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Extremely low frequency—antenna

  1. Jul 1, 2009 #1
    Hi!

    I would like to do an experiment by using a can size satellite(at 1km high from earth) to sense the extremely low frequency(elf) and measure the characteristic of this elf.
    So, I am searching for a suitable antenna to fix inside my can size satellite. But result is the antenna is extremely large as compared to my can size satellite.
    Any suggestion about the antenna? Or how to create a suitable antenna for this situation?
    I try to use loop antenna, but seem that it still larger than a can size.
     
  2. jcsd
  3. Jul 1, 2009 #2
    Bundle of ferrite rods.
     
  4. Jul 2, 2009 #3

    vk6kro

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    A half wave dipole for 100 KHz would have a length of about 1500 metres or a bit less than a mile.
    That would be about the smallest efficient antenna you could use.

    You can't use quarter wave antennas, because you can't make an earth connection.

    So, some less efficient antenna could be considered. A length of wire as long as possible could be launched from the satellite and the results can just be accepted.

    Something like the metal tape in tape measures could be used as an antenna. In a weightless environment, this would probably be strong enough. It could be wound out of the satellite by a motor, once it was in orbit.
     
  5. Jul 2, 2009 #4

    berkeman

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    The OP said the "satellite" was only going to be 1km high ("in orbit"...), so either that was a typo, or it is a low-altitude launched payload.

    But you bring up an interesting idea, vk6kro, maybe a long wire antenna either trailed by the can, or maybe even with a baloon on the end of it or something.

    ws0619 -- can your can/satellite trail a long wire?
     
  6. Jul 2, 2009 #5
    What is the source of the ELF, the earth, the atmosphere or something else?

    Would the FAA have anything to say about dangling a long wire antenna from 1000 m?
     
  7. Jul 2, 2009 #6

    berkeman

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    Whoopsies. Very good point!
     
  8. Jul 3, 2009 #7
    Wouldn't a loop of wire on a ferrite rod work well its what they use for AM radio,
    you would need a much longer wire though, with any telescopic antenna.
     
  9. Jul 3, 2009 #8

    vk6kro

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    Ferrite rods actually make very poor antennas but the radios that use them get away with it because broadcast stations use lots of power.

    For effective reception at frequencies below 100 KHz, the ideal antenna is a dipole or, failing that, as much wire in the air as possible. At least the elevated location should help reception.

    If the poster had the means to put the satellite a bit higher, maybe 100 KM, it might be possible to use such long wire antennas. Anything that can fit inside the satellite might not work very well at low frequencies.
    At 1 KM perhaps a hot air balloon with 20 metres of wire below it might be OK.
     
  10. Jul 3, 2009 #9
    I think wire is not suitable for my can size satellite, because I need to BUILD IN the antenna in the can.
    For the (elf),I need to sense from the atmosphere like magnetic wave which produce by earth.But the main thing is I need to do this research on a particular area(1km from earth) to sense any (elf) in that area.And analysis out the characteristic of the (elf).
    Thanks for you all to reply!I try to ask my University lecturer on this antenna, my lecturer said loop antenna is more suitable, but by using loop antenna calculator, the loop antenna still larger than the can size.
     
  11. Jul 3, 2009 #10
  12. Jul 3, 2009 #11
    Has to be ferrite or powdered iron otherwise eddy loses are high. And it can't be a metal can.
     
  13. Jul 3, 2009 #12
    Though he hasn't given the frequency, I was thinking it might be as low as a few tens of Hz. Ferrite or powdered iron wouldn't be needed at those frequencies.
     
  14. Jul 3, 2009 #13
    You can't use solid iron even at mains frequency. It would have to be laminations then as per the common transformer.
     
  15. Jul 3, 2009 #14

    berkeman

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    If you really want to do this experiment 1km high, the most appropriate method would be to trail a wire behind a small aircraft, and shut off the engine for the duration of the measurement (glide for a minute or two). The requirement of a can-size object and your frequency range are not very compatible, and at best you will have a very poor S/N ratio in your measurements (and hence, may not get any useful data at all).
     
  16. Jul 4, 2009 #15
    Thanks for your advice.I get your point.

    I am not using a small aircraft.How I will do it is like I will use a can size satellite and launch it by maybe using small rocket or balloon to reach 1km high then let off the can size satellite.During the comeback process, It support by para foil.If I use your way by trailing a wire behind, I think during the falling the wire won't be a straight line, and it will become helical.So, can this wire act as antenna?I was thinking of coiling the wire around the outer layer of the can size satellite,can this actually work?
     
  17. Jul 4, 2009 #16

    vk6kro

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    Are you listening for Whistlers?
    http://en.wikipedia.org/wiki/Whistler_(radio [Broken])

    If you have access to a balloon, maybe you could turn the experiment upside down and try it from the ground with the balloon holding your wire vertical for you. Then you would be able to see how sensitive your equipment needs to be before you commit to more expensive techniques.

    1000 cu ft of Hydrogen will lift 70 lbs
    1000 cu ft of Helium will lift 65 lbs
    1000 cu ft of hot air at 212 deg F will lift 17 lbs

    1000 cu ft is a spherical shape 6.2 ft in radius or 12.4 ft in diameter. That is a big balloon.

    The antenna would be ineffective if you coil it up.
     
    Last edited by a moderator: May 4, 2017
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