Відео

Thank you. You are correct, in the real world, things will not always work out so ‘perfectly’ as they did in my lab experiment. I think there were a few points I was trying to make with this video: 1. Learning with the use of AI can be effective, but a person needs to use a good deal of critical reasoning ability to cross examine the AI’s answers as they can sometimes be incorrect. 2. Common mode currents on the outside of the coax shield are not caused by an impedance mismatch at the antenna feedpoint. 3. Even though a dipole is a balanced antenna, a balun is recommended to ensure high common mode impedance at the antenna feedpoint. This helps to ensure that the currents on the legs of the antenna are indeed balanced. However, even with a well balanced dipole, common mode currents can still be present along the outside of the coaxial cable shield. This depends on the impedance presented by the shield at the feedpoint, which is influenced by things such as the coaxial cable length. For example, if the cable length happens to be 1/2 wavelength, the impedance at the feedpoint will be very low, resulting in significant common mode current on the shield.

This is much more difficult to do, and an area I have not yet explored much, however, based on the principles I have learned thus far, I would expect to have to spend a lot more time and attention to ensure the test jig and methods used are suitable for testing at these higher frequencies where inductive and capacitive reactance of the DUT and test apparatus will be much more sensitive to “proper” design. It would require a dive into researching techniques suitable at these frequencies.. and it sounds to me like you are about to embark on an electronics adventure doing so :-) Enjoy!

I have to disagree. In free space an electromagnetic wave propagates with the E and H (B) fields in phase. (H and B can be used interchangeably when the magnetic field is not in the presence of a magnetic material) On a transmission line, the electromagnetic wave propagating in the TEM mode is also in phase. Here is one reference for you to read: farside.ph.utexas.edu/teaching/316/lectures/node117.html#:~:text=Maxwell%20was%20able%20to%20establish,in%20both%20time%20and%20space.

Yes. I was able to track down the source. It was coming from two arcing insulators atop two power poles in the back alley. I was able to use a VHF/UHF yagi and my FT-857D on battery to listen using AM demodulation to locate the poles, and then used the MFJ ultrasonic dish to pinpoint the offending component on each of the poles. The local power company came out and replaced both the failed insulators and my powerline noise issue is now gone :-) The LOG antenna just didn’t do what I needed at my location. There are many positive experiences posted online regarding this antenna, however, so perhaps it’s just one of those things where what works for one operator may not for another, depending on circumstances unique to each QTH.

Yes, that little noise cancelling box works pretty good when fed a good noise source to phase the actual receive antenna against. It’s not a full cure for noise though, you are having to constantly fiddle with it, and the receive levels after noise cancelling are not as good as taking the signal straight off the receive antenna. Perhaps a better quality unit would offer increased performance. I was looking at the DX Engineering NCC-2 at one point, but I no longer need it. www.dxengineering.com/parts/dxe-ncc-2?seid=dxese1&gclid=Cj0KCQiA1p28BhCBARIsADP9HrPa72Qu_zjT8Tu5XEq6Ay4MobAu-wV6v3H-r-m26SS-pM0-KPPo_DsaArL5EALw_wcB

It makes the example simple and easy to understand. At 538 kHz the magnitude of the inductive reactance is equal to the magnitude of the resistance, which results in a voltage vs current phase relationship in this circuit of 45 degrees. The point of the video is to bring to people’s attention that even though we know the phase angle of this circuit is 45 degrees, why does the nanoVNA display a phase angle of 62.7 degrees? This video explains why, and what the relationship is.

@@ve6wo Thank you for that response. I'm trying to learn the nanoVNA I've just purchased. Your video and Comment response was very helpful. Sorry for my delayed answer. I've been distracted by the UA-cam Presidential Election activities.😁

Now that I haven’t looked at this video in a while I should go back and see if that section you mentioned makes sense.

