[time-nuts] Re: Query about List and about 10 MHz Distro

[Joseph Gwinn]

On Sun, 29 Aug 2021 13:56:42 -0400, time-nuts-request at lists.febo.com 
wrote:

Re: time-nuts Digest, Vol 208, Issue 24 (messages 2, 4, and 15) 


> ------------------------------2
Date: Sun, 29 Aug 2021 09:51:39 -0400
From: Bob kb8tq <kb8tq at n1k.org>
Subject: [time-nuts] Re: Query about List and about 10 MHz Distro
To: Discussion of precise time and frequency measurement
	<time-nuts at lists.febo.com>

Hi

Skin depth is what gets you with most coax at 10 MHz, even with the 
“double 
shield” stuff. Hardline with a nice heavy outer would do better. 
It’s not terribly 
practical around the typical lab. Might not be a bad choice for a 
“burry in the yard”
setup though. With normal cable, the losses from the soil at normal 
burial 
depths make a good shield, that makes dealing with it questionable 
even there ….

Bob

>> On Aug 29, 2021, at 9:36 AM, Robert DiRosario <ka3zyx at comcast.net> wrote:

What about double shielded RG-223, or Thinnet ethernet cable?
Most Thinnet has both a braided shield and a foil wrap.

Robert

> ------------------------------3
Date: Sun, 29 Aug 2021 09:21:32 -0500
From: Dana Whitlow <k8yumdoober at gmail.com>
Subject: [time-nuts] Re: Query about List and about 10 MHz Distro
To: Discussion of precise time and frequency measurement
	<time-nuts at lists.febo.com>

I believe that a significant problem in my case was not leakage 
through the
shield, but
rather common-mode currents on the cable.  My experiences were with 
RG-6
style cable,
namely a cable that mostly went around the outside of the house for TV
distribution.
I get all the TV I want with an indoor dipole, and I had had plans of 
using
that TV
cable for ham purposes.  One thing I have noted is that adding ferrite
common mode
chokes near the ends of the cable did help quite a  bit, just not 
enough.


> ------------------------------15
Date: Sun, 29 Aug 2021 13:29:26 -0400
From: Robert DiRosario <ka3zyx at comcast.net>
Subject: [time-nuts] Re: Query about List and about 10 MHz Distro
To: time-nuts at lists.febo.com

Andrew Sureflex (FSJ) is a lot more flexible then the LDF4 Healix, 
which 
is what you find a
lot of on ebay and at hamfests.

Here is some 1/4"  Sureflex, new:

Andrew Type N Male to Type N Male Sureflex Cable 30 feet 
F1RNA-PNMNM-30-L1 NEW!
https://www.ebay.com/itm/173863049072
US $30.00 + $13.02 Shipping.  For two pieces shipping goes to $16.33, 
for three $22.90

You can cut it with a hacksaw, do NOT use wire cutters.  Connectors 
can be
installed with just hand tools.

If you move the test equipment around it's not very practical, but if 
it's setup on
a bench or in a rack it will work.

Robert

Those large 10 MHz distribution systems typically used Heliax cable, 
which as a solid copper foil shield, with better than 100 dB/meter 
shielding effectiveness (SE).  Actually, I think that's the limit of 
their ability to measure SE. Back when we did underground nuclear 
tests, the cable connecting down-hole instruments to instruments a 
safe distance away were Heliax.

At power-line frequencies, the skin depth well exceeds the thickness 
of any reasonable shield, so the transfer impedance (between inside 
skin and outside skins) is the DC resistance of the shield.  

Power-frequency ground loops can be a problem, and common-mode chokes 
are not effective against this, but 10 MHz RF transformers intended 
for galvanic isolation are.  May also need a blocking capacitor to 
prevent transformer core saturation.

As for common-mode chokes, one can implement such things by winding 
insulated coaxial cable multiple times through a big ferrite toroid 
core.  The insulation jacket prevents the cable shields from shorting 
to one another.  This dodge also works for shielded twisted pair 
cable.  This dodge is discussed in the books on Balun Transformers, 
and is raised to a high art in the book on Coaxial AC Bridges.

Joe Gwinn