Warm Snap On Ferrite Beads

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N0KTB, Jul 31, 2020.

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  1. N0KTB

    N0KTB Ham Member QRZ Page

    Good day, QRZ.

    My antenna experimentation continues. My 1/4-wl vertical has been reconfigured for 15m for the past few weeks, including Field Day. Made several 15m contacts during that weekend, but no activity on SSB since. Most of the work has been on FT8. But, it has given me a chance to use this antenna as a platform to test om 15m since I already have a 20m dipole.

    I went to Home Depot and bought a 500-ft roll of 14-ga insulated wire to use for different antenna builds. The next step for me was to try to get a 40m antenna set up in my small backyard. I already have an approximately 12-ft elevated vertical with four matching elevated radials. After doing some reading and research, I wanted to make sort of a jack-ed up inverted-L using the 15m vertical as the vertical section. (Note: I understand this doesn't follow standard wisdom for inverted-L dimensions, this is an experiment.) After spending sometime in 4NEC2, I found a length of wire that I could attach to the top of the copper pipe antenna and string over to the opposite side of the yard. Pulled out the new NanoVNA and my wire cutters. Trimmed and watched the NanoVNA until I got low-swr in the 40m band.

    Sure enough, go to the rig and it tunes up nicely without using the built-in tuner. Made several FT4/FT8 and some CW contacts. I was receiving several + db reports on digital modes from all across the country. Even noticed my signal made it overseas and into SA on PSKReporter. I know the gain is straight up due to the antenna being low, but I can confidently say I am radiating a decent signal.

    Last night, I was operating digital modes on 40m. After I was done, I went to take down the antenna so I could mow this weekend. I removed the coax with the 5 snap-on Mix 31 beads at the feedpoint and they were significantly warm! This was well after the sun went down, so I am assuming they were heated up due to RF.

    Question: What is the significance of the warm ferrite beads? In the mechanical engineering world, heat = inefficiency. And, in the electrical world current + resistance = heat. However, I know in the RF world there may not be a direct translation. My theory is that since this is probably a wildly unbalanced antenna, there is probably current that is going down the shield of the coax. If the beads are warm, does it mean the ferrite choke is working or does it mean that I am still getting unwanted current on the coax?

    Thanks for reading. Look forward to your responses.

    Adam - N0KTB
     
  2. AI3V

    AI3V Ham Member QRZ Page

    In the exotic land of EME (moonbounce) contacts on vhf amd above, there is a phenomenon known as "imagination enhanced contacts"

    In the commercial and military world, the idea of adding random chokes and ferrite beads to a simple Marconi antenna is completely unknown.

    Your mind is playing tricks on you.

    Needing software and a vna to adjust a Marconi is silly.

    Rege
     
  3. SPEEDSKATER

    SPEEDSKATER QRZ Member

    Maybe some of the return current, is not passing thru the ferrites on it's path back to it's source.
     
  4. K7JEM

    K7JEM Ham Member QRZ Page

    When you reconfigured the antenna, did you also lengthen the 4 radials? Each radial needs to be around 33 feet or so for a 40M antenna.
     
    K0UO, N8ZL and KD6RF like this.
  5. N0KTB

    N0KTB Ham Member QRZ Page

    Thanks for the replies!

    I probably gave too much detail. Really I just have a couple of questions that could easily been generalized.

    1. Is ferrite bead heating real or perceived? I've seen other posts about beads warming up.

    2. My understanding is that the ferrite creates a high impedance path so the current will not go on the coax shield. If the beads are warm, is that a good sign or a bad sign?

    If current is passing on the coax shield, to me it means the antenna has a higher impedance than the coax shield with the beads. Is the answer more beads or "fix" the antenna system and/or feeding method?

    Edit: "fix" = impedance transformer, move the feedpoint up/down the antenna, add radials, etc.

    My question is not so much about my specific scabbed together antenna. That was just context.

    Thanks for your help and replies.

