Feedline chokes for mobile operation at 10m, 6m, 2m, 70cm

Discussion in 'Mobile Radio Systems' started by N0IOP, Oct 17, 2019.

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  1. N0IOP

    N0IOP Ham Member QRZ Page

    Thanks, have mostly memorized that, and built one of his HF chokes before he redid his cookbook. I used 3/8" superflex Helixax and ran it through three sets of epoxied together stacks of ferrittes. It's at the base of a 43' wire that goes up the side of my house. Between the choke and the wire is a switchable matching coil for 80 and 160 meters. There are radials. I have one of those cheap Chinese VNAs on order and may very well sweep the common-mode side of that whole lashup to see how the chokes worked out as built.

    The idea is that it's supposed to cover everything from 160 to 15 meters. I have local antenna ordinances and a limited amount of space so it's probably the best I can do for those bands. I still have to put some insulators on my roof before I can use it, quite possibly a project for tomorrow morning.
  2. N0IOP

    N0IOP Ham Member QRZ Page

    Kindly start your own thread for this conversation. Please.
    NH7RO likes this.
  3. N0IOP

    N0IOP Ham Member QRZ Page

    Thank you, Alan, I had.

    I would invite further commentary specific to the particulars of my situation.
  4. N0IOP

    N0IOP Ham Member QRZ Page

    Perhaps my opening post was unclear.

    What I'm asking about, in particular, is the the applicability of placing a number of ferrite suppression beads over the coax (sometimes called a "bead balun"), or alternatively multiple turns of coax through a suppression core, to suppress common-mode current. The same devices are sometimes called "current baluns." I suppose since both the input and output are unbalanced they might also be "current ununs" or perhaps just "ununs" although this terminology is far from being in universal use.

    The technique is the same whether the output goes to a balanced load, such as a half-wave dipole, or an unbalanced one, such as a quarter-wave. In other words, the output could go to an NMO mount (in my case), or perhaps an N connector (unbalanced), or binding posts for ladder line leading to some sort of balanced load.

    As much as I appreciate precision in prose and word choice, I'm not sure it matters much. The meaningful distinction is that this is a common-mode choke rather than a transformer where the input and output each have their own windings.

    Hope that helps.
    NH7RO likes this.
  5. W5LZ

    W5LZ Ham Member QRZ Page

    N0IOP - What you describe with the ferrite beads on coax is a choke, it is not a balun or unun. It's purpose is exactly the same as a coaxial choke, it 'strips' CMC off the outer shield. It's been around for a very long time. It's also not used much because it takes quite a few of those 'beads'. Instead of those ferrite beads it can also be done using a PVC pipe (a long one) running the coax down through it and stuffing the space between coax and the inner pipe with steel wool. That thing is even longer/worse than the ferrite bead thingy. They are just chokes in different form. They do not change states (balanced/unbalanced).
  6. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Anytime there is a load imbalance at the end of the feed line (coax in this case), there will be RF current flowing back to the source on the outside of the coax shield. In other words, common mode current. There are several ways to "choke" off this current flow. The most efficient one is to use a ferrite core. Since we're speaking of HF here for the most part, the correct core material is mix 31.

    Technically, a common mode choke is indeed a 1:1 balun, and it does "change states", albeit not in the verbal sense—BAlance/UNbalance.

    Things that don't work well, are ugly baluns (coax wrapped around an air core); string baluns (a bunch of beads with the coax running through them); and steel wool packed inside a piece of PVC which does work at all!

    The question remains, why do we need on in a mobile scenario? The simple answer is the large amount of ground loss we can't do anything about. This fact causes an imbalance at the load end of the feed line—the antenna in this case. Steve?
  7. N0IOP

    N0IOP Ham Member QRZ Page

    Right. Another way to look at it is that when there is common-mode current on the feedline, there is some other path being taken by the RF from the transmitter to the base of the antenna. Historically this has mostly been perceived and addressed as a "grounding problem" that is then solved by bonding the transmitter to the frame and body of the vehicle, and then bonding parts of the vehicle together. Commercial LMR and public safety have mostly "solved" the problem by moving away from low band and reducing transmit power.

    I think if I were only interested in 2m and up I would skip the choke because the ground losses with a roof mount quarter wave aren't great enough to pose a problem. I don't have practical experience with 6m or 10m but have to believe that there will be more common mode current because the vehicle roof isn't large enough to provide a sufficient ground (or counterpoise, choose your preferred word) at these wavelengths.

    Beads vs wound forms, depends, I read the Fair Rite data sheets, yes material 31 is probably the best for my situation. With the 1.25" beads maybe 20 or 30 beads will provide a choking impedance of around 5000 ohms at the frequencies under discussion. Three loops through a larger core would provide somewhat less than half that with the VHF/UHF choking performance then being reduced by capacitive coupling between the turns. The beads, sized for typical mobile coax, are under $1 each, and result in a choke of comparable cost and performance but smaller.
  8. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    All agreed. One item often missed with respect to lots of beads over the outside of the coax, is bandwidth. It is similar to ugly baluns, in that one band can be easily covered. But, if you want WIDE bandwidth, a single ferrite core is best.
    NH7RO likes this.
  9. AI3V

    AI3V Ham Member QRZ Page

    It is a complete and utter waste of time and money.

    It us unheard of in the commercial world.

    K7JEM likes this.
  10. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    I beg to differ! I could explain, but why bother?

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