electrical theory--- newbie question----

[Catbrain]

i am trying to understand how a coax cable works---
I thought I had a basic idea of how electrical circuits work. But I am confused.

Googleing says in a coax--- the core and braid carry the same currents. (ignoring common mode currents)
Googleing also says the braid is a ground potential. so how can any current flow in the braid if it is grounded. ?
I could see it working if the braid floated up and down opposite the core-- like a simple electrical cord feeding a lamp in your house.

By then when I look up why it is referred to as "unbalanced"--- it says the "signal" is carried by the core and the braid isn't carrying anything (seems awfully vague electrically---) If is is an AC current (which it has to be) there must be conservation of current around the loop forming the compete circuit. so both halves must carry the same current and the same "information".......

So what is it that is "unbalanced" ----- and how can the braid carry current if it is grounded ?
what am i missing here ?

Tim (i am studying for the tech test)

 

[k6cpo]

The answer is: IFM... "It's F***ing Magic."

 

[prcguy]

Look at it like 2 conductor lamp cord carrying current to a light bulb even though one of the wires is technically at ground potential (neutral side). Coax is doing the same thing with the braid/shield being one conductor and the center conductor being the other. It gets more complicated when you look at it from an AC or RF perspective where the currents are mostly confined to the outer skin of the center conductor and the inner skin of the braid/shield. The inductance of the center conductor and the capacitance between the center conductor and the inner shield will determine the AC impedance.

 

[Catbrain]

Thank you PRC !

I also asked my EE friend the same q. So-- it sounds like "unbalanced" idea is somehow coming from the fact that the two conductors are trying to have some capacitance and one is huge and the other is tiny- geometrically. (unlike the lamp cord or ladder line) i don't have a good intuitive idea about the electrical ramifications of that unbalance. That may be the "black magic" part that isn't obvious.

He was talking about using twin core coax---- does anybody do that in this field ? if not is it an electrical issue or a cost issue ?

Tim-- North-east TN ( no call sigh yet :-( )

 

[paulears]

The snag is that this kind of thing is always simplified to make it understandable to beginners, but the simplification in turn bends the physics sometimes. The electrician have a similar question when neutral and ground are bonded together in houses - the neutral current shouldn't flow because it's earthed, like the RF example. They then get asked why a meter across the earth and neutral at the other end often ends up with a voltage across them, which implies current is flowing, when it shouldn't!

 

[Catbrain]

thanks Paul------

Is there a long- but complete explanation somewhere to read ?

sometimes I would get physics students in lower level classes ask questions that the answers
required equations that required 2 more semester of calculus to understand. so you would have to simplify your answer and some times that was unsatisfying or foggy--- which is understandable.
but the same was often done to me when I would ask a question from the teacher.

just heard someone from England 2 nights ago on 7 mhz. about 8 -9 pm here eastern standard.
seems like by 10 pm the atmosphere magic mirror vanishes.

 

[Catbrain]

and I don't understand house wiring that way either-- because no one teaches a real explanation of neutrals and grounds here either. Its all simplified to the point it isn't self consistent. or it appears that way----
I guess they figure they just have to know how to do it-- even if they dont understand it fully. Dang- i hate that----

 

[prcguy]

On the balanced vs unbalance part, think of a dipole antenna fed with two conductor open line from a balanced tuner. Both outputs of the tuner would be isolated from ground via capacitive coupling or RF transformer and no part of the feed line or antenna is attached to ground. The antenna system including the feed line and antenna are considered balanced, the feed line conductors will typically be equal physical length and operate independent from any ground or ground return to the signal source. RF currents flowing on the balanced feed line will be equal and opposite at any given point along the balanced feed line.

With a coax fed dipole the shield of the coax will be connected to the radio chassis at some point which might include the microphone cable, speaker cables, power supply wires and chassis, neutral side of the AC and eventually earth ground. The coax feed line will not have equal lengths between conductors due to the shield being connected to other stuff where the center conductor will be more isolated. RF currents may not be equal at any given point along the feed line due to the possibility of currents flowing through the radio chassis, power supply connections and AC neutral wiring. Not to mention a separate ground rod some people attach to their radios.

Or think of the copper telephone lines from the central office to your home telephone if you still have one. Its a two conductor floating circuit all the way with no electrical connection to ground. If one or both telephone line conductors are grounded due to water or mistakes, you will usually hear a terrible hum on your phone line due to the circuit being unbalanced and now currents from power lines and other stuff can ride along with your voice on the phone lines.

Or how about your home stereo system using unbalanced RCA phone plugs for most things that connect to the amplifier. Connect your TV to the stereo and it may hum because the TV is fed by a cable TV coax which is grounded who knows where down the street but certainly at a different ground potential and every stereo and cable TV component will be trying to equalize the grounds between your house ground rod and wherever the CATV cable is grounded and AC current will be flowing on many of the cables including the RCA cables now having that AC ground current getting into the low level audio inputs of the amplifier.

