Common Mode Coax Choke Baluns 

+Mike Samra I made a mistake by stating that the SWR was changed. It is not. What I can consistently measure is that the common mode current is so much lower that it becomes hard to measure it with the sliding current probe. With no choke at the antenna to mitigate the common mode current, the reactance (not the SWR) becomes much more pronounced along the length of the coax (standing wave). That I am certain of. As for the antenna tuner, it does seem to be a pre-antenna-tuner. I still use my antenna tuner as I have never seen that it hurt anything, as you and I discussed on the phone. With that said, there are articles that suggest using a coax choke at the antenna, one in the middle of the coax and another at the transmitter. How could that possible go wrong ?? :-) You know I am joking. Without some sort of common mode choke at the antenna, the length of the coax is going to become critical, to within a couple of feet on 20 meters from a horrific amount of reactance to not much reactance just a couple of feet one way or the other. I can measure this. It is just (irresponsible?) to connect an unbalanced line to a balanced antenna without some sort of a matching device (which could include tuning the length of the coax). It also (theoretically?) reduces the noise pickup on the coax into the receiver. That is the way I see it.

Exactly. If the length of feedline effects the antenba, you have a poorly matched antenna to begin with. His test are valid, but the whole thing becomes unnecessary if the antenna is tuned...

Hi Mike, I'm not sure where your roller inductor tuner is located that you mentioned in this very old post, but lets say it's located in your shack. Your antenna tuner really does not "tune your antenna", it just transforms the impedance seen on the end of your coax that connects to your transmitter to an impedance that your transmitter likes (lets say 50 ohms) and this allows your transmitter to dump (transfer) as much power as possible into the transmission line (because the transmitter is now happy), but you have not done anything to change the actual impedance that exists out at the end of the transmission line at the antenna feedpoint so the SWR is still the same on the transmission line between your tuner and your antenna feedpoint, etc. On the other hand when placing a choke on the coax shield right at the feedpoint of the antenna you have eliminated an additional path of current which is the outside shield of the coax, so you indeed alter the impedance that exists at the end of the coax where the antenna is located when using a choke at that location, and therefore you have indeed altered the SWR at the feedpoint of the antenna as well as anywhere you measure SWR along the feedline. Measured SWR in the shack should not change when feedline length changes except for a slight reduction in SWR as the feedline gets longer due to attenuation of the feedline, and if SWR does change (neglecting attenuation) when measured in the shack as feedline length changes this means the impedance out at the antenna feedpoint is changing and it's changing because the length of the outside of the shield changes which causes the antenna to look like a different length because the outside of the coax shield is acting as an additional antenna element (which you typically don't want it to be). By using a choke at the antenna feedpoint you eliminate the outside of the coax shield from being an additional element of your antenna, and when you eliminate this additional element the SWR no longer changes with a change in feedline length (except for changes observed due to feedline attenuation as mentioned above). Therefore having a choke on the outside shield of the coax at the antenna feedpoint is not the same as using an antenna tuner in the shack. The antenna tuner in the shack will not alter the antennas radiation pattern as an example, whereas having a choke on the coax at the antenna feedpoint can indeed alter the radiation pattern because it eliminates the coax shield as a radiating element. 73, Don (wd8dsb)

@@clems6989 Hi Control Man. The outside of the coax shield becomes an additional antenna element if you don't use a choke on the coax out near the antenna feedpoint, and therefore it can and does alter the impedance out at the antenna feedpoint, and the amount of change varies as you change the length of the feedline. Using a choke on the coax shield out near the antenna feedpoint eliminates the outside shield of the coax from becoming an additional antenna element. 73, Don (wd8dsb)

Yes, it needs to right at the antenna and if the length of the coax (from the shack end) is measured with a device like a TDR, the length of the coax choke will show up simply as the single length of coax. Basically, it helps keep the coax leading from the antenna to the shack from becoming part of the actual radiating (or receiving) antenna . You do not want the coax to radiate power from the transmitter or pick up signals for the receiver. We don't have much of an issue nowadays with what we used to call TVI in the VHF analog days of TV transmission but I dare say if we had included these simple coax chokes back then, we certainly would have had less interference to the analog TV channels. Also, some will disagree with me but the length of the coax choke should be cut for each HF band. There is not one single length of coax wound into a coil as a choke that will work properly on all HF bands from 3 to 30 MHz. Hope this helps.

@@ElPasoTubeAmps i can measure the tdr distance, so are you saying with the CMC it works like a tuned length without acting like part of the antenna? Or did i loose the bubble. Also does the placement apply to EFHW ?

​@@richardchandler9027 Maybe what I mention is not really important about the total length of the coax but just in case you did measure it, the CMC appears as just length in the coax on the TDR. As far as EFHW or dipole or in my case a rotatable half wave dipole, I don't think it matters what type of antenna is up in the air. It seems to be a simple method to prevent the coax from being part of the radiating system. The antenna should work independently of the coax which, together, becomes the antenna system. You want your antenna to be independent of the transmission line - the coax. Also, in my case of only operating 20M, I cut the length of the coax (thus the need for the TDR) to some whole number multiple half wave length so I can be assured what I measure at the shack end of the coax is a good representation of what is actually at the antenna. If you cut your coax to some multiple of half wave lengths, don't forget to include the Vf (velocity factor) of the coax as the coax will be shorter than a free-space wavelength. Old coax is 66% and I think the new stuff like LMR400 is something like 80% - better check all that out before using those numbers. Remember that the longest wavelength of all electromagnetic waves is longest in free space and is shorter in any other conducting material be it light in glass or water or radio waves in coax. That is the way I remember it and air and vacuum is so close it is not necessary for us building antennas to worry about the difference. 300,000,000 meters per second is a perfect number for the speed of electromagnetic waves in free space and the Vf is taken into consideration when using other materials, in our case usually 50 ohm coax.

That's a special Southern ham radio antenna sorta like the common dipole... :-)

The so-called, choke balun is a method of putting a few turns of coax transmission line, very close to where the coax line connects to the antenna up in the air, in an attempt to prevent RF from coming back down the coax line and the coax line becoming part of the radiating (and receiving) antenna system. The goal is to make the antenna "stand-alone" in the antenna "system" as a receiving and transmitting item. We want to isolate the coax line, as a radiating item, in the antenna system. The antenna "system" is the whole enterprise from the antenna proper to the connection on the back of the transmitter or receiver. If we allow the coax to radiate and receive signals and become part of the radiating system, we will likely get RF back in our radio room, possibly cause RF interference to our neighbors and the coax will pick up noise which will interfere with our receiving signal. I hope all of this makes some sense. There are better ways to "choke" off the coax line and keep it from radiating and receiving RF but the coil of wire adds only the cost of a few feet of coax whereas a legitimate balun could easily cost 80-100 dollars.

Thanks. I still use it as a mobile rig on 20M. Only delivers 25 watts to the antenna but does surprisingly well into a ham-stick antenna mounted on the top of my camper shell. 73

I will do the best I can at explaining it as I understand it. When I put a non-reactive Bird dummy load on the end of the coax I get no common mode current and the SWR is 1:1 and nothing can be measured so I did, as you say, deliberately put some reactance in the load so there would be a standing wave on the line and we could measure it. There would be no standing wave if there were no reflected power. With a load (antenna) that is not perfect, and none are, there is always some, reflected power and some level of standing wave on the line and if our instrument is sensitive enough we an measure it. As the little meter was stock, it took 100+ watts to drive the meter. That is why I modified it. We want to make the coax line any integer number of 1/2 wavelengths so that what we see at our transmitter (and receiver) is nothing more than what the antenna is. With any amount of reactance at the antenna, the SWR will vary along the line. You can measure your SWR and then add in, say, 1/4 wavelength of coax and measure it again and it will have changed unless the load (antenna) is perfect. Try it. A similar condition to standing wave is when we use a TDR to see if our transmission line is terminated in its characteristic impedance. If you have ever played with a TDR and a variable load, you can see the reflected pulse go positive with a load higher than the Z of the coax or low if the load is lower than the Z of the coax. When the load is exactly the Z of the coax, the pulse disappears just as the standing wave would if you were exciting the transmission line and load with a sine wave as we do with the transmitter. This is what we see with a pulse and is the easy way to measure the length of the coax. A modern day ethernet network analyzer works great. You can build your own pulse from an RC circuit that differentiates a square wave. I have posted at lease one video on the TDR you might like. I did a number of measurements with my GR bridge that I didn't show on the video and it proves that the complex Z at the transmitter end of the coax varies with the length of the coax. As for the exact length of the coax choke... The "books" say it should be resonate somewhat above the frequency you are operating at. If my memory serves me well, somewhere around 80 MHz or so - at least according to that author. I intentionally tried to make my mono-band coax choke resonate just above 20 meters and I believe it came out to be around 18 MHz. Some authors say the length doesn't matter - somewhere between 10-30 feet, scramble wound, etc... other authors say the exact positioning of the winding's is critical. Who knows? To tell you the truth, if the choke was exactly resonate at the operating frequency, I am not sure if it would appear as series resonate or parallel resonate. It would make a huge difference. In any case, not only do I use the minimal choke at the antenna, I also have the length of my coax at exactly 3 wave lengths. The aluminum tubing the antenna is made of broadens the frequency range of the antenna and it is about 1.5:1 well below and above the 20 meter band. It is still just a dipole but it make a huge difference being able to rotate it. I hope I am generally correct and this makes sense. If it doesn't and you know where I am in error, I truly want to know and learn. David WA4QGA

