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How to tell the difference visually on BNC connectors
vk6zgo:
--- Quote from: TerraHertz on September 06, 2016, 02:22:37 pm ---Here's that pic I promised. The white cable is 75 ohm coax from a video installation - the old ABC studios at Gore Hill. You can be sure they were using the correct connectors. I have three big fruit boxes boxes full of these video cables, and I never noticed the connectors are not quite the same as standard 50 ohm BNCs (center in the pic.)
I'm feeling pretty stupid now.
Edit: I have to wonder though, if the connectors are actually significantly different in impedance. Or is the mechanical difference intended more to prevent accidentally using 50 ohm cables in 75 ohm systems? Since they won't fit.
Too bad I don't have some 75 ohm ones new in pack (and their mating gender), or I could crimp a pair on some 50 ohm cable and have a look with my HP 54121T TDR. It can see really small impedance discontinuities, like holding a fingertip near a controlled impedance PCB stripline.
--- End quote ---
At TVW7,when digital video was introduced in the 1990s,there was an emphasis upon using "true" 75 Ohm connectors in the new fitout.
I hadn't really thought much about it at the time,but evidently 50 Ohm BNCs were previously used extensively.
From memory, the same applied at my previous job with Telecom Aust Broadcast Branch (although they used Siemens 75 Ohm connectors for their jackfields).
At 5MHz the impedance mismatch of a few mm of connector in tens of meters of cable is negligible.
If it is necessary to discern the difference between a 50 Ohm & 75 ohm cable,it is more usual to look at the cable rather than the connector.
Hams use such horrors as PL259/SO239 so-called "UHF" connectors at upwards of 400MHz with no major impedance "bumps".
dmills:
Small cameras are usually still miniature coax, but sometimes the connectors change to be SMA/SMB or the like, occasionally LEMO, everything in the broadcast world is digital these days, analogue video is history.
The CCTV crowd has a digital video standard that is almost but not quite SDI, really tedious.
Increasingly we are seeing fibre used at the higher data rates, it is actually sometimes cheaper to install then coax because the data installers are familiar with it, where coax is more specialist.
For a security critical link I would be tempted to put a SDI->Fiber box at the camera then run armoured fibre and alarm on loss of sync at the receiving end. This also breaks any possibility of ground loops and earth voltage differences if you are running between buildings which massively reduces the potential for induced current damage from lightning.
'UHF' connectors are pure crap, quite apart from being a royal pain to terminate reliably, give me N type or 7/16th if it really matters.
Incidentally if ebaying for RF connectors, a good rule of thumb is that anything excessively shiny is probably a cheap knockoff and may very well have threads that are just slightly the wrong pitch (Been there, done that).....
Regards, Dan.
vk6zgo:
--- Quote from: dmills on September 08, 2016, 10:25:35 am ---Small cameras are usually still miniature coax, but sometimes the connectors change to be SMA/SMB or the like, occasionally LEMO, everything in the broadcast world is digital these days, analogue video is history.
The CCTV crowd has a digital video standard that is almost but not quite SDI, really tedious.
Increasingly we are seeing fibre used at the higher data rates, it is actually sometimes cheaper to install then coax because the data installers are familiar with it, where coax is more specialist.
For a security critical link I would be tempted to put a SDI->Fiber box at the camera then run armoured fibre and alarm on loss of sync at the receiving end. This also breaks any possibility of ground loops and earth voltage differences if you are running between buildings which massively reduces the potential for induced current damage from lightning.
'UHF' connectors are pure crap, quite apart from being a royal pain to terminate reliably, give me N type or 7/16th if it really matters.
Incidentally if ebaying for RF connectors, a good rule of thumb is that anything excessively shiny is probably a cheap knockoff and may very well have threads that are just slightly the wrong pitch (Been there, done that).....
Regards, Dan.
--- End quote ---
Have you ever come across a Chinese L27 connector?
They look very similar to a 7/16,but are a bit larger.----horrible things!!
Robert763:
Hi, I know this is old, but there is some information missing here.
The 75R BNCs being discussed are the modern "compatable" type.
The original 75R BNCs had full dielectric coverage and smaller diameter center pins. These have better match and less reflections than the modern ones but are NOT compatable with 50R or modern 75R connectors.
If you plug a male 50R or "new" 75R into a original 75R female it will deform the contacts and the female will no longer connect reliably to an original 75R.
An original 75R male won't make reliable contact with any 50R or "new" 75R.
Fortunatly original 75R connectors are rare. The center pin is noticably smaller diameter.
coppercone2:
also its teflon, i noticed if you want to get something tricky done with solder the 50 ohm melts but the 75 ohm does not and the 75 ohm I have are obviously visually different from the 50 ohm once you have both in hand
the 75 ohm looks airy and i regard it as a higher quality connector because it uses teflon rather then delerin, despite the fact that I prefer the 50 ohm impedance.
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