Interesting concept on RFI. Thanks Dave. Was thinking by knowing the coax inductance and capacitance (per foot) and frequency used may provide some help. Perhaps it can be applied into a new formula in determining the braid/shield needed to suppress the RFI, given that criteria. 73, AC3HT
That's pretty neat. I was wondering how it was going to work, then you pointed out it's basically one side of a capacitor and the other side is grounded, genius.
Thank you, Dave. This is an interesting video. From the last photo, the braid over the outer coax insulation appears not at all connected to the coax shield. So the exterior braid over the outer coax insulation forms a capacitor with with the shield and the outer insulation serves as a dielectric of this capacitor. OK. RF behaves strangely!!! Well now, I have something to ponder. Thank you. N0QFT
Great idea, makes me wonder if for indoor use like just in the shack, that maybe a section of tin foil about a foot long or so wrapped around the coax and then wrap that with black tape might also work. Just a thought I guess but this looks like something to try, makes sense. Thanks for your videos as always, keep up the great work! 73..wd0akx
@@PaulGriffith It's worth a try, I have some copper foil tape I bought to try to make a magnetic loop antenna, it would probably fold right around the coax. Always worth a try.
Kinky. I like it. Sound logic. Great explanation, Dave! Why not add a groundwire to the second coax shield, or would that effectively short circuit the two shields by putting them at the same DC potential? RF is different.
If I put some ferrite beads on the coax I add inductive reactance to CMC path but if I put a braid outside the coax (connected to nothing) I don't add capacitive reactance as you claim. Putting ferrite sleeves or beads on the coax works because it becomes a one turn inductor for CMC (no effects for the energy inside the cable). A conductor outside the coax and not connected to anything, can only... "disturb" CMC, nothing more. It looks like a bad "sleeve" or "bazooka" balun: it is "bad" because his base it is not connected to the coax braid.
Interesting idea. I wonder, is there a way to sniff for the escaping RF so you know exactly where to put the sleeve, or is just putting it near the antenna feed point good enough?
TIG welder ,RFI from arch starter sets off igniters of kitchen gas stove and may have killed electronics of 2, yes 2 oven/microwave built-ins. Can I capture RFI with a grounded sheath of coax net between my TIG welder and kitchen. Reminds me of Wave Guides in my Russian radar van in Vietnam era.
What isn’t clear to me is whether or not that shield is “peeled back” on the coax for one foot, or if it’s just added over the insulating jacket after being removed from a different piece of coax. If it’s not peeled back from itself, is he saying that there is no physical connection between this external shield and the internal coax shield?
It is very clearly explained. Take a fresh piece of coax cable about 2 feet length revive only shield after sliting the black plastic sleeve from coax. Then slide it over your antenna cable at the feed point or at the shack near to the radio.
I don't get it. Why wouldn't the RFI voltage simply capacitively couple to the added braid and still be there both in the coax shield and the braid? What's holding the braid at ground potential?
