Thanks, Jeroen.
Well on that basis, all the plugs I have here are indeed 50 ohms, then.
In fact I pulled a centre pin out of a new packet from the same batch
and it fits the sockets in the VNA perfectly. They all accept the pin
with a nice snug fit and what's more the sockets show no signs of ever
sustaining any damage/spreading/flairing/bending and that goes for
those on the T/R bridge, too. On reconnecting everything, however, the
problem still persists, although the 50 ohm load is now showing as very
close to a dead short; virtually the same spot on the Smith chart
overlay as when the socket is shorted, in fact. Once again, the other
loads do likewise and they all check out fine as 50 ohms with a DVM.
Very strange! I wonder if there's some setting I may have got wrong
somewhere. :-/

In article . com,
wrote:

On reconnecting everything, however, the
problem still persists, although the 50 ohm load is now showing as very
close to a dead short; virtually the same spot on the Smith chart
overlay as when the socket is shorted, in fact. Once again, the other
loads do likewise and they all check out fine as 50 ohms with a DVM.
Very strange! I wonder if there's some setting I may have got wrong
somewhere. :-/

Paul-

What would be the result of calibrating the system with an inductive
"short"? If it can be done, results of testing with a good 50 Ohm load
might be skewed.

Fred

Having had a look in my (junk)box with N-connectors I found that most
silver plated (professional) units have 50 Ohms stamped into the body
,whereas the nickel plated units either have no identification or a
specific manufacturer's code .
The Farnell catalogue you referred says these N-connectors are nickel
plated . Try to find any stamped code on your units . If any I'll be
happy to compare with those on my connectors.

Frank GM0CSZ / KN6WH
==========================================
Since posting earlier I've been on to the actual manufacturers and they
reckon these plugs are actually 50 ohms. So the supplier's insist
they're 75 and the makers say they're 50! I need to know for sure, as
one or the other is in error. How can one tell, by visual inspection
and or physical measurement, which type is which? There must be
something that's visibly different about the two types.
THanks.

Good thinking, Fred. That's theoretically possible, I guess. But I'm
carrying out these checks purposely at only around 5Mhz to minimise
such high frequency effects. You may well be on the right lines, though!

Jeroen Belleman wrote:
. . .
It's one of my long-time gripes about coaxial connectors that N and
BNC series of the two impedance levels are sufficiently compatible
to mate together, but not enough to do so without damage. In a lab
with someone ignorant of this fact, it's a source of endless trouble.

All the 75 ohm BNC connectors I have, have the same diameter center pin
in the connection region as 50 ohm connectors. The difference is in the
thickness of the dielectric around the pin in the connection region,
being much thinner in the 75 ohm connectors. The pin of the 75 ohm
connector is smaller only where it's in solid dielectric at the bottom
of the connector.

Roy Lewallen, W7EL

Roy Lewallen wrote:

Jeroen Belleman wrote:

It's one of my long-time gripes about coaxial connectors that N
and BNC series of the two impedance levels are sufficiently
compatible to mate together, but not enough to do so without
damage. In a lab with someone ignorant of this fact, it's a
source of endless trouble.

All the 75 ohm BNC connectors I have, have the same diameter
center pin in the connection region as 50 ohm connectors. The
difference is in the thickness of the dielectric around the pin
in the connection region, being much thinner in the 75 ohm
connectors. The pin of the 75 ohm connector is smaller only where
it's in solid dielectric at the bottom of the connector.

Roy Lewallen, W7EL

Hi Roy,

There's lots of room for confusion. The Kings tool FAQ implies the
center contact diameter got smaller when the pin was redesigned and
the old crimp die set would not work:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
Why won't my Kings 2025-X-9 die set work with the 2065-X-9
connectors?

With the redesign of the True 75 Ohm BNC connectors, the center
contact diameter got smaller to maintain a constant 75 Ohm
impedance through the connector. Consequently, the center contact
crimp dimension got smaller also.

