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Useful to spies and TV enthousiasts.
Take a large parabolic dish, aimed at your favorite geostationary satelite or earth-attached object. Cut a horizontal slice out of the dish, horizontal to the focal point. Now you have a very low profile reflector with decent surface.
This curved surface could
be integrated into the design of a building and become completely unnoticeable. With a large enough focal distance, it would even appear as flat.
Or it could be on little stands, stand two inch above a lawn to let you have C-band TV without upsetting your restrictive home owners association.
Similar HB Idea
Portable_20solar_20...igh_20concentration
Cut parabola into tiny pieces [AntiQuark, Sep 17 2006]
Fresnel mirror
http://images.googl...e=utf-8&sa=N&tab=wi
Google image search [AntiQuark, Sep 17 2006]
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Annotation:
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The feed will have to have a large aperture and will collect far more noise. |
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For some reason I pictured the Deathstar when reading this. |
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Don't know enough to question the science, but it'd make for an interesting coffee table design. |
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Many ship radars are horizontal-strips-of-parabola like this. I'm guessing it's so that they don't catch as much wind. And I've seen pictures of some very large parabolic radars which are a horizontal-strip design perhaps for structural reasons. |
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One thing to remember about antenna design is that for any dimension of the antenna that's small, the beamwidth is wide, and vis versa. So a horizontal-strip antenna will point in a precise compass direction, but it will cover a fairly large vertical slice. |
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[wiml] Isn't your second paragraph the reason for the shape of ship radar reflectors? |
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A dish doesn't need to be dished anymore: "A revolutionary flat panel radar "dish" is used to transmit and receive signals. Unlike more traditional radars and other polarimetric weather radars that use a parabolic shaped bowl to send and receive radar signals, this new system uses carefully sized engraved dipoles that have been etched into a specially prepared panel, much the same way a circuit board is manufactured." |
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[ldischler] the purpose of the paraboloid is to create extreme directional sensitivity. Can dipoles without reflector achieve the same? Does the system you're talking about use stacked dipoles? Can you bring us a link to read a little more? |
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[wiml] you're right about the ship radars. I think my hidden parabolic dish is trivially different in that it will have a focus that is not inline with the direction of the signal. (focus is horizontal to reflector, but signal higher up). |
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[AntiQuark] I wonder if a fresnel mirror would work well for radio signals with their wavelength that is relatively long compared to the scale of the fresnel segments. Also, I think the idea in the first link you posted was to use many flat mirrors, not parabolic or paraboloid mirrors. |
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Unless the wavelength was
significantly longer than the
difference in distance to the
antenna of the fresnel elements,
you'll probably encounter phase
smearing and other nastiness on a
fresnel. |
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A UK satellite TV company (BSB) had a
flat,
square dish that they called a squariel.
They were bought by Sky (who called
themselves B-Sky-B for a bit) but
soon reverted back to Sky and
discontinued the 'squariel'. I never saw
one
up close (let alone took
one apart) but assume that, under the
radio-tranparent flat casing, there was
a
fresnel dish. |
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[edit: Just looked at a picture of a
squariel online. There doesn't seem to
be boom sticking out of it so
neelandan's description (below) of a flat
antenna would seem much more likely.] |
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Flat antennas are arrays of dipoles with integrated delay elements arranged so that the signals all add in phase. |
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The gain will be slightly less than that of a parabolic reflector of the same projected area. |
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