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!
Hi. Do you have a similar video showing different ways that CM does get onto the outside of the coax? I was discussing with people about tuning the antenna with an SWR meter at the shack rather than at the feed-point and if the coax length of feed-line is important or not. What I have noticed is that different unbalanced antenna act differently depending on how they are tuned or loaded. I still believe that for mono band antenna it is still better to tune the coax length as well as the antenna length. Radial length can effect the tune of the antenna and it's all to easy for the antenna to start seeing lengths of coax as part of a radial or counterpoise system, same thing goes for a metal mast vs a glass fibre mast. In theory coax length should not matter however I find this is not the case in practice and it depends on the antenna. I've played with many different types of unbalanced vertical antennas and they can all too often show different swr results depending on coax length. One antenna I built has a 32 cm 1 turn inductor ring at the feed-point where the inner and outer of the coax see an electrical short but you tune here, as well as tuning overall length. You can also tune this antenna with the capacitive section that lies between the lower section where the inductor connects and the upper section of the main element where the ring reconnects. The length of the radials can also effect the tune, the shape of the ring the gap in the ring etc. The type of mast, the height of the mast, the proximity to other objects etc. Personally I don't think I or anyone else around me has suffered from common mode on my coax. I have tried 1:1 bifilar UNUN's and where there is nothing wrong with using one I have not really noticed any improvements when using it other than some antenna behave more stable regarding reflected power and sometimes the usable bandwidth is wider. Would love to see more examples of actual common mode on the coax and how it is effecting people weather the operator or the people in their vicinity. The way I see it regarding 1:1 Unun's or Balun's on mono band antenna is that if you need a 1:1 there is something wrong that could be resolved without a 1:1. One should be able to eliminate CM without a balun on a mono-band antenna. If your adding a 1:1 then you have not fixed the issue. On a side note. The other day i was wondering if i could build a rigid dipole where it is fed from the inside of the shielded leg. Say an aluminium tube 1/2 wavelength long with an insulator and feed-point at the centre. Coax would come from one end and up the inside to the feed-point. Is there any way I could make this design work without causing an in-balance? Perhaps a 2nd 2" insulator and then another section of aluminium tube for the coax to continue down until it's a bit further from the antenna and then say down a tree at 90 degrees once it is a few meters away from the actual antenna? I just wondered if there is a design like this that works as it would look much tidier and put less weight on the feed-point.
@@aerofart Thanks. So often people with experience in only one area, one band or who only use balanced antenna don't really understand the issues surrounding true antenna tuning. In theory is one thing but in practice it's another. If you want the best possible performance from a monoband antenna it needs to be tuned at the feedpoint and if the system sees a different tune once the coax is added back to the equation then there must be an issue. I can vouch to coax lengths changing the tune of an antenna because I've tried it over and over. If anyone is tuning an antenna on HF bands and it's an unbalanced vertical then just try adding a meter or 2 or 3 or whatever you have lying around. With some female female SO239 add some long patch cables, try a few different sizes and then check the SWR and the results will change. If the antenna is broadband it won;t show up so much but if it is high gain with narrow Q I gaurantee the differences can be huge. Ideally I want at least 95% of radiated power to leave the antenna also, once you know it's correctly tunes reception will be stronger just as you can hear when using a tuner to match an unmatched antenna, the received signals are stronger also.
Unfortunately I do not have any further videos specifically dealing with how CM currents wind up on the coax. Generally, from the knowledge I have accumulated thus far in my Amateur Radio adventures, if changing the coax length effects the SWR measured at the transmitter end of the coax/antenna system then there is an issue. Perhaps I will research and prepare another video to take a look at this subject 🙂
@@izzzzzz6 one of these days I hope to have enough knowledge, experience and budget to build an amazing antenna system. Until then, at least the learning curve seems to be getting flatter.
@@aerofart The thing is that radio, is that in theory and in practice things are very different. Too many people are believing that there are certain practices that must be followed however in reality some of these are fixes to solve problems due to compromises. Weather it be due to installation compromises or from trying to multi-band an antenna. Do what works best for you but at the end of the day if one is making compromises then we should expect a decrease in performance albeit a very small one sometimes. Some people have much worst things to worry about such as local power line noise that destroys all weak signals. My compromise will be trying the NR-1 external noise blanker.
