# Magnetic modulation/ induction

I've been looking for a viable means of communication under water (other than sonars), be it short or mid-ranged, and I have come across MM and MI. I am no engineer, so I need help with this, and I guessed you guys would be helpful.

In this case, the emitter would be located in an AUV (Autonomous Underwater Vehicle), so the power input to the comm system can't be too high, let's say no more than 20 or 25 WH/dm^3, and it should last for more or less 30 hours.

The question is: What would the maximum range of this system be?

• Is that watt-Hours per decimetre (cubed)? Can we have it translated to watts output power please? Dec 16 '15 at 8:56
• @Andyaka every hour it should consume 25 watts for every dm cubed the battery takes up. This means that for a 1dm^3 battery, the battery should be able to output 25 watts. If you were to need more wattage, you could increase the battery size, but keep in mind the AUV still needs to take measurements and move and stuff, so you don't want to use all the power in the battery or make the battery too big. If you need anything else ask me for it. Dec 16 '15 at 9:01
• Fresh water or sea water? Dec 16 '15 at 9:23
• @Andyaka ideally sea water :) Dec 16 '15 at 9:25
• Data rate? Hint, lower allows lower frequency, allows longer range. Dec 16 '15 at 9:36

## Is there a case for radio over magnetism underwater?

You can send and receive data using radio underwater but you will find that sea water and fresh water are quite different mediums: -

The graph is comparing seawater with Adelaide fresh water and yes, the graph is a bit poor quality but the formulas are here: -

• Attenuation (α) in dB/metre = 0. 0173 √(fσ)
• where f = frequency in hertz and
• σ = conductivity in mhos/metre (siemens per metre)

The graph tells you attenuation per metre and is taken from THIS document. So why persist with radio waves when you can do it magnetically?

Answer - In the short range "arena" magnetism wins but as distance increases the voltage induced in a receive coil falls as distance cubes (as does the E field when not conjoined to a H field like it is in a proper EM wave). If you transmit a radio wave then this is a proper EM field and individually E and H falls as plain ordinary distance. That's the beauty of radio - you appear to get something that is totally better than the components that it's built from.

See my answer to this unrelated question for some more detail and also this answer which has the following pretty picture: -

The picture above shows the formula for flux density at a distance from a transmit coil and note that as Z gets dominant over the coil radius the denominator becomes $2Z^3$ i.e. an inverse cube law.

So, my recommendation is to strongly consider a proper radio wave or at least compare the levels of signals you are likely to receive versus distance. Also, read the very excellent document with the bold link under the bullet points. You can learn a lot from it.

• Really nice answer, well explained! I'd upvote it if I could, but I ain't got no reputation yet. Anyways, the thing with proper EM waves is that they have an absolute crap range underwater, and because o'the fact that you can use compasses even over the water, I thought that MF (actual magnetic fields, not EM fields) would have a better (or absolute) penetration. Is there anyway you could open a chat with me even if I don't have enough rep? Dec 16 '15 at 9:55
• I'm unsure how to open a chat but it would be a timing problem for me for the next several hours anyway. Find my email on my profile is probably the most convenient way for me. Dec 16 '15 at 10:06
• I will upvote this for you, since the answer is good :) Most important note is that "as distance increases the voltage induced in a receive coil falls as distance cubes". Magnetic field is different from EM field only if it is a static field. As soon as you add any modulation, it becomes EM field, as the changing M field induces E field and changing E field induces M field. If the modulation is very slow (to treat it as static field) than the range is too small. That is what I was trying to explain but here the wording is far better. Dec 16 '15 at 10:16

As I understand...

Magnetic field if it is not in form (the part) of normal magnetic wave is very short ranged due to its round shaped form. Moreover it is useless for communication if it is not modulated.

The main idea of "uncurling" the field is to convert it to magnetic wave - which is performed by modulating the field, i.e. emitting the waves with antenna - simply speaking, by changing, modulating the field.

But in this case it is no different from problems radio-waves which "do not travel well under water as it is a good electric conductor". Changing field induces small currents in conducting medium and the energy of the field dissipates...

So you'd better explain what do you want to achieve.

• What I'd like is some sort of communication that works by modulating either intensity (relative intensity 4 for a 0 bit and rel. intensity 5 for a 1 bit, for example) or frequency, and get a receptor that would react to sudden changes in a magnetic field stronger than the earth's. Dec 16 '15 at 9:14
• It is not different from physical point of view "intensity or frequency" - in both cases you use varying magnetic field. And hence it is still no much difference from AM / FM radio waves. However you still haven't specified what distances you want to achieve - and what transfer speeds... Dec 16 '15 at 10:12
• The ideal range would be between 50 and 100m as to the transmission rate, around 25-30 kbits/s. Dec 16 '15 at 10:25
• Oh, I'm afraid this is not going to work. For static magnetic field this is too far. If you've seen magnetic cranes you can get the idea of magnet producing significant field for about a meter. Dec 16 '15 at 12:51
• Thanks @rodiongorkovenko, you've been very useful. :) Is there any way to close my own questions as "answered"? Dec 17 '15 at 8:17

I'll add this as an answer, rather than a comment to the OP and all of the answers.

You cannot transmit over a significant distance with a magnetic field.

There are two types of extreme situation that occur when using antennae to communicate with each other.

a) you have a predominantly dipole electric field, for instance a pair of seperated plates. This can be detected electrostatically nearby as a high impedance near field, that falls off as the cube of the distance, and doesn't propogate any power unless intercepted by a reciever. Further away, the electromagnetic 377ohm impedance far field wave that falls off as the square becomes the only signal detectable.

b) you have a predominantly dipole magnetic field, for instance a coil of wire. This can be detected magnetically nearby as a low impedance near field, that falls off as the cube of the distance, and further away as a 377ohm impedance far field electromagnetic wave that falls off as the square.

A long range transmitting antenna will be designed so that neither of these extremes are excited, as both will be inefficient, and will ideally launch a propagating electromagnetic wave. The non-propagating near field merely increases voltages or currents in the antenna which increase losses or limit the maximum power that can be used. Near-field magnetic chargers for cars and phones are intentionally short range and non-propagating in the far field.

The difference between near and far field has to do with the aperture of the antenna, and frequency of the signal. In the case of a sub the size of a person, once you are more than a person's length or two away, very little of the magnetic near field remains to be detected, and the only signal that is receivable will be the electromagnetic far field, that will be strongly attenuated by the water, with a frequency dependence, and a water conductivity dependence.