[time-nuts] TNC connectors

Chuck Harris cfharris at erols.com
Tue Jun 23 10:19:24 EDT 2009


Dave Baxter wrote:

> 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.

Connectors that deviate from standards were at one time pretty common.
The "Ex BT BNC's" in your junk box are undoubtedly an example.

I have run into carbon copies of the old PL259/SO239 UHF connectors
that were identical in all respects, including the nomenclature markings,
but were metric threaded.

And, I have run into alleged BNC's that looked like they were ok, but
wouldn't mate with anything I could find.  They were a silly millimeter
bigger in diameter.

> 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.

93 ohm coax is standard fare for automobile AM radios in the US.

Its claim to fame is its very low capacitance per foot.

> (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.

Unlikely.  The biggest issue would be edge smearing caused by the received
signal bouncing up and down the length of the coax.  This effect is minimized
because the mismatch is small, and the loss of the coax is pretty high.

   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.

What a thing to say!  Quad shielded RG-6 is the minimum quality you will
find for today's TV and cable systems.  Even RadioShack sells it.  It is
the same stuff used by the satellite TV guys.

   I'd guess the foam dielectric
> "Satelite LNB" cable would do.  It fit's 'F' connectors too.

That is because Satellite LNB's use Quad RG 6.  Normal TV coax.

-Chuck Harris




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