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Magnetoelectric antennas could transform how underwater robots talk

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Re: Magnetoelectric antennas could transform how underwater robots talk

#11

Post by adrian_b »

peter_d_sherman wrote: post #4

"At 36 kHz, the wavelength shrinks from roughly 8,327 m (27,320 ft) in air to just 170 m (558 ft) in freshwater..." Yes, waves apparently compress or expand depending on the medium they are in... I'm curious as to what the extremes of potential medium might be... on one end, we might have the densest of heavy metals and on the other, we might have the vacuum of outer space... Also, what role does/would temperature pl…

When a wave passes through different media, its frequency remains the same, but its velocity changes.

The wavelength is the ratio between velocity and frequency, so it changes proportionally.

If you multiply 36 kHz by 8326 m, you get a value only slightly less than the speed of light in vacuum, which is true for the propagation of electromagnetic waves in most gases.

On the other hand, with 170 m, you will get a speed of VLF radio waves in sea water that is much lower than in vacuum.

The speed of electromagnetic waves in most media depends strongly on frequency.

At frequencies corresponding with visible light, only in few materials the speed is lower than half of the speed in vacuum (i.e. the refractive index is greater than 2).

On the other hand, for low frequency radio waves, speeds that are 10 times slower or even 100 times slower than in vacuum are not unusual.

Re: Magnetoelectric antennas could transform how underwater robots talk

#12

Post by zipy124 »

adrian_b wrote: post #9
gus_massa wrote: post #3

I was wondering how this could make sense until: The result is an antenna that operates at very low frequencies, around 35–36 kHz, while remaining far more compact than the conventional electrical antennas that work at those same frequencies. They are using a super low frequency.

Very low frequency radio waves is the traditional means of communication with military submarines, while submerged. However, this required huge antennas and very high power transmitters, so this was used mainly to transmit short messages from a terrestrial station to submarines, for instance instructing them to send an antenna to the surface, for bidirectional communication at high speed. The innovation here is the u…

Yup, on the order of below 100 Hz usually https://en.wikipedia.org/wiki/Extremely_low_frequency .

Re: Magnetoelectric antennas could transform how underwater robots talk

#13

Post by fc417fc802 »

zipy124 wrote: post #12
adrian_b wrote: post #9

Earlier quoted context omitted.

Very low frequency radio waves is the traditional means of communication with military submarines, while submerged. However, this required huge antennas and very high power transmitters, so this was used mainly to transmit short messages from a terrestrial station to submarines, for instance instructing them to send an antenna to the surface, for bidirectional communication at high speed. The innovation here is the u…

Yup, on the order of below 100 Hz usually https://en.wikipedia.org/wiki/Extremely_low_frequency .

Below a certain point do you suppose my headphone jack can double as a transmitter?

Re: Magnetoelectric antennas could transform how underwater robots talk

#14

Post by Klaus23 »

raffael_de wrote: post #6

my first association here would be steering of torpedoes. the US Navy must have been on this for decades and very deep pockets.

Torpedoes are usually steered using fibre-optic wires, like the fibre-optic drones in Ukraine today, so there is no need for problematic low-frequency radio.

Re: Magnetoelectric antennas could transform how underwater robots talk

#15

Post by ferdzo »

fc417fc802 wrote: post #13
zipy124 wrote: post #12

Earlier quoted context omitted.

Yup, on the order of below 100 Hz usually https://en.wikipedia.org/wiki/Extremely_low_frequency .

Below a certain point do you suppose my headphone jack can double as a transmitter?

Technically, yes. Also a lot of people in the amateur radio community use the microphone in as a receiver for VLF transmissions.

Re: Magnetoelectric antennas could transform how underwater robots talk

#17

Post by bobmcnamara »

gus_massa wrote: post #3

I was wondering how this could make sense until: The result is an antenna that operates at very low frequencies, around 35–36 kHz, while remaining far more compact than the conventional electrical antennas that work at those same frequencies. They are using a super low frequency.

Thank Shannon!

Re: Magnetoelectric antennas could transform how underwater robots talk

#18

Post by redsocksfan45 »

Klaus23 wrote: post #14
raffael_de wrote: post #6

my first association here would be steering of torpedoes. the US Navy must have been on this for decades and very deep pockets.

Torpedoes are usually steered using fibre-optic wires, like the fibre-optic drones in Ukraine today, so there is no need for problematic low-frequency radio.

[dead]

Re: Magnetoelectric antennas could transform how underwater robots talk

#19

Post by mcswell »

adrian_b wrote: post #11
peter_d_sherman wrote: post #4

"At 36 kHz, the wavelength shrinks from roughly 8,327 m (27,320 ft) in air to just 170 m (558 ft) in freshwater..." Yes, waves apparently compress or expand depending on the medium they are in... I'm curious as to what the extremes of potential medium might be... on one end, we might have the densest of heavy metals and on the other, we might have the vacuum of outer space... Also, what role does/would temperature pl…

When a wave passes through different media, its frequency remains the same, but its velocity changes. The wavelength is the ratio between velocity and frequency, so it changes proportionally. If you multiply 36 kHz by 8326 m, you get a value only slightly less than the speed of light in vacuum, which is true for the propagation of electromagnetic waves in most gases. On the other hand, with 170 m, you will get a spee…

I tried to do a little (web) research on this. It is of course the reason a prism separates white light into its components. I didn't find out much about sea water, though.

And then there's "slow glass", in which the passage of light through half an inch of glass takes years; the subject of the short story "Light of Other Days" :).

Re: Magnetoelectric antennas could transform how underwater robots talk

#20

Post by XorNot »

raffael_de wrote: post #8
7952 wrote: post #7

Earlier quoted context omitted.

Navies are known to use low frequency radio to send messages to submerged subs.

But isn't torpedo steering still dependent on wire?

The issue is it doesn't really matter and radio isn't much benefit: you get much higher bandwidth, better reliability, immunity to ECM, and fiber-optic wires in Ukraine are over 50km long.

The exact application for this is autonomous underwater vehicles where what you would like to do is communicate quickly and without a tether in arbitrary scenarios - i.e. think a bunch of autonomous vehicles which might need to relay a message or communicate with dropped assets. Using radio in those scenarios solves the problem of a consumable (the wire), and also the problems associated with sonar like fouling of the array.

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