Compare that with a typical professional audio or PA system using balanced XLR or TRS connectors. All connections from microphones, preamplifiers, power amplifiers and everything else will have two conductors carrying the audio signals isolated from ground and usually transformer input with everything floating from ground. It doesn't matter if some equipment is chassis grounded or if the cable TV coax is connected to something in the system because no audio cable conductor is referenced to ground therefore no AC power currents will flow on them trying to equalize AC ground potentials.

I could probably think of more examples but I'll spare you.

 

[WA8ZTZ]

They then get asked why a meter across the earth and neutral at the other end often ends up with a voltage across them, which implies current is flowing, when it shouldn't!

A voltage from neutral to earth (ground) measured downstream at, say, a receptacle is the voltage drop on the neutral conductor.
The neutral and ground are bonded together at the source (service panel).
Therefore, if you connect a voltmeter lead to the neutral (white wire) at a receptacle and the other lead to ground at the same receptacle,
the ground wire (known as the equipment grounding conductor... usually green or bare) acts as an extension to the voltmeter lead.
Remember, the neutral and ground are bonded together at the service panel.
Thus you have the voltmeter connected across the neutral conductor from the source (panel) to the receptacle. If there is current flowing on the neutral conductor, a voltage drop will be developed on it from the panel to the receptacle and the voltmeter will indicate this.

E=I x R; where E is the voltage drop, I is the current on the neutral conductor, R is the resistance of the neutral conductor from the panel to the receptacle.

 

[robertwbob]

more confusion. current flows around outside of wire. so a question arises ,if i install 2 runs of coax,center wire multiple wires,the other solid core hooked to 2 identical antennas to 2 identical radios will 1 outperform the other?
ive been told it dont matter but i stopped using single center wire when i removed a run and used multiple wire center core. my range was a good 20% increase.
ps vswr was low on either coax

 

[paulears]

You are trying to jam together every bit of physics you can 'collect' - but without any context. You are also attributing performance by coincidence. The only thing that matters is the specification.
Look at this specification

Frequency MHz	Attenuation	Unit
50	6.5	dB/100m
200	10.3	dB/100m
400	16.3	dB/100m
550	18.5	dB/100m
750	21.6	dB/100m
850	25.5	dB/100m
1000	29.3	dB/100m

If you see another cable that at 200MHz has 24.7dB/100m, it's awful, correct?
How about another that at 200 has just over 2.5dB/100m?

One of these cables is very flexible and pretty tough, even when flexed quite a bit. One is quite stiff and doesn't like being bent into too sharp a radius, and another has the habit of half failing when bent around sharp corners, maintaining electrical performance but compromising screening. Worse still one is £25 for 100m, while one other is £200 for 100m?

Stranded vs solid is another common confusion area. I also suspect you've been reading up on skin effect, hence the mention about current on the surface. In AC circuits, of course it exists and increases in importance as frequencies get higher, but doe it really impact on lossy cables below 1GHz in any practical, measurable way to the average person? I doubt it. I don't have anything remotely accurate enough to measure it. It's clearly a real thing, but in the context we are talking about here, a bit of a red herring.

Some cable designs are inherently lower loss than others. Just run with that. We have said it before. If RG6 was the 'best' cable because it had low loss, then everybody would use it, and use impedance conversion for the antenna and radio end of the run - but they don't. System designers look at the system, calculate signal strengths, determine the cable loss, the antenna gain and the receiver sensitivity and work out what cable to use. RG6, taken up when we started doing home satellite, was just a simple to manufacture, low in copper cost cable that did the trick. It doesn't have any particular magic.

 

[robertwbob]

You are trying to jam together every bit of physics you can 'collect' - but without any context. You are also attributing performance by coincidence. The only thing that matters is the specification.
Look at this specification

Frequency MHz	Attenuation	Unit
50	6.5	dB/100m
200	10.3	dB/100m
400	16.3	dB/100m
550	18.5	dB/100m
750	21.6	dB/100m
850	25.5	dB/100m
1000	29.3	dB/100m

If you see another cable that at 200MHz has 24.7dB/100m, it's awful, correct?
How about another that at 200 has just over 2.5dB/100m?

One of these cables is very flexible and pretty tough, even when flexed quite a bit. One is quite stiff and doesn't like being bent into too sharp a radius, and another has the habit of half failing when bent around sharp corners, maintaining electrical performance but compromising screening. Worse still one is £25 for 100m, while one other is £200 for 100m?

Stranded vs solid is another common confusion area. I also suspect you've been reading up on skin effect, hence the mention about current on the surface. In AC circuits, of course it exists and increases in importance as frequencies get higher, but doe it really impact on lossy cables below 1GHz in any practical, measurable way to the average person? I doubt it. I don't have anything remotely accurate enough to measure it. It's clearly a real thing, but in the context we are talking about here, a bit of a red herring.