I might throw one more thing in here. I first tried two 50 ohm Bird loads parallel which gives me 25 ohms and into a 50 ohm coax I will have a SWR of 2:1. The Bird watt meter shows this. However, I can't explain it but, this 25+j0 impedance did not cause any common mode current that I could measure. Of course, I could have always made some dumb mistake at the moment but when I added the capacitance (reactance) I got the common mode current that I could measure.

Thanks so much for your response. Theory says that the swr is determined by the mismatch at the load, where it meets the coax feedline. (It is defined there, once and for all). And, except for losses in the coax, the swr is the same all along the line. This I am certain of. With a mismatched line the impedance as seen at the end of the coax changes. That is, the R and X varies, but repeats what was at the load every half wavelength. But there are lots of different R and X values that result in the same swr. I understand that some swr meters can't deal with the above and will report different values. Example: 25 ohms is 2:1 as you said. So is 100 ohms. A meter may read the 100 ohm case as 2:1, but have an error with the low impedance case. Large common mode on the line could also do the same thing. This MAY be why people try to get a 1/2 wavelength of cable, so the swr meter is seeing the Z at the antenna. If you have a good swr meter (a Bird? Although you have to do math to get swr for every reading) you should easily verify this. THEN, if you put in the reactance I am still stumped as to why this causes CM current but apparently it does. Now I am having second thoughts about your coil of coax moving the point of peak voltage of the CM along the line. It should definitely be counted in the transmission line length to determine the 1/2 wave distance from the feedpoint, as if you were to be phasing two antennas together you would need to account for the phase delay in the coil of coax. But in the case of the CM current and voltage the coil of coax may act differently since it is an inductance to the shield of the coax. Right? I think I will duplicate your test using a big clamp on toroid I have, to first build a current probe, and then go from there. Thank you.

I think the network TDR would include the choke length as I don't think the coil would affect that pulse signal in any significant way. We, for sure, both agree that each 1/2 wavelength everything repeats so we only see the antenna and not the effect the coax adds, at some particular frequency. If you try this, please let me know your results. It is always conceivable that I have made an error in my approach or measurement. The schematic can be seen on line for this device,. It consists of the ferrite core with 8-10 turns of #18 or #20 (not critical) wound on it with maybe a 100 ohm resistor across the winding. One side of the coil is ground and the other feeds a germanium diode with a cap back to ground on the output of the diode and this all goes to the meter - maybe with a pot in series with or across the meter. It's pretty simple but I couldn't find a ferrite core that would open up. I also said something wrong when I said resonate at about 80 MHz. That came out my brain from an article I recently read on "Improved anode parasitic suppression for modern amplifier tubes". You might like the article too. I think the idea for a "best performance" mono band choke is to have the resonate frequency somewhat above the operating frequency. How much is debatable. Per your comment, I am not sure the choke moved the point of peak voltage of the CM along the line. I was not very observant exactly where the peaks and troughs occurred. I don't believe it moved them. I think it just diminished the amplitude. I may have to watch my video and see if I can see if anything changed along the length of the coax with and without the choke. Seems like everything we do just raises more questions - but I like that and eventually we figure things out. One more thing - maybe I am thinking wrong, but - if the standing wave ratio is 1:1 there is no standing wave - isn't that right? A standing wave is caused by the mismatch between the coax and load. The reason I have real confidence in saying this is because I could not measure any current (standing wave) along the coax when the terminating load was, 50+j0. I can't explain why I could not measure CM current with a Z=25+j0 impedance. I assume I would get the same results with Z=100+j0 both giving me a 2:1 SWR. I could have had bad connections - who knows. Maybe I forgot to turn the transmitter on... :-) I agree that there are many different complex Z's that can result in the same SWR. There are some really nice online calculators that let you solve for the SWR for any value of R and X

Consider the symmetry (or asymmetry) of your two test loads. The paralleled dummy loads were near perfectly unbalanced. Additionally, the actual internal loads are completely shielded - no radiation. In the assembly of your dummy load & capacitor "antenna", do you have any exposed leads that could radiate and induce currents into the shield?