I have always thought that you should only use 50ohm RG58 cable for leads from one piece of kit to another, but I keep seeing RG59 cable listed as patch leads in such places as RS, what would be the difference in the two from say a sig gen to a scope or other stuff.
what is annoying me is that I can pick up a RG59 cable for about £1.50 but a similar one in RG58 is around £8.
Use 50ohm cable for 50ohm equipment. Use 75ohm cable for 75ohm equipment.
If one end is 50ohm and the other 75ohm, put a 50/75ohm impedance matcher in the relevant place with the relevant orientation[1]. A wideband passive
resistive 50/70ohm matcher will lose ~6dB power. If you can't tolerate 6dB loss, then there are narrowband and active alternatives.
Be very careful of inserting an Xohm cable into a Yohm socket; frequently the centre conductor and/or dielectric diameters are different, which might require "undue" force.
[1] use
http://chemandy.com/calculators/matching-pi-attenuator-calculator.htm for the resistor values, e.g. 45, 86, 2380 ohms. I've made one to connect a 75ohm SDR receiver to 50ohm systems.