[time-nuts] Re: 10 MHz Distribution Best Practices?

[David Bengtson]

Thanks. Triax is problematic as I'm going from BNC to BNC connector,
so I think that won't work for this application.

Dave

On Fri, Mar 24, 2023 at 1:46 AM Kitski via time-nuts
<time-nuts at lists.febo.com> wrote:
>
> I'll second John's recommendations, particularly double-shielded coax, but
> with a twist (pardon the pun).
> There are various grades of 'double shielded' coax - some with loose braid
> coverage and others with really tight weaves (loosely spec'd as 'coverage').
> Use 95% or forget about it.
>
> Next up (down ?) this rabbit hole is triax which by definition is double
> shielded. More to the point, the braids are insulated from each other.
> Another benefit is that BNC triax connectors are not that expensive.  As the
> outer braid is typically at 'mains/safety earth' potential, the independent
> inner braid can be used in a variety of hum-busting ways.
> Then another rabbit hole to peer down is re-wiring your AC mains facility
> with separate electrical earth and technical earths. Not for the
> faint-hearted this one.
>
> Mitigating leakage LF thru to UHF (and crosstalk) in and out of your
> facility are part and parcel of MIL requirements in sensitive
> establishments.  Measuring their effectiveness (BT-DT) is an interesting
> past-time.
>
> My 10c worth (currently 67c US).
>
> Kit
> Canberra, Australia
>
>
> -----Original Message-----
>
> A lot of things can be said in favor of low-noise distribution amps with
> good VSWR, good channel isolation, and good PSRR (not so fast, HP 5087A.)
> But over the years, I've learned that the three most important factors when
> it comes to piping 10 MHz signals around are:
>
> 1) Shield resistance
> 2) Shield resistance
> 3) Shield resistance
>
> I've spent a lot of time recommending double-shielded coax in the TimeLab
> manual and elsewhere, and I still stand by that advice, but what I've come
> to realize is that this is really just a proxy for low shield resistance.
> Good grades of single-shielded cable are basically as effective at HF as
> double-shielded cable.  To the extent your cable ground shield exhibits
> resistance, it's not a shield, it's a resistor.
>
> Avoiding ground loops is on the list too, but further down.  Never lift a
> ground to avoid a ground loop.  Use coax-to-coax baluns only when you can
> see a beneficial effect.  Focus instead on providing a shared low-resistance
> common ground  to your entire network -- ideally not the ground all the way
> back at the service entrance -- and rely on low shield resistance on the RF
> side to do the rest.
>
> Every installation is different and your mileage will most certainly vary,
> but this is my take on it.
>
> -- john
>
>
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