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> it inflates like a beachball, providing a stable parabolic-dish shape Except the surface of a beachball is spherical, not parabolic. A parabolic surface is n

> it inflates like a beachball, providing a stable parabolic-dish shape Except the surface of a beachball is spherical, not parabolic. A parabolic surface is not closed, so you can't inflate it like a beachball. So I'd like to know either: how they control the shape of the surface, so the mirrored portion is parabolic; or whether the antenna surface is actually spherical, and not parabolic at all. The portion of a parabolic mirror nearest the focus approximates to a sphere. If the deviation is less than 1/4 wavelength of the signal of interest, a spherical mirror will focus the signal perfectly, as if it were a paraboloid. A paraboloid is what you need for distortion-free imaging; this antenna is apparently used only for signalling, so perhaps accuracy doesn't matter, because only the gain is important. But this is a NASA publication, so I wouldn't expect them to say it's parabolic if it's really spherical.

fgeiger 3y ago
Maybe I misunderstand your point of spherical vs parabolic, but doesn't this paragraph explain it? > The concept turns part of the inside surface of an inflated sphere into a parabolic antenna. A section comprising about a third of the balloon’s interior surface is aluminized, giving it reflective properties. The balloon is approximately spherical but probably not exactly (actually, when looking closely, hardly any beachballs are spherical either). What is parabolic is the part of the balloon which is aluminized. Edit: fixed a typo
- denton-scratch 3y ago
  Yeah; I was wondering how they get the mirrored part of the balloon to take on a parabolic shape, rather than spherical; is that done by manipulating the balloon material somehow? Or are they settling for spherical as a good-enough approximation?
  - deleted 3y ago
    [deleted]
- persnickety 3y ago
  This references the problem, but doesn't explain the big question: how do they turn a section of a sphere into a section of a parabola? Beach balls might not be spherical, but my bet would be on ovoid rather than parabolic.
sandworm101 3y ago
A few internal wires/cables might allow the final inflated structure to be roughly parabolic. All sorts of wacky shapes can be made with internal stays. Also, at low pressures the outer shape of the balloon could be made parabolic. Not every balloon need be perfectly round. When inflated a balance occures between internal pressure and outer structure. Just look at any birthday baloon. They arent spheres. And perfection isnt needed for a radio antenna. It just needs to be better than one of the same mass.
- denton-scratch 3y ago
  > Just look at any birthday baloon. They arent spheres. Yeah, I wondered about that. I guess a birthday balloon is a pretty complicated curve; but I doubt it comes closer to a paraboloid than it does to a sphere or a plane, anywhere on its surface. But this thing isn't a telescope, it doesn't have to focus an image; it doesn't need to be a parabola, a sphere is fine for achieving highly-directional gain.
guenthert 3y ago
Seems like an error in the article. [1] states: "FreeFall’s antenna technology is unique because it is using a spherical reflector. In the past, antennas always used a parabolic antenna. A parabolic antenna focuses energy to a “single point” – while a spherical antenna focuses energy to a “focal line”. Parabolic antennas are symmetric about only one axis which severely limits field of view. It also requires precise pointing for high gain and has more complex packaging, deployment and on-orbit operations. The spherical antenna provides a wider field of view for antennas and high gain without re-pointing of an antenna. Combining a spherical antennas with inflatables is the key to achieving a large aperture in a simple lightweight system." [1] https://www.freefallaerospace.com/nasa-balloon/ https://www.freefallaerospace.com/nasa-balloon/
- Intralexical 3y ago
  Arecibo is/was spherical Ftr, presumably because a paraboloid could only sweep a single path through space. They aimed it by moving the receiver.
- denton-scratch 3y ago
  Thank you. I'm not up to date on telescope design (by at least 40 years), but the Cassegrain design used a spherical mirror, I think. It was used for wide-field astrophotography, and (again, I think) it has a curved focal field, so it needs a curved sensor. I'm certain that Cassegrain designs have been obsoleted. I'm just bragging about what an anachronism I am. [Edit] The Cassegrain design also requires a refracting lens in front of the mirror; and the lens is a peculiar shape. [Edit again] I may be thinking of a Schmitt. I've never played with anything more interesting than a Newtonian; I was just an amateur stargazer. All these fancy designs were made for professional astronomers. For me, they were strictly theoretical.

fgeiger 3y ago

Maybe I misunderstand your point of spherical vs parabolic, but doesn't this paragraph explain it? > The concept turns part of the inside surface of an inflated sphere into a parabolic antenna. A section comprising about a third of the balloon’s interior surface is aluminized, giving it reflective properties. The balloon is approximately spherical but probably not exactly (actually, when looking closely, hardly any beachballs are spherical either). What is parabolic is the part of the balloon which is aluminized. Edit: fixed a typo

denton-scratch 3y ago
Yeah; I was wondering how they get the mirrored part of the balloon to take on a parabolic shape, rather than spherical; is that done by manipulating the balloon material somehow? Or are they settling for spherical as a good-enough approximation?
- deleted 3y ago
  [deleted]
persnickety 3y ago
This references the problem, but doesn't explain the big question: how do they turn a section of a sphere into a section of a parabola? Beach balls might not be spherical, but my bet would be on ovoid rather than parabolic.

sandworm101 3y ago

A few internal wires/cables might allow the final inflated structure to be roughly parabolic. All sorts of wacky shapes can be made with internal stays. Also, at low pressures the outer shape of the balloon could be made parabolic. Not every balloon need be perfectly round. When inflated a balance occures between internal pressure and outer structure. Just look at any birthday baloon. They arent spheres. And perfection isnt needed for a radio antenna. It just needs to be better than one of the same mass.

denton-scratch 3y ago
> Just look at any birthday baloon. They arent spheres. Yeah, I wondered about that. I guess a birthday balloon is a pretty complicated curve; but I doubt it comes closer to a paraboloid than it does to a sphere or a plane, anywhere on its surface. But this thing isn't a telescope, it doesn't have to focus an image; it doesn't need to be a parabola, a sphere is fine for achieving highly-directional gain.

guenthert 3y ago

Seems like an error in the article. [1] states: "FreeFall’s antenna technology is unique because it is using a spherical reflector. In the past, antennas always used a parabolic antenna. A parabolic antenna focuses energy to a “single point” – while a spherical antenna focuses energy to a “focal line”. Parabolic antennas are symmetric about only one axis which severely limits field of view. It also requires precise pointing for high gain and has more complex packaging, deployment and on-orbit operations. The spherical antenna provides a wider field of view for antennas and high gain without re-pointing of an antenna. Combining a spherical antennas with inflatables is the key to achieving a large aperture in a simple lightweight system." [1] https://www.freefallaerospace.com/nasa-balloon/ https://www.freefallaerospace.com/nasa-balloon/

Intralexical 3y ago
Arecibo is/was spherical Ftr, presumably because a paraboloid could only sweep a single path through space. They aimed it by moving the receiver.
denton-scratch 3y ago
Thank you. I'm not up to date on telescope design (by at least 40 years), but the Cassegrain design used a spherical mirror, I think. It was used for wide-field astrophotography, and (again, I think) it has a curved focal field, so it needs a curved sensor. I'm certain that Cassegrain designs have been obsoleted. I'm just bragging about what an anachronism I am. [Edit] The Cassegrain design also requires a refracting lens in front of the mirror; and the lens is a peculiar shape. [Edit again] I may be thinking of a Schmitt. I've never played with anything more interesting than a Newtonian; I was just an amateur stargazer. All these fancy designs were made for professional astronomers. For me, they were strictly theoretical.