Hi Dave, another video with lots of confused statements.. At 1.25 you said clearly that the outside of the shield braid is at ground potential for DC. Absolutely right. And all things are happening only on the inside of the coax cable; again correct. Yes, all of the RF energy is contained in the dielectric material (or the space) between the outside of the centre conductor and the inside of the outer conductor. A dipole is a balanced device. The coaxial cable has three physical conductors including the outside of the shield braid. If the dipole radiators are deployed correctly with symmetry and in free space then there are no chances of a CMC (Common Mode Current) to originate and start to flow on the outside of the coax shield at the interface of the coaxial cable and the dipole radiators. But any asymmetry in length (of the radiators), proximity of the feeder cable to one of the arms of the dipole or any other RF absorbing structures near to one of the arms of the antenna or any other radiating antenna (in the same or harmonically related bands) in the nearby vicinity can all give rise to a CMC. During reception the "third conductor" ie, the outside of the coax shield can bring in RF trash and it will raise the noise floor. A CMC Choke can be some kind of reactance that should offer very high resistance to the CMC but should be allowing the differential mode current unimpeded. Usually we employ an inductive reactance to do the job to obtain a large value of reactance (largely the resistive part is preferred) over a wide frequency spectrum. Of course there's a compromised design for the entire HF spectrum. The W2DU's design consists of inserting number of ferrite cores (sleeves) over the end of the coax cable near the feed point. Probably he used #73 or 75 Ni-Mn material, I don't know. Next option is the "ugly BalUn" though not necessarily an ugly one to look (your suggestion of 9-10 turns of 9-10" in diameter). The drawback is that it might act like a "trap" for some frequency bands. And ultimately the ferrite loaded CMC Choke seems the best option. One can refer to G3TXQ for very good details of the selection of core, winding number and materials and very importantly the pure resistive portion of the choking that would be obtained in a particular contraption. Now let me come to the today's topic of using the "shield braid" of the coax cable to stop the CMC. Just by adding or sleeving a foot or so of the shield braid over the coax cable feeder is not going to do anything at all; not only to the CMC or anything else whatever it may be. In the VHF UHF bands you can add a 1/4 lambda long sleeve over the feeder cable with the open end of the cable soldered to the top end of the sleeve. This will act like a "skirt" over the feeder cable and will offer a high impedance to the flow of the CMC that tries to originate from the tip of the coax cable at the feed point. But here you made some confusion as I could not understand if or not the suggested (by the questioner) sleeve is simply left over the coax cable or is electrically connected. But as I understand you (and the questioner) said the sleeve acts like a capacitor (how) in air (!?) and will offer capacitive reactance to the flow of CMC. All your readers and I are totally confused. But you, this time, asked for comments hence I have made mine. De VU2RZA
Many years ago some old boy explained to me this method of stopping RF coming back down the coax, over the years I have tried this braid experiment when installing many different types of antenna.. I can honestly state with all my installs adding the braid made no difference whatsoever.
@@kannappanmpk yes, adding a braid or any copper tube as just a sleeve along the length of the coaxial cable is not going to do anything. After all the coaxial cable is meant for this specific special feature; whatever that's happening inside the cable does not know what is going on there in the outside world. The main setback with the open wire line is that you can not have anything (especially metals or other RF absorbing structures) in close proximity (at least around a distance of 5-6 times the width of the line) But having a sleeve of 1/4 lambda long connected at the top with the shield braid of the coaxial cable will effectively prevent the CMC flowing down the length. The "flower pot" antenna exploits this phenomenon. But then it's going to work for just one frequency or a narrow range of frequencies of the band. Hence it's used in the VHF UHF bands. De VU2RZA
Wait... don't you mean that you DON'T want feedback? Joking... see what I did there? Using coax shielding just might not give you feedback! Okay, I will try this on my next antenna experiment and let you know!
Especially odd since a typical braid is made of interwoven strands, so physically the inside and the outside of the coax shield are constantly exchanging places/roles. So there is no simple inside or outside, unless the signals are literally jumping from strand to strand every few millimeters and "square dancing" on them. Most peculiar, unsatisfying, and illogical, Dave.
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Most interesting idea -
yes, I am also interested in seeing others findings.
Thanks Dave for putting this one up
Interesting concept on RFI. Thanks Dave. Was thinking by knowing the coax inductance and capacitance (per foot) and frequency used may provide some help. Perhaps it can be applied into a new formula in determining the braid/shield needed to suppress the RFI, given that criteria. 73, AC3HT
That's pretty neat. I was wondering how it was going to work, then you pointed out it's basically one side of a capacitor and the other side is grounded, genius.
Thank you, Dave. This is an interesting video.
From the last photo, the braid over the outer coax insulation appears not at all connected to the coax shield.
So the exterior braid over the outer coax insulation forms a capacitor with with the shield and the outer insulation serves as a dielectric of this capacitor.
