75 ohm coax are actually better to use than 50 ohm. The voltage will be higher in the coax and have higher level above the noise floor.
That's only true if the antenna matches the impedance of the 75-ohm coax. Connecting 75-ohm coax to an antenna to a 50-ohm antenna isn't going to raise the voltage above the noise floor at all, unless you use an impedance matching transformer, or a preamp with a 50-ohm input and a 75-ohm output. Connecting directly, you lower the signal voltage somewhat because less of the signal from the antenna feeds into the coax, and more signal reflects back into the antenna.