Path: ...!3.eu.feeder.erje.net!feeder.erje.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 15:17:22 -0000 (UTC) Organization: A noiseless patient Spider Lines: 122 Message-ID: References:

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

<20240514d@crcomp.net>

MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Sat, 18 May 2024 17:17:22 +0200 (CEST) Injection-Info: dont-email.me; posting-host="92137f0690326acda829e37a5f3bca45"; logging-data="2993243"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19EDINevl+MmIGLEXlnOPR+" User-Agent: NewsTap/5.5 (iPhone/iPod Touch) Cancel-Lock: sha1:TdqBYx7PG4cjZ6TpdhmnJo8eDO0= sha1:4nL8xQC5e4+dTaMgKC3TP15gvB8= Bytes: 5976 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 -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics