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

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.

  • Then is it possible to understand why such a large frequency range was needed? Or were they used because of the compactness instead? I'm looking for the "root cause" if possible. The satellites shown in the four bulleted linked questions seem to have gotten by without them. – uhoh Sep 8 at 15:29
  • Got it, thanks! – uhoh Sep 8 at 16:39
  • I found that reference in Table C-1 in which I found in this answer – uhoh Sep 9 at 11:03

@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

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