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Well done Sir.

I had similar issues with the loop in my front yard but yielded better results with the loop in the backyard. I suspect it's related to the underground hydro and CATV running under the front of the property.

I really don't see the point of your segment at 18:34. You essentially only re-did your same previous calculation backwards, except with a bit of rounding error stemming from initial truncation of the result. I speculate that maybe you got confused, and actually wanted to find the value of what L should be if inserting the measured tank frequency, 1.344Mhz, rather than the calculated, 1.358. Doing that, I got a calculated L of 15.11uH. Otherwise, cool video on using nanovna to inspect component values and resonances. And I solved the above using the exact same calculator :) Funny enough your alligator readings got 2000khz closer to the tank measured frequency.

Good job!

Very good video

old fashioned, I use a desktop windows 11 to watch youtube. Dont tell me you can zoom using youtube - if you can I had no idea you could.

My view is that connecting coax to a balanced antenna is bad practice and should only use a balanced Z match using open wire line and there is no such thing as a ATU its a impedance matching device used to cancel capacative and inductive reactance. Using a so called ATU in the HF rig only cancel the reactance at the so 239 socket and does not tune the antenna to resonance as a lot of people seem to think.

Great video. It is especially difficult to imagine how much is 5/16 or 15/32 or similar. it is much easier to have millimeters, it is easier to visualize and compare

A balun is important when transmitted. If no balun is present power may be reflected and run on the outside of,the coax cable. It may transmit on its whole length, and you may feel rf when you touch the cable. Even if you send only a few watts, the little power on the outside of the coax is not useful and it is a result of a mismatch (balanced versus unbalanced) between the cable and the antenna .

we cant actually read the screen. on all your excellent videos the camera is too far away.

Hi, well done video. My question: what about initial calibration and other ones during the next steps of measures?

I thought that the length of a coax feed line had been shown to be irrelevant.

Great video. I have known of the impedance transformation using a transformer, but this is the first time I have ‘seen’ it in action. The NanoVNA “sees” the 300 ohm load as 43.6 ohm load.

Sounds like it said the cause of the mismatch causes TEM and common mode current. Mismatch is a result of radiator imbalance aka root cause. This is what I would expect of an end-fed antenna that is not an exact halfwave (adjusted for the usual velocity factor, etc).

Good presentation! What would have been a nice addition, with the displayed common mode current, place a ferrite bead(s) on the coax to see if any effect AND run the bead(s) up and down the coax to see if there are optimun points of rejection (if any?). Since you are at 100mhz, the feedline is probably in excess of 1/2 lambda so would be interesting to see if there are other bead placements that replicate the feedpoint?

Many thanks for doing the experiments for us. The key takeaway from this video are : 1. Antenna feed point impedance mismatch doesn't cause common mode current in the outer surface of coax shield. 2. A line imbalance at the feed point (when feedline is coax=imbalanced and antenna is dipole=balanced) will cause common mode current to appear in outer surface of the coax shield even when the impedance is matched. I have one query, what will be the common mode impedance, that determines that how much will be the common mode current on the outer surface of coax shield. What will be the process to determine that common mode impedance ? 73 DE VU3KFK

This is great timing for me. I'm looking at the optimum placement for a ferrite choke on an endfed single band antenna. Can you move your coupling device up and down the feed line to find the greatest magnitude of CMC, then measure from the feed point? It would be great to do this test with the single end of a dipole (1/4 wave), and even a half wave endfed antenna, and I would need to know the frequency. I mean, since you have this set up already, and short of myself making a sensing device and buying a scope.

I experiment with random antennas in my living room in the same room as the radio and even work 5k miles with indoor antennas. Very often though, working digital modes, the radio and PC will disconnect from each other when TXing. The 2 options I have are lowering power and adding clamp on ferrites to the coax. But some antenna designs will handle 100w and no RFI issue at all. Others can only handle 10w even with a perfect SWR before RFI causes havoc. I work 160m to 10m indoors just experimenting.

For some reason these "expert'' commentors just keep acting like they know better than the ARRL Handbook. And each time when one proves them wrong and embarrasses them, they still believe what they wanna believe... Like in lowering the swr by increasing coax length)

Impressive video, Gregg Messenger - VE6WO. Looking forward to your next upload. I tapped that thumbs up icon on your content. Keep up the fantastic work! Your demonstration of common mode currents on coaxial cables was enlightening. Have you considered the implications of antenna design on signal integrity in more complex installations?

One thing I noticed is that you have the coax running parallel to the radiating elements which may be contributing to RF wanting to go back down the line. Would be interesting to see the difference when the coax is perpendicular to the elements.

I would not say common mode signals on the outer of the coax. Common mode signals are on both conductors and in the same direction..

Excellent demonstration! 73

Thanks, You saved me a lot of time. I have been planning to perform that experiment for years.

