when do you look at coaxial cable capacitance? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: March 05, 2024, 10:11:50 am
content: What do people use the coaxial cable capacitance specification for?It seems for signals you always look at impedance.I know it might be useful to know to make a capacitor out of coaxial cable (unterminated). Like a stub. Or to know how much energy is in a cable if you disconnect it with a DC bias on it (i guess a long run can be dangerous to equipment if charged)Another place is I guess for low frequency signals work, where you need a shielded cable but the frequency is low (i.e. sonar). They sell low capacitance cable here, meant for work with ultrasound or whatever biology.What other situations would you look at the cable capacitance specifications?

author: Gyro
author_rank: Super Contributor
timestamp: March 05, 2024, 10:24:11 am
content: In my case, when it was part of a capacitive divider...https://www.eevblog.com/forum/testgear/high-voltage-x10-attenuator-for-my-gay-mta-memory-voltmeter/

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 05, 2024, 11:31:58 am
content: There are relationships at work there, are between the capacitance, the relative permittivity (aka dielectric constant), and the velocity of propagation.The Wikiedia page on Velocity Factorhttps://en.wikipedia.org/wiki/Velocity_factorhas a table of common cable types. Note that the "Open wire (ladder line)" approaches unity (100%) because it has little/no capacitance.Generally, cables with more capacitance will have a lower velocity of propagation.

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2024, 03:01:21 pm
content: When the coaxial cable is used without termination, then it can be modeled as a lumped capacitance.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 05, 2024, 03:20:58 pm
content: A short section of transmission line that is open circuit or high impedance load is treated as a small capacitor. This comes up when using coax for a shielded connection to high impedance sensors such as photodiodes, piezeo transducers, and strain gauges. The extra capacitance limits the bandwidth and noise of amplifiers, so you want to keep the cables as short as possible.For low impedance loads a short transmission line looks like an induuctor with similar limitations.

author: A.Z.
author_rank: Super Contributor
timestamp: March 05, 2024, 08:40:28 pm
content: just think "capacitor" and "statics"; a 33ft run of RG8 is already a capacitor, if it isn't properly shunt, never had an electrical shock from some coax connector?

author: p.larner
author_rank: Frequent Contributor
timestamp: March 05, 2024, 09:02:56 pm
content: ive used lumps of rg213 to resonate a loop on a spot freq a few times 73.

author: tszaboo
author_rank: Super Contributor
timestamp: March 05, 2024, 10:23:31 pm
content: Surge protection or considerations.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 05, 2024, 10:44:48 pm
content: When using it at low frequencies and impedances above Zo.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: March 06, 2024, 02:08:08 pm
content: Quote from: cosmicray on March 05, 2024, 11:31:58 amhmmm so cable that have higher capacitance per length always propagate faster?i wonder if dispersion of a pulse is less on ladder line too?

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2024, 04:50:44 pm
content: Quote from: coppercone2 on March 06, 2024, 02:08:08 pmNo: lower capacitance/length cables (e.g. air or Teflon foam) arefaster.Actually, that is a function of the dielectric constant (air = 1, solids > 1).Compare similar coax with solid polyethylene (RG-58/U) at 66% to lower capacitance Teflon foam (LMR195) at 88% propagation factor.

author: coppercone2
author_rank: Super Contributor
timestamp: March 06, 2024, 09:31:31 pm
content: how is air line vs a non insulated twin lead (also in air)?that also conceptually makes sense that there is no capacitor to charge or something like thatyeah i switched words

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2024, 10:14:27 pm
content: Air-insulated coax or twin-lead still have capacitance per length, as do all transmission lines, but plastic dielectric increases the capacitance for a given set of dimensions.However, when you look at the detailed math for coax, regardless of dimensions (which affect the characteristic impedance), the velocity is a function of only the dielectric constant: see the usual textbooks.The Wikipedia article has these equations, and points out that the speed is a function of the product of dielectric constant and magnetic permeability.See “Derived electrical parameters” in that article.The permeability is unity for normal coax, but the dielectric constant ranges from 1 (air) up to 2.25 for solid polyethylene.Teflon and foamed plastics are in between.Other plastics such as PVC (3.18) are lossier and not used for RF coax.

