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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Dressing RG6
Date: Sat, 18 May 2024 15:17:22 -0000 (UTC)
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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 



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