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From: John Larkin <jjSNIPlarkin@highNONOlandtechnology.com>
Newsgroups: sci.electronics.design
Subject: Re: Dressing RG6
Date: Sat, 18 May 2024 09:00:09 -0700
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On Sat, 18 May 2024 15:17:22 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>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 

A practical question is what might the coupling be between two close,
parallel coaxes.