"BNC" connectors

[Alan Bain]

These horrible things arrived on some 50 ohm RG58 BNC leads from, no not a Chinese vendor on ebay, but a well known UK supplier.

The central part is not tapered and not slit. As a result the outer connection is made by the bayonet and the connector wobbles on the socket. I suppose it might work at DC but it's hardly constant impedance and definitely not a BNC connector.

On complaining I was offered a refund, but couldn't get the message across that these are not BNC connectors...

[Station X]

Could it be down to the differences between the 50 Ohm and 75 Ohm versions of these connectors?

[Graham G3ZVT]

I believe 50 and 75 Ohm BNCs were "harmonised" decades ago. Not sure if that was a compromise too far.

[Alan Bain]

Good point. The cable is 50 ohm RG58 so I expected the connectors to match. Also I'm not very familiar with the 75 ohm connectors, but I do have a SA with 75 ohm inputs on BNCs (which unsurprisingly doesn't get much use - will go and look).
IEC 61169-8:2007 has a drawing for the 50 ohm connector and note 3 states "slotted and flared to meet test gauge according to 5.1.1"

[cmjones01]

In my experience, there are 75 ohm BNC connectors. They have a thinner centre pin, as you might expect, and the plugs and sockets have a different shaped plastic insert which doesn't cover the whole contact pin. The 75 ohm plugs will fit in to 50 ohm sockets but the thinner pin may not make a good contact, and putting a 50 ohm plug in to a 75 ohm socket risks expanding the contact so it'll no longer work well with 75 ohm plugs afterwards. The 75 ohm connectors are usually found where good impedance matching is critical, like on HD-SDI video connections. But they're comparatively rare, and OP's plug isn't one of those.

All that said, I've got loads of cables here which came from Pye TVT in Cambridge and were used in a professional video environment. The cable is clearly 75 ohms but all the plugs on the cables, and the sockets I've got from scrap equipment that came from the same place, are 50 ohm ones. They (and the broadcasters!) clearly didn't care about the impedance mismatch of the connectors.

Chris

[Radio Wrangler]

I know both 50 and 75 Ohm versions of the BNC.

The difference is that 75 Ohms has cut-back PTFE dielectric to do most of the impedance change, and the finger cage which contacts the centre pin is a bit smaller in diameter to do the rest of the impedance change needed. The fingered outer ring is the same, the bayonet is the same except there is usually a turned band on the knurling to make 75 Ohm ones more visible.

BOTH need springy fingering of the outer cylinder to form multiple paralelled sprung contacts for the outer.

What is in the photo is NOT a BNC and is NOT fit for purpose.

In the telecomms division of HP we had to do a lot of 75 and 50 Ohm work and the connectors on our equipment had to be to full Bell/Weco/GPO specs.

David

[Bazz4CQJ]

I think we all know that there are lot's of very poor RF connectors around.

I'm sure that there other good suppliers available, but I increasingly tend to think that RS is the first place to go for good components, and accept they will be "pricey". Finding "bargain" RF leads and connectors may happen, but seems harder as time goes by

As the OP has already pretty much condemned the whole Chinese nation for supplying (only?) bad connectors (they did put a soft-landing spacecraft on the far side of the Moon, so are capable of quality), I wonder if he'd care to share with us the name of the supplier and the price he paid for the goods? Perhaps he was hoping for a bargain when he should have known better?

B

[Oldmadham]

Originally Posted by cmjones01

In my experience, there are 75 ohm BNC connectors. They have a thinner centre pin, as you might expect, and the plugs and sockets have a different shaped plastic insert which doesn't cover the whole contact pin. The 75 ohm plugs will fit in to 50 ohm sockets but the thinner pin may not make a good contact, and putting a 50 ohm plug in to a 75 ohm socket risks expanding the contact so it'll no longer work well with 75 ohm plugs afterwards. The 75 ohm connectors are usually found where good impedance matching is critical, like on HD-SDI video connections. But they're comparatively rare, and OP's plug isn't one of those.

All that said, I've got loads of cables here which came from Pye TVT in Cambridge and were used in a professional video environment. The cable is clearly 75 ohms but all the plugs on the cables, and the sockets I've got from scrap equipment that came from the same place, are 50 ohm ones. They (and the broadcasters!) clearly didn't care about the impedance mismatch of the connectors.

Chris

At around 5MHz highest frequency component with video, the mismatch was negligible.
The 50 ohm connectors were somewhat more rugged, which also was a a good point in their favour.

With the advent of digital video, the studios "went all funny" and changed to 50 ohm connectors.

[emeritus]

My 1970's Suhner connector catalogue states that 50 and 75 Ohm connectors can be mated either way without mechanical damage. It also says that the same is not true for type N connectors.

[Alan Bain]

So, yes the supplier was Farnell, who I usually associate with quality products and they were the cheapest ones described as RG58 50 ohm BNC to BNC which seemed fine for 10MHz signals.

I did not intend to condem the whole Chinese nation (I have worked with some excellent Chinese engineers), but I accept that if I buy products from a "Chinese vendor on eBay" there is likely to be an element of chance involved as to whether the item superficially resembles that ordered, or is actually correct (e.g. connectors with the wrong threads, ICs that fail basic tests, transistors which don't meet the datasheet, assembled PCBs with solder blob shorts that were clearly not tested). That's just personal experience though.

[Bazz4CQJ]

If Farnell are now selling that kind off stuff, then we are most surely on a downhill trend which seems to be getting worse.

At various times, I've had problems with BNC, SO/PO259's and with SMA. The only safeguard I can see is to make light with spending the money .

B

[G6Tanuki]

This is the sort of thing that is best flagged up with the supplier's quality management people.

Point out that if they are BS5750 or ISO accredited then this sort of thing, if repeated, and if their quality auditors got to hear about it (yes, a veiled threat...) it could cause them some annoyance.

In times past I have had a panicked Quality Manager get back to me in minutes!!

If that doesn't work, zero stars on the various reputation places, or a reasoned negative posting on social media can get results.

'I would have expected better, much better, from a seemingly reputable supplier. Will not buy again'..

[newlite4]

Before I retired four years ago, I was constantly replacing BNC leads, maybe 50 at a time. Not too bad for the BNC to BNC leads, but very costly for the BNC to 4mm plug leads. The centre pin contact was always fine, it was the outer ground connection that would fail. The ptfe partition insulator would appear to shrink slightly so that the plated outer ring would not make reliable contact with the mating part in the female receptacle.
Neil

[Vintage Engr]

I have also had problems with some BNC plugs, which came from CPC. They were 50 ohm types for URM 76. The problem was obvious when I crimped one on to the correct cable, & the centre pin did not 'click' into position. On mating it with a socket, the inner was pushed back into the cable... When I checked another plug, I found that there was no groove in the pin, nor an annular ring in the PTFE for the pin to firmly locate into. I did raise the matter with the supplier & received no reply.

Having in my working life, crimped on thousands of various RF connectors, I have never had this problem previously. Having said that, most of the connectors I used were either Suhner, Kings, or Greenpar. I have had the odd centre-pin with no hole in the middle!

David.

[m0cemdave]

I suggest that Alan follows this up with their QC / QA department directly (they must have somebody) as suggested in post Nr. 12.
It looks like Farnell have been badly stitched up by their supplier, and they need to deal with it urgently or their reputation as a trusted supplier will suffer.