Best frequency of radio light to penetrate 1 meter of sea water?

[chemisthypnos]

We are attempting to penetrate 1m of water featuring dissolved ions ( assume salinity of sea water) using radio waves. Aside for using wavelengths of very low frequency radio or below, what would be the best frequency for achieving this task?

 

[LesRhorer]

This is from memory of many decades ago, But I think the lowest absorption in seawater is light around 580nm. I know for a fact yellow-green apparel can be seen more than 100 meters away in clear seawater, having sen it myself. From even further back,I vaguely seem to recall that some longwave UV also penetrates well.

 

[Baluncore]

We are attempting to penetrate 1m of water featuring dissolved ions ( assume salinity of sea water) using radio waves.

Will you radiate from air into water, or underwater to underwater.

Why? Imaging? Will you look at the reflection?

How much attenuation can you tolerate?
https://www.researchgate.net/profil...6400134714/RF-attenuation-in-sea-water-41.png

 

[tech99]

To obtain propagation into sea water notice that the skin depth is about 1m at a frequency of 500 kHz. Beyond this distance the attenuation wil rapidly increase. In the following paper, the author has used an immersed antenna to increase the distance of communication by utilising a surface wave at the fairly high frequency of 50 MHz. https://arxiv.org/ftp/arxiv/papers/1809/1809.06741.pdf

 

[DaveE]

I'm not sure what "radio light" is. But DARPA and the US Navy has successfully tested communicating with submarines with 532nm lasers. Blue lasers are attenuated less, but they are hard to build at high powers. Unfortunately, I think the results, like depth, are classified. At least they were in the 1980's when I worked on the laser for some of the tests. But, trust me, it works if you do it right with some pretty high tech optical stuff.

 

[berkeman]

We are attempting to penetrate 1m of water featuring dissolved ions ( assume salinity of sea water) using radio waves. Aside for using wavelengths of very low frequency radio or below, what would be the best frequency for achieving this task?

1 meter? Seriously? Just blink your flashlight.

For larger depths, use the suggestion from @DaveE above.

 

[DaveE]

https://idstch.com/military/navy/bl...arine-warfare-and-network-centric-operations/

 

[chemisthypnos]

Will you radiate from air into water, or underwater to underwater.

Why? Imaging? Will you look at the reflection?

How much attenuation can you tolerate?
https://www.researchgate.net/profil...6400134714/RF-attenuation-in-sea-water-41.png

It would be used for air to water and it would be for underwater imaging. That graph was very helpful. I think that we are going to try for 540 kHz wavelength. This should minimize the attenuation. We are trying to find out how large of an antenna we would need and how much power that would take if we used electrical loading and fractal antenna design for it. I see that there is a small one on Walmart for 1MHz and it seems pretty small.

https://www.walmart.com/ip/Sardfxul...le-for-Radio-Receive-Player-Module/1729339158

 

[Baluncore]

It would be used for air to water and it would be for underwater imaging.

What will you be imaging?
What resolution?
At what depth?

We are trying to find out how large of an antenna we would need and how much power that would take if we used electrical loading and fractal antenna design for it.

There will be a problem with the beamwidth required of your antenna array.
Beamwidth in degrees = 57° / wavelength in metres.
If you use 100 MHz which has a wavelength of 3 metres, then to get a 5.7° beam, your antenna array will need to be 30 metres across.

To do the same at 540 kHz where the wavelength is 555 metres, your antenna array will need to be 5.555 km across.

At 1 GHz with a wavelength of 300 mm, the array will be only 3 metres across. That is a good reason for using shorter wavelengths.

 

[Vanadium 50]

This is hopeless.

The resolution of your image will be of order one wavelength. (I am ignoring factors of 2 and π and suchj) The ability of the radio to penetrate the conductor is of order one wavelength. (I am again ignoring factors of 2 and π and suchj)

So if you are 1 m deep, the scale of what you can see is one meter.

You really want to go with the @berkeman solution and use light.

 

[Baluncore]

This is hopeless.

This problem of underwater imaging has been solved many times over the last century. Radio and optical systems are employed from many different types of vehicles.

Single-frequency transmitters do not benefit from fractal antennas. I'm sorry to spoil the fun, but I think it unlikely that an array of fractal antennas will be required.

It seems silly to reinvent a solution from first principles. Maybe if the OP identified the target, we could list the possible solutions currently employed.

 

[DaveE]

LiDAR is what you want for this application, I think.

 

[Baluncore]

LiDAR is what you want for this application, I think.

If there is mud, algae or fish in the water, first take a look at this 460 page book; "Light Absorption in Sea Water", by Bogdan Wozniak, Jerzy Dera (auth.) Springer-Verlag New York (2007)

 

[Vanadium 50]

fish in the water

If there are fish that can be imaged with 500 kHz radar, I think I want to let them be. Holy Jonah!

 

[chemisthypnos]

What will you be imaging?
What resolution?
At what depth?

There will be a problem with the beamwidth required of your antenna array.
Beamwidth in degrees = 57° / wavelength in metres.
If you use 100 MHz which has a wavelength of 3 metres, then to get a 5.7° beam, your antenna array will need to be 30 metres across.

To do the same at 540 kHz where the wavelength is 555 metres, your antenna array will need to be 5.555 km across.

At 1 GHz with a wavelength of 300 mm, the array will be only 3 metres across. That is a good reason for using shorter wavelengths.

We are attempting to image an aluminum target at the base of the pond at 1m depth. The resolution does not particularly matter as we are looking for a binary outcome ( either object is detected or the object cannot be detected). That is a very good point about the 540 kHz antenna length. We were thinking of using a 1MHz wavelength and using electric shortening to about 1/100th ( it should get the size of the antenna down to about 1m or so). Do you think this could work?

