The Radio Spectrum

By Sean Carroll | March 24, 2011 8:59 am

At every point in space, there is something we call the “electric field.” It’s a tiny vector, a quantity with a magnitude and a direction. If you want to measure it, just put an electron at rest at that point, and watch it start moving. The direction and size of its acceleration (over and above what we get from gravity) is proportional to the electric field. Typically, if you watch closely enough, you’ll see our little electron jiggle back and forth like mad. That’s because the electric field doesn’t just sit there; we are surrounded by an extraordinary superposition of all kinds of electromagnetic waves, pushing by us with different amplitudes and directions and frequencies. If you build the right type of gizmo with an appropriate collection of electrons, you can pick out just a single wavelength from amidst the cacophony. Voila! You are listening in on the electromagnetic spectrum.

In the modern world, there are an awful lot of devices out there communicating by shooting electromagnetic waves at each other. In particular in the radio frequency range (roughly between 10 kHz and 300 GHz), which has the nice property that its waves aren’t blocked by annoying things like walls or air. This means that everyone building such devices wants to produce waves at some part of the spectrum, and that in turn means that the right to do so is an extraordinarily valuable commodity. In the US, the Federal Communications Commission gets to decide who can do what at various different radio frequencies.

This state of affairs has come into the news once again, as wireless carrier AT&T has swallowed competitor T-Mobile; many people would be unsurprised if Verizon counters by swallowing Sprint, leaving us with a duopoly and possibly giving consumers the squeeze. Currently big chunk of spectrum is allocated to broadcast TV, which some are arguing is a waste, since you could stick a lot of mobile data devices in there and everyone has cable anyway.

All very fascinating, but somewhat over my head. I’m more of a theoretical kind of guy. I just wanted an excuse to link to this gorgeous chart (pdf), showing how the spectrum is currently allocated.

Click for much bigger and more legible pdf version. There’s a lot going on here; see the zoom-in of a tiny region near 30 GHz:

Nice to see that there is space carved out for scientific research, including radio astronomy. Those jiggling electrons have a lot of work to do, let’s hope they can keep everything straight.

CATEGORIZED UNDER: Technology, Top Posts
  • joe

    SETI has found alien life! Oh wait, that’s just AT&T encroaching on scientific frequencies.

  • Supernova

    Someone has to post this, so I will:

  • Matt B.

    Nice! They got the colour of magic in there. Thanks, Supernova.

  • teawithbuzz

    I have a big poster of this, that I drag out and share with my high school students when we get to the electromagnetic spectrum. It is nice to have it all big enough to read and also still see it all at once!

  • Emily

    I wonder what the high score would be for the question: how much of this chart could an individual expert reproduce from memory? Presumably there are allocation experts who really know this stuff, but it certainly looks daunting when it is presented like this.

  • Claire C Smith

    Thanks Sean….. but you’ve just broken my neck(second diagram!!!)

  • Joe

    Errr…. Since when exactly are COSMIC RAYS an EM wave? (See the overview of the EM spectrum at the bottom.)

  • Roderick


    Technically particle radiation, but generally still included in the EM spectrum.

  • Joe

    @ Roderick
    Not by the astronomers that I know…

  • Low Math, Meekly Interacting

    It’s amazing how crammed parts of it look, even over stretches of a whole log.

    I wonder: How much wireless data transmitted these days is compressed? How much of the available spectrum could be opened up if everything were compressed?

  • RandomGuy

    @Low Math: how lossy would the compressed stream be? Over a noisy medium like radio, less-than-maximal compression is used for error correction.

  • Low Math, Meekly Interacting

    Sure, but analog communication isn’t compressed at all, right? I think I read someplace you can fit the entire hi-def digital bandwidth of basic cable in the amount of spectrum one analog channel takes up, or at least something along those impressive lines. I would naively think even the most conservative lossless compression of digital signal would yield considerable improvements in EM real estate over any analog signal.

  • Charon

    And this is why radio astronomers want telescopes on the far side of the Moon. The one damn place left that’s actually radio quiet! (As long as you can deal with the Sun and the Galaxy, but those are much more minor problems.)

  • Anne

    Those spaces left for radio astronomy are less useful than one might hope. They left a little hunk around various important spectral lines, which is nice if you want to do spectral-line work, but the rest of us radio astronomers are now going for fractional bandwidths of 50% and higher – when our input signal is everything from 1100-1900 MHz, there’s no way we can count on having spectrum allocated. We just have to hope there’s not too much crud and that we can distinguish it from what we want to see.

    The real appeal of the far side of the Moon (or the near, for that matter) is not so much the radio silence as the low end of the spectrum – below about 20 MHz, signals don’t make it through the ionosphere. Our only idea of what’s out there at those frequencies comes from one short-lived satellite experiment, with a necessarily very small collecting area (and no directionality). There’s a lower frequency limit set by the interplanetary medium, but there’s still a lot of unexplored spectrum down there.

  • anne

    Responding to Emily, I seriously doubt that there are more than a few people with detailed knowledge of U.S. spectrum allocation. I expect that most of us with any level of expertise tend to be limited to our own narrow realm. Based on my experience, I can tell you a great deal about FM, including surrounding bands (VHF ‘low’ and aviation, mostly), but not much about anything else, except as it’s occasionally related to FM. For example, about 20 years ago, when digital audio broadcasting (DAB) was first proposed, many of us lobbied heavily to adopt the practice of most other countries, to use the L-bands. I swear, at one time I knew where in the spectrum the L-bands were, but I honestly don’t know now; I’d have to look it up. (I did. It’s 1452–1492 MHz.)

  • rfguy

    This is great! I design little collections of electrons (and protons and neutrons, of course) to detect the jiggling at certain frequencies: see here for example. I’m also a ham radio operator (but I have a few kosher radios around too). I like the zoom in at the 24GHz area, as that’s the upper limit of my equipment – I’d like to play around with electrons jiggling at 250GHz, but I can’t afford it…


  • dogregor

    Um. It is indeed a gorgeous chart. However, it unfortunately it does not come close to showing “how the spectrum is currently allocated.” If you’ll zoom in on the lower left corner, … what’s that? Oh. Copyright 2003.

    Things change. It was good then. Now, not so much.

  • Piper Patron

    Someone is using my brain’s and body radio signal to torture me. Is there some way to capture and trace by the originating signal and prosecute the criminals involved?

    Is this how doctors generate revenue? They can pump up my blood pressure and make my chest feel so heavy like an artificial heart attack or stroke. I think they have a blue-man diagram of my body on their computer screens somewhere and are able to restrict blood flow from specific points in my brain.

    Please help?


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Cosmic Variance

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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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