Recommend me the thinnest coaxial cable but I need the thickest shield. - Page 1
Forum section: electronics / projects

author: Frederico Acardi
author_rank: Contributor
timestamp: August 16, 2021, 11:42:04 am
content: Hello, I am looking for a coaxial cable that can shield against 60Hz (I know it will not be able to block it but the best possible shield).I need a thin version, and I don't care about the solid or standed core. I dont care about the price, but i will not buy overpriced copper tooWhat do you use in your builds ?Thank you.

author: dmills
author_rank: Super Contributor
timestamp: August 16, 2021, 11:47:57 am
content: At 60Hz, twisted pair will generally outperform coax if run as a balanced circuit.Some of the microwave hardlines have very small diameters with a solid copper tube screen, but the skin depth down there is about 8.5mm!

author: Frederico Acardi
author_rank: Contributor
timestamp: August 16, 2021, 11:54:10 am
content: Thank you, I can't run balanced at this place, that is why i wanted to find the best coax.

author: Berni
author_rank: Super Contributor
timestamp: August 16, 2021, 12:07:37 pm
content: You can also get double shielded triaxial cable in pretty small diameters. That way you can use the outer shield to divert most of the noise away, making the inner shield have a easier job.The stuff is pretty rare tho. Most of the triaxial cable you find is the thick kind that is more the size of RG6 and is mostly used in video broadcasting to keep down the noise for long runs. The thin stuff is closer to RG316 in size.

author: Benta
author_rank: Super Contributor
timestamp: August 16, 2021, 12:10:40 pm
content: How thin is "thin"?

author: Frederico Acardi
author_rank: Contributor
timestamp: August 16, 2021, 12:20:47 pm
content: 3 to 3.5mm is max. Thank you.Quote from: Berni on August 16, 2021, 12:07:37 pmThank you I will see what I found about these.What is the difference between double-shielded coax and Triaxal (Double-shield but isolated) if the amount of copper is the same ?

author: Gyro
author_rank: Super Contributor
timestamp: August 16, 2021, 01:43:18 pm
content: You've said what you're trying to shield against (ie. 60Hz), but unless I've missed it, you haven't said what you are trying to put through the coax. That's going to influence cable choice as much as shielding. What frequency, impedance, acceptable attenuation, etc.?

author: Frederico Acardi
author_rank: Contributor
timestamp: August 16, 2021, 01:53:46 pm
content: It's a high impedance signal so it's very sensitive, very close to power transformers.

author: Gyro
author_rank: Super Contributor
timestamp: August 16, 2021, 02:10:01 pm
content: If it's high impedance, then you probably want ptfe or Polyethylene insulated. If subject to vibration then you might want a low noise dielectric (graphite coated) type.EDIT: I know you said that it had to be coax, but don't write off overall screened twisted pair too quickly. This would give you the benefits of an overall electrostatic screen, with twisted signal and signal ground for maximum magnetic field rejection.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 16, 2021, 03:01:09 pm
content: With triax the inner and outer shields are separated. This allows you to connect them to the circuit differently so that induced current in the outer shield does not flow directly into the sensitive common node of your amplifier.Another technique with high impedance signals is to use the inner shield as a guard, driving it with a copy of the signal. This reduces the impact of capacitance and leakage on the signal wire.You still can't really shield 60 Hz magnetic fields. That is why balanced circuits such as twisted pair, twinax, or just two coax lines with complementary signals are used.

author: Benta
author_rank: Super Contributor
timestamp: August 16, 2021, 03:31:28 pm
content: RG-179 coax comes to mind. 2.54 mm Ø, very tightly woven shield and Teflon dielectric.5075 ohms. But not cheap.

author: Frederico Acardi
author_rank: Contributor
timestamp: August 16, 2021, 03:40:21 pm
content: Thanks to all of you.I will try to find the best rg179.Also Gyro how would you connect the shielded twisted pair, one conductor would be signal, the other inner conductor is tied to shield ?But in the meantime, if I find a triax cable, but still connect both shields together, does it provides any advantage against double-shielded coax ?Thank you.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 16, 2021, 03:54:41 pm
content: Excuse my impertinence, but why not just amplify or buffer the signal? I think electronics are cheaper than a high performance cable/connector combo.