The problem with the inability of the die set to crimp this new
connector is probably because it is the older, larger die
dimension. The Kings website has a Cross-Reference Search that
lists the applicable die set for most Kings connector part
numbers.

http://www.kingselectronics.com/Reso...4/Default.aspx

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
However, another manufacturer claims their new 75 ohm hardware is
interchangeable with 50 ohm connectors:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
TRUE 75 Ohm BNC & Connectors

Where Controlled Impedance is a Necessity

Emerson Network Power Connectivity Solutions is introducing its
premium line of TRUE 75 Ohm BNC connectors and adapters. These
flexible cable 3 Piece BNC Connectors are essential for analog and
digital high frequency signals.

They have been designed around MIL-STD-202 for up to a 3GHz
frequency range, and are constructed of high performance materials
including a machined nickel plated over brass housing, a teflon
dielectric and gold plated brass contacts. These connectors are
designed to eliminate distortion and impedance mismatching caused
by using 50 ohm connectors on 75 ohm cables, and they safely
inter-mate with standard 50 Ohm BNC connectors.

http://emersonnetworkpower.com/webap.../pdfs/True-75-
Ohm.pdf

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
If your junkbox is anything like mine, it may have stuff purchased
20 years ago mixed with more recent purchases. So it might be
worthwhile to check carefully before mating different hardware.

Regards,

Mike Monett

Mike Monett wrote:

So it might be
worthwhile to check carefully before mating different hardware.

So the thought finally strikes me. How do you check the female socket
dimension? Do you stick a male pin in and see how much friction there is?
That could bend the contacts and damage the pin.

How about checking the male pin diameter when it is already installed on a
connector? There's not enough room for calipers to reach the pin.

I often find the BNC connectors on my scope go intermittent due to a loose
center pin. Often the only choice is to use a pair of pliers to squeeze the
contacts together again. The damage appears to have been caused by some
off-brand 50 ohm terminators that require an unusual mount of force to seat
on the connector. But the repair doesn't last. Once the female pin has been
forced open, it quickly goes intermittent again. This is a big problem
since the scope connector is usually a special item that is difficult to
get.

Regards,

Mike Monett

Mike Monett wrote:

Hi Roy,

There's lots of room for confusion. The Kings tool FAQ implies the
center contact diameter got smaller when the pin was redesigned and
the old crimp die set would not work:

. . .

However, another manufacturer claims their new 75 ohm hardware is
interchangeable with 50 ohm connectors:

. . .

If your junkbox is anything like mine, it may have stuff purchased
20 years ago mixed with more recent purchases. So it might be
worthwhile to check carefully before mating different hardware.

For sure. It looks like it's not safe to mate a 75 ohm connector with
anything, even another 75 ohm connector, without checking pin diameter.

Roy Lewallen, W7EL

Maybe your "N" female connector has "splayed" leaves on the center
contact. This sometimes happens when someone tries to screw a PL-259 UHF
connector in place by accident.

wrote:

Okay, thanks. I must admit it is symptomatic of the load's center pin
not quite making contact for some reason, even though the load is fully
screwed home. But the same thing happens with 3 different loads!
I'm using the T/R test set, by the way, although I don't see how that
could explain the problem. I don't have the full S-parameter bridge but
am in the market for one.




--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

Roy Lewallen wrote:

For sure. It looks like it's not safe to mate a 75 ohm connector with
anything, even another 75 ohm connector, without checking pin diameter.

Roy Lewallen, W7EL

Or even 50 ohms, for that matter. But how do you check the pin diameter on
an existing connector? A caliper won't go in there. Also, the diameter
could be correct but the pin could be assembled incorrectly and stick out
too far. This will damage the female pin.

Or maybe you know of some other non-destructive method to tell if you can
mate an existing male and female coax connector without damage?

The vulnerable part is the female pin. Once those legs get splayed there's
no way to put them back. All it takes is one event to effectively ruin a
brand-new scope.

Regards,

Mike Monett