Radio Signals through the 'Surface'

[caiman]

Hi
There are a couple of threads on the forum at the moment concerning PLB's.Can somebody please clarify for me,'Can radio signals such as emitted from a submerged PLB, transmit to a recieving station above the surface, from underwater?'In other words,if you set off your PLB from it's belt pouch on your LJ whilst in the water,will the signal be recieved while the pouch is under the surface?I am willing ,and wanting, to be corrected,but I thought that, if say a submarine wished to transmit to a recieving station above the waves,then it would have to transmit to a bouy with an aerial sticking out under the water,connected to an aerial sticking out into the air,to relay to surface recieving aerials?
This is not just about PLB's.For instance,can you radio control a submerged ROV from aboard a boat, without the transmitting aerial penetrating the surface of the water?
Thanks for any input.
Cheers

 

[William_H]

Radio from underwater

Basically no you can't get a signal from underwater to the surface.
However this depends on frequency and depth.
here in West Australia a huge communication station was set up by US navy to communicate with submarines under water. The power was huge and the frequency about 16 khz. Yes barely above audible freq if was vibrations not radio. I don't know at what depth it worked. I believe it is kind of out of use now. (not sure) This signal could also be used for navigation also now superceded by Sat Nav. (GPS)

I am sure if I wanted to set off a PLB I would get it out of the water above my head. I wouldn't feel confident with it under water. olewill

 

[Ubergeekian]

Can somebody please clarify for me,'Can radio signals such as emitted from a submerged PLB, transmit to a recieving station above the surface, from underwater?

No. Radio waves are generally rotten at penetrating conducting materials. The only way you can get any significant transmission is to use very low frequencies indeed - and even then submarines use antennae towed just below the surface.

The other downside is that you need vast amounts of transmitting power and an antenna which is of the same order of size as the wavelength. The French VLF antenna is about 50km east of Poitiers: it broadcasts on 18.3kHz which is a wavelength of around 16km and has aerials several miles across. When I flew my glider over it the audio variometer started playing music at me ...

 

[caiman]

Thanks for the replies.Again please correct me,but this means a non floating PLB has to be held/mounted above the surface for it's signal to be recieved?You can't just 'pull the pin and wait for the 'Paraffin Budgie' on a non floaty PLB?This seems daft.If it's mounted on your waist strap/in your pocket,it's not going to be recieved if activated while in the water.Even if on a lanyard,if you let go of it, it will sink and be ineffective.
This for me, is another'arrow in the quiver' for a floating HH DSC VHF.At least if you let go of it while activated,it's signal will still be recievable.
Does anyone make a floaty PLB?That would be the answer.I would have thought that being able to float 'hands free' would have been a critical design feature for the operation of an emergency location device for use on the water.I appreciate that you could remove the PLB from your belt/pocket and stick it(velcro or whatever)on the outside of your inflated LJ/BA,but that makes something else to remember/do in a time of great stress and danger.
It seems to me that,not only do we want pockets on the outside of our LJ/BA for convienience,we also need one somewhere for mounting a PLB where it will best be able to transmit above the surface.
Another question,is it the 'miniscus' on the surface that prevents the radio waves from penetrating?
Thanks for your help.
Cheers

 

[sarabande]

no, meniscus is to do with surface tension, and what happens when a liquid meets a wall, such as that in a beer glass. Some curve up, and some curve down.

If you play around with helium3 you find that the stuff climbs up the wall and over the top. At horribly horribly low temperatures, though.

 

[alant]

Hi
There are a couple of threads on the forum at the moment concerning PLB's.Can somebody please clarify for me,'Can radio signals such as emitted from a submerged PLB, transmit to a recieving station above the surface, from underwater?'In other words,if you set off your PLB from it's belt pouch on your LJ whilst in the water,will the signal be recieved while the pouch is under the surface?I am willing ,and wanting, to be corrected,but I thought that, if say a submarine wished to transmit to a recieving station above the waves,then it would have to transmit to a bouy with an aerial sticking out under the water,connected to an aerial sticking out into the air,to relay to surface recieving aerials?
This is not just about PLB's.For instance,can you radio control a submerged ROV from aboard a boat, without the transmitting aerial penetrating the surface of the water?
Thanks for any input.
Cheers

This one will go down to 75m, http://www.seamarshall.com/406MHznogood.php, but some (McMurdo) are only rated for 10m immersion http://www.mcmurdo.co.uk/products/product.html?product_type=2&product_sector=5&product=98.

That's not transmittor depth.

 

[SHUG]

The dielectric constant of water is about 80 and this means that any electromagnetic fields are reduced to 1/80th of their value in air. Salt water also has reasonable conductivity which also reduces the fields substantially. So its no go for PLB transmission from below the surface.

