Hello dear antenna enthusiast,
I would like to receive and emit EM signals with a dipole antenna in the range of **433.9 to 434.5 MHz in fresh water.
So I chose a half-wavelength dipole sleeve-antenna design that I made by myself out of coax cable (part of the coax is folded back and acts as the counter pole. So one pole is the inner conductor of the coax cable and the counter pole is the outer layer of the coax stripped back and soldered onto the coax cable in the middle of the antenna. Similar to the one in the video... https://www.youtube.com/watch?v=hK0KX0YcvH0
However 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 (SWR in dB) at 433 MHz has a pole length of 80mm and with shorter poles the peak moves to the HF-area (as expected). You can see one of the reflected power plots in dB below, the antenna was in the middle of a 4mx8mx2m fresh-water tank connected to a VA by a 4m thin coax cable.
So I have now following questions...
- Why is one antenna pole 80mm instead of 19mm, what would you expect?
- Do I have to look at another physical value with my Network Analyzer or other gadget to be able to judge the performance of my antenna (max. emitted power), how does normal values look like?
- Is it typical for a Dipole to have only one SWR peak at one frequency?