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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Dressing RG6
Date: Thu, 16 May 2024 11:41:19 -0400
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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

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com