Great video! ChatGPT can be a useful tool if you are careful. Otherwise, it’s like asking a mediocre high school student that has studied Wikipedia for a few minutes.
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.
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.
In the case of a 1/2 wave end fed antenna, yes, you do see high impedance at the feedpoint. If you were to ground the other end of that EFHW… you would see a very low impedance at the feedpoint. The 1/2 wave reflects whatever impedance is present at the load to the source. In the case of the coax cable outer shield, the source where the energy is originating is at the antenna feedpoint end, and the load is at the radio, which is typically ground… thus a very low impedance path is seen at the antenna feedpoint for common mode currents to enjoy!
I thought you were talking about a 1/2 wave dipole fed with coax. The coax shield sees 1/4 wavelength of antenna and if the coax is 1/4 wave long we have another dipole of sorts.
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
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?
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.
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.
super good video and good experiment. As you imply elsewhere in your view in a perfect case there is no "common mode" induced per se as your demonstration shows. My bet though, is that if you up the gain on your scope you'd see that your demonstration case, as in most real world scenarios, wasn't perfect and you would have seen some current running on the outside of the coax. Also, braided coax isn't perfect so there is some "leakage", hence why we use hardline in our lab when necessary. You left me wondering what your recommendation was how the unbalance coax was connected to the dipole - maybe I missed it, were you recommending a balun into an unnon? I guess I never saw the update to your experiment last year on this related topic. DId you make it, or did I miss it? Thanks for your outstanding videos! de N1baq
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.
Hi Gregg. I can’t tell how much I enjoy your videos. They are so well made end a pleasure to watch and learn from, and your areas of interest are spot on (relevant). The technique you used to measure the outer shield currents is quite clever. I was wondering if a IR camera like the ones used to see hotspots in electronics circuits could ever be sensitive enough to detect heating of the coax as a result of CMC. Wouldn’t it be nice if we could just SEE these radio waves and currents the way we see things on the visible portion of the electromagnetic spectrum? It would be nice to see how CMC would apply in the case of an EFHW. Seems to me such an analysis might be trickier because in such case you would have to consider the counterpoise methinks. I employ an EFHW in my setup and have no choke at the feedpoibt, only one at the radio and wonder if having the choke elsewhere or using a counterpoise might improve the performance of my system. I get really good signal reports and am always able to reach any station I can hear all with just 100W out of the radio.
Thank you for the kind words :-) I have often wished this same thing, that I could just see the electromagnetic waves and observe with my eyes what is going on the same as we observe light, which also happens to be part of the electromagnetic spectrum… maybe one day we will have visors (like the one worn by Geordi in Star Trek TNG) 😮 that will allow us to view a much broader spectrum with our eyes.
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? )))
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.
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.
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).
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.
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 agree. It’s not until one pushes it in the correct direction that it eventually gives an accurate answer… however, I find it very helpful to quickly sort through reams of textbooks and to distill the location of information down very quickly. This would have taken me weeks to do manually. I can now locate and read technical books and publications on my chosen subject very quickly. I still have to use my brain to try to form accurate conclusions, and to use critical thinking ability to question if my conclusions add up though.
I asked it why it gave the wrong answer and it said it made an incorrect assumption.. then I asked it if it would use this conversation to learn.. it said… no. Hahaha!
"Common Mode Currents On Your Coax?" Pet peeve alert: That's not "common mode". I don't know what to call it other than "current on the outside of the coax". Common mode is when a current flows on *both conductors* ,of a balanced pair of wires, in the same direction. Coax isn't balanced and the currents in the inner conductor and outer conductor are not flowing together. en.wikipedia.org/wiki/Common-mode_signal It is called "common" because the induced voltage or current is *common to both conductors* and because of that can be eliminated in various ways, usually by a balun or differential transformer.
