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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: Dressing RG6
Date: Wed, 15 May 2024 23:25:21 +0200
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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