BNC Connectors - A Cautionary Tale

[VK3QI]

Admin edit: Originally posted within a Summer FD topic, but excellent information worthy to be copied into a topic of it's own


BNC CONNECTORS - A CAUTIONARY TALE

Not sure if this post should go here (Contesting) or in the (Components Category)

In order to try and simplify (eliminate mistakes) setting up of our microwave equipment, we have tried to standardise the use of various connectors.

(1) 12 volts = 6 pin Molex
(2) 28 volts = 12 pin Molex
(3) GPS locking (10MHz) = BNC
(4) 2mx IF = PL259/SO239
(5) Power controls/switching = 9 pin Molex
(6) 1.2/2.4 GHz = N connectors on FSJ4-50 cable
(7) Rotator cables = Amphenol C16 7 pin connectors and caravan cable
(8) 3GHz and upwards = SMA

Last Spring FD was very windy and we noticed that at times, the 3/5/10 GHz transverters seemed to drop level and/or drift off frequency. (intermittent)

So before this Summer FD I investigated the issue, trying to reproduce the problem and was able to do so.

The issue concerned the BNC connectors used for the 10MHz GPS lock signal AND the 145 MHz IF into the transverters (DEMI) that use BNC sockets.

I discovered that the BNC plugs that I was using were a loose fit in the BNC connectors = intermittent problems.

After a lot of research and discussion with knowledgeable persons, it transpires that there are 2 standards for the pin size on BNC male plugs.

I AM NOT TALKING 50 ohm versus 70 OHM - WE KNOW ABOUT THAT!

NO, it appears that there are both 0.9mm and 0.7mm pins being used by the various manufacturers (predominantly Chinese sourced). In my case, the 0.7mm pins were being plugged into USA manufactured sockets that required 0.9mm pins. Ordinarily this would not be a problem, but with gale force winds moving cables around, the plugs could get into positions where they were not contacting properly.

The solution was to replace the BNC socket with an SMA socket for the IF out/ and up the level of the 10MHz GPS lock signal and all is now well, as evidenced last weekend in the Summer FD.

I have tried all sorts of manufactured BNC leads and various suppliers BNC plugs, and there is no clear pattern as to why some are a tight fit and others not.

SO IF YOU ARE USING BNC LEADS IN CRITICAL APPLICATIONS LIKE RECEIVE IF OUTPUTS, CHECK FOR RELIABLE CONNECTIONS.

Cheers

Peter VK3QI (VK3ER/p)

[VK3ZTE]

This I think, explains many of the multitudinous issues I've experienced around the shack over the last couple of years of getting an entire home brew station up and running. The various signal processing functions around here for tx, rx, freq locking and you name it, are separated into many discreet boxes/enclosures in order to isolate them for easier development and fault finding. However, almost all the interconnects are via BNC and I've lost count of the number of different BNC leads and connectors that I've replaced. Now I begin to understand why I've had issues. many thanks Grant VK3ZTE

[VK3ZAZ]

wobble any input lead on any TV PA and the power out goes up and down

Check the bnc failures on ITELCO 20KW FM services and invariably you will find the BNC plug on the input to the 4CX20000A from the 400 watt IPA will have arced.
The Italians managed to put 400 watts through a BNC at 100 MHZ for some reason only known to the Iltalians
All two way radio suppliers sent new BNCs out with two way radio kits.
I had a trick I would put a hand held on a bnc lead and fling it around in an arc and it wouldn't let go, all the techs would duck for cover.
The only BNC I have ever used that never caused a PA failure was the BNC SQ CUT from AACME three pieces only.
Combined centre pin, sadly NLA



BNC YUK

[VK2XSO]

I AM NOT TALKING 50 ohm versus 70 OHM - WE KNOW ABOUT THAT!

Have you actually found a different in 50 & 75 ohm pin sizes in BNC connectors. I ask because I haven't.

There's a clear difference in N type connectors with 50 and 75 ohm, where a 50R male will physically break a 75R female.
A 75R male of course makes a poor connection to a 50R female.
And just to confuse matters they also make 75R N type connectors (to suit the coax) with 50R pins.

I use top shelf BNC connectors like Huber & Suhner so I never have connection problems unless it's something like a dodgey socket.
ACME and Amphenol BNC 50 vs 75 ohm BNC connectors are the same. The connection pin size is always the same, but the coax crimp or clamp is different.
RG58 vs RG59 crimps are different sizes.

But it doesn't surprise me that somebody somewhere has messed up on the pin size, either deliberately to save money or accidentally through poor quality control.

[VK3QI]

Trash,

Like you, I always thought there were two variations of the BNC (50 and 70) but they were compatible physically.

But after much searching around and checking various BNC plugs, solder type, crimp type, preprepared audio leads (some RG59, some RG6, some RG58)
and some really old plugs from pre computer network days and what I have found is that they are all over the place as far as fit is concerned!

In particular, one area of big differences is the number of cuts in the metal ring surrounding the inner insulator. Some have 6 or more, others have 2 or 4, others look like they have none and have just been crimped on.

As Chas 3PY said:" the great thing about standards is that we have so many to choose from!" is very true in this case.

I don't know the answer to the problem other than checking each and every connector, but I thought it salutary to mention my experiences of late.

P.S. Steve ZAZ 400 watts at 100MHz through a pin that size - trust the Italians!

