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Radio reception under water.
Out of curiosity I did an experiment to find out how well medium wave radio signals could be received under water.
Underwater antenna - version 1
For the test, this antenna is build, the tube contains a 10x200 mm ferrite rod.
A coil with 10 turns is wound around the tube, this must pick up the magnetic field of the radio station.
The coil and ferrite rod are to be put under water.
This design is based on my wideband ferrite antenna, and is almost the same as the version 1 described there.
The wires of the coil are twisted and connected to a BNC connector placed above the water, the twisted wires are 75 cm long.
From the BNC connector, coax cable is used for the connection to the spectrum analyser.
The wire is insulated and made of one piece, so there is no direct contact with the water.
Detail of the coil, the ferrite rod is in the tube.
I measured the antenna diagram of this antenna, and found out it was not working well, even above water the diagram was not as it should be.
This diagram shows the reception level of a strong medium wave station for various orientations of the antenna.
The blue line is an example of the diagram like it should be, but this underwater antenna gave the result indicated with the orange line (measured above water).
And this was not useful for my experiment.
The cause of this wrong antenna diagram is the 75 cm twisted wire, this is picking up too much of the electric field of the radio station, and this completely changes the antenna diagram.
Underwater antenna - version 2
To improve the antenna, the twisted wire is replaced by a piece of coax cable.
The coil is now connected to a piece of coax cable.
This improves the antenna diagram very much, the antenna now has two minima in sensitivity in the direction of the axis of the ferrite rod.
And two maxima in the direction perpendicular to this.
Above water it works good enough for me, in fact we now have the same antenna as wideband ferrite antenna version 1.
But when the antenna was put under water, the level of received signal suddenly increases by more then 10 dB
Turning the antenna under water, or even removing the ferrite rod out of the coil didn't change this strange high level of received signal.
I suspected the direct contact of the water with the solder joins of the coax cable to be the cause of this problem.
And therefore the next step is to make the antenna waterproof.
Underwater antenna - version 3
Around the antenna I wound some bubble wrap and tape.
Then the antenna was put into an old bicycle inner tube.
The two cut open ends of the bicycle tube stay above water, so the antenna stays dry.
With this "waterproof" antenna under water, there was still an increase in received signal compared to the measurement above water.
And turning the axis of the antenna toward the transmitter didn't give much reduction in received signal.
Although it works better then version 2, it seems not to work good enough at the moment.
The problem is probably that the water is at another RF voltage level then the
ground of the spectrum analyser.
This RF voltage difference couples capacitively to the coil under water, and this gives a common mode current in the coax cable.
To solve this, I looped the coax cable 5 times through a (big enough) ferrite core.
This gives a high impedance for the common mode currents, so they are reduced in amplitude.
The differential mode current (the one we want to measure) is not affected by this ferrite core.
Another improvement was to connect the screen of the coax at the antenna side
to ground (I can better say: to water).
The final test setup, with the antenna under water.
The coax cable is wound on the orange ferrite core, and the blue wire connects the screen of the coax to the RF potential of the water.
Now finally the antenna works well, both above and below water.
With the antenna above water, the received signal was measured from local medium wave station "Groot Nieuws Radio" on 1008 kHz (100 kW at 41 km distance).
There was little difference in measuring indoors or outdoors, with the antenna hold in the air or laying on the ground, the difference in received signal was in this cases within 1 dB.
Putting the antenna 0.5 metre below water only gives a small reduction of received signal, about 1.5 dB.
Under water the antenna still has it's directional properties, when the antenna is turned with the ferrite rod pointing towards the transmitter there is a clear dip in received signal.
This picture shows the received spectrum of a medium wave station, with the antenna above and below water, and also one measurement with the antenna under water turned for minimum reception.
Only look at the amplitude of the peaks in the centre of the picture, this is the amplitude of the carrier.
The peaks left and right of the carrier are caused by the audio modulation of the transmitter.
This measurement was done a few days before this last strong medium wave station in the Netherlands was turned off.
The conclusion of this al is: it is possible to receive medium wave radio under water (0.5 metre deep), with only a small reduction of received signal.
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