How to tell the difference visually on BNC connectors

[G7PSK]

Is there any way to tell the difference between 50 & 70 ohm BNC connectors where there is no markings as to the impedance on them. Without test gear that is.

[vlf3]

Not easily unfortunately... the design can vary internally, centre core pin or shoulder screen within the
insulated spacer; it's a very small diameter milling that set's the through path impedance, but visually
can be very hard to tell.

If your going to use an unknown ! 75 Ohm BNC for Tx on a short feed-line, it's miss-match should not be too great
a VSWR.  However, for much longer feed-lines and if placed halfway in the run, it would show-up with a high VSWR...
at the end of the day, fit them and check the reflection on short Tx bursts.

Used on Rx, will not be a problem.

[xygor]

In all the ones I have seen, In the 50-ohm, the dielectric extends to flush with the end of the pin.  In the 75-ohm, the pin protrudes from the dielectric.  The is true for both the plug and the jack.

[edpalmer42]

Is there any way to tell the difference between 50 & 70 ohm BNC connectors where there is no markings as to the impedance on them. Without test gear that is.

Sure.  Go to Wikipedia and look at the pictures of 50 & 75 ohm connectors.

http://en.wikipedia.org/wiki/BNC_connector

They can typically be connected to each other without damage but, of course there could be reflections depending on the frequency of the signals.  There is a legend that sometime in the dim, dark past there was one brand of BNC connector that would cause damage if it was interconnected to a connector of the other impedance.  I don't know the details.

FYI, the situation with N connectors is completely different.  If you try to connect 50 & 75 ohm N connectors you can damage the center pin or have a really poor connection because the diameter of the pins is different.

Ed

[vlf3]

The Wiki image would depend on the manufacturer... my experience with British BNC's 75-50 Ohm some time ago now, their was no visual dramatic design change, as shown; well you live and learn. 

[opablo]

Just to help others save a valuable click, here is the image from wikipedia...

Sure.  Go to Wikipedia and look at the pictures of 50 & 75 ohm connectors.
http://en.wikipedia.org/wiki/BNC_connector
...
Ed

[G7PSK]

I have some BNC's that I know are 50 ohm and some that are supposed to be 75 ohm but neither are marked and both look like the 50 ohm's shown in the wiki photo.

[edpalmer42]

I have some BNC's that I know are 50 ohm and some that are supposed to be 75 ohm but neither are marked and both look like the 50 ohm's shown in the wiki photo.

I've seen cables with 50 ohm connectors on 75 ohm cable.  Usually it's on cheap video cables for CCTV systems.  Maybe there isn't any visible difference on those systems due to the relatively low frequency and/or limited resolution of the cameras.

Check any of the big connector companies like Amphenol, Pasternak, etc.  and you'll find that the connectors look just like the Wikipedia picture.  I'll believe them before I believe the One Hung Low Cable Company!   

Ed

[IdahoMan]

Now, where to find 75ohm BNC adapters(BNC-BNC, BNC-RCA, BNC-N, BNC-F)..? Anybody have sources??

Many sites that sell them, for video application (CCTV), don't even list the impedance! Some do, but the product picture looks like a 50ohm.

As for Pasternak, unless it's plated in pure 99.9% platinum, I'm not paying $17.00 for a BNC-BNC adapter.

[XOIIO]

Just to help others save a valuable click, here is the image from wikipedia...

Sure.  Go to Wikipedia and look at the pictures of 50 & 75 ohm connectors.
http://en.wikipedia.org/wiki/BNC_connector
...
Ed

Still needed to click on the thumbnail :p

[TerraHertz]

Just to help others save a valuable click, here is the image from wikipedia...

Ah ha. Learn something every day. Till this moment I had no idea there were different impedance BNC and N standards. I'd just assumed for BNC people put up with the 75/50 mismatch in video systems, and that N connectors only ever used 50 ohm cable.

Checking, I find I have some 75 ohm BNCs that are different to that photo. Will post a pic later. Got to go out now.

[dexters_lab]

i have seen 50ohm BNCs which look like the 75ohm versions too, that wiki picture is not the whole story

[IdahoMan]

i have seen 50ohm BNCs which look like the 75ohm versions too, that wiki picture is not the whole story

And vise-versa? Here's a site that is selling claimed 75ohm BNC adapters, but they look 50ohm..?

