| Electronics > Beginners |
| Difference between coax cable and shielded cable |
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| Berni:
Well in terms of the shield braiding there is a ton of difference between nice high quality cables and cheap crap. Some are nice thick tightly wound ones on the nice cables or sometimes there is essentially a few strands of it wrapped around with gaping holes in between, some include a foil under the braid some not etc... There are also the double shielded triaxial cables for when you really want to shield things well and proper. But yeah for single conductor shielded cables id say the only reason its not called a coax cable is that they are not built according to any coax cable standard that prescribes the dimensions, materials and of course the all important characteristic impedance in Ohms. If a cable can't be trusted to be 50 Ohm (or whatever else you need) then you can't use it as RF cable. But you could certainly use real RF coax as a single conductor shielded cable if you wanted. Another extreme example of such cables are the cables used to connect the 3 phase motor drive to the BLDC motor in electric vehicles. They are huge 50mm2 copper and up wires that are surrounded by a metal shield to keep the wires radiating out all the motor controller switching noise and causing interference. |
| tkamiya:
I wonder if anyone with VNA would be willing to characterize a good quality shielded 1 conductor cable....? Then compare it against say RG174. Unfortunately, this is well outside of my capability. |
| Berni:
--- Quote from: tkamiya on October 16, 2019, 06:28:42 pm ---I wonder if anyone with VNA would be willing to characterize a good quality shielded 1 conductor cable....? Then compare it against say RG174. Unfortunately, this is well outside of my capability. --- End quote --- You probably wouldn't see much interesting. As long as its good even cable you would get a RF cable with a weird impedance of like 63.8 Ohm. It might have more loss or less loss than RG174 depending on what type of dielectric is used inside. The weird impedance would make it difficult to use for RF applications because most RF things are designed to have certain commonly used impedance such as 50 Ohm or 75 Ohm. So in order to drive the cable you would need a impedance matching transformer that takes 50 Ohm and boosts the voltage slightly to drive the 63.8 Ohm cable and on the receiving end you need another one of those transformer but connected in reverse so that it drops the impedance from 63.8 Ohm back down to 50 Ohm. If you don't do impedance matching and connect it directly then RF power will bounce back when it enters the cable so not all of the RF power will even make it into it, and once it reaches the end of the cable and hits 50 Ohm again more RF power will reflect back instead of going into the load. This reflection can happen multiple times making the signal bounce around, getting in phase in some spots and adding or out of phase and subtracting, as a result making a cable with a very wibbly wobly freqency response. |
| vk6zgo:
--- Quote from: T3sl4co1l on October 16, 2019, 08:28:58 am --- --- Quote from: tkamiya on October 16, 2019, 05:35:38 am ---I see.... that's very true. OK, then what's the difference between single center conductor shielded cable and a coax? --- End quote --- Who even makes such a thing yet does not market it as coax? You could have one-conductor cable made up with foil screen and/or braid just like a multiconductor cable, but this would only have the effect of cheapening the insulation (e.g., using crummy old PVC instead of low loss PE or PTFE) and worsening the shield resistance or effectiveness, and probably giving poorer impedance (but that's already understood given the lossy PVC insulation). :) Related note, I've heard of ferrite-loaded hookup wire, which gives reasonable attenuation (filtering -- absorption) at high frequencies. It's a distributed ferrite bead. Really oddball, mil spec I think. No electric-field shielding (as you get from a foil or braid shield), nor magnetic shielding beyond the surface layer. Tim --- End quote --- Single conductor shielded cable used to be common, but maybe, not so much now. That "single" conductor is not concentric with the shield, as in coaxial cable, & the cable's impedance is not well specified.. In many cases, if twin conductor shielded cable is available, people will use it, instead, either ignoring one conductor, or paralleling the two. Alternatively, they may use thin coax, just because it is widely available & durable, without actually needing the special characteristics coaxial cable brings. The connectors commonly used with,"single ended" audio are not the most rugged, either.. An example:- When I worked at the TV Studio, we had some "standalone" TV projectors, which we used to make available, free of charge, along with a VCR, to various community groups, as part of our commitment to being "good corporate citizens". These needed one BNC to BNC video cable, & two RCA to RCA audio cables. The former was easily found, but the latter was not normally used in the Studio, where audio was normally balanced, not "single ended". We, perforce, had to "make them up". The store had twin audio cable, & some "Consumer grade" RCAs, so we would do so. One problem was that the "twin" cable was quite a bit more bulky than the cable the RCAs were intended for, & after one or two outings, the latter would fail, & need to be replaced. They also needed to be specially stored, so they wouldn't be lost, something which happened regularly, so we would need to make new ones, all for no profit to the company! Eventually, we'd "had enough", so we fitted BNC to RCA adaptors to the VCR & projector. Now we only needed to provide three BNC to BNC coax cables. |
| Berni:
Nothing wrong with replacing RCA with BNC. The RCA cables used for composite video are 75 Ohm and the BNC cables found in a TV studio are also going to be the 75 Ohm kind. There is actually a problematic combination of 75 Ohm and 50 Ohm BNC connectors. I forgot what way around it is but i think plugging a 75 Ohm male into a 50 Ohm female BNC can damage one of the connectors because some of the dielectric inside them gets in the way. |
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