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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Dressing RG6
Date: Sat, 18 May 2024 18:16:48 -0000 (UTC)
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John Larkin <jjSNIPlarkin@highNONOlandtechnology.com> wrote:
> 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.)
>>  
> 
> A practical question is what might the coupling be between two close,
> parallel coaxes.
> 
> 
> 
> 

Since I now have a 3-GHz VNA, I might have a try measuring that. The
coupled amplitude goes like a cosine, so it’s easy to calculate how much
interaction length you need for 100% coupling from just one measurement. 

Cheers 

Phil Hobbs 

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