Very Low Frequency EM Waves (now: Magnetic Launcher)

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bob012345
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Is it possible to make very low frequency EM waves, say 10 KHz, without an antenna the size of Texas? Something more human sized? Thanks.
 

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  • #3
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How much power? That makes all the difference.
 
  • #4
berkeman
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(Great minds think alike!) :smile:
 
  • #7
berkeman
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Say 1 KW. How does it scale?
To transmit any substantive power, you will need an efficient antenna, which means on the order of λ/4 to λ/2.

What application do you have in mind? If you try transmitting that kind of VLF power and you live anywhere near the ocean, you will probably get a visit from a blue van pretty quickly. Quiz Question -- Why?
 
  • #8
bob012345
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To transmit any substantive power, you will need an efficient antenna, which means on the order of λ/4 to λ/2.

What application do you have in mind? If you try transmitting that kind of VLF power and you live anywhere near the ocean, you will probably get a visit from a blue van pretty quickly. Quiz Question -- Why?
I'm guessing signals would follow the water around the surface and interfere with Naval communications. Is that close? I'm interested in knowing or finding a mechanism to transmit a strong Lorentz force between objects at a significant distance. I think low frequency interactions would be easier to deal with. One application could be manipulating objects in low earth orbit.
 
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  • #9
berkeman
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I'm guessing signals would follow the water around the surface and interfere with Naval communications. Is that close?
Mostly that VLF is used for some submarine communications, so I don't even know if you could get a license to transmit in the VLF band.
 
  • #10
bob012345
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Mostly that VLF is used for some submarine communications, so I don't even know if you could get a license to transmit in the VLF band.
I wouldn't want to be accused of trying to pass information to the Russians.
 
  • #11
tech99
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Is it possible to make very low frequency EM waves, say 10 KHz, without an antenna the size of Texas? Something more human sized? Thanks.
As an illustration, the 16 kHz transmitter (callsign GBR) at Rugby, UK, now defunct, used a 1MW transmitter and achieved a radiated power of about 20kW. The antenna masts were 820 feet high and the area of the transmitting antenna was about a square mile.
If you do some numbers, I think you will find that to overcome the high levels of atmospheric static at these frequencies, even with very slow data, you will need a kilowatt transmitter and a very long antenna, say 500m long. This might achieve a milliwatt or so of radiated power.
 
  • #12
bob012345
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To transmit any substantive power, you will need an efficient antenna, which means on the order of λ/4 to λ/2.
As an illustration, the 16 kHz transmitter (callsign GBR) at Rugby, UK, now defunct, used a 1MW transmitter and achieved a radiated power of about 20kW. The antenna masts were 820 feet high and the area of the transmitting antenna was about a square mile.
If you do some numbers, I think you will find that to overcome the high levels of atmospheric static at these frequencies, even with very slow data, you will need a kilowatt transmitter and a very long antenna, say 500m long. This might achieve a milliwatt or so of radiated power.
Thanks. So transmitting traditional VLF EM waves seems like it won't be feasible for what I interested in. I'm not so interested in VLF waves per se as slowly changing E/B fields at a distance which is why I asked about VLF frequencies. What about mixing/beating two masers with a few KHz difference?
 
  • #13
berkeman
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Thanks. So transmitting traditional VLF EM waves seems like it won't be feasible for what I interested in.
Can you say more about the application you have in mind? Maybe there is an alternative, depending on distance, obstructions, etc. You could consider using audio or ultrasound or optical communication in the 10-20kHz range, for example...
 
  • #14
bob012345
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Can you say more about the application you have in mind? Maybe there is an alternative, depending on distance, obstructions, etc. You could consider using audio or ultrasound or optical communication in the 10-20kHz range, for example...
I know a guy (not me) who's obsessed with the idea of a magnetic launcher. I don't think his ideas of a long range magnetic 'beam' will work but it got me to thinking about a Lorentz force over a long distance between physical objects. In order to have a Lorentz force, you need fields and in order to get fields at a distance you need waves or beams in some form. So, I want a substantial field transmitted over a distance that can interact with a receiver of some type that is active, meaning it provides either a charged surface or current for the fields to interact with rather than just a passive detector. Sort of like using strong EM waves to accelerate charged particles in new compact accelerator designs but over a distance and acting on a larger object.
 