author: tggzzz
author_rank: Super Contributor
timestamp: March 06, 2024, 11:07:56 pm
content: Quote from: coppercone2 on March 06, 2024, 09:31:31 pmThere isalwaysa capacitance. Consider the capacitor formed by the earth and moon...https://electronics.stackexchange.com/questions/111582/capacitance-between-earth-and-moon#111594

author: coppercone2
author_rank: Super Contributor
timestamp: March 06, 2024, 11:10:30 pm
content: less*i got the wrong connector from ebay now digikey sent me the connector without the fucking nut and collar and i cant post strait

author: MarkT
author_rank: Frequent Contributor
timestamp: March 06, 2024, 11:29:35 pm
content: Quote from: coppercone2 on March 06, 2024, 02:08:08 pmNo, the propagation speed is basically the speed of electromagnetic wave propagation in the dielectric, independent of geometry or capacitance. Air-cored coax is the fastest.Things are slightly more complicated in reality, and the above assumes non-magnetic conductors.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 06, 2024, 11:41:16 pm
content: Quote from: coppercone2 on March 06, 2024, 02:08:08 pmNo. Velocity factor only depends on the dielectric constant. Capacitance depends on both dielectric constant and geometry. A high impedance line will have lower capacitance per meter than a low impedance transmission line of the same dielectric, but the velocity factor is the same.Ladder line has a high velocity factor because the dielectric is mostly air, not specifically because it has low capacitance. If you made a ladder line with parallel strips instead of thin wires the capacitance would be higher but the velocity factor would be the same.

author: David Hess
author_rank: Super Contributor
timestamp: March 06, 2024, 11:51:00 pm
content: Quote from: ejeffrey on March 05, 2024, 03:20:58 pmJust to expand on that, when the transmission line is not terminated, then there are reflections back and forth. The level of the reflections gradually decreases as more energy is lost, and the average voltage in the transmission line charges to that of the source as if the transmission line was a capacitor. The curves are identical.If the transmission line is made longer, then the reflections take more time traveling back and forth, so it takes more time for them to lose energy, and the average voltage settles more slowly, just as if the capacitance is larger. It is pretty easy to see on an oscilloscope if you care to look.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2024, 01:41:43 am
content: Quote from: ejeffrey on March 06, 2024, 11:41:16 pmOr, most specifically, jointly with relative permeability. There are only some contrived scenarios, and the occasional wave propagation through magnetic materials (and more generally, free waves and optics, of course), where one needs to consider permeability and permittivity together; practical TLs are pervasively made with mu_r = 1 materials, hence it can be ignored in most all cases.Usually, such applications are something obscure, like wire/cable with a strong filtering quality thanks to the high-frequency lossiness of the ferrite material. In other words, a distributed ferrite bead sort of material. Occasionally, one can employ this to effect, like using magnetic saturation of the loading material to launch shock waves down such a line -- a magnetic pulse compressor. But those are more often crafted from particular material stackups, rather than bulk fillers.Quote from: David Hess on March 06, 2024, 11:51:00 pmAs I summed up / hinted at in my earlier message, the low-frequency approximation of a transmission line is an inductor for Z < Zo, and capacitor for Z > Zo.What is "low"? When we don't have to worry about reflections.It's a surprisingly powerful and deep statement in so few words; but it takes some contemplation to realize that power. One can (and should!) demonstrate it using simulations, and derive it from basic theory.For moderately higher frequencies, or Z ~ Zo, we can use both, i.e. an LC section, to model it up to the first resonance, say; it quickly gets unwieldy as more and more harmonics need be modeled, and we find a growing discrepancy between lumped-equivalent (rational polynomial valued expression) and wave (TLs have impedance/transfer functions with trig functions in them, relating electrical length to phase angle). The limiting case of course is the Taylor series expansion of same, but convergence to within some margin of accuracy need not be rapid. At some point, we prefer the 1-D wave solution (i.e., RLC + TL lumped equivalent circuits) instead.Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: March 07, 2024, 05:43:34 am
content: Quote from: A.Z. on March 05, 2024, 08:40:28 pmNope, never!Coax can be useful as a "gimmick" capacitor.

author: tggzzz
author_rank: Super Contributor
timestamp: March 07, 2024, 08:40:57 am
content: Quote from: T3sl4co1l on March 07, 2024, 01:41:43 amPrecisely.For some people it is worth pointing out that you don't have to worry about reflections when a signal's edge is longer than the transmission line. And then emphasising that it is the edge duration not the signal's period.

author: David Hess
author_rank: Super Contributor
timestamp: March 07, 2024, 06:26:19 pm
content: Quote from: T3sl4co1l on March 07, 2024, 01:41:43 amI brought it up because if you have a fast rise time pulse generator, it is something you can see for yourself on a moderately fast oscilloscope.

author: mawyatt
author_rank: Super Contributor
timestamp: March 07, 2024, 07:21:07 pm
content: Recall before ~1960 some research into the leading edge of a pulse propagating at identically the speed of light in a vacuum thru a cable. There was a book on Wave Propagation and Phase Velocity from an IBM researcher that briefly discussed this effect, they were referred as "Forerunners", first and second types.Don't know if this ever became useful, but certainly interesting at the time!!Best,

author: coppercone2
author_rank: Super Contributor
timestamp: March 07, 2024, 09:16:48 pm
content: I want to read that

author: mag_therm
author_rank: Frequent Contributor
timestamp: March 07, 2024, 09:43:37 pm
content: https://www.allaboutcircuits.com/technical-articles/how-to-eliminate-reflection-noise-in-your-next-pcb-design/Some animations relating to pcb track discontinuities.