how many ferrite beads for RF choke on coax?

Discussion in 'General Technical Questions and Answers' started by KP3FT, Aug 26, 2009.

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  1. KP3FT

    KP3FT Ham Member QRZ Page

    Hi all,
    I need to choke off some RF that's apparently coming down the outside of our coax at our FM radio station (not a pirate station, it's legit, WJIH-LP in Oneonta, NY) . I'm not sure how many and what type of ferrite beads to use on the coax. The setup is:
    Frequency is 95.9 MHz. Two circular-polarized 50-ohm dipoles stacked 1/2 wavelength apart. Main feedline is Andrew 50-ohm hardline coax that terminates into a "T" connector. Two 75-ohm (Belden RG-11/U) cables branch from the "T" and go to the antennas. The outside diameter of the 75-ohm cables is a bit over 3/8" outside diameter, like 13/32".
    Is there a formula for determining the correct value of ferrite and the number of beads used? The SWR is very good, so the coax lengths, etc. are fine. I have noticed however a very large fluctuation in SWR (so large as to be unacceptable) when I move one of the short 75-ohm cables away from the boom and just a few inches down into the vertical plane of the antenna. Does this indicate RF on the shield of the coax? I ruled out bad connections, etc. so it's definitely the placement of the coax that causes the fluctuation. Thanks and 73!
    Jeff KP3FT
  2. ZL1UZM

    ZL1UZM Ham Member QRZ Page

    Just keep the coax out of the field of the antenna. Any metal (the braid of the coax) will detune the antenna.
    I don't think you have a problem unless I'm missing something.
  3. AG3Y

    AG3Y Guest

    Why would you want to move those feeder stubs away from the boom ?

    I don't recall ever seeing ferrite beads being used on a broadcast antenna system. Of course I could be wrong, and they could be being used all over the place, now, but I am inclined to believe that that is the situation. I'm sure that someone will chime in, if I am wrong.

    It sounds like you need to hire a consultant to give you the right answers. I would not be trusting any information that you would receive off the internet ( not even here ! ) to be definitive!

    Good luck ! 73, Jim
  4. KP3FT

    KP3FT Ham Member QRZ Page

    I noticed the effect only when experimenting to see why we keep getting changing SWR readings day to day, thinking it was perhaps a bad connection or something. It is presently fastened where it should be. I thought that it might be a clue as to whether or not the coax was getting RF on the outside of it or not. There's not really any essential difference in the commercial broadcast dipoles and any dipole we hams make. They all will either have RF coming back down the coax or not. I can't see anything in the design of the dipole to indicate it would have any 1:1 choke balun effect. There's nothing unusual about it; no capacitors, no gamma or T-match, no balun, just directly fed. These are manufactured and supplied by an FM broadcast antenna maker. Obviously I will also contact them to see what they say, but I have found nothing at all on the internet regarding FM broadcast and baluns. I very well may solve the problem by asking some hams, especially since the information I'm looking for does not involve changing antenna design or placement, but instead only adding ferrite beads which couldn't hurt anything even if it doesn't help.
    Last edited: Aug 26, 2009
  5. WF8O

    WF8O XML Subscriber QRZ Page

    FM Broadcast Antenna Problems!

    During that time, I certainly was able to utilize experience I have gleeaned as a ham. However, the problem you describe should be assessed by a competent tower maintenance crew. They will put a signal on the line and sweep the line to determine accurately just where the problem resides. Andrew coax is usually gas filled, often with nitrogen though sometimes with dried air. There could be a problem with contamination inside the coax. Also, the larger versions of coak are assembled in sections with "bullett" connectors and appropriate fittings. Sometimes those interconnections can become faulty even intermittent. I would call in a professional antenna service. I have never heard or seen the use of ferrites on the outher of the coax, but I am always willing to learn something new. Good luck.


    Mike Narges
  6. G4ALA

    G4ALA Ham Member QRZ Page

    Quarter Free Space Wavelength


    The purpose of the choke ferrites is to create inductance to all but differential signal mode in the coax. The inductance is created locally to the ferrite ring or clamp. Little local "strangulation points" are created. By spacing the choke ferrite appropriately, coax outer current resonance, and hence radiation, can be avoided.

    On the outside of coax, the wave travels at the speed of light. The wavelength of the wave is the same as the free space wavelength. Your signal gives a wavelength of 3.14 metres.

    To prevent common mode current antinodes (maxima) occuring, any spacing less than 1.57 metres would work, say 1.0 metres. A best solution would be every 79 cm (31 inches) which is more or less a quarter of the freespace wavelength.

    As to which type of ferrite, well, anything which works at 100MHz or higher.

    73 and good luck


  7. OM1ZZ

    OM1ZZ Ham Member QRZ Page

    the impedance X (ohm) of the ferrite beads stacked in series for that purpose should be about 1000-1500ohm at given frequency. Have a look on the datasheet for the ferrite bead and do find the impedance (ohm) of a single bead at the given frequency (at 100MHZ e.g. 100ohm for 1 turn or pass). The manufacturer has usualy a table ready (impedance in ohm vs. frequency for 1,2,3 turns via the bead). So you need 10 beads to get 1000ohm (linear equation). You may pass the coax more times (more turns) via the bead (but max 3times typically, as than the capacitive part prevails and the impedance drops) - the impedance goes with power2 of the impedance of 1 turn (3 turns = 900ohm for one 100ohm bead). You may stack e.g. 3 beads in series and use 3 turns of coax via the stack = (100x3)x(3^2)=2700 ohms at given frequency. The attenuation of the common mode signal will be e.g. for 50ohm coax aprox. 2700ohm/50ohm=50 which would be a good result.
    73 Igor.
  8. AG3Y

    AG3Y Guest

    "Main feedline is Andrew 50-ohm hardline coax that terminates into a "T" connector."

    He is NOT going to be looping that stuff through a ferrite donut!

    As I said before, he needs to hire a consultant for a professional installation. There is only so much that transfers over from amateur radio practice!

    73 Jim
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