Since we’re being pedantic, it doesn’t need to flow on both conductors. Any net (unbalanced) current is common mode. You could pass a signal on just the shield or just the center conductor. The naming convention is a bit wonky - common not meaning on both, but rather not differential mode.
@@briansauk6837 "Since we’re being pedantic, it doesn’t need to flow on both conductors." Yes, it does. But this may be like me trying to stop a rising tide. "common not meaning on both" That is exactly what it means; same voltage and phase on two conductors. A similar phenomenon exists in network engineering. GoDaddy comes along and re-defines "subdomain" as what until then had been "host". Subdomains actually exist; but aren't what GoDaddy says they are. But so many people use GoDaddy as their engineering knowledge source.
I wonder if you could please clarify… what’s not common mode? The definition I use is: “A CM current is one that has the same value and direction in both wires (or on the shield and center conductor)” - (ARRL Antenna Book 25th ed. P 24.28)
@@thomasmaughan4798 That is a fair comment on the nomenclature. That’s why I described it as wonky. In this case, there are no practical differences in observations or measurements as to whether or not the net current is split on both conductors, or rides on just one side. So the original term is admittedly being co-opted for cases that result indistinguishably.
@@briansauk6837 " So the original term is admittedly being co-opted for cases that result indistinguishably." Yes, exactly. I regret there isn't apparently a word to describe this phenomenon; so I'm sort of stuck with "common mode" if I want to be understood. When I first encountered it, I wondered how is it possible for a current to exist on the outside of a coax that differs from the current on the inside of that exact same conductor (the shield). The answer is *skin effect* and has considerable implications for radio especially microwave. You CAN actually have a different current on the outside from the inside, in fact, you can even have a different current on the left side of the outside or the right side of the outside (at millimeter wavelengths) and that's the principle of the waveguide.
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!
Hi. Do you have a similar video showing different ways that CM does get onto the outside of the coax?
I was discussing with people about tuning the antenna with an SWR meter at the shack rather than at the feed-point and if the coax length of feed-line is important or not. What I have noticed is that different unbalanced antenna act differently depending on how they are tuned or loaded. I still believe that for mono band antenna it is still better to tune the coax length as well as the antenna length. Radial length can effect the tune of the antenna and it's all to easy for the antenna to start seeing lengths of coax as part of a radial or counterpoise system, same thing goes for a metal mast vs a glass fibre mast. In theory coax length should not matter however I find this is not the case in practice and it depends on the antenna. I've played with many different types of unbalanced vertical antennas and they can all too often show different swr results depending on coax length. One antenna I built has a 32 cm 1 turn inductor ring at the feed-point where the inner and outer of the coax see an electrical short but you tune here, as well as tuning overall length. You can also tune this antenna with the capacitive section that lies between the lower section where the inductor connects and the upper section of the main element where the ring reconnects. The length of the radials can also effect the tune, the shape of the ring the gap in the ring etc.
The type of mast, the height of the mast, the proximity to other objects etc.
Personally I don't think I or anyone else around me has suffered from common mode on my coax. I have tried 1:1 bifilar UNUN's and where there is nothing wrong with using one I have not really noticed any improvements when using it other than some antenna behave more stable regarding reflected power and sometimes the usable bandwidth is wider.
Would love to see more examples of actual common mode on the coax and how it is effecting people weather the operator or the people in their vicinity.
The way I see it regarding 1:1 Unun's or Balun's on mono band antenna is that if you need a 1:1 there is something wrong that could be resolved without a 1:1. One should be able to eliminate CM without a balun on a mono-band antenna. If your adding a 1:1 then you have not fixed the issue.
On a side note. The other day i was wondering if i could build a rigid dipole where it is fed from the inside of the shielded leg. Say an aluminium tube 1/2 wavelength long with an insulator and feed-point at the centre. Coax would come from one end and up the inside to the feed-point. Is there any way I could make this design work without causing an in-balance? Perhaps a 2nd 2" insulator and then another section of aluminium tube for the coax to continue down until it's a bit further from the antenna and then say down a tree at 90 degrees once it is a few meters away from the actual antenna? I just wondered if there is a design like this that works as it would look much tidier and put less weight on the feed-point.
Good questions.
