreflector (b) Move the secondary focus closer to the subreflector P.Napier EVLA Feed/FE PDR, 12 Feb02

A Larger Subreflector Advantages Smaller feeds Better long wavelength performance Disadvantages Large cost for small gain Existing feeds do not work 300 MHz feed does not work Phase II subreflector rotator more difficult Greater risk to high frequencies P.Napier EVLA Feed/FE PDR, 12 Feb02

ATA Wideband Feed ATA Feed: 0.5-11 GHz, subreflector subtends 85 deg 40 cm 120 cm P.Napier EVLA Feed/FE PDR, 12 Feb02

Use of ATA-type Feed on EVLA Antenna ASSUME ATA-feed 0.3-18 GHz K, Ka, Q Band receivers used on new feed circle P.Napier EVLA Feed/FE PDR, 12 Feb02

Advantages/disadvantages of ATA-type Feed Lower cost Disadvantages 0.75 G/T loss at zenith, 0.5 G/T loss at 45 deg elev High instrumental polarization Linear polarization – transition complexity Increased beamsquint Current feeds do not work Need feed focussing P.Napier EVLA Feed/FE PDR, 12 Feb02

P.Napier EVLA Feed/FE PDR, 12 Feb02