In the question What's the spiral pattern on this satellite? I show this image of a spiral-shaped pattern on the spherical surface of a (presumably) vintage satellite or model thereof. @OrganicMarble's answer links to an article which identifies this as a logarithmic spiral antenna.

In this answer I show this image of a project ELINT satellite aka GRAB-1 (for an interesting story, read further there) which @OrganicMarble pointed out is the little one on top, and the one with the spiral pattern is another satellite of the same type as that above.

I'm wondering why these spiral-shaped antennas were actually necessary. Does it have to do with limitations of the electronics, or the nature of the RF signals, or the spherical shape of the spacecraft, or something else like "packaging"?

Compare to images of earlier spherical satellites in these questions and their answers, which all used several straight rod antennas sticking out from the sphere

Question: Why were the antennas on the spherical surface of some early satellites spiral-shaped?

  • Let's not forget Lunokhod's "Unicorn horn". – SF. Sep 11 at 10:11
  • @SF. I love it! It's not a spherical unicorn, but it's nice nonetheless. – uhoh Sep 11 at 10:14
up vote 6 down vote accepted

Those are spiral antennas.

Spiral antennas belong to the class of frequency independent antennas which operate over a wide range of frequencies. Polarization, radiation pattern and impedance of such antennas remain unchanged over large bandwidth.[3] ... Spiral antennas are reduced size antennas with its windings making it an extremely small structure.

So, an antenna that can operate at a large frequency range with a nice predictable performance, and with a small footprint. All desirable qualities for an antenna to be used in a spacecraft.

For the Transit navigation satellites, I'll have to make a guess. The early Transits were launched on Scout rockets which meant severe constraints on size and weight. That makes an antenna that can be painted on the sat's outside surface an attractive option. Their function doesn't seem to dictate a wide frequency range: it broadcast on two fixed frequencies.

@Hobbes' answer is correct about the what, but not the why.

Radio waves -- just like light -- are electromagnetic waves. Because they are transverse waves of the electric and magnetic fields, they can be polarized. There are two ways (*1) to polarize E-M waves: linear polarization and circular polarization.

Linear polarization

  • The electric field vibrates in one direction perpendicular to the direction of propagation, but with an amplitude that fluctuates periodically.
  • Light example: polarized sunglasses
  • Radio examples: dipole antennas; television, radio, Wifi, Apollo high-gain antenna

Circular polarization

  • The electric field maintains constant amplitude, but the direction changes as the wave propagates.
  • Light example: polarized 3-D movie glasses
  • Radio examples: spiral antennas, helical antennas, Apollo scimitar antenna

linear (top) and circular (bottom) polarization

When E-M waves reflect off smooth surfaces, they can become linearly polarized. The best example of this is glare, which is light that reflects off a road, snow, or ground. In this case, the electric field perpendicular to the surface is reduced by destructive interference, and so we say the reflected light is polarized parallel to the surface. By wearing polarized sunglasses that are vertically polarized, you can block the glare, yet still see the ambient light.

polarization by reflection

The same destructive interference by reflection can also happen with radio waves. Television, radio, and WiFi are transmitted by dipole antennas that create a radio wave polarized in the direction of the antenna (vertical in the case of TV and radio). However, when the signal reflects off certain geographic features, you can get cancellation of the linearly-polarized signal. This is why some locations have a hard time getting TV or radio signals. However, this problem doesn't happen with circularly polarized signals.

Another issue with linear polarization is that when you rotate the transmitter (or receiver), the amplitude of the signal changes. If you tilt your head while wearing polarized sunglasses, the glare will start to appear. You could make a 3-D movie with linearly polarized glasses (one eye horizontally- and the other eye vertically-polarized), but you would see double-images if you tilt your head. It's also why the antenna on your WiFi router can be rotated into various orientations. This is another problem that doesn't happen with circularly polarized signals. Thus, 3-D movie glasses use circular polarization.

The disadvantage of spiral antennas is that they are inefficient. They tend to spread their signal in a wide beam, which means much of the energy of the radio transmission is being broadcast in the wrong direction. As @Hobbes notes, they can transmit on a wide bandwidth; you don't want to do this as it requires more transmitter power.

Putting a spiral or helical antenna on orbiting spacecraft is a smart idea. It creates circularly-polarized radio waves, which means the orientation of the antennas don't matter. That's important for an orbiting spacecraft, which is constantly changing its orientation relative to positions on Earth. There is also the advantage of circularly-polarized wave being less susceptible to interference by reflection. You can see a picture of a helical antenna on a ground-tracking station in my answer here.

In contrast, spacecraft beyond orbit -- think Apollo or Voyager -- have fairly stable orientations, and so they use more efficient linearly-polarized antennas. That's why Apollo had so many dang antennas: some for when the orientation was stable (*2), and others for when the orientation was changing (*3).


(*1) Elliptical polarization is a third way to polarize E-M waves, but they add nothing to this discussion.

(*2) CSM to Earth (trip between Earth and moon); LEM to Earth (on the moon); the S-IVB ("third stage") to Earth; and the ALSEP experiments left behind.

