I would like to emit as much RF energy in the frequency range of 433.9 to 434.5 MHz into surrounding fresh water with a 100mW transceiver.
The antenna is in the middle of a 4mx8mx2m fresh-water tank connected to a VA by a 4m thin coax cable.
So I took a simple half-wavelength dipole sleeve-antenna design. I calculated the 433 MHz half-wavelength in fresh water as round about 19mm, but when I compare several antennas of the same type with different pole length, the one that has a peak in the reflected power plot at 433 MHz has a pole length of 80mm and for antenna variants with shorter poles the peak moves to the higher frequencies.
- So I ask myself now why is my resonance antenna length longer than my calculated value?
- Could it be that the RF field will be disturbed by the air around the test tank and if how would this be related to the length?
- If I would like to emit as much RF energy as possible into the water, what do I have to respect in my antenna design?