how many ferrite beads for RF choke on coax?
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!
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