End Fed OCF Antennas

End Fed OCF Antennas
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Off Center Fed (OCF) antennas are very convenient as they will allow multiple bands of operation with a simple wire antenna which is fed off center. For example, the Palomar Engineers 4010 OCF is a very popular antenna and consists of an off center fed dipole with sides of 55 feet and 11 feet and it will work all bands from 40-6 meters.The End Fed OCF uses the same dimensions but the wire section is 55 feet as in the OCF and the coax outer braid is used for the “other” short side of the OCF dipole. We place a choke on the coax at the 12 foot distance from the matching unit and we essentially have an OCF antenna with its great frequency range, but with only a 55 foot wire length instead of the 66 feet required by the regular OCF. You feed the antenna from one end which may be more convenient for some installations. This antenna will work on 40-30-20-17-15-12-10 and 6 meters. Most bands are under 3:1 SWR and easily tuned by your transceiver’s internal antenna tuner or external tuner used with an amplifier. (see BULLET-4006 for specific details and SWR curves).

These antennas are MUCH BETTER PERFORMERS than End Fed Half Wave antennas because they work the WARC bands, DO NOT HAVE A DANGEROUS HIGH VOLTAGE at the antenna connection and are SHORTER to fit in smaller spaces yet provide more band coverage up to 6 meters!
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Ferrite beads are used for RF decoupling and parasitic suppression. When placed over a wire, cable or coaxial cable they suppresses common mode current flowing on the wire or wire bundle or the outside of the coax shield but does not affect the signal inside the coax cable or wire (differential current).

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.

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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

FB56 FB102 FB103 Relative Sizes - Slip On Ferrite Beads

Left to right sizes: FB56 (ID = .25″), FB102 (ID=.505″), FB103 (ID=.77″)

Application note for using Slip On or Snap On ferrite beads for RFI/EMI suppression: How many chokes do I need?

SLIP ON Sizes for Heliax and Coax Cable

Shown below in the table are the SLIP ON and SNAP ON products for various popular Heliax and Coax cables. These are the correct size for making sleeve baluns.

Cascaded Common Mode Chokes/Line Isolators/1:1 Baluns

Here is a simple idea that is cost effective and simple to make using FB102-31 ferrite beads with RG-58 (1/4″) size cable. Take a FB102-31 bead and insert 4 turns of RG58 spreading them out 360 degrees to minimize the capacity effects. 4 turns give 16X the choking impedance of a single bead. So let say you are having a problem with 7 Mhz interference. Typcial choking impedance at 7Mhz for the FB102-31 per the chart above is around 70 ohms for a single turn through the center of the bead. With 4 turns we have 16 x 70 = 1120 ohms and with two chokes on the cable in series we would have 2240 ohms which is pretty good for $11 in beads. See picture below for example of how these beads are used. This winding technique is also applicable to FSB-1/2 Snap On Ferrite Split beads which will accommodate 3 turns of RG-58 coax which already has a connector attached.

Need even more choking impedance? Use a larger diameter toroid like a FT240-43 (best above 10 Mhz) or 31 mix(best below 10 Mhz) and put 12-14 turns of RG58 on the toroid as shown below. Our Super Choker is very similar to the toroid choke example shown below but uses multiple FT240 toroids and is useful for larger 1/2″ size cable like the RG-213, RG-8, etc. You can order the FB102 beads and the FT-240-31 from info on this page and the FT-240-43 on the Ferrite Toroid Cores page.

RG-58 Multi-Turn Chokes/Line Isolators Examples
RG58 4 turns on 2 FB102-31 for 2000 ohms choking impedance	RG58 12 turns on FT240-43 for 4000-6000 ohms choking impedance

Got a question about using ferrites? Drop us an email at Sales@Palomar-Engineers.com and we will be glad to answer your question.

Handy Wire Size reference for ferrite bead sizes needed

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