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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: Dressing RG6
Date: Wed, 15 May 2024 00:08:26 +0200
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On 5/14/24 23:46, Phil Hobbs wrote:
> John Larkin <jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>> On Tue, 14 May 2024 19:22:12 -0000 (UTC), "Don" <g@crcomp.net> wrote:
>>
>>> Phil Hobbs wrote:
>>>> Phil Hobbs wrote:
>>>>> Don Y wrote:
>>>>>> I've several short (a few feet) lengths of RG6 that I
>>>>>> would like to "strongly coerce" into assuming a particular
>>>>>> dressing.
>>>>>>
>>>>>> Securing the cables to a stationary surface isn't practical
>>>>>> without significantly lengthening them and distorting
>>>>>> their "natural" routing.
>>>>>>
>>>>>> But, ISTM that I should be able to slip each cable into
>>>>>> a comparable diameter copper (?) pipe and then use traditional
>>>>>> tools to bend that pipe into the appropriate configuration.
>>>>>> I'd have to observe constraints like minimum bend radius
>>>>>> but are there other issues that I might "discover" down the
>>>>>> road?
>>>>>>
>>>>>>
>>>>>
>>>>> You’re planning to make a random- length shotgun balun.
>>>>
>>>> Bazooka balun.
>>>
>>> The parasitic capacitance created between coax and its metal armor can
>>> open a Pandora's box of potential problems.
>>>
>>> Danke,
>>
>> 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.
> 
> Cheers
> 
> Phil Hobbs
> 

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.

Jeroen Belleman