Need A Low-Mass Antenna In Space? Just Blow It Up!

A parabolic antenna is a simple enough device, a curved reflector designed to focus all the radiation from the direction it’s pointed into a waveguide or antenna at its feedpoint. They’re easy enough to make for a radio amateur, but imagine making one for a spacecraft. It must fold into a minimal space and weigh almost nothing, both difficult to achieve. An engineering academic doing work for NASA, [Christopher Walker], has a new way to make the parabolic surface that solves the spacecraft designer’s problems at a stroke, it forms its parabolic reflector on the inside of an inflatable structure. In this way relatively huge reflectors can be built in space, with easy folding and very little weight.

The linked article describes the antenna as spherical and its accompanying photograph certainly looks pretty spherical to us. This does however present a problem, because a circle and a parabola are not the same. Thus a purely spherical shape might approximate to a parabola at low angles but would remain a not-very-good antenna because of the relatively small usable area of the sphere it would present. The same problem affects photographers, in that their lenses will present some level of spherical aberration as they tend away from a parabolic shape.

The NASA article is frustratingly vague on the details of how they intend to solve this problem, but we’re guessing that they might design their balloon with carefully shaped panels such that it reaches the desired profile when under pressure. Think for a moment of the techniques that dress designers use to create 3D shapes in fabric, or that metal fabricators put into structures for hydroforming. It’s also possible that a feedhorn could be designed to correct for the effect of a spherical reflector, but perhaps that might be a less simple solution.

Oddly this isn’t the first time we’ve looked at the shape of a parabola.

28 thoughts on “Need A Low-Mass Antenna In Space? Just Blow It Up!

      1. A parabolic antenna is a passive reflector with a sensor at the focal point.
        A passive reflector is a parabolic reflector with an antenna at a distant focal point.

        Sure the parametrics are different (shape) but there is little else that is different.

    1. This may be the answer. The GATR inflatable antennas are spherical, but have two chambers inside at slightly different pressures. The chambers are separated by the reflective material, which becomes truly parabolic as the higher pressure from the front chamber presses it toward the back chamber. It’s an ingenious design and appears to work well.

  1. The picture might be spherical but there is no need at all for the balloon to be spherical, and I’d bet a lot of money that it isn’t. I mean, just look at the fantastical configurations hot air balloons are capable of.

  2. If you consider the common capability to create “inner air springs” in some air mattresses, it should be fairly simple to fashion and anchor a metalized fabric/plastic structure inside of a ball such that when said ball is inflated to a spherical shape, the structure within will be drawn into a parabola. The structure within the ball becomes the radio dish; the spherical shape of the ball is then inconsequential, so long as the ball material is transparent to the rf wavelength in use.

    1. Orbital debris and micrometeoroid risks aside, it goes without saying that any improvements to better approximate a parabola will increase the payload weight, especially the springs or panels mentioned above. Even a multi-chambered designs with supporting structures adds weight, e.g. air mattresses or the dummy tank and artillery units being used in Ukraine as well as WWI and WWII.

      What I think is possibly missing from the reduced weight debate is the gas used to fill up the shape. Perhaps it isn’t very much volumetrically, but I hope their weight savings aren’t predicated on something like the ISS already having a ready gas supply. At least I assume the weight of gas cylinders required for inflating should be inlcuded in the cost reduction analysis and not simply taken for granted.

  3. I wonder if it would make sense to have an outer layer of another material (e.g. a fabric) which is impregnated with a substance that stabilizes (e.g. UV curing resin). This would improve reliability when hit by micro-meteroids or debris.

    1. Wow, good idea! But I think most UV curing plastics will degrade after some long exposure to high UV environment (like in space). Might still work longer than simple membrane due to debris strikes.

  4. Use a pcb, or flex cable, and etch your antenna, either a yagi, or even a jbeam or jpole, probably aren’t going to push more than 50watts out a space satellite

    With 10+dB of gain…

    Smaller, and made of light weight materials, and use a tuning stub you don’t need to worry about ground planes and radiators especially since ground doesn’t exist in space.


    Formerly kj4pmk

      1. Yeah if it’s geo synchronous and has constant sun power

        I’m referring to a deep space satellite you send outside earths orbit, so you cant have kws of solar power and push a kw when your rtg has to power more than a radio transmitter and has to power computers and steering the craft/satellite

        The best rtg will do is maybe a kw per generator so you’d need 2 or 3 just to have over a kw worth of power supply, might get a kw per rtg, if you have the money to spend

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