author: Berni
author_rank: Super Contributor
timestamp: August 16, 2021, 04:03:01 pm
content: If you connect the two shields together you get the same thing as the two shields being on top of each other. In that case its better to just buy a cable with thick shielding, possible foil+braid shield at the same time.The advantage of having the outer shield separate is that you can divert away unwanted interference to some sort of high power low impedance ground, keeping the inner shield carrying the actual ground signal cleaner. Getting noise on your ground is just as bad as getting noise into your signal. If your ground jumps around by 1mV then the device on the other end will think the signal is jumping around by 1 mV. So perhaps the outer shield might connect on only the driven side to prevent it carrying the noise to the device on the end of it. You might have the outer shield connect the metal case of the two devices together, but the inner shield is carrying the actual ground signal that is allowed to float a few volts in respect to the case ground. In some cases the inner shield is driven with a guard voltage to prevent cable insulation leakage when the signals in question are extremely high impedance such as GigaOhms.All of this is part of engineering, it is not just simply about selecting the "best cable" and calling it a day. It's more about how can you use the cables properties more effectively with the device on the end of it.That being said shielding protects from electrostatic interference. So unless your shielding is a centimeter thick walled iron pipe it won't really do much for magnetic fields. This is like expecting a compass to stop working inside a Faraday cage. So why the heck are people raving about this twisted pair cable? If you can't just stop magnetic fields this easily why would a piece of twirly wire scare them off? Thing is that twisted pair wire has an extremely low loop area between the conductors because the twist makes the loop area swing between positive and negative values, causing it to cancel itself out. And what do magnetic fields need in order to induce a voltage? Loop area! This is the magic bullet that makes them so great for it. Of course they have a weakness against electrostatic fields since the wires are just left exposed, but there are braid shielded twisted pair cables out there too, combining the best of both worlds.

author: Frederico Acardi
author_rank: Contributor
timestamp: August 16, 2021, 04:13:43 pm
content: Thank you I dont want to modify the equipment, but I admit I did not even thing about it. So thank you.

author: Gyro
author_rank: Super Contributor
timestamp: August 16, 2021, 04:28:34 pm
content: Quote from: Frederico Acardi on August 16, 2021, 03:40:21 pmNormally the shield would be connected to chassis of whatever is at each end and signal ground would go from signal source to signal destination ground references. Alternatively, the shield would be connected at one end only, to the lowest impedance ground that you've got (normally the source).Hard to say without more detail of what's going on, and possibly some experimentation for quietest results.

author: bd139
author_rank: Super Contributor
timestamp: August 16, 2021, 04:36:58 pm
content: Convert to low impedance differential signal first

author: Gyro
author_rank: Super Contributor
timestamp: August 16, 2021, 04:40:46 pm
content: Quote from: bd139 on August 16, 2021, 04:36:58 pmPreferably.

author: Damianos
author_rank: Frequent Contributor
timestamp: August 16, 2021, 06:37:37 pm
content: A high impedance signal, near power transformers, that needs coaxial line to connect it and needs shielding against 60 (or 50?) Hz, seems a little vague to me.High impedance and coaxial line or even twisted pair doesn't match well!Depending of the signal properties there may be more than one solutions...If it is an RF signal use high pass filtering, transformers... If it is DC use low pass filtering... If it is coded it may be transmitted optically...What is this mysterious signal? Shape (sinusoid, pulses, level,...), voltage range, impedance, frequency range, relation to ground, distance to transmit it...?

author: Scrts
author_rank: Frequent Contributor
timestamp: August 16, 2021, 06:41:30 pm
content: Look up RG174 type of cables with double shielding and braid. For example LEONI Dacar 462-6

author: jonpaul
author_rank: Super Contributor
timestamp: August 16, 2021, 08:02:39 pm
content: You need to understand the basic of Differential mode vsd common mode noise and the mechanism of coax vs twisted pair balanced.The coax will not help if the culprit is magnetic coupling.Please study the theory of noise coupling and shielding, there are many app nots and text gbooks on this issue.Kind Regards,Jon

author: bdunham7
author_rank: Super Contributor
timestamp: August 16, 2021, 11:22:44 pm
content: If you could tell us exactly what the signal is going through the cable, including what exact equipment you already have (that you don't want to modify) and what issues you are having, we could be more helpful.I'm sort of working on something similar (as far as I can tell) and I need to use a coax for a sensing application (low frequency) and I want to eliminate as much noise and interference as possible. I have similar size restrictions. I found one very promising candidate from Cooner Wire, but they quoted me $17,000 for a spool so that was out. My final three candidates were RG174--the common and cheap one, then LMR-100 and RG180. I'm pretty sure they will be pretty much useless around unshielded power transformers unless the sensing circuitry filters out the 60Hz--which in my case it will. If yours can't, then things will be difficult.

