One sees perennial discussion in ham circles of compatibility of ordinary 50Ω and 75Ω versions of the BNC (Bayonet Neill–Concelman) connector, in particular the risk of damage in mating a 50Ω and 75Ω pair.
But are there incompatible connectors commonly in circulation.
These discussions often seize on the different dimensions 0.7mm and 0.9mm.
Above shows measurement of the centre pin diameter of a Kings BNC connector (for RG58), it is 1.339mm… nothing like 0.7mm or 0.9mm. (Amphenol Connex 2001) gives the centre pin diameter as 1.32-1.37mm.
The 0.7mm and 0.9mm has has nothing to do with the mating interface, it is the crimp cavity diameter for the inner conductor, and a different diameter is required for different cables (eg RG58 vs RG59). Clearly this is an important compatibility issue, the crimp cavity MUST suit the inner conductor for reliable crimps, but this dimension is not critical to the mating interface.
All conductor diameters, crimp cavities, and die cavities are critical to reliable crimps for all cables and all connector types.
In practice, ordinary 50Ω dimensioned connectors are used (with appropriate crimp cavities) interchangeably with 75Ω applications with very low Insertion VSWR.
(Amphenol Connex 2001) states:
75 OHM SERIES
Within the internationally standardized BNC mating face dimensions, a perfect 75 ohm characteristic impedance cannot be realized. However, at frequencies up to 1000 MHz, the small impedance deviation is negligible for practical applications. Connex true 75 ohm connectors with a typical VSWR reading of 1.06:1 at 2000 MHz are identified by an asterisk following the part number.
All 75 ohm BNC connectors and 50 ohm BNC connectors are intermateable without restrictions
- Amphenol Connex. Nov 2011. BNC coaxial connectors. http://www.amphenolconnex.com/media/downloads/5648/bnc.pdf (accessed 14/01/2015.