@@aerofart Thanks. So often people with experience in only one area, one band or who only use balanced antenna don't really understand the issues surrounding true antenna tuning.
In theory is one thing but in practice it's another. If you want the best possible performance from a monoband antenna it needs to be tuned at the feedpoint and if the system sees a different tune once the coax is added back to the equation then there must be an issue. I can vouch to coax lengths changing the tune of an antenna because I've tried it over and over. If anyone is tuning an antenna on HF bands and it's an unbalanced vertical then just try adding a meter or 2 or 3 or whatever you have lying around. With some female female SO239 add some long patch cables, try a few different sizes and then check the SWR and the results will change. If the antenna is broadband it won;t show up so much but if it is high gain with narrow Q I gaurantee the differences can be huge. Ideally I want at least 95% of radiated power to leave the antenna also, once you know it's correctly tunes reception will be stronger just as you can hear when using a tuner to match an unmatched antenna, the received signals are stronger also.
Unfortunately I do not have any further videos specifically dealing with how CM currents wind up on the coax.
Generally, from the knowledge I have accumulated thus far in my Amateur Radio adventures, if changing the coax length effects the SWR measured at the transmitter end of the coax/antenna system then there is an issue. Perhaps I will research and prepare another video to take a look at this subject 🙂
@@izzzzzz6 one of these days I hope to have enough knowledge, experience and budget to build an amazing antenna system. Until then, at least the learning curve seems to be getting flatter.
@@aerofart The thing is that radio, is that in theory and in practice things are very different. Too many people are believing that there are certain practices that must be followed however in reality some of these are fixes to solve problems due to compromises. Weather it be due to installation compromises or from trying to multi-band an antenna. Do what works best for you but at the end of the day if one is making compromises then we should expect a decrease in performance albeit a very small one sometimes. Some people have much worst things to worry about such as local power line noise that destroys all weak signals. My compromise will be trying the NR-1 external noise blanker.
Thanks, You saved me a lot of time. I have been planning to perform that experiment for years.
Great video! ChatGPT can be a useful tool if you are careful. Otherwise, it’s like asking a mediocre high school student that has studied Wikipedia for a few minutes.
I agree. It can be a useful tool.
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.
Very interesting video and subject !!
Excellent demonstration! 73
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..
Thanks for this excellent video.
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.
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!
In the case of a 1/2 wave end fed antenna, yes, you do see high impedance at the feedpoint.
If you were to ground the other end of that EFHW… you would see a very low impedance at the feedpoint. The 1/2 wave reflects whatever impedance is present at the load to the source. In the case of the coax cable outer shield, the source where the energy is originating is at the antenna feedpoint end, and the load is at the radio, which is typically ground… thus a very low impedance path is seen at the antenna feedpoint for common mode currents to enjoy!
I thought you were talking about a 1/2 wave dipole fed with coax. The coax shield sees 1/4 wavelength of antenna and if the coax is 1/4 wave long we have another dipole of sorts.
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
I thought the same thing today… Done.
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?
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
You showed only the open and properly terminated case. What happens when you short the output?
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.
th-cam.com/video/AGYehX1zPN8/w-d-xo.htmlsi=JmLzonOILUYBccL4
super good video and good experiment. As you imply elsewhere in your view in a perfect case there is no "common mode" induced per se as your demonstration shows. My bet though, is that if you up the gain on your scope you'd see that your demonstration case, as in most real world scenarios, wasn't perfect and you would have seen some current running on the outside of the coax. Also, braided coax isn't perfect so there is some "leakage", hence why we use hardline in our lab when necessary. You left me wondering what your recommendation was how the unbalance coax was connected to the dipole - maybe I missed it, were you recommending a balun into an unnon? I guess I never saw the update to your experiment last year on this related topic. DId you make it, or did I miss it? Thanks for your outstanding videos! de N1baq
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.
Hi Gregg. I can’t tell how much I enjoy your videos. They are so well made end a pleasure to watch and learn from, and your areas of interest are spot on (relevant).
