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FREQUENCY: Radio to Infrared threshold

  1. Apr 8, 2010 #1
    Hi folks,

    EM radio signals range in frequencies from a couple Hz up to approximately EHF (30-300GHz)...

    As we exceed the 300GHz threshold, we begin to approach the infrared section of EM spectrum, and furthermore into the visible light area of the spectrum.

    My question is, what happens on the transmitter side of things if you begin pushing frequencies out past 300Ghz?...Does the antenna itself begin to glow infrared, and eventually emit light?

    What physical implications would arise if a radio tower pushed frequencies in the visible range?

    In other words, how would passing the radio frequency threshold manifest from a physical perspective?

  2. jcsd
  3. Apr 8, 2010 #2


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    Welcome to the PF. That's a good and interesting question, IMO. I think, though, that you should be looking at the ways that those frequencies are generated physically. How high have RF oscillators reached? Generating photons via electron orbital transitions is fundamentally different from generating photons via RF currents in antennas (at least I think it is -- interesting physics question).
  4. Apr 8, 2010 #3
    They do get up into hundreds of GHz range now.

    Have you ever heard of skin depth? In classical EM, materials have a property called skin depth that limits how far a wave can travel in it before it attenuates into nothing. The skin depth is frequency dependant. Higher frequency means shorter skin depth. That means that all materials have a physical frequency limit in classical field theory. Higher conductance also lessens the skin depth just so you know.

    Quantum mechanics changes things. Materials can actually allow higher frequencies to pass on through. Take glass for example. It's transparent to visible light.

    Quantum mechanical effects like transparency don't become prominent until the frequency starts getting up towards the visible range. This is the opposite of field theory which is more relevant at lower frequencies. There's a kind of a dead zone where frequencies are too high for us to easily build classical electronics with conductors (because of skin depth among other things) but the frequencies are still too low to use any quantum properties (like transparency in a fiber optic cable). It's not like a smooth transition up the frequency scale. We're a long way from ever building a single device that can transmit 1Hz to 1,000,000,000,000,000Hz
  5. Apr 9, 2010 #4
    Thanks for your replies, still looking for lots more insight, people please post up! :smile:

    That's precisely the question: "Is it possible to generate photos via RF currents in antennas"...

    And how would the light be emitted?
    Would it follow much the same principles of EM fields (for a cylindrical conductor, emit in the radial direction)?

    @Okefenokee, I know of the skin effect - the skin depth is actually the distance at which the original current drops to 1/e (time constant) of it's original value...It's typically set up by eddy currents in materials.

    What is the skin-depth of something like Air, or Empty Space (vacuum)?

  6. Apr 9, 2010 #5


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    There is 'gear change' as you go from RF to Light sources / amplifiers. RF sources tend to be coherent and your average 'glowing thing' will be producing incoherent radiation.
    I would consider lasers to be a sort of intermediate device. The radiation they produce has a high degree of coherence and they can be used as coherent amplifiers. (So you could see them "glowing" in response to a received signal).
    In an electrical amplifier circuit, the electrons, which interact with the received signal, are interacting with the bulk of the material because the energy of the photons (if you want to look at it that way) is appropriate to the energy levels / populations in the material as a whole. Single photons of higher energy interact with individual atoms and it is only when you produce the 'population inversion' under laser conditions that the bulk of the material starts to behave in the same coherent way as a transistor amplifier.
    Considerations such as skin depth, as mentioned preciously, come into it, of course, and you also find differences in the actual implementation of amplifiers and detectors which deal with different RF frequencies. Coils, transmissions lines and waveguide cavities all have different structures but serve similar functions at the appropriate frequencies.

    This provokes the question about what is the maximum frequency at which you could expect coherent amplification. Gamma Ray Lasers (Gasers)?
  7. Apr 9, 2010 #6
    The official microwave region starts from 0.3 GHz and goes to 300 GHz. And 300 GHz to 3000 GHz is called the far-infrared, but is generally refereed to as the Terahertz radiation.

    There are many ways to generate that high frequency, the easiest and the most widely used is frequency multiplication method. Basically the frequency of a lower frequency signal (a radio or a microwave) is multiplied in successive steps to reach a higher frequency using special diodes.

    Another method used is heterodying two infrared lasers to obtain their difference frequency which can be in the terahertz region, or the high microwave end.

    Third method used are specially designed tubes called "clinotrons" which are similar to magnetron in microwave ovens and they can generate frequency up to 500 GHz with alot of power.
  8. Apr 9, 2010 #7


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    One problem with radiating light from a conventional antenna would be the antenna size required.
    Red light has a wavelength of 0.0000007 meters so a half wavelength at that frequency would be half of that. This is VERY SMALL, so other techniques have to be used.

    There is a continuous range of frequencies in the Electromagnetic spectrum and the arbitrary boundaries you see in books are just that. Arbitrary. Nothing magical happens at the boundaries.
    If "photons" are generated at Infra Red frequencies, then they are generated at microwave frequencies. They seem to be just a different way of looking at electromagnetic waves, but I don't want to reignite any "particles" vs "waves" debate.

    If you did get Infra Red radiating from an antenna, you might feel warmth from it if the signal level was high enough, but this happens at microwave frequencies already.
  9. Apr 9, 2010 #8
    For truly empty space the skin depth will be infinite. Air has a non-zero but very small conductivity so the skin depth is practically infinite (Do you want me to actually calculate it for some frequency?). Of course, we can't really build any signal generator or antenna out of air. More about air, if the wavelength is small compared to the average distance between air molecules then the photons will slip through the air like it wasn't even there. Occasionally, a photon may strike a molecule then scatter.

    I like the way sophie put it. There's a fuzzy boundary on the frequency spectrum where you have to switch gears. On the low frequency side, classical EM gives us a good model to predict things and generally we can build classical electronics to suit our needs. On the high side, quantum mechanics rule and we have to build something like a laser if we want a transmitter.

    BTW, I have a question for any physicists. Is a really long wavelength (low frequency) radio signal made up of smaller virtual photons emitted by individual electrons? I read that virtual photons are proposed to explain the interaction of static electric fields so I was wondering if a radio wave is like a Fourier sum of virtual photons.
  10. Apr 10, 2010 #9


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    The Hydrogen Atom model for describing the interaction of Photons with atoms has generated a lot of confusion. It isn't, strictly, the electron that is interacting with a photon - it is the System. When energy is absorbed, the relatively light electron can be regarded as having shifted its orbit (crudely) but that's just for convenience and, in any case, applies to a particular range of photon energies (frequencies) - around the optical region. Gamma rays result from Nuclear energy changes. The energy changes associated with other frequencies of EM may involve molecules vibrating, rotating or changing shape. When you get to 'RF' energies, it isn't the interaction with a single electron that counts but the effect on the whole system of many electrons and the positive ion cores. Also, in dense materials, you get bands of energy and not discrete energy levels.
    The photon energy for RF is extremely low, (=hf) of course and there would be no way of associating a single photon with any particular 'physical' effect so I think it is always treated as a continuum, afaik. This another part of my "gear change" idea.
    An EM wave will always have a finite time/distance limit, as it is turned on and off at some stage. That means that it can be regarded as a range of frequencies, about a centre frequency (the Fourier transform). That is a classical view. Trying to discuss the actual nature of the photons involved is very problematical. I can't for instance, see any reason why or how photons need to be described as tiny bullets - their energy content is will defined but nothing in QM seems to insist that they should occupy a particular region in time or space. As time has no meaning for something that is travelling at c, one should avoid talking in terms of how time and distance should affect a photon - 'from the photon's point of view'.
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