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What happens when a wavelength is longer than the length of coax?

  1. Jun 9, 2014 #1
    We are all taught that the TEM modes in coax have no low frequency cut-off. So this would suggest that nothing different happens for very long wavelengths that are longer than the coax itself.

    However, in this rather shocking paper here: http://vixra.org/pdf/1403.0964v5.pdf

    ...it is suggested that long waves do not exist in a transmission line! Am I reading this paper correctly?

    I agree if the ends are shorted, there can be no standing waves longer than the length of the cable (obviously), however, the above paper appears to be talking generally for all circumstances. Am I reading this correctly?

    Any idea exactly where this paper goes wrong? What is the most succinct way to counter the claim in the paper?
     
    Last edited by a moderator: Jun 9, 2014
  2. jcsd
  3. Jun 12, 2014 #2
    I think long wavelengths means it's practically DC.
     
  4. Jun 12, 2014 #3

    nsaspook

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    Last edited: Jun 12, 2014
  5. Jun 13, 2014 #4

    f95toli

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    I am not even sure what the issue is. Coaxial cable can obviously be used for whatever frequency you want up to (and sometime over) the modding frequency; there are sometimes practical issues with attenuation etc but those are not fundamental.
    Whether or not you talk about "waves" when the wavelength is much longer than the cable is just a question of semantics; Maxwell's equations are always valid so there is nothing stopping you from applying them even if you are technically in the lumped element regime.

    I assume I don't have to point out that coaxial cables are very often used for very low frequency signals (all the way down to DC), this is (hopefully) something everyone has experience of (most people get to play with an oscilloscope at some point in school)
     
  6. Jun 13, 2014 #5

    AlephZero

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    I think the paper is about the semantics of the English language, not about physics.
     
  7. Jun 13, 2014 #6

    nsaspook

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    The fact that only a small portion of the 'wave' is present on the short cable at any one time only means the distributed lump element effects of the electrical characteristics of the cable are minute if losses are low and the cable is short not non-existent. The changing signal takes a finite amount of time to travel from point A (original signal) to B (time delayed cable modified signal) so there is a transmission of EM wave energy in the cable dielectric that also interacts with the charge carriers in the conductors that can be measured as the signal levels change across the wire. The root question of the papers seems to be can the directionally of the wave energy across the cable be used to detect the key sequence sent by modulation of noise sources. Because the key sequence information is sent by the EM waves of the noise sources on the cable, detection should be possible in a keying system that uses a lossy cable beyond a few meters by tapping the cable a two distant points. The tricky part is how to isolate the large common wave components in both signals so you can see the small difference.

    http://arxiv.org/pdf/1303.7435.pdf
     
    Last edited: Jun 13, 2014
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