Common-mode currents, again

[lars]

Hi folks

I've been asking about antenna problems on other threads, and the subject of common-mode current comes up from time to time. It seems that, with coaxial cable, it is possible to have a situation where there is an instantaneous net flow of current either towards the antenna, or away from it, rather than equal and opposite currents in the core and screen of the coax, which would be preferable.

Fair enough, but...

1. Where do these common-mode currents actually go? They can't just vanish into space, right? They've got to get back to the transmitter somehow. But how?
2. I've heard it said that common-mode currents flow in the outside of the coax braid, while balanced, differential currents flow nearer the dielectric. Maybe so, but why? I'm prepared to believe that at RF there can be opposing current flows in different parts of the braid -- although the physics of this situation eludes me -- but why would this problem affect the braid, and not the core? And where does this current go when it reaches the end of the line?

Comments welcome.

Lars.

[m0lsx]

It is not uncommon to hear of people getting slight shocks on their lips from microphones. But the more common result of common mode is to change the VSWR as the microphone is touched or as due to what it runs close to etc.
If you think about how the PL-259 is fitted, the braid goes to threaded part, the Chassis. So the Common mode goes to earth.
Coax is not balanced feedline. The flow on the braid & in the centre core are not equal & opposite. 50 Ohms is achieved by separating the core & the braid at a set distance from each other, normally in most coaxes using plastic. Thus bend them too tightly & you could alter the impedance by damaging that insulator.
Common mode is another form of power being returned to the transmitter.
There are some very very short HF antennas which some people swear by. A 180 meter antenna is about 3 feet tall But put an RF choke on the coax & the antennas tend to catch fire.
My logic would be that Common mode effects the braid not the centre core, as one you cannot have two different currents running in different directions & because the braid goes to chassis & thus is the earth & thats what current always tries to do, take the easiest path to earth. But that would be a guess, as I have never really thought to much about common mode.

[G4RMT]

Oddly, you can have different current paths running at the same time in the same conductor - both AC ones and DC one. It's one of those 'magic' things.

I remember back in school (a long time ago) that the really simply circuit stuff we were doing - battery, cable, lamp, back to battery could share a common piece of cable. You could even swap the polarity of one of the batteries and it still worked.

Lars is quite right about these currents having to go somewhere and they do have to have the return path - but this needn't be a chunk of metal, like a wire. It's a bit tricky to explain - but the concept is important - my work isn't with RF it's with entertainment - and we have grief with getting data to things like lights. We stuff data down a piece of coax, and often the lights will go berserk - spinning around, flashing and doing very odd and totally unwanted things. The problem is reflections. The data (in a current stream) goes down the cable, gets to the very last socket, as the data travels in and out of each light - and has nowhere to go - the paths suddenly stops. (similar to aerials, in a way). Instead of stopping, because the coax we use has an impedance (110Ohms, rather than radio's 50), it acts like a mirror and reflects the current back to the source. In our case, this messes up the data - the lights don't know that they are getting two arrivals of data, and get confused. Our solution is very, very simple. We terminate the cable with a simple 110 Ohm resistor. The mirror effect vanishes, and the lights behave. The cable behaves like a transmission line.

In RF terms a length of coax cable with nothing on the end is called a 'stub'. They have properties - capacitive or inductive, depending on the length of the stub - related to the wavelength. We use stubs all the time in radio - you can short the furthest end, or leave it open depending on what you want to do. They're great for making interference filters, or notching out a very strong local signal if you are a scanner user, for example. In operation, the current IS on the inner and outer conductor, but the inner one is shielded by the outer, hence why the outer one does more damage if it radiates.

The other thing to remember about current is that it's linked to voltage by Ohm's law. As current increases, voltage goes down and vice versa. We're dealing with AC so with any aerial system some parts will be carrying low current with high voltage, and other parts will be low voltage with high current. People do talk about things catching fire, as Alan says. If your aerial has been shortened using coils and other tricks, then one part of it can easily be at a very high voltage - remember the early days spark transmitters that got as far as America? You can get a burn from some aerial designs. Other designs need very thick aerial components because in these ones, the current can be high.

Last thing is to remember that transmission lines don't need to use cable at all. Radar uses waveguide. A square or rectangular section piece of metal. Nothing inside, and the RF bounces it's way along the inside (like optical fibre).

The circuit in the question always exists, or current can't flow - exactly how it returns is the clever bit.

The one thing that has always caused me problems is this notion of how they go both ways at the same time - it seems very wrong, and worse still, the old examples of how electrons work in cables seem to fail. I remember it being explained to me by stuffing marbles into a tube - stuff one in one end and another falls out the other end. Works great with DC, but falls over when we talk RF and reflections!

[essexRXer]

I don't know if this is of any relevance, but I got shocked a few times when running new coax to my TV sets in the house. I think it was down to double insulation because all sets are Class II

[G4RMT]

Oddly, that's quite a common one, I've done the same myself! I've never investigated but the problem I think can be caused by many of the distribution units sticking volts up the spout to power the external pre-amp that is sometimes fitted - these never have a separate power cable. When you attach them, you short the coax down to earth, there's a little spark. Next time, I'm stick a meter on it. Could be just changing up from a capacitor in the power supply, so no real current - or I guess it could be a real fault?

[m0lsx]

Could it not be the fact that Coax is a capacitor? And Capacitors can do this? I know the voltage needs to appear from some where, but would we treat other capacitors like we treat Coax?

[G4RMT]

Never thought about that - but I wonder of you could charge up a length of coax? It certain has the properties, but interesting thought?

[lars]

Thanks. I think this statement sort-of gets to the heart of where I don't understand:

"If you think about how the PL-259 is fitted, the braid goes to threaded part, the Chassis. So the Common mode goes to earth."

Is a coaxial feed considered unbalanced only because, for safety or EMC reasons, the screen is conventionally grounded? Or is it because of the geometry of the cable? While accept that the current _density_ in the core and screen might be different, even in balance, the total current in ideal situations should be equal and opposite in the core and the screen, no? So what is actually unbalanced?

[m0lsx]

Is a coaxial feed considered unbalanced only because, for safety or EMC reasons, the screen is conventionally grounded? Or is it because of the geometry of the cable? While accept that the current _density_ in the core and screen might be different, even in balance, the total current in ideal situations should be equal and opposite in the core and the screen, no? So what is actually unbalanced?

Coax is not balanced feeder, in that there is not an equal & opposite in the centre core & the outer braid. Where there is in Balanced feedline.

[lars]

I think I have to disagree with this, albeit reluctantly...

If the currents in the core and screen are not equal and opposite, then surely that means there is a net flow of current along the feed (in one direction or the other)? But a net flow of current along the feed is, by definition, a common-mode current, and isn't that exactly what we want to avoid?

My point is that any difference between the total core and screen currents _must_ be common-mode, because a common-mode current is defined to be the surplus current in a pair of conductors that are assumed to carry balanced currents. If the core and screen currents are not assumed to be balanced, even in ideal circumstances, then I don't think even the concept of "common-mode" has any validity.

I accept, however, that the term common-mode current might be used in a different way in RF to what I am used to. But if a common-mode current in RF _isn't_ a net flow of current along the feed, then how is it defined?