[time-nuts] TNC connectors

Dave Baxter dave at uk-ar.co.uk
Tue Jun 23 05:59:00 EDT 2009

> Message: 9
> Date: Tue, 23 Jun 2009 09:28:44 +0100
> From: "David C. Partridge" <david.partridge at dsl.pipex.com>
> Subject: [time-nuts] TNC connectors
> I'm thinking of buying a Panasonic VIC100 timing antenna with 
> a 50 ohm TNC connector (I assume) from fluke.l on eBay, as the room I
> for my den has moved to one without a view to the south, so I can't
use the 
> puck antenna mounted just outside the window any more.
> The T-Bolt manual recommends 75 ohm coax, so will a 75 ohm 
> TNC mate with a 50 ohm TNC without problems?   Though I suppose for a
> run of cable, 50 ohm will be OK?
> PS Where do you get the (1" diameter?) threaded poles these 
> antennas screw onto?
> Cheers
> David Partridge

Hi Again David.  Hope all's still well with that DMM...

According to Wikipedia, a 75r TNC will mate OK with a 50r version.

However, it's years since I saw a 75r version, so I don't know for sure.
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 ???

However.....   Scratching about the www for a minute or two...

a 75r TNC data sheet is here.

and the 50r version is here.

The centre pin's are very similar it seems!

Unfortunately, if you have to source one from these people it'll cost an
arm and a leg.  We used to be able to ring up the distribution centre in
Bicester, then run over to get some parts at good cost/time etc, now
even we have to go through a third party in Kent (I think is where they
are/were) who hold no stock and have very long lead times, and even
higher costs!

I'd be tempted to just fit a 50r TNC plug (common enough) and go with

I'm also looking at a Thunderbolt, but mainly to get a reliable 1PPS
feed for a NTP server that I've yet to figure out how to get going.
(Just not enough of the right sort of time to figure out this sort of
stuff)  After I blew up the only GPS RX I had with a 1PPS output, due to
a (commercially built, with no crowbar) PSU failure.

If I could use the 10MHz output for a receiver LO reference, and
soundcard clock locking too, that would be a bonus.

Not looked at a TB manual yet (are they downloadable?) but it does have
a 'F' connector on the front from what I've seen of the pictures, and
those are generally used with 75r solid centre conductor coax.  50r
solid centre conductor cable can be used with F's too however that I do
know.  But at 1500MHz any matching problem may be an issue, F's are
generally only rated up to 1GHz anyway, though "usable" to several GHz,
they say...

Does anyone know what the actual design input Z of the TB's GPS RX is?
As connectors, they are not much better/worse than "UHF" types.
Mechanically poor is an understatement, and ideally they need the
correct torque to secure them to be reliable.  (In a past life, I
briefly worked for a Cable TV equipment company.)

In any case, there will be less of a problem at the antenna end with a
mismatch, than at the RX end of the cable.   Source mismatches are much
less troublesome than load mismatches, when a long cable is used, so if
indeed it needs to use 75r, then that would be best.   "Long" in this
context, being relative in terms of signal wavelength.  At 1500MHz the
wavelength is only some 0.2m (in free air too, it will be even shorter
in the cable) so just about any practical physical length will appear

At the moment, I'm up to my eyes in dead multi kW RF amps, and creaky
firmware in other (unrelated) products, that & friends/family are
continuing to keep me very busy with various PC woes, plus a possible
trip to the US is on the cards, as a result of one of the multi kW amp
failures.  It never ends!...

Regards to All.

Dave B.

PS:	1" 'Threaded' poles?   A marine chandlers perhaps?   Or just
"wing it" with a cheap ally' TV type pole, and a bit of creativity with
some silicone rubber!  (The non corrosive stuff)

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