Thermal effects on cables
A google search for digiwave turned up cheap coax at walmart. I don't know
how well copper clad steel holds up in the kHz, low MHz.
On Mon, Jan 23, 2017 at 5:01 PM, Bob Camp kb8tq@n1k.org wrote:
Hi
Back a long time ago the people I was working with spent time looking
at the impedance of a variety of coax cables. The data they came up
with on some varieties of cable would suggest that cable is not an outlier…
Bob
On Jan 23, 2017, at 3:08 PM, jimlux jimlux@earthlink.net wrote:
On 1/23/17 9:16 AM, Bob Camp wrote:
Hi
Nothing is ever simple if you dig deep enough:
here's my guess on the curious case cable: it was mismarked by accident
at the factory (e.g. 93 ohm coax, marked as 50 because someone forgot to
push the button on the marking machine), and dumped in the scrap bin, and
then sold by someone who bought the surplus and just read the markings.
Or, even sold as a whole spool, likely at a discount, to a
"knowledgeable buyer" who was willing to take it, mismarkings and all.
mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/
mailman/listinfo/time-nuts
and follow the instructions there.
oI On 23 Jan 2017 17:02, "REEVES Paul" Paul.Reeves@uk.thalesgroup.com
wrote:
Hi David,
Hi Paul
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.
No, that is incorrect.
There are two complications with coax, which mean that you can't trust the
"normal" equations at very low or very high frequencies.
- LOW FREQUENCY
An accurate equation for the impedance of the cable is
Zo=sqrt ((R+j w L)/(G+j w C))
w = 2 Pi f
R = Resistance per metre (ohms/metre)
C = Capacitance per metre (Farads/metre)
G = Conductance per metre (Mho/metre)
If the frequency is high enough (above a few MHz),
j w L >> R and
j w C >> G
so the R & G terms are insignificant and one gets the usual equation we know
Zo=sqrt (L/C)
At low frequencies the assumption that the resistive losses (R&G) are
insignificant is no longer valid, so impedance rises at frequency
decreases.
At DC a bit of coax is just a capacitor..
- HIGH FREQUENCY
At high frequencies!the best of9 higher order modes can propagate. The
reasons for this are more complicated to explain, but are a result of the
breakdown of the assumption of the boundary conditions used to arrive at
the impedance.
The maths of this effort is more complicated, needing Bessel functions.
You might well have problems converting the larger diameters down to a
suitable size for the connectors at the higher frequencies though....
Yes, but small connectors are used for the very same reason small cables
must be used.
I thought that the HP cabling for the 8510 series VNAs was air spaced but
I might well be wrong - I just tried not to damage them :-)
You might well be right about the air spacing. Of course there needs to be
some support for the centre conductor, but it might be beads like in 3.5
mm connectors. I really don't know.
Regards,
Paul Reeves
Dave, G8WRB.
On 23 January 2017 at 17:29, Tim Shoppa tshoppa@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
and 16453A dielectric material test fixture
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
Hi Attila,
I quite agree, I should have remembered that.... but I tend to use fairly small coax anyway at those frequencies as a matter of course and it didn't register as something that needed saying. My fault!
It can be quite difficult to get people to notice the difference between SMA and 3.5mm - I have often had to repeatedly remind some of our test technicians of the difference as they tend to think they are interchangeable with the unfortunate results you describe :-( It is just as well that the 3.5mm cal kit for the HP8510 (still a fine VNA, if lacking a few of the new bells and whistles) is a work of art with DTIs for checking the mechanical setup of the connectors - they are not just for show!
Regards,
Paul Reeves
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@febo.com] On Behalf Of Attila Kinali
Sent: 23 January 2017 19:33
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Thermal effects on cables
On Mon, 23 Jan 2017 15:13:39 +0000
REEVES Paul Paul.Reeves@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 _______________________________________________
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.