Common-mode currents, again

Forum for CB, PMR, Ham, Business users, GMRS, Repeaters and all other TX Chat

Re: Common-mode currents, again

Postby G4RMT » Wed Feb 15, 2017 9:19 pm

This subject always gets into trouble every time it's discussed on the net. I suspect the problem is that there is always a notion of a complete circuit to enable current flow, but with coax, which by nature is unbalanced, the problem is current on the outside of the shield. The circuit would normally be on the inside - an important distinction, so one paths is on the outer of the centre conductor, with the return on the inner of the shield. This concept of RF current travelling on the surface rather than the cross sectional area of the conductor (as in DC, and very low frequency AC) is the one that the Hi-Fi brigade use to bolster up their speaker cable claims - when skin effect only happens in HF and above RF systems.

It gets even more confused by VSWR measurements - where everyone accepts the concept of reflected power, as in travelling the other way- which we spend ages reducing. My own viewpoint on common-mode current is that it's what's left after the return within the cable. I don't think that's a very scientific explanation, but in my head , it works for all the usual uses of common mode as a term. We have the usual circular out and back current flow within the feeder coax, but because of the area imbalance, the surplus is carried on the outside - and we need to block it with one of the usual tools - like ferrite chokes.

I've read some of the papers on the subject, but frankly the level of the maths is way above me - so I am left with simplifications that I can live with.

I'm not sure we're really able to discuss this sensibly, as we all have our own simplifications, and this is what clash. The real science doesn't, but I'm not up to understanding it - yet.
G4RMT
 
Posts: 1105
Joined: Sat Feb 07, 2015 5:45 pm

Re: Common-mode currents, again

Postby lars » Thu Feb 16, 2017 10:28 am

I don't think that's an over-simplification:

"My own viewpoint on common-mode current is that it's what's left after the return within the cable. I don't think that's a very scientific explanation, but in my head , it works for all the usual uses of common mode as a term."

In fact, my understanding is that this is the very definition of common-mode current. If you have a feed that is constrained to be balanced at the source end (the transceiver in this case), then you ought to have (say) at some instant in time 1 amp out and 1 amp back. If it turns you have 1.5 amps out and 0.5 amp back at that same instant, then then you have 0.5 amps common-mode current (away from the source, in this case) impressed on the 1 amp differential current. Of course, all these currents are time-varying, so it's awkward to talk about some particular current value.

But having a definition of something doesn't explain how it arises -- that's where I get stuck.

People keep telling my that I need a balun to connect my coax feed to a dipole (actually, not everybody says that, but most people do). But the function of the balun remains obscure to me. As I said, I don't understand why a coax feed is "unbalanced" in the sense I understand that term -- in the sense that, in ideal conditions, there is the same current flowing out as flowing back. Typically the balun has some impedance-matching properties _in addition to_ any "balancing" properties it might have; so how can we figure out what the balun is really doing here?

I've seen people demonstrate (on web sites) that connecting coax directly to a dipole creates a situation in which the coax radiates RF energy, which is typically deemed to be a Bad Thing and the result of the unbalanced nature of the feed. However, has anybody tried replacing the coax with a balanced feeder that has the same degree of impedance mismatch as the coax would have? That would help to understand whether the problems are really caused by the balanced/unbalanced thing, or by an impedance mismatch that the balun incidentally corrects.

Incidentally, FWIW, I have a PhD in physics. Math doesn't frighten me in the slightest. But I've often wondered whether the kind of math you can really do with antennas has any bearing on what goes on in the real world. It's a bit like trying to explain chemical reactions in terms of quantum theory -- it's possible, in principle, but for any chemical system much more complicated than a hydrogen molecule, we simply don't have the mathematical techniques to handle the complexity. I suspect that the same sort of problem applies to practical antennas.
lars
 
Posts: 142
Joined: Fri Jun 10, 2016 9:03 am

Re: Common-mode currents, again

Postby G4RMT » Fri Feb 17, 2017 8:29 am

If your maths is that good then there are plenty of papers on the subject, but the thing that really gets me thinking is that for this all to make sense, I think you must get used to the notion that the 'circuit' needed for current to flow does not always have to be a piece of cable - after all, if it radiates, there too a circuit is in operation.

