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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: Dressing RG6
Date: Sat, 18 May 2024 14:19:49 +0200
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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