Some cable designs are inherently lower loss than others. Just run with that. We have said it before. If RG6 was the 'best' cable because it had low loss, then everybody would use it, and use impedance conversion for the antenna and radio end of the run - but they don't. System designers look at the system, calculate signal strengths, determine the cable loss, the antenna gain and the receiver sensitivity and work out what cable to use. RG6, taken up when we started doing home satellite, was just a simple to manufacture, low in copper cost cable that did the trick. It doesn't have any particular magic.

Well you said no way to measure it..im staying with multiple wire for current flow because of more surface in 6 small wires than 1 big solid wire.
But there is questions n doubts.
Add this in copper carrys more current flow than aluminum. Thus copper coated aluminum coax center conducter.
Braided wire best ground wire.stands to reason braided wire or twist as in center wire should work on edge of better.but i just know what i experinced with both coax.
But 1 thing that dont help much is
recycled copper. It has particles of other metals i question. You solder some wire its harder needs more heat to get a bond. And i worked at Eagle Pitcher soldering space craft batterys. We seen dirty copper we rejected it because it took too much heat to bond

 

[paulears]

No I said there is no way most of us have the equipment top measure it. Stranded vs solid for most people is simply to do with flexibility and failure likelihood. For years, the video industry have been solid conductor based - 75Ohm solid core for 12G-SDI. Just a different application. Frequency response is key - and nobody there is remotely talking about skin effect or the other things that are worrying you. You haven't even thought about the layers. centre conductor, dielectric, then braid then foil, or foil then braid?

You say you are staying with stranded (nobody calls it multiple wires) because of the surface area? You cannot measure that kind of difference without lab gear! It's just so small as to be for all practical purposes pointless. Power companies use aluminium for cost reasons, it impacts on current capability sure, but their reasons are practical ones. The hifi folk trying too use gold for audio quality vs the power companies using cheaper aluminium. A few feet of gold vs a few miles of aluminium. Perspective and context rule.

 

[MUTNAV]

Pretty sure skin effect IS a factor at DC. (especially at high voltages).

Thanks
Joel

 

[robertwbob]

No I said there is no way most of us have the equipment top measure it. Stranded vs solid for most people is simply to do with flexibility and failure likelihood. For years, the video industry have been solid conductor based - 75Ohm solid core for 12G-SDI. Just a different application. Frequency response is key - and nobody there is remotely talking about skin effect or the other things that are worrying you. You haven't even thought about the layers. centre conductor, dielectric, then braid then foil, or foil then braid?

You say you are staying with stranded (nobody calls it multiple wires) because of the surface area? You cannot measure that kind of difference without lab gear! It's just so small as to be for all practical purposes pointless. Power companies use aluminium for cost reasons, it impacts on current capability sure, but their reasons are practical ones. The hifi folk trying too use gold for audio quality vs the power companies using cheaper aluminium. A few feet of gold vs a few miles of aluminium. Perspective and context rule.

Lmr 400 is what you just described about lectric companys using aluminum .thats why im using stranded center wire coax.it delivers more power.
Lmr 400 is what i sent to junk.

 

[k7ng]

Lmr 400 is what you just described about lectric companys using aluminum .thats why im using stranded center wire coax.it delivers more power.
Lmr 400 is what i sent to junk.

Eh?
I've put a lot of LDF (solid center conductor) coax up towers and it seems to last decades if properly supported.

 

[robertwbob]

Eh?
I've put a lot of LDF (solid center conductor) coax up towers and it seems to last decades if properly supported.

ive had diffrent brands of it and never seen decent signal.dug out old coax from the 70s and was like nite n day diffrence.only thing i can say for it,its cheep

 

[paulears]

Im not sure that skin effect actually makes any sense when we are talking about stranded vs solid core coax. As stranded cable is designed for flexibility and bending radius, the fact that it is in physical contact with the other strands ruins the maths and the science. The op collected data on single core insulated cable and transferred it to the inner stranded single core of a coax cable. These are effectively a single conductor not multiple ones. Performance is different of course but within limits, does it matter for most uses? SDI video, especially 12G, down 50 instead of 75Ohm cable doesnt work very well at all, but RF wise, all we really worry about is loss, not rounding off the edges of square waves!

 

[prcguy]

Im not sure that skin effect actually makes any sense when we are talking about stranded vs solid core coax. As stranded cable is designed for flexibility and bending radius, the fact that it is in physical contact with the other strands ruins the maths and the science. The op collected data on single core insulated cable and transferred it to the inner stranded single core of a coax cable. These are effectively a single conductor not multiple ones. Performance is different of course but within limits, does it matter for most uses? SDI video, especially 12G, down 50 instead of 75Ohm cable doesnt work very well at all, but RF wise, all we really worry about is loss, not rounding off the edges of square waves!

SDI video equipment is designed for 75 ohm cable and if the equipment was designed and matched to 50 ohm cable it would work about the same.

 

[paulears]

Yep - absolutely, but it's not. Just so easy to swap the RF ones from radio mics with the video ones.