Re: Common Mode Current Choke Test Rig, Theory Of Operation

Smitty, N6MTS

Are there any ACTUAL RF engineers on the list who can check my intuitive understanding of #4 below?

Andrew:  I've uploaded a new version of the document.

I really like your point on #3 below.  I made the interplay between CMCC and Ground much more clear and explicit.

I'm with you on #4.  I've never had this explained to me, but intuition feels like this is the right explanation (see below), but I'm not comfortable enough with my understanding (yet) to publish it in that document.

My Intuitive (read: might be rubbish) Understanding:
The "50(ish) ohms" of a dipole is not literal resistive impedance between the two leads of the feedline.  It's just the ratio of voltage:current that flows into the antenna and that gets radiated into space.  So, you can think of it as an impedance between the antenna, and space.  It's often called "radiation resistance" when comparing it against literal resistive properties of the wire doing the radiating.

But, if it's an impedance between the antenna and space, then each half of the antenna MIGHT have a DIFFERENT impedance between it and space.  If, for example, one side of the dipole is 50+1j, and the other half of the dipole is 49-2j, then not only do you have a mismatched antenna, you also have a misbalanced antenna: each half has a different radiation impedance.

Figure out what the voltages and currents are in each half of that antenna into their respective radiation impedances.  The difference between each half causes common mode reflection, fixed by CMCC.  The part that's left over after removing the difference causes the differential mode reflection, fixed by a matching network (aka tuner).

We don't want to stop the differential mode current, we want that to flow.  So the only thing we can do is match 50+0j to whatever the differential mode impedance actually is.  (And, for literally the first time as a ham, I think I'm beginning to understand why 450 ohm ladder line deals with mismatched antennas better.)

We don't WANT the common mode current, so I don't mind throwing a big-ass impedance in series with the common mode current, to stop it from flowing.  And that's what the CMCC is doing.

SO!  The point being, if my intuitive understanding is correct
  • Common mode current reflections, the type improved by a CMCC, are caused by the difference in radiation impedance on each half of the antenna.
  • Differential mode current reflections, the type improved by a matching network and detectable as high SWR, are caused by the common radiation impedance to both halves of the antenna, the part that's left over when you remove the difference.
There's almost certainly some vector math in there somewhere to calculate those radiation impedance differences and "commonalities."

Is anyone who is actually trained in this sort of thing reading this?  If so, can you correct me if this is rubbish?


On 7/6/22 12:16 AM, Smitty, N6MTS wrote:
All great feedback, thank you. I’ll apply those changes tomorrow. 

I woke up the cat who’s currently/until recently sleeping on my chest, by full-on snort-laughing at comment number 2. I’ll definitely fix that. 😂


On Jul 5, 2022, at 11:47 PM, Andrew Rodland <andrew@...> wrote:

A few bits of general feedback.

1. "affect" should be "effect" near the end of 2.3.1.

2. "Analyzer", not "Analizer". Don't want to give people the wrong idea.

3. In section 3, I would adjust the nuance of choking vs. grounding just slightly. They're both great! They both make the other work better! A choke increases the impedance of CMC going towards your radio, but grounding the shield gives it a low-impedance path to somewhere harmless. Usually it's easier to add more choke than add more ground, but ideally you want them both to be at least "pretty good", because the overall effect is like a classic voltage divider, only more RF-y.

4. In 3.1.3, there's a kind of advanced topic that skirts really close to this. High SWR is not high CMC, but the myth does come from somewhere! High SWR *usually* (though not always) means high |Z|, and high |Z| means that you need a relatively higher choking impedance for the choke to be effective. Why? Well, I could give three completely different halfway-right explanations, which is another way of saying I don't understand it well enough myself. Nonetheless it's true. Ever try operating an 80m doublet on 40m via an ATU, and keeping RF out of the shack?

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