Combination of a 50 ohm cable with 75 ohm connector OR 75 ohm cable with 50 ohm connector is a case of stepped transmission line, cable forming one section, connector forming the other setion. The combined impedance of stepped transmission line depends on the ratio of lengths of the two sections. Since connector length is a fraction of length of the cable, resultant impedance is nearly same as that of the cable. Please see the calculation below which I have prepared in 2016, showing resultant impedance of a 75 ohm RG6 cable connecting 50 ohm mcx connector of generic DVB-T is 74.13 ohms, almost that of the RG6 cable alone.
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