Common Mode Current, Where does it come from & how to fix it? (013b)
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- čas přidán 4. 07. 2024
- In the first video we understood what common mode current really is.
In this video I explore where it comes from in practical terms. I each case I answer the question, "What can I do about it?"
00:05 Introduction Sources of Common-Mode Current & What can we do about it?
00:21 SOURCE #1: Feeding a Balanced Antenna with an Unbalanced Feedline
01:37 HOW DO YOU FIX THAT?
02:45 SOURCE #2: Mismatched Antenna Leg Impedances
04:12 HOW DO YOU FIX THAT?
05:36 SOURCE #3: R.F. Radiated from an Antenna
07:07 HOW DO YOU FIX THIS?
08:15 SOURCE #4: Common-Mode Current in an adjacent Feedline
09:18 HOW DO YOU FIX THIS?
09:56 IMPORTANT NOTE REGARDING THE OUTER SKIN OF YOUR COAX: It acts like a random length antenna!
11:20 ---chokes teaser
11:31 Final Comments and Toodle-oots!
Ah, it's becoming more clear. I need to watch the whole series of videos. Thanks again for making them.
You are very welcome! 🙂
Very nicely presented. Thank you.
Thank you! I'm glad you found it helpful! 🙂
excellent Ralph. I liked how visuals were superimposed on to the video. Very professional .
How'd you like the "unbalanced" feedline easter egg at the beginning? (I couldn't resist - every time I reviewed past that part of the video, that's what went through my head.)
@@eie_for_you Ralph, you captured my expression when my lips once touched an old D104 with rf on it :)
Awesome video, thank you sir! 73s
Thanks, man! You are very welcome! 🙂
👍
The coax is only ground referenced at its origination at the transmitter. Past that, it's a transmission line, and as such will have a voltage. The antenna has to be a circuit, which means current that flows out on the center conductor must have a return path. That means during operation the coax shield has *different* voltages with respect to ground at various points and times. It cannot be ground along the transmission line because there cannot be a voltage difference between two ground points. So it's an error to say the outside of the coax outer conductor is GND where it couples with the antenna.
The common mode current arises because the coax is actually *three* conductors, coax inner, inside of coax outer, and outside of coax outer. If return currents travel on the outside, they do not balance the fields inside the coax, and we get radiation from the coax, distortion of the antenna pattern, changes to impedance, dogs and cats living together. If we choke the outside, return currents *must* pass on the inside of the outer conductor, we have field cancellation, no radiation from the feed line, and the antenna remains balanced.
Well, all I can say here is that a **LOT** of research went into the creation of this video/presentation. These are not my own thoughts as emanating from me alone. They are a compilation of the knowledge of a *significant* number of *reliable* sources.
@@eie_for_you I agree with many of your conclusions, and have really enjoyed the series. Please don't think I'm trying to be rude here -- just wanted to weigh in with the common mode explanation that has proven more useful to me in this journey.
And it is corroborated by another list of "reliable" sources too, of course, just to make everything murkier! It's extra fun when two respected experts disagree on something and call the other one an idiot :-).
Anyway, thanks for putting these videos together; I've picked up a few great nuggets from your presentations, and have been working through your channel.
@@yakovdavidovich7943 First, I apologize for for my somewhat curt initial response. No excuses on my part. Just understand that it had been "one of those days" and open minded communication on my part was significantly impeded because of it.
On second reading, with sanity having returned, I really appreciate what you had to say it makes perfect sense to me. I like the "dogs and cats living together" part. It made me smile!
Always feel free to share your observations, insights and experience with me. I really *DO* welcome it! 🙂
Transmission lines are indeed strange beasts (often hard to tame). If a coax feed happens to be about 1/2 wavelength long at the frequency of the current it carries, and is at ground at one end, then the other end-where the center conductor emerges-will essentially be at both DC and RF ground potential. If at 1/4 wavelength, it will essentially be at DC ground potential and something like an RF open circuit (infinite Z). For everything in between, well… take a guess. I love that you definitively explained how outer shield current is not strictly common mode current. Got my EE degree in 1971 and never could square coax cable behavior with the definition of common mode current, etc. Thought I was just not getting it. Thanks. Millions of dollars are spent preventing and eliminating common mode noise that enters digital and other precision equipment through the incoming AC power connection. Great topic for another video.