author: Frederico Acardi
author_rank: Contributor
timestamp: August 17, 2021, 08:31:35 am
content: On most forum going into the details deviate from the asked question, that is why I wanted to stick to the topic, but I will give information if it can help.I have and unshielded cable running very close from power transformers, the cable is inside an aluminum chassis (but still picks the 60hz hum from the PT !), and sits right under the transformer left, when I move the cable away the hum gets lower (but not enough) so I thought if I can move it away and shield it, it would be acceptable, but I know 60hz will not go out, but I wanted to try. The signal is preamplifier, going to power-amp. The preamplifier is in another box. I cant add buffers because it will "eat away the pure tone of the tubes" (not my word, its not my equipment).I also wanted to add it need to be thin enough to be slided between the circuit board and the chassis, without de-soldering the board, and I have about 4mm from the side of the board to the chassis.Thank you.

author: bd139
author_rank: Super Contributor
timestamp: August 17, 2021, 02:52:57 pm
content: I would find some better customers

author: Frederico Acardi
author_rank: Contributor
timestamp: August 17, 2021, 03:35:47 pm
content: Too bad he is the friend of my brother in law, almost family to him, always thereAnyways I could not find any store in europe that sell RG179 double shielded, the only one I could find is rs-online, but I dont need 25meters just to test........Do you have any source ?

author: jonpaul
author_rank: Super Contributor
timestamp: August 17, 2021, 04:30:44 pm
content: Hello again: Kindly suggest a bit more research on this old but interesting topic.For an audio system perhaps high end, you can shield the transformers and add a leakage flux band to reduce external magnetic field. Most high end use toroidal power mains trsf.The coax may not help much.Most professional systems use balanced audio on XLR, 600 Ohm, with transformers to solved such issues.On Great engineer and speaker on the topic is Pat Brown of SynAudCom:https://www.prosoundtraining.com/pat-brown-bio/RANE app note:https://www.ranecommercial.com/kb_article.php?article=2107Belden cable apps from Steve LAMPENhttps://www.belden.com/blogs/broadcast/balanced-lines/Jon

author: bdunham7
author_rank: Super Contributor
timestamp: August 17, 2021, 05:41:43 pm
content: Quote from: Frederico Acardi on August 17, 2021, 08:31:35 amOn the other hand, leaving out vital information results in a so-called X-Y problem where someone asks about implementing an improper or ineffective solution to a problem and misses out on wider-ranging ideas that could be helpful.So in your case you have a tube audio system of some sort with a hum problem. You clearly can't use any sort of filtering like you could if it was DC/LF or HF and you don't want to use any sort of buffers, not that they would help any in this case. Your signal is audio-band in the 1VRMSregion. I'm assuming you are using an unbalanced interconnect system where one side of the audio signal is at chassis ground.I recently overhauled an old AM/FM tube radio and I set out to modify it so as to eliminate hum, among other things, by switching to a regulated DC supply. I won't go into all the gory details here since most of what I did would not be practical from your description, but I did manage to eliminate all audible hum--as long as the chassis was on my workbench. Once I restuffed it into the cabinet, I had some hum--interestingly it hums even before the tubes are warm. I have some additional work to do!So here are some potentially useful ideas:First, is the noise a deep, dull 60Hz hum, a sharper 120Hz hum or is it a buzzy/clicky noise that occurs with a 60Hz rate?Second, you can try it, but coax is unlikely to shield much at 60Hz. Still, with these things a lot of it comes down to experimentation and moving thing around and you might get lucky.Third, as has already been suggested, a flux band on the power transformer may be more helpful than anything else. This is especially true if you have the deep, dull 60Hz tone. If it is already in a can, then that won't be feasible.Fourth, even though you can't use any thicker cable, you can try shielding the cable with a metal shield in various locations. Without a photo I can't suggest more. I have small sheet sections of mu-metal from old oscilloscope CRT shields that occasionally come in handy, otherwise you can try some sort of ferrous metal.Fifth--again no photo, but the best thing you could probably do is radically reroute those input wires somehow.You are most likely to solve your issue with a lot of experimentation.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 17, 2021, 07:15:01 pm
content: Audio signal transformers might be acceptable. They will do far more to disturbed the pure sound of the tubes than opamps with nearly unmeasurable distortion but might still be considered acceptable.

