From: Ian Walker
Subject: Re: How to check for a 50 or 75 ohm connector
Date: Sun, 22 Dec 2002 23:48:54 +0000
NNTP-Posting-Date: Mon, 23 Dec 2002 00:00:55 +0000 (UTC)
User-Agent: Turnpike/6.02-U ()
In article , DarkMatter
>On Sun, 22 Dec 2002 21:31:08 +0000, Ian Walker Gave
>>In article <email@example.com>, DarkMatter
>>>On Sun, 22 Dec 2002 15:15:52 +0000, Ian Walker Gave
>>>>Wrong! Whilst 50 and 70 ohm N (BNC) type connectors are both N (BNC)
>>>>type they are incompletely described by calling them N (or BNC) type.
>>>>You must also specify the impedance for correct electrical (and
>>>>mechanical in the case of N type) performance, the type of cable
>>>>providing it is suitable for the connector is irrelevant (i.e. you can
>>>>have RG-174 on one side of an N system and RG-203 on the other).
>>> You prove in your own statement that it IS indeed the cable that is
>>>the determining factor.
>>Only in that the properties of the system determine the type of cable
>>that must used (RG-174, RG-58, RG-203, Su-104, etc.), both the cable and
>>the system determine the connector that must be used (BNC, N, C, K,
>>PC3.5, PC7, 2.4, 1.0, etc.). If the connector side is not designed to
>>have the same impedance as the cable then you will get a reflection from
> Wrong. SOME BNC connectors have differences in the media between
>the grounding shroud, and the center pin. That media is usually air.
>Some use a hard dielectric.
They might be mostly air as in the case of precession low loss N type,
but they must have some hard dielectric otherwise you would not be able
to maintain concentricity with a consequent degradation in return loss
and an increased risk of damage which would further degrade
>The differences in losses or impedance
>will only show up at high GHz level frequencies and are all but
>negligible in many if not most commercial applications for such
Perfectly true that the effect is generally negligible in many
commercial applications; but not in all.
> Note that the current industry uses newer, smaller connectors for
>today's modern Ghz applications as BNC is too bulky in today's
Not only are the smaller connectors less bulky but they provide a better
performance above 1GHz.
>>If you have only got a bag of unidentified plugs you will have to
>>measure the inner and outer diameters and, along with a guess at the
>>dielectric calculate the impedance.
> All you have to do is determine the cable type that it was meant to
>be terminated with. That cable's impedance determines the impedance
>of the jumper, and the correct connector to use for it. The connector
>itself MUST be identical on the PLUG/JACK side so that anyone can plug
>into any jack when both are declared as BNC. Sheesh, this is so
>simple, I can't believe you don't get it.
There are too many types of cable to make this practical, it would even
be possible to have cables with identical inner and outer diameters but
different impedances by choice of dielectric. I have not looked for
cables of this type, but then again that is how 50 and 75 ohm BNC
connectors are implemented. Is it sooo difficult to understand, if the
inner and outer must be the same diameter to allow non-destructive
mating then the only way of making the impedance different is to change
the dielectric between them. I can not believe you do not understand
> IT IS the CABLE that determines the jumper's impedance, and the
>connector is of very little sway in that regard. That FACT that one
>cannot use a RG-6 BNC connector on an RG-174 coax proves that. You
>can make either impedance jumper, and BOTH will still plug into any
>BNC connector. If someone wants to claim "Oh no. It won't plug into
>an "N" type BNC connector." then the idiot is too blind to grasp the
>fact that he is referring to a different connector entirely.
> BNC is BNC is BNC. Impedance is cable specific. Pretty simple.
> Another proof is in simple numbers. A 100 foot length of cable is a
>capacitor, for sure. A half inch connector is a negligible amount of
>that run length. If you remember ANY of your training, you will know
>that such tiny additions are typically rounded off.
We are not talking about capacitors but transmission lines. If I connect
two 50 ohm BNC cables via a 75 ohm coupler it will exhibit a worse
return loss than if I use a 50 ohm coupler. If I really feel bored and
have nothing useful to do tomorrow then I might perform this experiment
with an SNA, I won't confuse you with a VNA and a sliding termination.