Inflatable "dish" for the field day fanatic

[VK7XX]

This must rate fairly high on the "cool stuff" list. http://www.gatr.com/ and check out the 'latest videos' links on the left..

[vk3auu]

There is some very crafty geometry in this concept. The part of the sphere facing the satellite is opaque to RF and the other side has some reflective coating. The focal point is exactly at the surface of the part facing the satellite, at least for small apertures, but if you want more gain, you just use a bigger balloon.

David
VK3AUU

[VK3ZJ]

Great idea. Expandable fabric , pump up the dome and presto increase the gain. No good a my QTH though. My grand kids would think it was a great JUMPING Castle

[zl4plm]

actualy looking at the feed I thought it was for UHF SATCOMS not uwave

then I noticed in other pics amps and an LNB at the focus!

Very nice!!!!

prob works fine for emergency use

after all I used to use a mil manpack for SATCOMS on UHF that was no more than a glorified umbrella covered in fine copper mesh with a cross dipole at the "focus" worked great!

so just a step forward!



nice idea!
73 Simon ZL4PLM

[vk3py]

Hmmm..... there's got to be more to this than we see in the photos. A hemisphere does not have a focal point. For a focal point you need a parabolic curve. I wonder if there's a parabolic dish hidden inside those spheres? Maybe sewn-in as part of the sphere's internal structure. The set-up video clip doesn't show sufficient detail to make this clear.

Chas
VK3PY

[vk3auu]

While you are correct that a complete hemisphere doesn't have a focal point, the area opposite the feed point for a few (I can't remember how many) degrees is a very close approximation to a parabola and so can be used in the place of a true parabolic reflector.

David
VK3AUU

[vk3py]

Aha, a plausible explanation comes forth. Thanks David. I guess they've traded some efficiency for convenience.

Pondering this further after my posting, I seem to recall that the Arecibo hole-in-the-ground radio telescope is actually a spherical reflector, not parabolic.

73,
Chas
VK3PY

[VK2GOM]

The difference in figure between a parabola and a spherical surface is fairly negligible.

I have seen spherical telescope mirrors that gave a reasonable view. And that's at the wavelength of light... never mind 3cm radio waves

The actual parabolic correction is small at best, and is of course related to the focal length of the mirror/dish; shorter focus being greater correction to arrive at the true parabola.

I have all the formulae for parabolic correction related to first-surface reflecting optics, so can calculate the difference for a microwave dish if required.

73 - Rob VK2GOM / G0MOH

[vk3auu]

If I remember correctly, Arecibo is probably a catenaroid, which also approximates a paraboloid over a cosiderable arc as well.
That is the shape you get if you hang a long chain.

David
VK3AUU

[VK5ZD]

Hi

The reflector is a parabola. See the FAQ section of the GATR web site.

73
Iain

[vk3py]

As Iain says, it is indeed a parabola - who'd have thought to read the FAQ's, eh? I must confess I didn't see the link on my first visit to the GATR website.

They state:

The ball simply acts as a pressure vessel to help us achieve the parabolic shape of the dish. After inflating the ball to the correct pressure, we increase airflow to the upper hemisphere, above the dish, thereby pushing it down into the parabolic shape.

Presumably there's a flexible diaphragm between the two hemispheres which deforms under air pressure to form a parabola.

The FAQ's also state:

Does the blower have to stay on?
It does, or else the dish would collapse when the pressure above and below the dish equalized. Of course, whatever components you’re supplying the satellite signal to will also need power and the blower requires very little power to keep the dish inflated. So it can be maintained for long periods on a battery or solar power if desired, though we typically use a generator or “wall” power.

So, there you go. You live and learn.....

Could be tricky holding the ball still on a windy hilltop during a field day!

Oh, Dave, while in FAQ mode, I checked out the Arecibo observatory's home page. It has this to say about the reflector:

Those who see the Arecibo radio telescope for the first time are astounded by the enormousness of the reflecting surface, or radio mirror. The huge "dish" is 305 m (1000 feet) in diameter, 167 feet deep, and covers an area of about twenty acres. The surface is made of almost 40,000 perforated aluminum panels, each measuring about 3 feet by 6 feet, supported by a network of steel cables strung across the underlying karst sinkhole. It is a spherical (not parabolic) reflector .

I particularly like "enormousness"!

Chas
VK3PY

[vk3auu]

Well, you live and you learn. I wonder how Arecibo managed to produce a spherical reflector, because the cables would naturally hang in a catenary (which is a hyperbolic function). They must have other cables underneath it pulling in the appropriate places to produce the spherical reflector. I wonder what percentage of the total surface is illuminated at any one time because of the limitations of the spherical shape.

Incidentally, if you want to make a quick and easy paraboloid, all you need to do is get a large circular vessel, uniformly stretch a piece of rubber across i and then evacuate it. Same deal as the aforementioned inflatable dish.

David
VK3AUU

[vk4tu]

According to Wikipedia (not the most reliable source, I know) there's a valid reason for a spherical reflector;
The telescope is a spherical reflector (as opposed to a parabolic reflector). This form is due to the method used to aim the telescope: the telescope's dish is fixed in place, and the receiver is repositioned to intercept signals reflected from different directions by the spherical dish surface. A parabolic mirror would induce a varying astigmatism when the receiver is in different positions off the focal point, but the error of a spherical mirror is the same in every direction.