The balun thing is thankfully simpler. You can indeed feed a dipole with balanced cable - remember the stereos that were popular in the 70s and 80s? They all came with a bit of ribbon feeder with a dipole for 100MHz at one end, then a length of the same material as the feeder and two spade terminals. The receiver had two screw down connectors for it. Balanced all the way to the receiver - and then inside the receiver was the balun to match it to the receiver front end, which needed an unbalanced signal. Most had 75Ohm inputs on the rear too.

In my entertainment work we have almost similar things - but with audio. We have unbalanced (and in this case high impedance) equipment and we have microphone, which are balanced and low impedance. To connect the things together generally a transformer is the obvious choice and does the balanced to unbalanced conversion. In my video work, it's now common to stuff video down Cat5 or Cat 6 cable - which is balanced and performs very well compared to coax cable - again, to do this conversion we use a balun. Dipoles, like microphones are naturally balanced devices. Coax isn't, and for as long as I remember, a balun was included in every TV aerial connection box - a thin finger of metal, with some wires wrapped around it. Doing the essential conversion for a proper path to the receiver.
G4RMT
 
Posts: 1105
Joined: Sat Feb 07, 2015 5:45 pm

Re: Common-mode currents, again

Postby lars » Fri Feb 17, 2017 10:44 am

One thing I've learned from my 30-odd years in the electronics/computing industry is that we tend to do things because we've always done them :) I've also discovered that even after 30 years I can do reasonable work without fully understanding a lot of what I do. I don't understand, for example, how we can get megabits per second data transfer down a twisted pair of wires, when I've seen very thorough theoretical analysis that proves that this is impossible. None of that stops me using the Internet, however.

I'm aware that AC currents can be carried in space, and that this gets easier as frequencies increase. After all, a simple capacitor is a great insulator at DC, having no conductive path. How current gets from one limb of a dipole to the other is non-straightforward (at least to me), but you can use Maxwell's field equations to show that this happens, and even predict the impedance, etc -- in simple cases. I strongly suspect that anything more complicated than an ideal dipole is impossible to handle using purely theoretical techniques, and we need computer modelling and such-like. My feeling is that if there _were_ manageable theoretical interpretations of real antenna configurations, there would not be so much uncertainty in this subject area.

Whatever happens at AC, however, the inescapable fact is that our transmitters are powered at DC. With a battery, there's no way to take more current out of one terminal than you put back in the other, not even for an instant. That means that no current can "leave" the antenna, unless an equal and opposite current re-enters the equipment from somewhere. And that in turn means that no common-mode current can flow down the feedline, away from the source, unless there is some way for it to return. That way might be through electromagnetic coupling -- there need not be a DC current path -- but it has to exist somewhere, somehow. If it doesn't, you end up with the impossibility of unbalanced _DC_ currents in the supply.

It may be that using coax to feed a dipole makes common-mode problems worse than using a balanced feeder; but I'm not at all convinced that describing the coax as "unbalanced" is an explanation for this. There has to be more going on; I just don't understand what it is.
lars
 
Posts: 142
Joined: Fri Jun 10, 2016 9:03 am

Re: Common-mode currents, again

Postby m0lsx » Fri Feb 17, 2017 11:07 am

G4RMT wrote: I think you must get used to the notion that the 'circuit' needed for current to flow does not always have to be a piece of cable - after all, if it radiates, there too a circuit is in operation.



I often use a light bulb to show a circuit in action. I then remove the bulb holder, connect two lengths of wire to make the bulb the centre of a cut to length dipole, key up my 70cm handheld & show polarity in action too.
I then fold the wire & show wavelength in action, as the bulb dims & goes out as the wavelength changes.
Buy a database from Kimmy JS19 via http://ukscanningdirectory.co.uk/
Or do Google search of this forum via https://www.google.com/cse/home?cx=partner-pub-6291336405621919:2662881632
73 De Alan (M0LSX.)
http://www.qrz.com/db/M0LSX"
User avatar
m0lsx
 
Posts: 5368
Joined: Wed Nov 21, 2012 9:14 pm
Location: Norwich. TG21.

Re: Common-mode currents, again

Postby G4RMT » Sat Feb 18, 2017 5:08 pm

Never thought to do that one! Cheers.
G4RMT
 
Posts: 1105
Joined: Sat Feb 07, 2015 5:45 pm

Previous

Return to The TX'ers Forum

Who is online

Users browsing this forum: No registered users and 1 guest