Path: ...!weretis.net!feeder9.news.weretis.net!feeder8.news.weretis.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Phil Hobbs Newsgroups: sci.electronics.design Subject: Re: Dressing RG6 Date: Sat, 18 May 2024 18:16:48 -0000 (UTC) Organization: A noiseless patient Spider Lines: 136 Message-ID: References:

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

<20240514d@crcomp.net>

<01kh4j9dsbnrm7c6drktrf48u9vsiji5go@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Sat, 18 May 2024 20:16:49 +0200 (CEST) Injection-Info: dont-email.me; posting-host="0da68a6e09cac624f1ee6d3da85782c0"; logging-data="3060477"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18zxkvoMOM40ILdkN4D1vz5" User-Agent: NewsTap/5.5 (iPhone/iPod Touch) Cancel-Lock: sha1:M3vTfXslVRWW3IdH6mkueVOWGjc= sha1:opNO3H8WK2n7hcz+piRI6UADNyw= Bytes: 6821 John Larkin wrote: > 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, thats 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, thats 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.) >> > > 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