Hi jordi,
A “standard” MMCX connector is, by definition, a 50Ω part. Only your diagram on the right is a standard MMCX connector. They do not come in a 75Ω option.
Samtech makes a couple connector series that they call “high-vibration” versions of “MMCX” connectors, but they look different and are not compatible with standard MMCX connectors. They make them in both 50Ω and 75Ω, so the different form-factors you noted are not related to impedance, but rather they are a different type of connector altogether. You can find these and standard MMCX series products from them listed on the Samtech website here.
- MMCXV (50Ω)
- MMCX7 (75Ω)
This TechForum post covers more about the 50Ω version.
So, when searching our website for MMCX connectors, make note of the “Connector Style” and select either the standard “MMCX” products or the Samtech-unique MMCX (V) series, as shown below:
Footnote: At the time of this writing, the 75Ω MMCX7 series parts are listed as being “Connector Style” of “MMCX” rather than “MMCX7”. This should be corrected shortly.