OK. RF behaves strangely!!!
Well now, I have something to ponder.
Thank you.
N0QFT
Great idea, makes me wonder if for indoor use like just in the shack, that maybe a section of tin foil about a foot long or so wrapped around the coax and then wrap that with black tape might also work. Just a thought I guess but this looks like something to try, makes sense. Thanks for your videos as always, keep up the great work! 73..wd0akx
It probably would work that way but its best to do it near your antenna to keep rf out of the house/shack
I was wondering if adhesive metal foil would work.
@@PaulGriffith It's worth a try, I have some copper foil tape I bought to try to make a magnetic loop antenna, it would probably fold right around the coax. Always worth a try.
Kinky. I like it. Sound logic. Great explanation, Dave!
Why not add a groundwire to the second coax shield, or would that effectively short circuit the two shields by putting them at the same DC potential? RF is different.
If I put some ferrite beads on the coax I add inductive reactance to CMC path but if I put a braid outside the coax (connected to nothing) I don't add capacitive reactance as you claim.
Putting ferrite sleeves or beads on the coax works because it becomes a one turn inductor for CMC (no effects for the energy inside the cable). A conductor outside the coax and not connected to anything, can only... "disturb" CMC, nothing more.
It looks like a bad "sleeve" or "bazooka" balun: it is "bad" because his base it is not connected to the coax braid.
If you had some copper or aluminum tape, do you think that would work as well?
Very interesting Dave . How far down the feedline do you insert this shield ? 1/4 wave ? Or any random distance from the feed point ?
Look for "bazooka" or "sleeve" balun: nothing new!
Wrapping the coax in thin brass sim sould have the same effect, along with being a lot easier to fit to the cable.
Interesting idea. I wonder, is there a way to sniff for the escaping RF so you know exactly where to put the sleeve, or is just putting it near the antenna feed point good enough?
Look for "bazooka" or "sleeve" balun: nothing new!
TIG welder ,RFI from arch starter sets off igniters of kitchen gas stove and may have killed electronics of 2, yes 2 oven/microwave built-ins. Can I capture RFI with a grounded sheath of coax net between my TIG welder and kitchen. Reminds me of Wave Guides in my Russian radar van in Vietnam era.
Why not just use aluminum foil in that case?
What isn’t clear to me is whether or not that shield is “peeled back” on the coax for one foot, or if it’s just added over the insulating jacket after being removed from a different piece of coax. If it’s not peeled back from itself, is he saying that there is no physical connection between this external shield and the internal coax shield?
There is no physical connection between shields. It's coupled capacitively.
@@louisseaman8455 that helps explain things! Thanks
It is very clearly explained. Take a fresh piece of coax cable about 2 feet length revive only shield after sliting the black plastic sleeve from coax. Then slide it over your antenna cable at the feed point or at the shack near to the radio.
Is there a specific length of added external braid based upon frequency(s)??? It appears he has about 1 foot or so? Art W1SWL
Look for "bazooka" or "sleeve" balun: nothing new!
I don't get it. Why wouldn't the RFI voltage simply capacitively couple to the added braid and still be there both in the coax shield and the braid? What's holding the braid at ground potential?
I believe he meant the inner braid was at ground potential.
Hi Dave, another video with lots of confused statements..
At 1.25 you said clearly that the outside of the shield braid is at ground potential for DC. Absolutely right. And all things are happening only on the inside of the coax cable; again correct. Yes, all of the RF energy is contained in the dielectric material (or the space) between the outside of the centre conductor and the inside of the outer conductor.
A dipole is a balanced device.
The coaxial cable has three physical conductors including the outside of the shield braid.
If the dipole radiators are deployed correctly with symmetry and in free space then there are no chances of a CMC (Common Mode Current) to originate and start to flow on the outside of the coax shield at the interface of the coaxial cable and the dipole radiators.
