Nizar,

Are you trying to use 75 ohm coax with a transmitter designed for 50 ohm? If so you should be not be using the DiSlord impedance setting which changes the system impedance to 75 (or 77) ohms. You want the SWR measurement to be based on what the transmitter will see.

Here is an example for you to consider. Take a 1/4 wave vertical antenna with a very good ground system (multiple radials) cut to resonate at 14 MHz. At resonance the feedpoint impedance will be 36 ohms plus the ground resistance - lets say 4 ohms. So if you measured at the antenna feedpoint with the NanoVNA you would see 40 +j0 at resonance and an SWR of 1.25. Next we add 7 meters (1/2 wavelength) of RG-58A/U coax and measure at the far end. We get an impedance of 40.9 +j0.194 and an SWR on 1.24. Now if the length is increased to 10.5 meters (3/4 wavelength) we get an impedance of 62.2 -j1.97 and an SWR on 1.23. So everything looks good.

Next we try 7m of RG-59 as the coaxial transmission line. We get an impedance of 41.8 -j0.02 and an SWR on 1.20 based on 50 ohm Zo and 1.81 based on a Zo of 75 ohms. This looks good as well. However if the length is increased to 10.5 meters (3/4 wavelength) we get an impedance of 134.2 -j2.7 and SWR of 2.69 based on 50 ohm Zo and 1.77 based on a Zo of 75 ohms. This is not desirable to have the SWR based on 50 ohms get worse because we changed the cable length.

Observations
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- When the SWR is calculated based on the impedance of the coaxial cable it goes down as the length of the cable is increased. This is due to loss in the cable. The Return Loss increases with cable length.
- When the 75 ohm cable is used the SWR based on a 50 ohm Zo calculation increased to 2.69 when we went from a half wavelength (7M) to 3/4 wavelength (10M). This also means that a transmitter designed for 50 ohms would see a higher SWR and the foldback circuitry (most solid state Tx use this) would cut back the power output.

Discussion
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A transmission line can be viewed as an impedance transformer. At half wavelength with a lossless line you "see" the impedance at the far end and the SWR will be the same. As the length is changed the impedance will change but the SWR will remain constant IF calculated based on the characteristic impedance of the transmission line used. With a lossy line the SWR will decrease as the line length is increased. If a 75 ohm line is used and the measurements are made using a Zo of 50 ohms there will be a considerable change in SWR as the line length is changed.

I am not suggesting that a 75 ohm transmission line cannot be used with a 50 ohm transmitter. On the contrary. Often times a specific line length can be used to transform a high or low feedpoint impedance to close to 50 ohm at the transmitter end. One can also use a "tuner" with 75, 300 or 450 ohm transmission lines to make an impedance match to 50 ohms.

What I am stating is that it is not a good idea to use a 75 ohm cable of unknown length and then trim the antenna to get close to 50 ohms and a low SWR at the transmitter end of the cable. One should also not change the NanoVNA menu setting to a different system impedance in the NanoVNA and then trim the antenna. The reasons are that the radiation efficiency of the antenna could be reduced and what will happen if you change the transmission line length.

All calculations above were done using TLDetails which is freely available on the Internet. Screenshots of my calculations are below.

Roger