    Adam - N0KTB
     
    Last edited: Aug 1, 2020
  6. SPEEDSKATER

    SPEEDSKATER QRZ Member

    If the center conductor Send current and all of the Return current, passes thru the ferrite, then the currents cancel out and the ferrite is happy.
    But of some of the Return current finds another path back to transmitter then the ferrite overheats.
    This is a Common mode, Differential mode thing.

    K9YC, Jim Brown writes:
    Total Field
    A ferrite sees the instantaneous algebraic sum of the fields produced by the currents in
    all the conductors that it surrounds (or that are wound through it). If, for example, a ferrite sur -
    rounds two conductors carrying currents that are equal and opposite, (for example, "hot" and "return"
    of a power circuit, loudspeaker line, or RF transmission line), the total field will be zero.
    When the currents are not equal and opposite, the ferrite sees the field resulting from that difference
    (that is, the common mode current). This means that we can use relatively small ferrites to
    suppress small common mode currents on paired cables that are carrying large differential currents
    as long as the ferrite surrounds all the conductors.
     
    N0YPD likes this.
  7. K4SAV

    K4SAV Ham Member QRZ Page

    It means that a lot of common mode current is flowing down the coax shield and the impedance of your beads is not enough to stop it. If you didn't change the length of the radials, then likely the coax shield is acting as the dominant radial. To fix the problem you can add some 40 meter radials or you an add a bunch more ferrites.

    Jerry, K4SAV
     
    K0UO, N8ZL, K7JOE and 4 others like this.
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    When ferrite beads used as a common mode choke heat up it means there's insufficient choking impedance, insufficient ferrite volume for the power being run or both. It's generally not a good thing particularly if they get very hot. If they get hot enough during sustained transmissions the choking impedance and resulting SWR back at the rig can change and if they really get hot you can exceed the Curie temperature and permanently damage the ferrite material.

    Specifically if there's excessive common mode current flowing on the coax then the impedance of the common mode path (the coax shield) is less than the impedance of the antenna's RF Return (i.e. radials/counterpoise). You can typically fix the problem either by improving the antenna's RF return, such as adding radials or by increasing the amount of common mode choking on the feed line. In a string of beads choke you can increase the choking impedance by using more series beads or using a different ferrite mix more appropriate for the frequencies of interest.

    You can typically do a better job of increasing common mode choking by using a multi turn choke through a sufficiently large toroid rather than a string of beads choke as the choking impedance roughly goes up with the number of turns squared. Increasing the size of the choke toroid also increases power handling and decreases noticeable warming as it's generally hard to choke ALL of the common mode current so there's often still some choke loss and related heating of the ferrite, a bit larger choke can help dissipate that loss heat better without risk of choke damage or shifting SWR at the rig.

    If you increase the choking impedance of the choke (either by adding more series beads or by going to a multi-turn choke) and the antenna system SWR changes dramatically then it's a good indication that your coax shield was serving as the main RF return and you really do need additional radials/counterpoise out at the antenna itself. IOW, if you successfully choke out the common mode on the coax and see a big increase of SWR back at the rig then your basic antenna system is lacking adequate RF return.
     
    K0UO, N8ZL, M7TTC and 3 others like this.
  9. AJ5J

    AJ5J Ham Member QRZ Page

    You can heat up your ferites all you want as long as you are kind to your ferrets---they will let you know in a hurry, otherwise!
    ferrets on coax.jpg
     
    K7KBN, WR2E and N0KTB like this.
  10. N0KTB

    N0KTB Ham Member QRZ Page

    Thank you for the replies. Ok, so the key is common mode current. I am reading up on common mode and trying to get an understanding. Obviously, the fact my ground radials are not 1/4-wl at 40m is probably the reason for the common mode current, like many of you have pointed out.

    I can understand the algebraic sum canceling out so the ferrite sees a net ZERO current. Makes sense. I think I am going to have a follow up question, but I need to take some time to re-read the posts and look at some of the sources that are available out there.

    I really appreciate your responses and input. Thank you!

    AJ5J, no ferrets or ferrites were harmed during these experiments!

    Adam - N0KTB
     
    N5PNZ, K7JOE and AJ5J like this.

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