But any asymmetry in length (of the radiators), proximity of the feeder cable to one of the arms of the dipole or any other RF absorbing structures near to one of the arms of the antenna or any other radiating antenna (in the same or harmonically related bands) in the nearby vicinity can all give rise to a CMC.
During reception the "third conductor" ie, the outside of the coax shield can bring in RF trash and it will raise the noise floor.
A CMC Choke can be some kind of reactance that should offer very high resistance to the CMC but should be allowing the differential mode current unimpeded. Usually we employ an inductive reactance to do the job to obtain a large value of reactance (largely the resistive part is preferred) over a wide frequency spectrum. Of course there's a compromised design for the entire HF spectrum.
The W2DU's design consists of inserting number of ferrite cores (sleeves) over the end of the coax cable near the feed point. Probably he used #73 or 75 Ni-Mn material, I don't know.
Next option is the "ugly BalUn" though not necessarily an ugly one to look (your suggestion of 9-10 turns of 9-10" in diameter). The drawback is that it might act like a "trap" for some frequency bands.
And ultimately the ferrite loaded CMC Choke seems the best option. One can refer to G3TXQ for very good details of the selection of core, winding number and materials and very importantly the pure resistive portion of the choking that would be obtained in a particular contraption.
Now let me come to the today's topic of using the "shield braid" of the coax cable to stop the CMC.
Just by adding or sleeving a foot or so of the shield braid over the coax cable feeder is not going to do anything at all; not only to the CMC or anything else whatever it may be.
In the VHF UHF bands you can add a 1/4 lambda long sleeve over the feeder cable with the open end of the cable soldered to the top end of the sleeve. This will act like a "skirt" over the feeder cable and will offer a high impedance to the flow of the CMC that tries to originate from the tip of the coax cable at the feed point.
But here you made some confusion as I could not understand if or not the suggested (by the questioner) sleeve is simply left over the coax cable or is electrically connected. But as I understand you (and the questioner) said the sleeve acts like a capacitor (how) in air (!?) and will offer capacitive reactance to the flow of CMC.
All your readers and I are totally confused. But you, this time, asked for comments hence I have made mine.
De VU2RZA
Supper Dr வெள்ளைக்காரன் யூ டுயூப் விட்டு ஓடப்போரான்
@@kannappanmpk unnecessarily negative observation
Many years ago some old boy explained to me this method of stopping RF coming back down the coax, over the years I have tried this braid experiment when installing many different types of antenna.. I can honestly state with all my installs adding the braid made no difference whatsoever.
@@kannappanmpk yes, adding a braid or any copper tube as just a sleeve along the length of the coaxial cable is not going to do anything. After all the coaxial cable is meant for this specific special feature; whatever that's happening inside the cable does not know what is going on there in the outside world. The main setback with the open wire line is that you can not have anything (especially metals or other RF absorbing structures) in close proximity (at least around a distance of 5-6 times the width of the line)
But having a sleeve of 1/4 lambda long connected at the top with the shield braid of the coaxial cable will effectively prevent the CMC flowing down the length. The "flower pot" antenna exploits this phenomenon. But then it's going to work for just one frequency or a narrow range of frequencies of the band. Hence it's used in the VHF UHF bands.
De VU2RZA
@@SunilAruldas !!??
De VU2RZA
Wait... don't you mean that you DON'T want feedback? Joking... see what I did there? Using coax shielding just might not give you feedback! Okay, I will try this on my next antenna experiment and let you know!
I'd run the coax through a length of 1/2" iron pipe, since copper braid doesn't pose any impedance to magnetic forces.
It's a simple "bazooka" or "sleeve" balun: nothing new and it work better using copper!
Especially odd since a typical braid is made of interwoven strands, so physically the inside and the outside of the coax shield are constantly exchanging places/roles. So there is no simple inside or outside, unless the signals are literally jumping from strand to strand every few millimeters and "square dancing" on them.
Most peculiar, unsatisfying, and illogical, Dave.