Measuring common mode current

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4LKR, Aug 5, 2018.

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  1. KC8VWM

    KC8VWM Ham Member QRZ Page

    A FS meter will give you an indication of something occurring on the feed line, but it won't provide you with a qualitative result. It will be like measuring a moving target.

    Maybe that's good enough, maybe not.
  2. W5DXP

    W5DXP Ham Member QRZ Page

    I hope we are talking about measuring common mode current on a coax line. Balanced line is difficult to measure. Here's a circuit from the internet:


    You don't have to use a snap-on toroid. I just slip a 10 turn toroid over the coax on one of my coax jumper cables and use that cable to take a relative measurement of common mode current. I check it out first with a dummy load. You can calibrate the output if you want to - I don't bother.

    The rectifier circuit couldn't be simpler. Anyone who wants a PC board on which to mount those components, send me an SASE to PO Box 604, Chandler, TX 75758, and I will mail one back to you.

    EasyCM.png If one doesn't need an absolute value of common mode current and a relative measurement will do, the meter can be replaced by a multi-meter which almost everyone already owns.

    It costs money and takes days to order and receive a meter from MFJ. In that time you can have one operational and already be using it.
    AJ6KZ, AE7US, KD0CAC and 1 other person like this.
  3. K0OKS

    K0OKS Premium Subscriber QRZ Page


    Interesting point about the common mode of course being a wave.

    Is there any advantage to positioning the choke at a particular point along the common mode waveform? In other words, would it perform better at the peak, or zero crossing, or what?
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Frankly, most people worry too much about common mode problems these days . . .

    What band is your dipole for? If you make sure your coax feeder is a quarter wavelength long, you don't need any kind of Balun, as you won't have any Common Mode current.

    Roger G3YRO
  5. W4LKR

    W4LKR Ham Member QRZ Page

    It will be for 40m. I had considered making it a segmented dipole for 40m and 20m, but probably just 40m to start with.

    I'll be operating QRP mostly for SOTA with this dipole, so I want as much of my meager 5W to radiate as possible.

    I didn't know a quarter-wave feedline would prevent common mode current on the coax shield. So in my case, you're saying a 10m feedline would do the same job as the balun?
  6. W4LKR

    W4LKR Ham Member QRZ Page

    Thanks for the diagram, that does look simpler than some of the other plans I've seen online.

    I'm not understanding a relative measurement vs absolute. How would I go about interpreting a relative reading vs an absolute reading?
  7. W4LKR

    W4LKR Ham Member QRZ Page

    Let me ask a different question...

    Palomar sells ferrite beads of varying sizes and mixes. Their website emphasizes that, in order to select the best mix and number of beads, you need to know the frequency of the common mode current and NOT the operating frequency of the equipment. All I know is the operating frequency (let's say 7.100 MHz). I having trouble googling for the right terminology here. Is there a formula or rule of thumb that tells me the frequency of the common mode current assuming I'm operating at 7.100 MHz ?

    I am also confused about the number of beads I need. I've read other threads where the OP is trying to add up enough beads for 1,000 or 1,500 ohms. Where does this number come from?
    Last edited: Aug 6, 2018
  8. W6JJZ

    W6JJZ Premium Subscriber QRZ Page

    It's not hard to measure common mode rf current on either coax or balanced lines. Ditto re absolute current as opposed to relative current.

    Cecil provided the basic circuit. Or for a similar circuit, see Owen Duffy at . If you wind the inductor on a split snap-on ferrite core (as Owen did), just put either the coax or both sides of the balanced line in the core's center and close the core. Specifically for balanced lines, if the core is one made for 1/2" coax, it will handle 300 ohm line easily. For wider spaced lines, squeeze the wires together to 1/2" where they go through the core, to fit within the core. That will create a very short characteristic impedance change in the line, but won't have much of an effect on the measurement at hf frequencies.

    As for absolute current measurements, you need only get an accurate rf voltage measurement across the resistor R1. The formula is I = N * V / R. (N is the number of turns on the core, and R is the value of R1.) If you use 10 turns and a 50 ohm resistor, the rf current is the rf voltage divided by 5. You can use a simple diode voltage probe to measure the peak rf voltage. I use 400,000 ohms in series with the diode probe's output to give rms voltage using a cheap (i.e., free) Harbor Freight multimeter with 1 megohm input impedance on the voltage ranges. (4 meg with a good multimeter with 10 meg input impedance.)

    Roy Lewallen described this (except for the multimeter addition) in a posting on the old Google antennas newsgroup in 2003, which you can find here:!s...adio.amateur.antenna/yItrLRTVOsE/wUgoSAsfzuYJ

    The insertion impedance of the ferrite core will create a bit of inaccuracy, as Roy explains (and one could take it into account, but it's generally small enough to ignore).

    In his article, Owen provides a useful calculator for deriving differential and common mode currents and phase relationships from three measurements on open line (one side, the other side, and both together through the core).

    In the common mode meters I've made, I've put the split core ferrite (#43 material) in the jaws of a plastic clamp attached with double-sided tape, and have mounted the detecting circuitry on a scrap of perfboard attached to the clamp. I've split the series resistance between the plus and minus leads to the multimeter, to help keep rf from the meter (verrrry sensitive to RF--but then it was free). I do all the measurements at QRP levels.

    As others have noted, the magnitude of the common mode current will vary along the length of the feedline.

    Charlie, W6JJZ
    AI3V likes this.
  9. K8JD

    K8JD Ham Member QRZ Page

    I tried a 1:1 BalUn
    What reason u have to add a balun? Have you got RFI problems in the house ? Unless a dipole is installed perfectly with NO surrounding objects you will have some common mode current, usually it will cause no problems.
    I would leave a working dipole alone unless there was a problem with RFI ETC. A dipole used in a portable and uncontrolled enviornment will usually work OK and get a signal out .

    When the BalUns became popular a decade ago I ordered the nice MFJ balun with a so239 connector on one end and a hanging hook eyebolt on the other.
    Replaced the old frayed wire on my 80M dipole and added the Balun. It made it easy to hoist back up into the tree and connect the coax quickly with an added PL259 plug.
  10. W5DXP

    W5DXP Ham Member QRZ Page

    No question, a common mode current choke works best where the common mode current is maximum (where the impedance is the lowest). If one doesn't know where the current maximum point is, two chokes 1/4WL apart work pretty well.

    You can calibrate your system by running a known RF current through the toroid. I have never bothered calibrating - I just reduce the relative reading as much as possible.

    The other frequencies may be interference from other sources or due to harmonics. What we hams are normally worried about is common mode current on our transmitted frequency. For this discussion, we probably don't need to worry about any other frequencies.
    Last edited: Aug 6, 2018
    K0OKS likes this.

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