 

[bbg]

I think even with a floaty PLB you would need to hold it up above the surface somehow. I don't think they are designed to float in a transmitting position (unlike EPIRBs).

I am exprimenting with how much foam I need to add to my McMurdo in order to make it float. Answer so far seems to be "not much".

 

[pteron]

Have you seen the test at equipped.org? McMurdoNewFastFind

Very interesting stuff and quite surprising how big the antenna is.

Edited to add: if you look at the deployment video you'll realise you won't be activating it one handed on your belt.

 

[Ubergeekian]

The dielectric constant of water is about 80 and this means that any electromagnetic fields are reduced to 1/80th of their value in air.

When EM waves move from a region of one relative permittivity (dielectric constant) to another the tangential component of the electric field E stays constant, as the potential gradient must be the same along both sides of the interface. The normal component of the electric flux D is also constant, assuming no charge on the surface, so only the normal component of E changes.

Similarly the normal component of magnetic flux B is constant (flux lines are continuous) and the tangential component of magnetic field H is constant (no surface currents).

Stick all that together and the effect of changing material properties is to change the relative sizes of the normal and tangential components of the E and H fields, leading to a change in direction of propagation but no attenuation of power.

Salt water also has reasonable conductivity which also reduces the fields substantially. So its no go for PLB transmission from below the surface.

You're right there. It's conductivity that's the killer, not refraction.

 

[Bilgediver]

Thanks for the replies.Again please correct me,but this means a non floating PLB has to be held/mounted above the surface for it's signal to be recieved?You can't just 'pull the pin and wait for the 'Paraffin Budgie' on a non floaty PLB?This seems daft.If it's mounted on your waist strap/in your pocket,it's not going to be recieved if activated while in the water.Even if on a lanyard,if you let go of it, it will sink and be ineffective.
This for me, is another'arrow in the quiver' for a floating HH DSC VHF.At least if you let go of it while activated,it's signal will still be recievable.
Does anyone make a floaty PLB?That would be the answer.I would have thought that being able to float 'hands free' would have been a critical design feature for the operation of an emergency location device for use on the water.I appreciate that you could remove the PLB from your belt/pocket and stick it(velcro or whatever)on the outside of your inflated LJ/BA,but that makes something else to remember/do in a time of great stress and danger.
It seems to me that,not only do we want pockets on the outside of our LJ/BA for convienience,we also need one somewhere for mounting a PLB where it will best be able to transmit above the surface.
Another question,is it the 'miniscus' on the surface that prevents the radio waves from penetrating?
Thanks for your help.
Cheers

The PLB has two requirements for RF exchange if a GPS is fitted. First the GPS has to see the satellites in the sky and receive the signals which are extremely weak and even heavy rain clouds in the sky can degrade them so a few mm of water will kill them. The GPS may even take a while to acquire the satellites if it was last switched on a great distance from where it is now activated.

Secondly the PLB transmits a signal on 406MHZ which is UHF. As others have said when submarines transmit from under water they use very low frequency radio signals VLF . UHF is at the other end of the spectrum and is unlikely to penetrate the thinnest layer of water.

Regarding a floating hand held. It might be wise to check how it floats as many I have seen float upside down so also can not be relied to transmit when in the water.

It would seem whatever you have it isn t much good unless you can hold it clear of the water. If you visit various web sites relating to users and manufacturers this point is confirmed. Having the equipment in a pouch on a belt is quite useless!!!!!!!!

One other point which I believe is more or less sorted now is that these units are sold for a marine environment. It has been the case that where they are used by hill walkers or white water rafters then there is a danger of the alarms not being treated as emergencies due to the fact that a mariner is unlikely to be in this location.

If you do buy a PLB then make sure it is correctly registered with the MRRC at Falmouth which I understand is a similar process to registering an EPIRB.
This ensures that the control centre knows who or what they are looking for.

 

[alan_d]

This for me, is another'arrow in the quiver' for a floating HH DSC VHF.At least if you let go of it while activated,it's signal will still be recievable.

I think you might find the range of a HH VHF floating on the surface disappointing. The range is related to antenna height, so even in a flat calm it won't be good, and worse in waves/swell. These things are less of a problem for PLBs as their transmissions are picked up by satellites.

 

[maxi77]

I would just like to point out that the VLF radio for submarines is one way only from land to the boats, partly because of the size of aerial required, and partly because it is very difficult to insulate the aerial from the groundplane underwater.

VLF radio waves do though pentrate the surface of the sea allowing submarines to receive it whilst submerged. Interestingly we could also get Decca and Loran A signals from floating aerials