  • #15
tech99
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The Lorentz force is alternating so it will not propel the projectile.
There is a possibility of using radiation pressure from a narrow beam source such as a laser, but the force is very small even with immense power. Additionally, a Laser bream gradually diverges.
How would a projectile steer itself to remain in the beam? It's like balancing on a pencil point.
 
  • #16
bob012345
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The Lorentz force is alternating so it will not propel the projectile.
There is a possibility of using radiation pressure from a narrow beam source such as a laser, but the force is very small even with immense power. Additionally, a Laser bream gradually diverges.
How would a projectile steer itself to remain in the beam? It's like balancing on a pencil point.
Thanks. Beamed propulsion is becoming quite a mainstream concept now in advanced propulsion circles and people discuss beams over planetary distances but they are very weak and require immense power which is why a Lorentz force based propulsion concept would be vastly superior. As for the Lorentz force alternating, yes, of course it does and that's exactly why I wanted a lower frequency source so the target currents or charges could match the phase of the field to keep the force in one direction.
 
  • #17
berkeman
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Thanks. Beamed propulsion is becoming quite a mainstream concept now in advanced propulsion circles
Links please?
As for the Lorentz force alternating, yes, of course it does and that's exactly why I wanted a lower frequency source so the target currents or charges could match the phase of the field to keep the force in one direction.
Could you show us the math behind what you just said please? Thanks.
 
  • #18
bob012345
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Links please?

Could you show us the math behind what you just said please? Thanks.
https://www.centauri-dreams.org/2016/06/24/a-photon-beam-propulsion-timeline/

https://www.nasa.gov/sites/default/files/atoms/files/roadmap_to_interstellar_flight_tagged.pdf

https://www.nasa.gov/feature/deep-in-directed-energy-propulsion-for-interstellar-exploration


I'll explain the concept further. A plane wave approaches an antenna, for our purposes just a wire carrying current, such that as the magnetic field passes by the wire a Lorentz force acts on the wire. If the current is constant the Lorentz force alternates with the field averaging out to zero. If the current varies slowly, the force will average out to zero. If the current changes at the same frequency as field, the Lorentz force is always in the same direction. I call that phase matching. Imagine the math of two current carrying wires interacting with constant current. The forces attract or repel depending on the directions of the currents. Let one current be constant and the other change sinusoidally. Let the varying wire be your source and the fixed your ship. The force nets to zero. Now match the other current and the force stays in the same direction, whichever that was. Now replace one with a propagating field from a distant source through a wave. It's the same concept as long as one can recreate a similar field at the wire (which may require 22nd century engineering).
 
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  • #19
berkeman
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If the current changes at the same frequency as field, the Lorentz force is always in the same direction.
So you want to have a resonant antenna as the target to try to maximize the unidirectional force, it sounds like. Interesting idea, and it would be good to see the math. Remember that the physical size of an efficient antenna is on the order of a wavelength. Maybe you could work on the calcs for a large array of resonant antennas fixed to a sail-type structure to see what kind of drive you could get in space...
 
  • #20
bob012345
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So you want to have a resonant antenna as the target to try to maximize the unidirectional force, it sounds like. Interesting idea, and it would be good to see the math. Remember that the physical size of an efficient antenna is on the order of a wavelength. Maybe you could work on the calcs for a large array of resonant antennas fixed to a sail-type structure to see what kind of drive you could get in space...
Thanks. I'll try and get a little more mathematical basis for the idea on paper. Thanks for the suggestion.
 
  • #21
Tom.G
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Yes, interesting. What is the source of the phase-controlled current in the wire?
 
  • #22
bob012345
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Yes, interesting. What is the source of the phase-controlled current in the wire?
Thanks. The original concept is an independent current source matching the known phase of the incoming wave. The refined idea is to allow the wave to energize a current, as usual, and then phase delay and orient that current to match and interact with the incoming magnetic field.
 
  • #23
Tom.G
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OK, what primary source supplies power to the 'independent current source'? Solar, Nuclear, Chemical, Magic, or...?
 
  • #24
bob012345
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OK, what primary source supplies power to the 'independent current source'? Solar, Nuclear, Chemical, Magic, or...?
It could be solar panels for example if one was talking about space applications in the inner solar system. I also suggested above the energy could ultimately come from the same source generating the wave in the first place. What are getting at?
 
  • #25
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