[time-nuts] Thermal effects on cables

Attila Kinali attila at kinali.ch
Mon Jan 23 14:33:09 EST 2017

On Mon, 23 Jan 2017 15:13:39 +0000
REEVES Paul <Paul.Reeves at uk.thalesgroup.com> wrote:

> Surely the impedance of the cable is only affected by the ratio
> of the inner conductor and outer conductor diameters modified by
> the internal dielectric constant, nothing to do with the frequency of
> operation. You might well have problems converting the larger diameters
> down to a suitable size for the connectors at the higher frequencies
> though....

If you are only looking at the impedance, then yes. But once you get
to high frequencies, you get also multi-mode behaviour of the coax cables
and connectors, which leads to dispersion. That's why people hardly use
N connectors for GHz frequencies, even though the connector itself would
allow it. For VNAs where even small phase shifts/instabilities due to
multi-mode behaviour/dispersion are a no go, the connectors are usually
3.5mm (basically a precise version of the SMA), 2.9mm, 2,4mm etc 
and go down to even 1.0mm which can be spec'ed up to 110GHz.

BTW: the 3.5mm connector is one that you will find on many instruments
that go beyond 1-2GHz. Unfortunately it looks exactly the same as an
SMA connector and will mate with one. Even more unfortunately, mating
it with an SMA connector will scar the connector and most likely move
it out of spec (ie degrade it to a simple SMA connector). If it's just
an adapter, you've only lost a bit of money (in the order of a few €100).
If it was the connector of your VNA/oscilloscope/..., you might need to
send it in for an expensive repair.

			Attila Kinali

You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

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