@@ve6wo Also it would be intresting to know if the VNA will give more accurate values of the L and C when measuring at the intended frequency of the filter, ie at/around 1.35 Mhz

@@srmeister1he mentioned at the beginning that his method may not be appropriate if the application frequency is much higher

I haven’t tried zooming with a desktop PC, but with the iPhone or iPad one can simply use finger gestures to pinch and zoom in on a video.

I agree mostly, but there is a bigger picture. Please take a look at Walt Maxwell’s book, “Reflections III” www.w4wb.com/Reflections_III.pdf

What kind of device are you viewing the videos on? I used my iphone just now and I was able to zoom in on the screen and read it.

The behaviour is complex. If there is a good match at the antenna, then the length of the coax from the point of view of the radio transmitter is irrelevant. This is with regard to the TEM wave travelling down the inside of the coax. The outside of the coax shield is it’s own separate conductor, and if the length of the grounding system at the station, and the shield combined, results in a situation where the RF sees a 1/2 wavelength conductor (or multiples of 1/2 wavelength), then the common mode impedance at the feedpoint will be very low. If there is no common mode choke there then large amounts of common mode currents are likely to make their way onto the outside of the coax shield.

This is an excellent question, and one that I was asking myself as I was preparing these recent videos. I have never tried to make this measurement, and have not yet run across any articles dealing with this subject in my reading. In theory, a person might be able to use a nanoVNA to take the measurement… I will keep this in the back of my mind and see what comes.

Yes, it's not going to lower the SWR at the feed point, but changing the length of the coax can make the transceiver happy... it will see something closer to 50ohms, for example. This is because the feed line transforms the impedance when the load and source are not of the same impedance. I'm positive you already know this.

Yes, I tried rotating the dipole and arranging the coax so that it was more ideal. It did not seem to make any difference in this experiment, so I arranged things so it was easier for me to fiddle while holding the camera. Haha!

In any case, the end result was the same. The use of a balun at the feedpoint stopped common mode currents from making their way down the outside of the shield, which also resulted in a much more stable antenna system.

​@@ve6woShure, because it's coupling all around, and You showed it so nice when touching something around - Feeding a symmetric radiator from a asymmetric Coax feed is always a big mess when not taking care with a BALUN element between. Congrats for Your demo experiment. Done very simple and very easy to understand. The most funny thing was the AI fail - It shows that the AI of today is far away from logical thinking - Today AI is mostly Bullshit Input = Bullshit output. Very cool Demo all the way down from the Mismatch effects until showing the real cause of coax shield currents. Congrats once again - Perfect👌👌👍👍

Thanks for the video. I watched the previous one too and now you have posted this in response to the question asked in the previous one. I believe you may want to make one more again to clarify some more doubts. As already pointed out by a commentator the main reason the CMC is impinged on the shield of the coaxial cable is that the feeder runs parallel to one of the dipole elements. It's advised not to run the cable parallel to any one of the arms of the dipole at least a quarter lambda long of the frequency of operation. Here you are operating at 100 MHz. Secondly, you should purposely cause an imbalance by varying the length of one of the dipole elements to bring about the appearance of the CMC in an otherwise no CMC or to cause a change in the magnitude of CMC. Thirdly, the effects of the installation of the CMC choke (1:1 Guenella current choke Balun) at the feed point (as you did) and one more choke before it gets into the oscilloscope (the detector) may also be demonstrated. Fourthly, absorption/reflection, scattering of the electromagnetic energy by other materials especially metallic structures in the near fields is well demonstrated. The good old article by Roy Lewellen, titled, "what baluns do and how they do it" is a very good read till today, I believe. You may very well device some more intuitive ideas to make it very clear that impedance mismatch causing varying degrees of reflection from the feed point towards the source (the radio) is a phenomenon restricted to the dielectric material of the coaxial cable; it doesn't affect anything outside of the cable. Or does it? Any amount of CMC has nothing to do with what's happening inside the cable, within the dielectric material between the outer surface of the center conductor and the inner surface of the shield. You might succeed in showing that there is no relationship between the CMC and the high SWR (when the load happens to be purely or nearly purely resistive) But, when the load happens to be highly reactive and the magnitude of the CMC is large, it would certainly skew the SWR read at the shack end of the coaxial cable. De VU2RZA

I have never built an air core choke, so I cannot comment from first hand experience, however… 1. The principles are the same, only the core material is different (air core as opposed to ferrite core). 2. I would guess it would, however, air core inductors do not produce as much choking impedance for the same number of turns as a choke wound on a ferrite core. 3. It will have no detectable effect on the signals traveling along the inside of the coax, unless the minimum bend radius is not respected (likely to cause a change in the characteristic impedance of the cable).