(*3) CSM to Earth (Earth or lunar orbit); LEM to CSM; LEM to Earth (ascending/descending); lunar rover to CSM.

  • I don't believe circularly polarized beams "spread faster" than linearly polarized beams. Every antenna design has its own radiation pattern, but spreading is not really caused by circular polarization. Same with the bandwidth; it's a function of the antenna design, not the circular polarization. – uhoh Sep 11 at 10:10
  • @uhoh: True, helical antennas are circularly-polarized but directional. I've fixed the answer. – Dr Sheldon Sep 11 at 10:21
  • Thirdly, let's confirm that this particular, unusual, unique configuration, where a sphere is completely covered in spiral, really does have circular polarization to begin with. I'm not convinced that this particular design falls neatly into the category of helical antennas. – uhoh Sep 11 at 10:35

From this related answer:

See A Broad-Band Spherical Satellite Antenna (paywalled)


tl;dr:

As pointed out in @Hobbes' answer already:

  • Broad range of frequencies by being an angles-only shape without characteristic length
  • isotropic with no strong nulls, worst case is -10 dB.

Some of the spherical satellites with spiral patterns, from: Artificial Earth Satellites, Designed and Fabricated by The Johns Hopkins University Applied Physics Laboratory

Satellite                      Transmitters
   1-A     17-Sep-1959      54/108 MHz, 162/216 MHz, 108 MHz (TM).
   1-B     13-Apr-1960     162/216 MHz,  54/324 MHz
   2-A     22-Jun-1960     162/216 MHz,  54/324 MHz, 108 MHz (TM), and NOTS infrared scanner on 107.9 MHz
   3-A     30-Nov-1960     162/216 MHz,  54/324 MHz, 108 MHz (TM)
   3-B     21-Feb-1961     162/216 MHz,  54/324 MHz, 136 MHz (TM), and 224/421/448 MHz (SECOR)

After these, the spacecraft changed shape and the spirals were not used. According to Spacecraft Design Innovations in the APL Space Department (Johns Hopkins APL Technical Digesl. Volume 13. Number 1 (1992)):

THE QUADRIFILAR HELIX

The Space Department needed to develop several novel antennas to radiate the Doppler signal to the ground. The first Transits used an APL-invented spherical projection of a logarithmic spiral (Fig. 3). When later spacecraft departed from spherical shape, the design switched to the aptly-named "lamp shade" antenna. That antenna, however, suffered from poor polarization and a null in its gain pattern at nadir. A new antenna was clearly needed-one that could provide broad beam coverage with good circular polarization yet be small compared with the 2-m wavelength and the spacecraft body itself. Charles C. Kilgus answered this need by inventing the resonant quadrifilar helix, first flown on Triad (and visible in Fig. 7)8

8Kilgus, C. C., "Resonant Quadrifilar Helix Design," Microwave 1. l3 (12), 49-54 (1970).


From the US patent US3,034,121, May 8, 1962:

The requirement for a broadband antenna, covering a frequency range of at least four to one and providing radiation characteristics which would produce no nulls greater than l0 db with respect to the maximum in any plane, and at any angle with respect to the antenna within a spherical boundary, was generated by the development of a special spherical aerial vehicle. This requirement was so imposed that regardless of the attitude of the vehicle in space, the received signal variation would not be greater than 10 db due to antenna pattern changes.

The design of the antenna constituting the present invention is based on the established properties of the class of broad band antennas using equiangular spiral slots or conductors. Such an antenna has the property that its shape is specified entirely by angles, so that its performance is independent of wavelength. A familiar example of such an antenna is the infinite biconical horn.

The logarithmic spiral has the property that an angle between the tangent tangent to the curve and its radius vector is a constant and greater than 90 degrees. Such a spiral is shown in FIG. 1. A spiral slot antenna can be constructed by plotting two spirals slipped in angle with respect to. each other, as shown, in FIG. 2. Similarly, as shown in FIG. 3, a double spiral slot can be produced by plotting two spirals (FIG. 2) 180 degrees with respect to each other. In FIG. 3 the feed points are shown at X-X.

The present invention contemplates the application of a logarithmic spiral antenna to the surface of a hemisphere rather than to a flat surface or sheet as in conventional practice. More specifically, and as shown in FIG. 4 of the drawings, the present invention utilizes a pair of concentric logarithmic spiral antennas applied back to back The sphere is shown at 10 and is mounted on a suitable support 12 which also carries the feed line, to be described hereinafter. passes through the equator of the sphere, indicated at 14, and is "thus positioned normal to the polar axis of the sphere. Suitably secured to or formed on each half of the sphere is a pair of concentric spiral logarithmic antenna's elements 16 and 18 separated by dielectric filled in FIG. 3.

It continues, and later says:

When the two spirals which are fed independently at the poles are fed in phase, by equal length lines, from a common point and the radiation components are in phase all around the equator, the radiation pattern is a figure of revolution about the polar axis.

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