 

[hutchphd]

Is the target (optically) shiny?
Are you trying this as a prototype for some more difficult problem?
A modulated optical source and synchronous detector should have no trouble with this. $10²
What am I missing?

 

[chemisthypnos]

Is the target (optically) shiny?
Are you trying this as a prototype for some more difficult problem?
A modulated optical source and synchronous detector should have no trouble with this. $10²
What am I missing?

The target ( aluminum) should be shiny in the radio spectrum. Visible light cannot be used in this scenario as the pond is opaque to visible light.

This is a prototype for a more difficult problem.

 

[Baluncore]

We are attempting to image an aluminum target at the base of the pond at 1m depth.

This is a detection, not an imaging exercise. Aluminium is a very good conductor, independent of colour or finish.

What are the physical dimensions of the aluminium target?
What material forms the bottom and sides of the pond?

Is the pond filled with seawater or with freshwater?

 

[chemisthypnos]

This is a detection, not an imaging exercise. Aluminium is a very good conductor, independent of colour or finish.

What are the physical dimensions of the aluminium target?
What material forms the bottom and sides of the pond?

Is the pond filled with seawater or with freshwater?

The dimensions of the target can be regarded as a 1 meter long glass canister full of aluminum finely cut to a powder for which each particle's diameter is 5 nanometers. Assume that the bottom/sides of the pond are made of a material that is transparent to radio light.

The pond is filled with seawater.

 

[hutchphd]

The dimensions of the target can be regarded as a 1 meter long glass canister full of aluminum finely cut to a powder for which each particle's diameter is 5 nanometers. Assume that the bottom/sides of the pond are made of a material that is transparent to radio light.

The pond is filled with seawater.

Why is the seawater opaque to "light"?

 

[Baluncore]

The dimensions of the target can be regarded as a 1 meter long glass canister full of aluminum finely cut to a powder for which each particle's diameter is 5 nanometers.

Are the canister contents conductive from end to end, like a 150 MHz dipole, or are the 5 nanometre spheres oxidised and insulated, like in a paint?

 

[tech99]

I my humble opinion the target will not be detected using radio waves due to the isolated small particles.

 

[Vanadium 50]

5 nanometers.

Nanometers? That changes everything.

This is not aluminum like people think of it. It's not even a powder like people think it is. It's more like dust. Each particle has maybe a few thousand atoms.

I doubt this is a good conductor, as the surfaces will have oxidized. At 5nm it may all have oxidized, which will make it safer. As it is, it is highly, highly flammable. Possibly explosively so.

For safety's sake I would start with a much, much, much smaller setup. I'd also look really hard for alternative materials.

 

[chemisthypnos]

Why is the seawater opaque to "light"?

The pond seawater is opaque to visible light because it is a simplifying assumption of the thought experiment and helps focus attention on the radio aspect of the problem..

 

[chemisthypnos]

Are the canister contents conductive from end to end, like a 150 MHz dipole, or are the 5 nanometre spheres oxidised and insulated, like in a paint?

The canister is not assumed to behave like a dipole. I think that 5nm aluminum powder would still be conductive. For the sake of the thought experiment, assume that the aluminum is not oxidized. Paint would be a good analogy.

 

[chemisthypnos]

Nanometers? That changes everything.

This is not aluminum like people think of it. It's not even a powder like people think it is. It's more like dust. Each particle has maybe a few thousand atoms.

I doubt this is a good conductor, as the surfaces will have oxidized. At 5nm it may all have oxidized, which will make it safer. As it is, it is highly, highly flammable. Possibly explosively so.

For safety's sake I would start with a much, much, much smaller setup. I'd also look really hard for alternative materials.

It should be a reasonably good conductor even at that size as experiments with nano chaff have shown: https://iopscience.iop.org/article/10.1088/1742-6596/1176/6/062002/pdf

We assume that the aluminum has not oxidized ( there are very good reasons to make this assumption that I would rather not elaborate on as it is outside the scope of the thought experiment).

Having aluminum of that size would be very explosive. In the real world, it will not be ( for very good reasons) so do not be concerned about that detail. I would ignore that detail for the sake of the thought experiment.

 

[Vanadium 50]

It is of course difficult to help someone who is keeping secrets.

The material in that paper is some unspecified metal plated on glass, up to 40x larger than your application here. I have no idea what it is, but it's not 5 nm aluminum spheres.

Aluminum forms an oxidation layer a few nm thick. Whether or not this completely oxidizes the material surely depends on the details of the material and how it was manufactured. But it is clear that there will be a lot of oxide since you have a lot of surface layer. It is not so clear that aluminum of this form is even a good conductor - it's 35 atoms across, so it won't necessarily have a proper conduction band.

But this is all moot. If you want help solving a problem, but don't want us to know what problem it actually is, so you are describing a fictional but hopefully similar problem, well, I wouldn't expect very good answers.

 

[hutchphd]

This is a prototype for a more difficult problem.

Why don't you tell us the real problem, and we can stop with the twenty questions.

 

[berkeman]

It is of course difficult to help someone who is keeping secrets.

Why don't you tell us the real problem, and we can stop with the twenty questions.

Thread paused for Moderation...

 

[berkeman]

Having aluminum of that size would be very explosive. In the real world, it will not be ( for very good reasons) so do not be concerned about that detail. I would ignore that detail for the sake of the thought experiment.

Please check your Private Messages (PMs). If you want this thread reopened to get assistance with your project, you will need to much more open in your posts here, especially when we have safety concerns. Thank you.