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2021, 11:00:02 pm
content: RG-316 is available in a thicker double shielded version so RG-174 probably is also. RG-316 is the Teflon version of RG-174.

author: Frederico Acardi
author_rank: Contributor
timestamp: August 18, 2021, 08:49:17 am
content: Thank you guys !I will try the coax, if it's not good a transformer will be nice idea.I just need a last help, I saw the spec of shield in dB/m. Does it mean a 90 dB/m shield is better than a 60dB/m shield ?Should I go for the highest dBm shield available ? or is it only useful when using the shield as conductor ?Thank you all for the help.

author: EPAIII
author_rank: Super Contributor
timestamp: August 18, 2021, 08:48:35 pm
content: The shield on coaxial cable will block the E component of E-M but will do little to block the M component. You can try coax, but I do not think it will work. I think your best bet would be to install a magnetic shield between the transformer and the cable. Get some sheet steel and try a piece there. A tin can that is actually steel (vegetable or fruit can from the grocery) can make a good shield for the magnetic component.One unasked question here is HOW LONG will this cable be. Any cable, including coax, will have losses. But then, this is audio, so that may not matter much.If this really is audio, and the length of the cable is relatively short (under one meter) then perhaps you can use a shielded AUDIO cable. Microphone cable comes to mind. But there are many, many shielded audio cables. Some have a single inner conductor while others have two for a balanced audio circuit. I have often used a two conductor, shielded audio cable which was already at hand, to run single ended (single conductor) audio. I just connected the two inner wires in parallel. It worked great.If you only need a small length of cable, many consumer devices like microphones and earphones will use shielded, audio cable. You can often buy an inexpensive microphone or other device and just sacrifice it for the cable. I have never been to Spain, but there must be consumer electronic places there.

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2021, 05:21:31 pm
content: Quote from: EPAIII on August 18, 2021, 08:48:35 pmIt works for the same reason twisted pair works to shield against magnetic flux; close proximity of the conductors prevents the flux from getting between them, so it affects both equally.

author: Frederico Acardi
author_rank: Contributor
timestamp: August 20, 2021, 09:26:11 am
content: So if i understand correctly, the best coaxial cable is not a standard one, but actually a twisted pair in a shield ?Like this :

author: Berni
author_rank: Super Contributor
timestamp: August 20, 2021, 09:51:04 am
content: Quote from: Frederico Acardi on August 20, 2021, 09:26:11 amYep this is the kind of cable i suggested at the end of the post all the way back on Reply #13

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2021, 10:41:22 am
content: The coax helps against the magentic component with close proximity and symmetry of the inner and outer conductor. This only works if the shield is also the return wire / ground and no other ground to cause a ground loop. A thinner diameters helps - extra thick shielding is not makting a big differentce at 50/60 Hz.Magnetic shielding would be better at the transformer, helping field to get back in a short way.With a transformer also take into accout mechanical coupling: the transformer will produce hum and cables can reactor like a microphone and convert sound to voltage, expecially with a high impedance signal.

author: Frederico Acardi
author_rank: Contributor
timestamp: August 20, 2021, 10:43:28 am
content: Yes Berni I remembered you reply, that is why I said that.Is 4 conductor twisted better than 2 ? I know it will have more capacitance, but I am talking about EMI only.Thank you.

author: Berni
author_rank: Super Contributor
timestamp: August 20, 2021, 11:09:10 am
content: I don't think 4 conductors would be any better, and such a cable is probably pretty rare.The 2 twisted conductors in a shield is commonly called a "Twinax" or "Twinaxial" cable, you don't quite find it everywhere but it should not be too hard to find it.