IM

[dmills]

At the sorts of frequencies and power levels most folks use BNC for it is not usually a huge deal (Outside of broadcast, where for mainly historical reasons we cart 3Gb/s serial digital video around on the 75 ohm versions).
Sometimes someone gets an attack of the stupid and tries to do 6Gb/s or even 12Gb/s over the same cables, that is when the money for Pasternack or Huber & Schurner makes sense.
CCTV of the composhite type is all so low frequency that you would have to really work at it to get a connector impedance discontinuity to cause a problem.

I had to laugh at the notion of a specifically 75 (or 50) ohm BNC to RCA, them things tend to be nothing in particular impedance wise.

The things that matter more on BNCs IME is using the right one for the cable and the right tooling when crimping the things, a poor crimp will cause far more pain then a 50 ohm BNC in a 75 ohm system.

The good stuff for RF is always expensive, as it is in every field, have a look at VNA calibration kits or precision attenuators if you want to see really silly pricing.

Regards, Dan.

[helius]

It should be kept in mind that at the frequencies carried by baseband analog video, you would need a very long cable before the reflections from the 50 ohm-75 ohm mismatch were visible. The way that video systems are designed, BNC connections are rarely used for RF.

[dmills]

Modern video uses high speed digital, and I have had a cable fail because the installer has placed a zip tie every meter along a run over a suspended ceiling and had pulled them so tight they crushed the foam dielectric slightly.

TDR revealed the problem, but the eye diagram looked really funky.

Still rather deal with SDI then terminating Triax on Lemo.

Regards, Dan.

[TerraHertz]

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.

[macboy]

50 ohm and 75 ohm BNC are 100% physically compatible, it is perfectly safe to mate then to each other. This is absolutely not the case for N type where different dimensions will result in damage to one or both.

It is the presence or lack of dielectric between the pin and shield that results in different impedance. The BNC connectors on the white cable in Terrahertz's photo are definitely not 50 ohm, there is too little dielectric to bring the impedance down that far.

[IdahoMan]

At the sorts of frequencies and power levels most folks use BNC for it is not usually a huge deal (Outside of broadcast, where for mainly historical reasons we cart 3Gb/s serial digital video around on the 75 ohm versions).
Sometimes someone gets an attack of the stupid and tries to do 6Gb/s or even 12Gb/s over the same cables, that is when the money for Pasternack or Huber & Schurner makes sense.
CCTV of the composhite type is all so low frequency that you would have to really work at it to get a connector impedance discontinuity to cause a problem.

I had to laugh at the notion of a specifically 75 (or 50) ohm BNC to RCA, them things tend to be nothing in particular impedance wise.

The things that matter more on BNCs IME is using the right one for the cable and the right tooling when crimping the things, a poor crimp will cause far more pain then a 50 ohm BNC in a 75 ohm system.

The good stuff for RF is always expensive, as it is in every field, have a look at VNA calibration kits or precision attenuators if you want to see really silly pricing.

Regards, Dan.

Thanks Dan for posting that explanation.

How effective is the shielding on coax cable? Shielding is very important for my use. It has to protect against malicious mischief: Someone using special equipment to detect it or disrupt/jam/destroy it.

As for RCA*.. Yeah, laughable, but a lot of these little CCTV cams have either a BNC or a yellow RCA plug on the end. I want to assemble an adapter kit (BNC to RCA/F/N/, F-to-RCA, etc.) for testing. Just wanted everything to match for good organization.

For a permanent setup, I'd use proper BNC properly crimped.. or solder the coax end straight to the two wires on the back of the cam in well-secured enclosure.

*BTW, what is the alternative for video when coax is not an option (such as a wearable lapel cam)?
s/pdif-rca?


Sincerely,
IM

[helius]

Coaxial cables are rated for a specific shield coverage. 97% to 99% are pretty common. You can easily disrupt the signal if you are close to the cable, but it does a good job of screening typical interference. "Quad shield" type cables have more layers of screening and admit less interference.

[vk6zgo]

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.

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]

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.

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.