The technique you used to measure the outer shield currents is quite clever. I was wondering if a IR camera like the ones used to see hotspots in electronics circuits could ever be sensitive enough to detect heating of the coax as a result of CMC. Wouldn’t it be nice if we could just SEE these radio waves and currents the way we see things on the visible portion of the electromagnetic spectrum?
It would be nice to see how CMC would apply in the case of an EFHW. Seems to me such an analysis might be trickier because in such case you would have to consider the counterpoise methinks.
I employ an EFHW in my setup and have no choke at the feedpoibt, only one at the radio and wonder if having the choke elsewhere or using a counterpoise might improve the performance of my system. I get really good signal reports and am always able to reach any station I can hear all with just 100W out of the radio.
Thank you for the kind words :-)
I have often wished this same thing, that I could just see the electromagnetic waves and observe with my eyes what is going on the same as we observe light, which also happens to be part of the electromagnetic spectrum… maybe one day we will have visors (like the one worn by Geordi in Star Trek TNG) 😮 that will allow us to view a much broader spectrum with our eyes.
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? )))
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.
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
Great explanation! Now a question -- How do antennas with multiple folded dipoles, like the DB-224, decouple the dipoles?
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.
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).
I thought that the length of a coax feed line had been shown to be irrelevant.
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.
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 agree. It’s not until one pushes it in the correct direction that it eventually gives an accurate answer… however, I find it very helpful to quickly sort through reams of textbooks and to distill the location of information down very quickly. This would have taken me weeks to do manually. I can now locate and read technical books and publications on my chosen subject very quickly. I still have to use my brain to try to form accurate conclusions, and to use critical thinking ability to question if my conclusions add up though.
Your making it smarter Stop.
I asked it why it gave the wrong answer and it said it made an incorrect assumption.. then I asked it if it would use this conversation to learn.. it said… no. Hahaha!
"Common Mode Currents On Your Coax?"
Pet peeve alert: That's not "common mode". I don't know what to call it other than "current on the outside of the coax". Common mode is when a current flows on *both conductors* ,of a balanced pair of wires, in the same direction. Coax isn't balanced and the currents in the inner conductor and outer conductor are not flowing together.
en.wikipedia.org/wiki/Common-mode_signal
It is called "common" because the induced voltage or current is *common to both conductors* and because of that can be eliminated in various ways, usually by a balun or differential transformer.
Since we’re being pedantic, it doesn’t need to flow on both conductors. Any net (unbalanced) current is common mode. You could pass a signal on just the shield or just the center conductor. The naming convention is a bit wonky - common not meaning on both, but rather not differential mode.
@@briansauk6837 "Since we’re being pedantic, it doesn’t need to flow on both conductors."
Yes, it does. But this may be like me trying to stop a rising tide.
"common not meaning on both"
That is exactly what it means; same voltage and phase on two conductors.
A similar phenomenon exists in network engineering. GoDaddy comes along and re-defines "subdomain" as what until then had been "host". Subdomains actually exist; but aren't what GoDaddy says they are. But so many people use GoDaddy as their engineering knowledge source.
I wonder if you could please clarify… what’s not common mode?
The definition I use is: “A CM current is one that has the same value and direction in both wires (or on the shield and center conductor)” - (ARRL Antenna Book 25th ed. P 24.28)
@@thomasmaughan4798 That is a fair comment on the nomenclature. That’s why I described it as wonky. In this case, there are no practical differences in observations or measurements as to whether or not the net current is split on both conductors, or rides on just one side. So the original term is admittedly being co-opted for cases that result indistinguishably.
@@briansauk6837 " So the original term is admittedly being co-opted for cases that result indistinguishably."
Yes, exactly. I regret there isn't apparently a word to describe this phenomenon; so I'm sort of stuck with "common mode" if I want to be understood.
When I first encountered it, I wondered how is it possible for a current to exist on the outside of a coax that differs from the current on the inside of that exact same conductor (the shield). The answer is *skin effect* and has considerable implications for radio especially microwave. You CAN actually have a different current on the outside from the inside, in fact, you can even have a different current on the left side of the outside or the right side of the outside (at millimeter wavelengths) and that's the principle of the waveguide.