Off Center Fed Dipole Notes

Here are some typical OCF dipoles that are successful designs

160-6 meter OCF antenna layout

(using a double tuned stub for better 20 and 40 meter performance)

Here is the 160-6 meter SWR graph using 200 feet of RG213 feed line and an average height above ground of 35 feet:

80-10 meter OCF antenna layout

 

For the PAL-OCF8010 with 200 feet of RG213 feed line and an average height of 30 feet:

 

For the PAL8010OCF, the impedance is very well behaved in a limited, easy to match range throughout the 1-30 Mhz range and, in fact, that antenna also works very well on 6 and 2 meters with very good gain.  No antenna tuner is used and switching bands is as easy as changing the band on the radio.  Super convenient and quick – great for contest operation!

It is well worth the effort to use the 80-10 OCF if you have the room as SWR’s are less on many bands and you do get the use of 75/80 meters also.

 

 

40-10 m OCF antenna layout

On 40-20-17-15-12-10 meter models we use a resonance compensator to pick up the 15 meter band not present on many competing OCF antenna products. We have complete OCF antennas and also the impedance transformers and chokes as separate products (see below for various options with and without vertical radiators).

For the PAL-OCF4010 with 100 feet of RG213 feed line and an average height of 30 feet:

Here is a comparison of the above two graphs for SWR between the 4010 and 8010 OCF antennas:

 

 

 

 

No vertical radiator is used in the OCF antenna.  Note that if you want to use a vertical radiator (ala Carolina Windom), then you will need to use a 4:1 voltage balun (Model CB-4-5000DCV), rather than a current balun so that some of the RF fed to the antenna feed point will go down the feed line through the vertical element and give some vertical polarized output.  At the bottom of the vertical element you will need a feed line choke with high choking impedance (such as the CB-1-1500 or CB-1-5000 ) to stop the current from going down the rest of the coax back to your radio and causing RFI with the neighbors and getting into your radio room.  At the radio end of your coax line add a common mode noise filter (CMNF-500, -1500, or -5000) to remove the common mode noise picked up by the coax braid as near field radiation from the antenna and neighborhood devices.

 

Already have an OCF antenna but need a new or replacement common mode feed line choke to stop the flow of RF down the feed line and give better swr readings?  Use the CB-1-1500 or CB-1-5000 for higher choking Z and power up to 5KW.

 

 

INSTALLATION

For best results support center at 30+ feet and ends over 15 feet.  Angle of feed each side must be great than 45 degrees or 120 – 180 degrees total between each side preferred.  Any length of 50 ohm feed line ok but longer feed lines over 50 feet may show reduced SWR on some bands due to losses in feed line and soil conductivity, nearby objects, etc. Due to local ground conditions, antenna height and feed line length, SWR may vary from samples shown and an antenna tuner may be required or some bands to bring SWR at end of feed line to acceptable levels.

 

Separate OCF Components (you provide antenna wire and coax feed line)

WITHOUT Vertical radiator options:

CB-4-1500OCF – 4:1 impedance transformer + feed line choke in 1 enclosure – 1500 watts PEP

CB-4-3000OCF – 4:1 impedance transformer + feed line choke in 1 enclosure – 3000 watts PEP

CB-4-5000OCF 4:1 impedance transformer (5KW PEP) + 1:1 feed line choke (5KW PEP) – 6″ x 6″ x 4″ enclosure

CB-6-5000OCF 6:1 impedance transformer (5KW PEP) + 1:1 feed line choke (5KW PEP) – 6″ x 6″ x 4″ enclosure (for OCF over 70 feet at center)

Technical Note: You can not effectively use or add a vertical radiator to an OCF antenna that uses a 4:1 current balun.  If you want to use a vertical radiator you need to make the choice for a 4:1 voltage impedance transformer (see options below).

WITH vertical radiator options:

Here is a typical setup for an OCF Dipole with vertical radiator (note that 4:1 impedance transformer must be a Voltage Balun to force a portion of the antenna current down the radiator):

CB-4-1500OCFV  4:1 impedance transformer (1.5KW PEP – voltage balun) + CU-1-1500 1″1 feed line choke (1.5KW) at bottom of radiator coax line

CB-4-5000DCV – 4;1 voltage impedance transformer for vertical radiator OCF antennas.  Use CU-1-5000SO feed line choke at bottom of vertical radiator to stop RF radiation on feed line.