@@ve6wo So, if I'm using an unbalanced antenna where I might get some current flowing on the outside of the coax, a choke would stop that? Than what happens to that current? Does it get reflected back to the antenna?

A choke will not block all of the common mode current, but it will greatly reduce it. That energy that would have been flowing down the outside of the coax shield is now mostly redirected towards the antenna with a bit of energy dissipated in the choke as heat. The nature of an unbalanced antenna, like an off center fed dipole, means that the current distribution on the legs of the antenna will never be balanced, but with the choke forcing most of the energy onto the antenna elements, it will rebalance it’s self to whatever ratio as dictated by the antenna element lengths.

One other point to make is that even a balanced antenna like a dipole will have some common mode current that flows back down the outside of the coax shield. It is inevitable. The amount of current allowed to flow back along the coax as common mode is determined by the common mode impedance as seen by the EM wave when it arrives at the antenna feedpoint.

@@BryanTorok www.hbphoto.com/Radio/Baluns_101.pdf

A short is the same as an open, simply at half a wavelength distant

@@donepearce Than it will not hurt to show it.

I didn’t think to do this as the open and the short both reflect all of the energy back down the coax like a mirror.. however, for clarity, I maybe should have.

@@reggindog3436 Ok, I revisited the experiment. ua-cam.com/video/AGYehX1zPN8/v-deo.htmlsi=JmLzonOILUYBccL4

It has been a few years since I put up one of those antennas. However, if I remember correctly the harnesses use specific length of coax that is not 50 ohms and also they have have ferrite beads on the outside of the coax.

@@phystimn You are correct. These are E and H field probes. Purchased to make near field relative measurements of electric and magnetic fields. Amazon: a.co/d/aqYFdIj

I would need to research toroid materials and see what would be required to build a choke that is effective at 100 MHz. The balun I used is one that I ordered (I didn’t build it) and it’s good from 150 kHz to 350 MHz, so it worked well in this demonstration. In my ham shack, I typically hang out on the air at 3.7 MHz, so everything I have as far as construction materials is for low HF frequencies.

​@@ve6wo something simple to try is a short piece of coax wound in a coil and attach that to the base of the antenna. 5 or 6 turns, maybe, but I don't know the diameter. It would be interesting to see them if there are any common mode currents along the coax.

I don’t like those kinds of chokes, but maybe playing with them on the bench will result in me changing my mind? Haha! We’ll see what happens.

Agreed, but they're a fast and cheap solution out in the field, or even for testing. It should make for an interesting test!

This is a PanaVise 301. A staple in most electronics labs and tech benches :-) Here are some Amazon links: a.co/d/787MQBU a.co/d/iRJmit6

I cheated on the balun… it’s from aliexpress. The 150kHz-350MHz version. It seems to work very well! I just found this on AliExpress: C$13.03 | 100KHZ-6GHZ RF Differential Single-Ended Converter Balun 1:1 ADF4350 ADF4355 10MHZ-3GHz FOR FOR HAM radio Amplifier a.aliexpress.com/_mq5KgeP

The balun is from aliexpress. The 150kHz-350MHz version. It seems to work very well! I just found this on AliExpress: C$13.03 | 100KHZ-6GHZ RF Differential Single-Ended Converter Balun 1:1 ADF4350 ADF4355 10MHZ-3GHz FOR FOR HAM radio Amplifier a.aliexpress.com/_mq5KgeP