So, how does the typical choke wound on the outside of a piece of PVC pipe work? Does it reduce common mode current on the outside of a coax? What effect, if any, does it have on currents (reflected or otherwise) on the inside of the coax?

You showed only the open and properly terminated case. What happens when you short the output?

I'm not into ham radio, but this was very interesting and informative. As far as AI goes, maybe it's a matter of version-ChatGPT o1-preview said there are no common mode currents on the outside of the coax.

Great explanation! Now a question -- How do antennas with multiple folded dipoles, like the DB-224, decouple the dipoles?

I have had similar wrong answers related to RF questions I put to Chatgpt. Once I pointed out that the answer was wrong and what the correct answer was, AI came back with an apology and agreed that my answer was correct. Seemed more lifelike intelligence than artificial.

Ha, that's so typical ) These gialons like - User: question, ChatGPT: answer, User: incorrect, because ..., ChatGPT: yeah, indeed, .... It also then sometimes continues as: User: so, which reply is correct? ChatGPT: the first one, User: but you said the second one!, ChatGPT: yes, the second one, User (crying): so which one? )))

Could you please add some links explaining the types of the probe (connected to the scope) used to measure the signal on the antenna legs and on the coax shield? They look like some tiny coils, perhaps measuring the magnetic field near the surface of the conductors and this way sensing the current, but is this interpretation correct?

Was hoping you'd show an example of common mode current flowing on the outside of the coax shield with a dipole, then how the choke fixes it. Rushing away to try that common mode current detector... Good video, thanks, subscribed!

Could you repeat this demonstration using a common mode choke in place of the balun?

An excellent practical demonstration, which I've bookmarked for the occasions where I need to help someone understand the practical applications of baluns and common mode chokes.

Hi Gregg, what is the link to that excellent vice ? .. 73

I've been playing around with using Microsoft's CoPilot to program an Arduino. Results are very similar to yours and likely for most folks. Some of what it does is really clever, and some of it is, well, the opposite, and in the end, the code rarely works. Because you didn't write it, troubleshooting takes a lot more time as you have to familiarize yourself with AI's code before you can fix it and that takes time. By the time you get done figuring out what it did wrong, often it would have been quicker to write it myself. I may get 1000x more stupid errors like missing ; [ } etc. but at least the fundamental code is solid. I've had times where after figuring out it's mistake and telling AI what it did wrong, it will repeatedly make the same mistake in it's 'corrections' to the point I just give up, make the correction, then feed it back into AI. I don't know how much of an effect correcting AI helps to improve it's accuracy but I'll continue to try as it should, in theory, get better with time. I've also found that you have to be careful how you phrase a question. What we take as implied AI probably has no idea of the implication and will interpret what you're asking differently for example. Asking it the same thing in different ways, or the same thing on different days and you'll get different answers. For now, like most things in life, buyer beware, what you get may not be what you thought. It can't be relied on to be correct, anything that matters should be verified thru different sources.

That was excellent. Thanks Gregg!

An excellent demonstration. Thank you.

A schematic of the balun you used would be helpful. Seeing what your super mini balun looks like would be a major help to build a balun for my (crazy to tune) HAM Stick dipole (SWR on my crazy dipole runs about 4:1 making a tuner an absolute necessity). As it stands, I can only imagine what my common mode current is. PS - enjoying your videos on the CMC problems.

Interesting. Would have like to seen the details and construction of the balun. Maybe you could make a video about it? You make very clear videos.

Very well done and informative. I'm cautious using Chatgpt for virtually anything these days. Only if you bully it can you get anywhere close to the correct answers, as your example showed. This means that typically you must already know somethlng about the topic at hand, else you'll be mislead by chatgpt. I discovered this in early AI inquiries within my profession, and chatgpt generated fantasies for answers (completely made up court cases).

👍 73

The reason you find difference using the alligator clips cable probably is due his characteristic impedance, bnc connector included. Check starting from there: does it use a 75 ohm bnc version?

Great experiments. And of course we would need a test fix for smd as well...

At 14:24 you said 1/2 wavelength. Is that correct? The impedance of end feeding 1/2 wavelength would be very high. Good video for sure!

Very good content. I personally don't trust Chat GPT at all. It makes a nice story but often incorrect. So it is a waste of time. Actually it is quite harmful, as people are incorrectly informed. Only when you know the topic well you see you need to push Chat GPT several times to get the correct answer. And every time it says, yes you are right bla bla bla. Wikipedia construction is then a much better choice to look up something fast. It is at least reviewed by a lot of people. You can find then the links to more and deeper information if you want.

I am disappointed that you did not connect an asymmetric dipole to the end of the cable. It would have been a ten minute job (if that) to whip one up. Perhaps a follow up video? Doc - K8DO

Great show very interesting test 73

Very interesting video and subject !!

Your making it smarter Stop.