Ferrite bead question

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KF7ES, Jan 1, 2019.

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  1. KF7ES

    KF7ES Ham Member QRZ Page

    I have not used ferrite beads on my coax in the past but I would like to start. I am moving my shack from a spare bedroom to a corner of the basement and moving all the antennas to the field behind the house. This seems like a good time to make some upgrades.

    I have four transmitting antennas:

    1) Cobweb (10-20) w/ 50' LMR-400 feed line, legal limit.
    2) 5/8 stainless whip w/ground plane (144Mhz) w/50' LMR-400 feed line, 80 watts max
    3) 10-160 ZeroFive vertical w/120 radials w/ 150' LMR-400 feed line, legal limit
    4) Fan dipole 40/80, 1/2 wave NVIS at 15' w/ 100' LMR-400 feed line, legal limit

    I'm not interested in making any air chokes, I'd rather just use ferrite beads if they will do the same job. I've read about them in the ARRL Handbook and on a few websites, but I'm still a little unsure of how many beads I should put on each feedline. Any feedback is greatly appreciated.

    Thank you.
    KU3X likes this.
  2. CX3CP

    CX3CP Ham Member QRZ Page

    Hi, happy new year !

    I don't use any ferrite on my feeder lines, I don't see what the practical purpose is. ... also can you figure out how to put a ferrite bead in a 7/8 " Andrew cellflex ??? such ferrite size !!!
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    PU2OZT, NL7W, AK5B and 1 other person like this.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    For HF antennas it's usually better to wind several turns through a larger ferrite core than to string a number of beads in a row. Either method can work but it can take a lot of ferrite beads to create the same common mode choking impedance of a larger core with multiple windings. The choking impedance basically goes up with the number of windings squared so it would take roughly 36 ferrite beads clamped on the coax to equal winding the coax 6 times though a larger core of the same material.

    In addition to the good information on Steve, G3TXQ's, site linked above (RIP Steve), this is a good guide to the reasons why common mode choking can be useful and how to build your own chokes: http://audiosystemsgroup.com/2018Cookbook.pdf

    BTW, if your Cobweb is of G3TXQ's design and uses a 1:4 balun then it already has a good feed point choke built in.

    And FWIW, I usually include current mode baluns and or common mode chokes in most antennas I've built in the past several decades but if you don't actually have common mode issues such as RFI in the shack or noise pickup from local electrical noise sources then you might not notice any difference at all when you choke your coax runs. It's pretty inexpensive insurance against those types of problems but don't be surprised if you choke all your coax feed lines and don't see any changes unless you're currently suffering from noticeable common mode problems.
    PU2OZT and AK5B like this.
  5. KU3X

    KU3X Ham Member QRZ Page

    If you don't know why one would use ferrite on coax, you need to try to get hold of the ARRL antenna book and start doing some research.

    Search the internet for, "Common Mode". That's a good starting point.

    You can put ferrite on hard line. It's easy. They make various sizes with inside diameters will will fit on almost any coax or hard line. They even make snap on beads if you don't want to slide beads on from the end. Even if you have 1-1/2 inch hard line, they make ferrite that fits it.
    I've have ferrite with an inside diameter of 2-1/2 inches ! It's not cheap, but you can get it.

    NL7W, AG5CK and AK5B like this.
  6. AA5CT

    AA5CT Ham Member QRZ Page

    This might be of interest or use - "Amateur Radio (G3TXQ) - Common-mode chokes"

    The following chart presents the results of impedance measurements made on a variety of common-mode choke implementations across the frequency range 1MHz to 30MHz. Amateur frequency allocations are indicated approximately by the vertical grey bands.

    PU2OZT and NL7W like this.
  7. K9GAS

    K9GAS Ham Member QRZ Page


    This is a good site. They have some good info and bead information, ferrite mix, how many you need etc.....
    I've built my slip over unun's using their products. They have good stuff... 73 Jerry
    PU2OZT and NL7W like this.
  8. KP4SX

    KP4SX Premium Subscriber QRZ Page

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  9. AK5B

    AK5B Ham Member QRZ Page

    Just make sure that you don't try to wrap any ferrets around your coax; they won't appreciate it one bit. Those little suckers have sharp teeth and can really bite!

    I think this is the main reason so many of us switched to the much more benign ferrite-core toroids a long time ago. Mixes 43 & 61 are your friends.
    N4FFL, KP4SX and AG5CK like this.
  10. NL7W

    NL7W Premium Subscriber QRZ Page

    Agreed. Their slip-on bead chokes work. I used the BA-8 common mode chokes on my vertical's coax cable. https://palomar-engineers.com/antenna-products/1-1-balun-kits
    KF5ES should use a choke on his vertical's coax feedline -- it needs that choking action more so than the other antennas. Personally, I would choke the vertical's feedline well beyond the radial field, if not twice. The first time near the antenna's feedpoint, and again well beyond the radial field, but before the where the feedline passes through the bldg penetration or entrance panel.
    Last edited: Jan 2, 2019

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