Cheers

Peter VK3QI

[VK3AQZ]

When comparing the dimensions of the component parts of a BNC connector, the dimension that needs to be measured is the outer diameter of the receptacle socket (female) which accepts the pin. Once the pin is inserted into the female receptacle, the diameter of the pin becomes irrelevant. The impedance is the ratio of the outer diameter of the pin receptacle to the inner diameter of the serrated outer connection of the plug. This is not always obvious as most people just look at the pin when in fact they should look at the inner conductor consisting of the pin plugged into it's socket.

I have also measured pins of different diameters, and it seems that receptacle hole diameters vary according to manufacturer. Where I worked in the TV industry, we tried to purchase the plugs and the sockets from the same manufacturer. This was to ensure that the pin matched the hole.

Policing this was difficult as various people purchased connectors and not everyone understood the problem.
And in some cases neither did the parts suppliers. In our case we had 75 ohm connectors for the television equipment and 50 ohm for the RF gear like the comms, microwave links, transmitter, and some test gear. So 50 ohm and 75 ohm patch leads and cables would get mixed up by techs who did not understand, or were just to busy to worry about it. And in some cases the difference did cause some issues particularly, with the higher video frequencies, and also some RF test gear.
As far is the impedance step goes, it generally only matters if the discontinuity is greater than about 1/10 of the wavelength of the signal passing through the connection. For most applications the discontinuity makes little difference.

The more significant problem was the variation in pin diameter causing unreliable contact due to the mixing up of plugs and sockets from different manufacturers. in many cases you could not avoid it as some of the gear came with sockets from unknown suppliers.
For example a Sony vcr would have different BNC connectors to, say, an Ampex, or RCA machine. And our cables might have Huber and Suhner or Amphenol male connectors. So it was hard to maintain a standard. No - I am not a great fan of BNC connectors - Type N or even SO/PL connectors for me.

[VK3ZAZ]

Have you actually found a different in 50 & 75 ohm pin sizes in BNC connectors. I ask because I haven't.

ABSOLUTELY

I work at TV site and as anyone will remember all the leads hanging on wall at LOOK-OUT HILL in the lead cupboard got mixed up.
Grey NEC leads with TRIAX will be 75 ohm
Fly leads to TANBERG TT1220 sat rx will be 75 ohm with rg6 cable
Fly leads on Ericson are RG6 75 ohm.
Video patch leads with 75 ohm bnc sockets
RF patch leads on BNC jack field for FM 50 ohm
RF leads with silicon triax 50 ohm
and it even extends to N Type yep they make N type in both impedances.
So if you scored to TV PAs and they came with GREY BNC leads or N they are probably 50 ohm for RF

BUT

They might be the grey VIDEO patch leads which will be double screened and have screw assemble BNC they will be VIDEO cables
75 OHMS..
Rule of thumb BLUE WHITE GREY VIDEO
GREY BLACK RF
MAYBE



And at Lookout Hill even PL259 in both impedances
Were hanging there complete with 50 and 75 ohm adaptors
I even have a 75 ohm connector marked ABRV3 that's 75 ohm and its an adaptor
and a PL259 which tyou can solder a single wire onto a solder tip 75 ohms.

PINS and brands
AWA sent out BNC with every two way.. GRRENPAR normal, ACME sometimes, and every other brand.
I used to throw them all in drawer no years on I have steady supply of BNC plugs, they are still hard to assemble and they still fail.


Wont even begin to talk about BNC WAN LAN PC

But my terminators often unmarked are 50 and 75 ohm..
BNC pins are different size 50/75 ohm to accommodate the centre conductor
A 75 wont fit in a 50 or maybe its other way around

And as for the NOISY and INT connectors
on real OLD TV sites the patch leads when removed showed a bright unexposed nickel on the socket as the air oxidised the Nickle exposed and not unexposed so if you wiggled the connector it helped reseat it if you didn't you often got a minus 1db change and consequent alarm.
I can remember driving a 400k round trip to wiggle a bnc connector onan ITELCO TV TL at Apollo Bay in 2002 that had INT -1db alarms it was the BNC input lead from the IPA to the PA and it was corroded where exposed and bright under the BNC plug.

The ITELCO at DUNDAs carrying 400w in BNC burned out the centre pin to centre lead connection it arced through took ages to find.
An R and S TX in Ballarat has that problem now but its the weight of the Grey duax on the BNC plug which is RG213 size and puts huge strain on the bnc chassis feed through which is still same size as RG58.. Bad design..
I hate BNC N type adaptors..

[VK4WDM]

Hi Steve

How can you tell the difference between 50 and 75 ohm type N? I have quite a few older ones I got from junk sales etc.

73

Wayne VK4WDM

[VK3QI]

Steve, Wayne, Trash and others,

I am glad that ADMIN moved my post to this category.

Hopefully those reading it will take on board what has been written and double check when using the ubiquitous BNC connector.

In my particular case, it seems that DEMI (Down East Microwave) were using a USA manufactured BNC female sockets that required the slightly larger diameter pin on the male plug. Most of the male plugs available in Oz (probably now from up north of the equator) seem to be the smaller pin. Consequently the intermittent I was noticing.

Cheers

Peter VK3QI

[VK3QI]

Wayne,

You used to be able to tell the difference, but not any more - too many variations between manufacturers!

Cheers

Peter VK3QI

[VK2KPK]

even SO/PL connectors for me.

Be aware that there are two thread types for these ...

[ADMIN]

I am glad that ADMIN moved my post to this category.

Peter, copied, not moved.
Looks like the 30 seconds of effort in doing this was worth it!

All too often, many good posts get buried in topics with an unrelated topic title, which not be seen by many others.