12G SDi over coax is a hard problem to solve; my normal practise is to not run 12G over distance (you can find all my original figures for measuring verious cable types here) - in fact anything much past twelve metres and you become very dependent on the quality of the sending and receiving equipment. Best practise is once you've left the MCR you should be running it over single-mode fibre.
Anyway, you see people claiming things online "I've heard you can can run 12G 50m using this kind of cable etc etc" but even the most expensive coax using all the tricks (Nitrogen-inflated dielectric etc) can't defeat physics and any transmission channel has a bandwidth/distance product. Testing I did with Belden 1694A showed it can reliably carry 3G to about 60m and so since 12G is two octaves more bandwidth down similar quality cable it will go a quarter of the distance.
Of course you may be able to do better with Sony, Tektronix, Barnfind, or Leader gear at each end, but most people are using Blackmagic etc. with the associated poor return-loss.
So, here are some test results using a Leader LV5600 with 12G physical layer measurements and test signals. The cable is Belden 4505R (data sheet) which is Belden's best spec'ed high bandwidth 75ohm coax (with a price to match!) and yes, I did use the 12G BNCs and the correct crimp tool...!
Interesting to note that they do not specify maximum distance to stay inside SMPTE-2082, but they do mention that at 12Ghz the cable attenuates at just less than 1dB per metre - if that doesn't tell you what you need to know...!
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