Re: Dressing RG6

Jeroen Belleman <jeroen@nospam.please> wrote:

On 5/16/24 17:41, Phil Hobbs wrote:

On 2024-05-15 17:25, Jeroen Belleman wrote:

On 5/15/24 16:27, John Larkin wrote:

On Wed, 15 May 2024 11:03:22 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:
 

On 5/15/24 01:33, Don wrote:

Jeroen Belleman wrote:

Phil Hobbs wrote:

John Larkin wrote:

Don wrote:

 
<snip>
 

The parasitic capacitance created between coax and its metal
armor can
open a Pandora's box of potential problems.

 
Capacitance between the coax outer and the copper pipe? Proper coax
shouldn't have any external field.

 
If the whole system is really coaxial, that’s true. Leaky
shields, ground
loops, and so on, will modify that.
 
Depending on the application, you may or may not care.
If the whole system is really coaxial, that’s true. Leaky
shields, ground
loops, and so on, will modify that.
 
Depending on the application, you may or may not care.

 
I've been putting coax inside copper tubes or braids to measure
and/or reduce the transfer impedance (leakage). I did that to
measure small signals in a particle accelerator, which typically
has kicker magnets and RF cavities with kA currents and kV
voltages nearby.
 
A colleague developed a special low transfer impedance coax
cable for this sort of application. It had two screens with
intermediate magnetic shielding. It was unpleasant to work
with, because part of the magnetic shielding was a steel
spiral foil tape that was razor sharp. But it worked really
well.

 
Empirical observation always trumps theory for me. Did you ground [1]
the copper tubes or braids?

 
Both ends were connected to the connector shields. The point of
the exercise was to reduce transfer impedance, which at low
frequency (<1MHz) is simply proportional to screen resistance.
 
Jeroen Belleman

 
Two parallel coaxes can make an attenuator.
 
What was the coupled frequency response like?
 

Ah sorry, this message didn't seem to get sent...
 
At low frequency, the transfer ratio was simply the ratio
of screen resistance over characteristic impedance. At medium
frequencies, a few octaves roughly around 1MHz, there was a dip,
and above that a steady rise of about 10dB/decade.
 
Not all cables behaved the same. RG58 is poorly screened and
doesn't have the dip. UT141 had a very deep dip.
 
Details at
<https://jeroen.web.cern.ch/jeroen/coaxleakage/leakage.shtml>.
 
Jeroen Belleman

 
Very interesting results, Jeroen.  Thanks for posting them.
 
Is the MF resonance due to the inductive and capacitive coupling
cancelling each other?  (They're 180 degrees out of phase, of course.)
 
The frequency is way too low to be a transmission line effect in a 1-m
length.
 
Cheers
 
Phil Hobbs
 

 
The original data came from an HP3577 and I recorded only the
magnitude. Since this looks like a resonance, that's also what
I'd expect.
 
I can't easily go back and look again. I did this in 2009, and
I'm now retired. At the time, I was trying to make a choice for
cables connecting beam trajectory pick-ups in the CERN PSB to
their pre-amplifiers.
 
I suppose -but did not verify- that the dip is a resonance of
the outer inductance with a parasitic capacitance of my setup,
with the screen resistance as the damping element. I can't quite
make it fit that model though. The screen resistance doesn't
differ enough between, for example, UT141 and RG58 to explain a
deep resonance for the former, and its total absence for the
latter.
 
Jeroen Belleman
 

Plus you had some pretty frou-frou RG58 there, with foil and two braids.

The normal stuff is one tinned-copper braid with about 80% coverage.  You
can probably make a directional coupler with a pair of patch cords and some
heat shrink. (I should try that.)

Cheers

Phil Hobbs



--
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics