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.
Measurements
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.
