The simplest, most cost effective feed line choke is a ferrite ring installed as shown in the picture (at the antenna feed point) to stop feed line radiation or at the radio end of the coax to suppress common mode coax noise current on the coax braid. Improves antenna pattern by preventing your dipole from becoming a tripole Suppresses pesky coax braid radiation and RFI Reduces coax braid common mode current up to -30 dB ( 6 "S" units) on receive Use one choke for each antenna feed line Add a double female barrel connector to act as an "extension" of the existing coax. Magnetic Loop Users: use 1 JC-1-1500 at the antenna feed point to keep all of transmit power on antenna and another at radio end to remove common mode noise received on the coax braid. Need the ring only and not the jumper, select F240-31-1 as the part #. Question: Assuming the JC-1-1500 uses mix 31, why is the effective range 1.8-65MHz instead of 1-300MHz? Answer: Look at measurement chart to see the answer - it takes at least -12 dB of common mode rejection to see/hear a noticeable difference with and without choke. Yes the spec for mix 31 say it is good to 300 mhz but at that frequency the CMRR is very small and hasn't much of effect on common mode rejection. Also when you form a choke with coax your are actually creating a parallel tuned circuit at a certain resonant frequency determined by the coax capacitance of approximately 28 pf/foot and the inductance of the coil. The graph shown that the max CMRR is roughly -30 dB between 1-21 MHz and slightly less as you go higher in freq (the resonant freq of the JC-1-1500 is roughly 10 MHZ and broad banded by the use of the ferrite). Above 6 meters CMRR becomes low so as not to be very effective. If you want to use the choke on higher bands you need to change ferrite mix and the number of turns of the choke so the center resonant freq is higher. Ferrite are in reality frequency dependent resistors and the specs say they have a resistive component above 1 mHz and under 300 mHz but the resistance (or CMRR) has to be high enough to be effective. We publish the effective range so you know what to expect when used.
Simple Jumper Feed Line Choke
$19.95
For RFI common mode suppression use, mix 31 is effective from 1-300 Mhz, mix 43 works from 25-300 Mhz, Mix 61 is for 200-2000 Mhz, and mix 77 favors .1-50 MHz. These frequencies are those of the interfering signal to be eliminated, not the operating frequencies of the equipment to be protected. See Mix Selection for other applications.
NOW WITH QUANTITY PRICING FOR CLUB , VOLUME PURCHASERS!
Bead dimensions are shown in the picture below and in the table as A, B, C
Ferrite Bead Dimensions
Application Note: Use impedances for each bead at your desired frequency to select number of beads needed for desired Z. For example if you have a 50 ohm RG-8 coax cable and want a choking impedance of 500 ohms at 10 Mhz, you can check the FB102 column (since FB102 will pass RG-8 through its center ID) at 10Mhz and find that Mix 31 has a Z of 108 ohms/bead and Mix 43 has a Z of 91 ohms/bead. For 500 ohms we would need 5 mix 31 beads or 6 mix 43 beads to have greater than 500 ohms. Mix 31 has better response below 10 Mhz but above 10 Mhz mix 31 and mix 43 are very close. If you need more Z, just use more beads in series on the cable or use more windings thru a larger ID bead (e.g. USE FB102-31 for 4 turns of RG-8X for 16X increase in Z per bead – see pictures below for examples). Use Mix 61 for VHF and above for RFI/EMI suppression and below 30 Mhz for multi-ratio impedance transformers (baluns/ununs). See Mix Selection for other applications.
For extra large cables use the FB400-31 which has an inside diameter of 3 inches (76mm).
Frequency Range Comparison Chart
(FB56-xx size)
Multi-turn Common Mode Choking Impedance Increase Example (FB56-43)
Ferrite Bead relative size comparison
Left to right sizes: FB56 (ID = .25″), FB102 (ID=.505″), FB103 (ID=.77″)
SLIP ON Sizes for Heliax and Coax Cable
Cascaded Common Mode Chokes/Line Isolators/1:1 Baluns
 RG58 4 turns on 2 FB102-31 for 2000 ohms choking impedance |
 RG58 12 turns on FT240-43 for 4000-6000 ohms choking impedance |
Handy Wire Size reference for ferrite bead sizes needed
————————————————–/———————————————————
Connector already on cable? See Snap On Ferrite Beads for convenient installation on cables with connector already installed.