Relation of AC to broadcast electromagnetic waves

Discussion in 'General Electronics Chat' started by Denesius, Oct 8, 2014.

  1. Denesius

    Thread Starter Member

    Feb 5, 2014
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    Ok, so couple of buddies hit me up with a question I couldn't address, so I'm turning here, even though this may not be the right forum:
    Output stage of a radio (we were discussing avionics): AC power of 5 watts at 125-Mhz is pumped into a power line (the 50-ohm cable), that terminates in a loop (the antenna of appropriate length). As the electrons zip back & forth, the change in direction of the resultant magnetic field creates an electromagnetic wave that then propagates outward, for many miles (line of sight or bouncing around based on freq, but irrelevant to discussion).

    Why doesn't common household AC transmit a similar wave for thousands of miles?
    The frequency is lower, but the electrons are still being whipped around
    The power is many orders of magnitude higher
    The power lines are not tuned to the frequency, but surely there's lines out there that happen to match, and even a mismatch should propagate a hefty signal.
    Yes, you do get 60 cycle hum in a radio off station, but hardly what you'd expect for the power output. Considering the output power, an appropriate receiving antenna a few miles away should still be putting out sparks!
    I'm not confusing a loop of wire within the magnetic field of the power line picking up current-I'm referring to the distant transmission of a 60-hz radio signal.
    What's missing?
    Any thoughts?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    have you calculated the resonant length of a 60 Hz antenna?

    People illegally tap into high tension power lines all the time using their broadcast power, get caught, and pay fines or more.
     
  3. JDT

    Well-Known Member

    Feb 12, 2009
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    It does.

    Correct. BTW: The electrons themselves, don't travel at the speed of light (or anywhere close). At 60Hz they probably move around a bit. At higher frequencies (such as the 125MHz you mention) they just "wobble" slightly. The wave, however passes down the line at about 60% the speed of light.

    The power travelling through the power lines is higher but as far as E.M. radiation is concerned it's mainly the current (magnetic field) and the voltage field that matters.
    Although the power lines are often very long, they are also very close together (compared to the wavelength of 60Hz) so the magnetic and voltage fields tend to cancel out.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Never thought of that. I know 30MHz is in the 10m band. Hence 60MHz is 5m.
    60Hz would be 5,000,000m = 5000km
    Quarter wavelength = 1250km
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    it would if the power lines werent designed to prevent it. both the 3 phase lines and two phase lines act as a ballanced transmission line, which does not radiate much. the single wire power lines are broken up with transformers to prevent a resonant length which would be bad for power transmissiion, making them poor radiators, as well as the low height of the lines.
     
  6. Denesius

    Thread Starter Member

    Feb 5, 2014
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    The way I understand it, current is a measure of the number of electrons passing thru a cross section of the conductor. In a closed DC circuit, there is a net movement of electrons from one electrode to the other. Are you saying in an AC circuit there is no net movement?
    I understood that the electrons 'moved' at a significant percentage of the speed of light during 1/2 cycle, then reversed during the other 1/2. In a normal length conductor, there's plenty of time for an electron to move from one end to the other, before reversing direction. For example, at 125MHz, at 60% of light speed (180KM/s), a theoretical electron would whip 2.8KM in one direction before reversing.
     
  7. Denesius

    Thread Starter Member

    Feb 5, 2014
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    You addressed my next question! If the power lines do radiate power as an electromagnetic wave, then isn't that the greater power loss during transmission, rather than line resistance?
     
  8. Denesius

    Thread Starter Member

    Feb 5, 2014
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    I think what the power stealers are doing is putting a coil in line with the transmission line's magnetic field, and taking inductance power. I once looked at purchasing a house with a high tension power line in the back yard. Wife worried about electrocution hazard, and I was, like 'honey, we can wind a coil of wire around the leg of the transmission tower on our property & have free electricity'. Hmmm.....
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    No, that is the speed of the electromagnetic field. The electrons themselves move at drift velocity, which is in the order of millimeters per second. http://en.wikipedia.org/wiki/Drift_velocity
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the fields of the two and three wire lines cancel. and the one wire types are broken into non resonant lengths to reduce radiation. a lot of the losses are actually capacative losses and corona loss.
     
  11. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Electrons are really slow in wires.
    http://webcache.googleusercontent.com/search?q=cache:rTslF-4YoGYJ:https://www.bpa.gov/PublicInvolvement/CommunityEducation/CurriculumActivities/CurriculumDocuments/ride_the_surprisingly_slow_electron_express.doc &cd=1&hl=en&ct=clnk&gl=us&client=iceweasel-a

    Electrons facilitate the transfer of energy almost instantly but the electrons by themselves don't transfer energy.
     
  12. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    if i rectify my AC and then charge a capacitor, if the electrons are not "flowing", what exactly is building up the charge inside that capacitor?
     
  13. Kermit2

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    Feb 5, 2010
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    a better question might be what causes the extreme and rapid rise in temperature when too large of an AC current tries to flow through a power cord? electrons moving so slowly do that?
     
  14. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    The total number of electrons in the circuit remains the same but they are separated via the circuits electric field energy that moves at close to light speed using the electrons as charge carriers. Electrons from one plate slowly move into the connecting wiring from the plate and electrons from the other connecting wire slowly move into the other plate and creating a electric charge between the plates that stores 'from charge separation' electrical energy as a voltage potential inside the capacitor.
     
  15. nsaspook

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    It's electrons colliding with other electrons (electrical resistance) and the kinetic energy of their slight movements being converted to heat energy (random thermal EM energy) instead of moving freely at the drift speed in response to the flow of electrical field energy around them.
     
  16. alfacliff

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    Dec 13, 2013
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    electrons do not move at the speed of light through wires. it is a bit slower, due to the inductance and capacitance of the wires, and the "drunkards walk" motion, the electrons do not go streight, but bounce from molecule to molecule on their way. compared to the reversal of ac current, it seems real fast, tho.
     
  17. nsaspook

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    The inductance and capacitance of the wires is a field effect that is modified by a fields interaction with charges. The electrons do move very fast at room temperature fermi but it's completely random so the total velocity vector is zero in any direction without a external field to bias the direction slightly in a wire.

    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html
     
    Last edited: Oct 8, 2014
  18. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    If you look again at the equation for the drift velocity, you see that the charge of one electron is 1.6*10^-19 coulombs, but there is 8.5*10^28 of them in a cubic meter of copper. That means that you got large amount of electrons, but you need only small number of them to pass the charge of 1 coulomb. Thus they need to move only veeeery slowly to get the flow of 1A or 1 coulomb per second.
     
  19. alfacliff

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    Dec 13, 2013
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    yes it is a field effect, but if you check on transmission line theory, it is also called velocity factor. look up delay line, a series l shunt c componant that delays signals through it. the series l is the inductance of the wires, and the shunt c is the capacitance to ground, which over a few miles really adds up.
     
  20. nsaspook

    AAC Fanatic!

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    Sure it can be explained easily using transmission line theory equations but the basics of every transmission line is a energy guide using field equations.
    http://ocw.mit.edu/resources/res-6-...ch-spring-2008/textbook-contents/chapter8.pdf
     
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