[time-nuts] Thermal effects on cables

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 23 January 2017 at 17:29, Tim Shoppa <tshoppa at gmail.com> wrote:

>
> Funny how people always want to put the words "dielectric" and "constant"
> right next to each other but we know it isn't constant :-)
>
> Tim N3QE
>

Yes. I will have to look into this, as I see some quite widely different
values quoted for the dielectric constant of PTFE. I have an HP 4291B
impedance/material analyzer

http://www.keysight.com/en/pd-1000000857%3Aepsg%3Apro-pn-4291B/rf-impedance-material-analyzer?cc=US&lc=eng&lsrch=true&searchT=4291b

and  16453A dielectric material test fixture

http://www.keysight.com/en/pd-1000000508%3Aepsg%3Apro-pn-16453A/dielectric-material-test-fixture?cc=US&lc=eng

To make a measurement of the permittivity of a material, the calibration
procedure is a bit complicated, needing 6 or 7 standards, but at one point
you need to use a dielectric of known thickness and known permittivity. I
believe it defaults to 2.1, which is supposed to be what the bit of PTFE
Keysight supply is. Needless to say I don't have that, and I'm not spending
the best part of £500 for a bit of PTFE that's 1 mm thick. But I've seen
values of Er of PTFE quoted from 2.0 to 2.2.

Dave