Path: Xl.tags.giganews.com!local-1.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Sat, 18 May 2024 16:02:05 +0000 From: John Larkin Newsgroups: sci.electronics.design Subject: Re: Dressing RG6 Date: Sat, 18 May 2024 09:00:09 -0700 Organization: Highland Tech Reply-To: xx@yy.com Message-ID: <01kh4j9dsbnrm7c6drktrf48u9vsiji5go@4ax.com> References:

<20240514b@crcomp.net> <66h74j1vfmbjvvl98jk1k017pimtinv2l5@4ax.com>

<20240514d@crcomp.net>

X-Newsreader: Forte Agent 3.1/32.783 MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Lines: 125 X-Trace: sv3-w5B3m+jEeI0VJuEI6JocQNiKrtsti4mFV2XCqzeWMHpQ6At6z9ALDeIJxMrmqUJD4tOaFtzLDeRCT2Q!05DONpDzh+ydl5v72fnl28QiAKhp3ALP5HI7xiG43t8RvdmFYeXnR+25NkmNf5oyCns5eRY2ZtW4!lhjZwQ== X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 6221 On Sat, 18 May 2024 15:17:22 -0000 (UTC), Phil Hobbs wrote: >Jeroen Belleman 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 >>>>> wrote: >>>>> >>>>>> On 5/15/24 01:33, Don wrote: >>>>>>> Jeroen Belleman wrote: >>>>>>>> Phil Hobbs wrote: >>>>>>>>> John Larkin wrote: >>>>>>>>>> Don wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>>>>>> 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 >>>> . >>>> >>>> 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 A practical question is what might the coupling be between two close, parallel coaxes.