author: cvanc
author_rank: Frequent Contributor
timestamp: August 20, 2021, 11:40:40 am
content: LOL, the story is getting more entertainingIn today's episode of WORKSHOP BLUES - "Family matters!"Good luck, man... I've used RG174 for audio and it's great but I've never put it right near a hum source to see what would happen.

author: bdunham7
author_rank: Super Contributor
timestamp: August 20, 2021, 01:53:52 pm
content: Quote from: Frederico Acardi on August 20, 2021, 10:43:28 amCanare Quad microphone cables are highly regarded in the professional audio industry for reducing EMI interference, but they are all a lot fatter than your requirement. If it turns out you can cram them in there somehow, I don't think any cable is going to do a better job of rejecting your power transformer noise. The capacitance shouldn't be an issue for a standard audio interconnect circuit if it adheres to, or is even remotely close, to the standard configuration of impedances.https://www.sweetwater.com/store/detail/StarQWT-200--canare-l-4e6s-star-quad-bulk-microphone-cable-white-200-foot

author: cvanc
author_rank: Frequent Contributor
timestamp: August 20, 2021, 02:12:19 pm
content: Oh yeah, star quad cable! That might be the good way to go here. The smallest star quad I know of is Belden 1804A, it's 2.9mm, does that work?https://catalog.belden.com/techdatam/1804A.pdf

author: bdunham7
author_rank: Super Contributor
timestamp: August 20, 2021, 02:17:48 pm
content: Quote from: cvanc on August 20, 2021, 02:12:19 pmI had no idea they made a version that small. Those conductors must be pretty delicate. That's definitely the winner here for this application.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2021, 01:39:53 am
content: Quote from: Kleinstein on August 20, 2021, 10:41:22 amI sometimes use twisted pair or whatever small coaxial cable I have available to get to a signal on a switching converter for measurement while rejecting interference from leaking magnetic flux. Both seem to work about equally well although coaxial cable is easier to attach to an oscilloscope probe and likely rejects electrostatic interference better.High frequency transformers are sometimes wound with coaxial cable to minimize leakage inductance which is the same problem.

author: spostma
author_rank: Regular Contributor
timestamp: September 14, 2021, 07:38:51 pm
content: maybe you could use some microphone cable?

author: coppercone2
author_rank: Super Contributor
timestamp: September 14, 2021, 08:24:11 pm
content: you can try to use some steel armored cable if you can find it maybe, I am unsure of what it would do, just keep in mind most MC cable is aluminum, you need to find steel type. You can also try to route it through a steel conduit maybe. its not what most people deal withhttps://www.ecmag.com/section/systems/steel-conduit-holds-steadyhttps://www.zoro.com/zoro-select-flexible-steel-conduit-100-ft-l-5502-30-00/i/G0493473/feature-product?utm_source=google&utm_medium=surfaces&utm_campaign=shopping%20feed&utm_content=free%20google%20shopping%20clicks&gclid=Cj0KCQjwkIGKBhCxARIsAINMioI4SU85P2N-IBZZmO3XcFKHULnXKcsZ1z1jUUPm0Htng9vQnZ0pO5kaAtikEALw_wcB

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2021, 08:56:14 pm
content: Quote from: spostma on September 14, 2021, 07:38:51 pmMy first choice for microphone cable is RG-174 unless I need a shielded pair, and RG-174 tends to be tougher than common microphone cable.

author: coppercone2
author_rank: Super Contributor
timestamp: September 14, 2021, 10:06:07 pm
content: how is armored steel cable for shielding anyway?Hard to imagine it inside of equipment but when I think of big bunch of DC switchers in one giant box , it does not seem like such a extravagant solution anymore..thats when I don't like WAGO as much, because inside of a equipment box, you can go for wago distribution, or possibly full steel shield distribution, using wire nuts inside of steel distribution boxes.. all off the shelf and adjustable, vs a box inside of a box with a power distribution WAGO backbone (heavy mechanical engineering), the idea being steel mini boxes inside of aluminum big box

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 14, 2021, 10:12:42 pm
content: if one wants to go down the coax route and shielding is an important factor, the only way are semi-rigids like RG402otherwise in high voltage, high current or strong magnetic fields a fiber optic connection could be the best way to go; simple POF would do just fine without horrendous costs