Last Sunday's trials with TX antenna in air and water.
My thanks to John (for his original suggestion in 2011) and Rob for his ongoing work in Holland:
https://www.robschuckman.nl/
Rob's calculation and description guide here:
https://www.robschuckman.nl/over
Measuring the Lake water conductivity. 488 micro Siemens/m. (Sadly there was no shortage of "plastic measurement containers" floating in the Lake!)

NanoVNA and RX antenna. (550mm dipole without Balun but with an extra inductor (2 turns around an FT82-43 Toroid in series at the RX and TX end.)

NanoVNA and TX antenna photographed just before the antenna is "dunked" in the Lake. (See the screen grab SWR plots at the bottom of this post.)

Radiometrix RX with RSSI display and RSSI data logger which stored value every 5 seconds.
TX ANTENNA IN AIRFirst measurement. TX in air with Telescopic Antenna. Range 15m. Average RSSI -75.

Second measurement. TX in air with Telescopic Antenna. Range 30m. Average RSSI -76.

Third measurement. TX in air with Telescopic Antenna. Range 40m. Average RSSI -80.
TX ANTENNA IN WATERFirst measurement. TX antenna dunked in water. (I forgot to collapse the telescopic antenna! Range 5m. Average RSSI -73

Second measurement. TX antenna dunked in water. (I forgot to collapse the telescopic antenna! Range 15m. Average RSSI -88

Third measurement. TX antenna dunked in water. (I forgot to collapse the telescopic antenna! Range 30m. Average RSSI -83

Fourth measurement. TX antenna dunked in water. (I forgot to collapse the telescopic antenna! Range 40m. Average RSSI -80

Plot from the spreadsheet. This shows RSSI versus time during the trial. (Note the high (low?) RSSI values when antenna was disconnected.
As I understand RSSI , -70 is OK but -80 is not good. So the underwater TX antenna needs more efficiency or power input.
COMPARISON OF TEST TANK WITH LAKE:Home test tank. (Wine bottle (empty!) shows scale.) It would be much easier and quicker for trials if a test tank was representative of a large body of water. The next few nanoVNA plots test this.

This is a nanoVNA plot for the RX antenna in the Lake:

This is a nanoVNA plot for the RX antenna in the test tank:

This is a nanoVNA plot for the TX antenna in the Lake:

This is a nanoVNA plot for the TX antenna in the test tank:

SWR (Standing Wave Ratio) is the key measure for the antennas because it shows how well the antenna is (impedance) matched to the RX and TX.
Rob's comments, after reading this post:
"Try to get the SWR figure below 1.5 You're currently in the range of 2.5 to 3.5 which is too high."
The plots look quite similar which means that test tank results are comparable.
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