> 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
Beach balls might not be spherical, but my bet would be on ovoid rather than parabolic.
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.
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.
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.