[time-nuts] TNC connectors

Dave Baxter dave at uk-ar.co.uk
Tue Jun 23 08:40:23 EDT 2009

> Message: 3
> Date: Tue, 23 Jun 2009 07:54:02 -0400
> From: Chuck Harris <cfharris at erols.com>
> Subject: Re: [time-nuts] TNC connectors
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Message-ID: <4A40C25A.7080403 at erols.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> Dave Baxter wrote:
> > Doing that with BNC's will lead to a flaky connection, as 
> the centre 
> > pin on the 75r ones is much smaller.  The other way round (50r plug 
> > into 75r
> > socket) will damage the socket.  As TNC's are very similar 
> to BNC's ???
> You are thinking of the incompatibility between 50 ohm and 75 
> ohm *N* connectors.
> 75 ohm BNC are use the same center pin hardware as 50 ohm 
> BNC.  The only difference is in the white plastic (teflon) 
> insulator.  In the 50 ohm BNC, the center insulator goes all 
> the way to the tip of the center pin.  In the 75 ohm BNC, the 
> center insulator is abbreviated.
> HP/Agilent uses 75 OHM BNC's on several of its devices that 
> have switchable impedances.  For example, the 3586C.
> -Chuck Harris

Indeed looking as some more data sheets.

However, I have somewhere in the deed box at home, some Ex BT BNC's,
marked up as 75 Ohm, that have very much smaller center pin's than the
common or garden 50r types, as well as less plastic in there too.

We once had a Novell computer network in the office, that used 93r coax
cable, and 50r BNC's, at 100MBPS.  Using Thomas Conrad cards I seem to
remember.   We still have the real of 93r coax.

(It was very much faster end to end, and much more stable than the
TCP/IP based Microsoft network we have now, but I digress.)

In general, yes indeed size for size, 75r coax will be less lossy than
50r types, but if the RX does present a nominal 50r load, they may be
some potential issues.  However, if the antenna is an active type, it's
probably not worth fussing over.  Enough people have used what's
recomended I guess and found no problems.  Just use good quality cable,
not the so called "Low loss" TV coax.  I'd guess the foam dielectric
"Satelite LNB" cable would do.  It fit's 'F' connectors too.

Cheers All.

Dave B.

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